Clinical manifestations

Presentation and course

Botulism classically presents with prominent cranial nerve palsies, followed by symmetric, descending, “flaccid paralysis” in an afebrile patient with normal mentation. The prominent cranial nerve palsies are loss of visual accommodation, causing blurred vision, external ophthalmoparesis, dysarthria, dysphagia, and facial weakness (61; 75). Limbs become weak over 1 to 3 days, potentially leading to quadriplegia. Deep tendon reflexes are normal or decreased. Patients often develop a dry mouth and fixed pupils. Weakness of respiratory muscles may require intubation and mechanical ventilation in up to two-thirds of patients (79). At hospital admission, up to 40% of patients may have respiratory symptoms, and these patients show a shorter median incubation time (20). Using defined criteria, Rao and colleagues collected data on 332 cases (2002-2015) from the CDC for which botulism antitoxin was provided (79). Chatham Stephens’s systematic review of the literature (1932-2015) included 402 cases. The most common signs and symptoms in these two large series were dysphagia (65% to 86%), weakness (50% to 85%), blurred (36% to 80%) or double (49% to 76%) vision, dysarthria (78%), and ptosis (37%) (20; 79).

Autonomic symptoms result from impaired acetylcholine release at peripheral cholinergic nerve terminals and include blurred vision, dry mouth, reduced sweating, constipation, orthostatic hypotension (80). Topakian and colleagues found significant abnormal sudomotor function and heart rate variability/blood pressure response to standing in five of five patients with foodborne botulism (101). Postprandial hypotension was seen in 40% of foodborne botulism versus 3% of controls (67).

Aside from infant botulism (discussed elsewhere), foodborne and wound botulism are the most common forms. Other syndromes include adult intestinal toxemia botulism, iatrogenic botulism, and inhalation botulism.

Foodborne botulism. After ingestion of food containing botulinum toxin, the mean incubation period is 2 days, with a range from 0.5 to 6 days (61; 32). Gastrointestinal symptoms develop within 12 to 72 hours of ingestion: abdominal pain, nausea, and vomiting. In general, the longer the incubation period, the milder the symptoms.

In a review of cases of botulism types A and B in the United States (n=55), over 70% of patients complained of dysphagia, dry mouth, double vision, dysarthria, fatigue, and arm weakness, and, on examination, were alert with arm weakness and ptosis (50).

In general, intoxication from type A is more severe than from types B, E, or F, although the frequency of clinical signs and symptoms varies from outbreak to outbreak (108; 35; 40).

Wound botulism. Patients with wound botulism develop neurologic signs and symptoms similar to those in foodborne botulism. The incubation period ranges from 4 to 51 days (45). Most, but not all, wounds appear infected. Individuals with wound botulism usually lack the gastrointestinal prodrome seen in foodborne infections. A rise in wound botulism cases has been seen in people who inject drugs (70). “Skin or muscle popping,” whereby users inject contaminated heroin subcutaneously or into muscle, increases risk for botulism by creating an anaerobic environment of necrotic tissue in which BoNT can be readily formed and released. From 2001 to 2016, 291 of 353 wound botulism cases in the United States reported to the Centers for Disease Control and Prevention, 291 were from California (15). Two larger series reported 15 cases of wound botulism in Texas and nine cases in California in persons who inject drugs. The majority were men (83%) and 83% had skin abscesses. The most common symptoms were dysphagia, generalized weakness, dysarthria and visual problems; 88% required mechanical ventilation (78; 72). Black tar heroin, which is a cheaper, acetylated morphine, has been associated with increased injection site vein loss and soft tissue infections, thereby increasing abscess formation and clostridial spore proliferation/contamination (99).

Adult intestinal toxemia botulism or adult intestinal colonization botulism. This entity is rarely reported and, therefore, not easily classified. It can present with nausea, vomiting, abdominal pain, constipation, and abdominal distension, lasting for weeks to months. Patients with bowel surgery or anomalies, or recent use of antimicrobials, are at risk, although adult intestinal toxemia botulism has rarely been reported in healthy individuals (39). It has been proposed that an abnormal gastrointestinal tract and altered bacterial flora enable Clostridia organisms (C botulinum, C butyricum, C baratii) to colonize the gut (24). The in vivo toxin produced is then absorbed through the gut to produce botulism that may have a more protracted onset and duration. Recovery typically occurs over months. However, of the 25 cases Harris and colleagues collected from the literature, eight ultimately died (39). Relapse of symptoms after antitoxin treatment has been reported, presumably from continued gastrointestinal in vivo toxin production (96). Key observations from several cases can help confirm adult intestinal toxemia botulism. These include prolonged excretion (10 to 100+ days) of viable Clostridium spores or neurotoxin in the stool, a high level of botulinum neurotoxin in the feces compared to serum, and persistent recurrence of disease symptoms (39).

Iatrogenic botulism following medical or cosmetic administration. Both botulinum A and B toxins are widely used as therapeutic and cosmetic medicines. The 50% lethal dose for an adult is about 3000 U of botulinum toxin, and therapeutic botulinum doses rarely exceed 400 U (12). In 18 cases of iatrogenic botulism from botulinum neurotoxin type A (BoNT/A), age range from 16 to 74 years, all patients had been treated for underlying neurologic disorders (62). The most common symptoms were dysphagia (56%) and upper, greater than lower, limb weakness (39%). Twenty-eight percent of cases had diplopia; speaking difficulty; and face, tongue, or neck weakness, with two cases having spontaneous remission over months. Eighty-six botulism cases caused by cosmetic injection of BoNT were diagnosed (04), with symptoms of headache, dizziness, insomnia, fatigue, blurred vision, eye opening difficulty, slurred speech, and dysphagia occurring up to 36 days after BoNT injection, (mean 2nd to 6th day postinjection). The dose of BoNT was negatively related to latent period. All patients were treated with antitoxin and discharged within 20 days. Cases of severe botulism have been reported following cosmetic procedures with unlicensed, highly concentrated botulinum preparations (23). In the case of off-label intragastric use for weight loss, clinical courses were long (29).

Inhalational botulism. Botulism may be acquired via inhalation on the battlefield, following inhalation of cocaine, in laboratory technicians performing postmortem exams on BoNT-exposed animals or in farmers or construction workers (59; 87). The signs and symptoms of inhalational botulism are similar to those from ingestion of the toxin, except for the absence of gastrointestinal symptoms. They include difficulty swallowing, extremity weakness, speech problems, abnormal extraocular movements, and moderately dilated pupils. The incubation period for inhalational botulism is similar to that from ingestion of BoNT, ranging from 24 to 36 hours to several days, rarely as early as 4 to 12 hours post-exposure (26). Unusual outbreaks, large numbers of affected victims in different locations, or lack of a common food source should lead the physician to consider bioterrorism or biological warfare and promptly notify local health authorities.

Prognosis and complications

Recovery is dependent on dose and toxin type. Rapidly progressing cases are more likely to require intubation. BoNT types A and E typically lead to a faster onset than type B (48). Patients who died had a shorter reported median incubation period and a shorter reported median time from illness onset to hospital admission (20).

Therapy with equine antitoxins became available in the early 1970s. Antitoxin cannot neutralize toxin once it is bound to the nerve receptors, and, therefore, does not reverse weakness. Antitoxin prevents progression of the symptoms, but this may take up to 12 hours following administration. The clinical evidence for botulinum antitoxin efficacy in humans is based on retrospective analyses of small numbers of patients and animal studies. Although the evidence is limited, it is believed that early treatment, especially within 24 hours, is likely most effective in preventing progression of weakness. Although antitoxin can reduce the length of hospital stay and mechanical ventilation by several weeks, recovery of strength is over weeks to months, requiring development of new neuromuscular junctions (33). With adequate ventilatory assistance, tracheostomy, and improved intensive care support, botulism fatality rates are generally less than 10% (26). Significantly fewer deaths from botulism have been reported from the 1970s continuing up to 2015 (20). In literature series from 1980 to 2015, the case fatality ratio was 9% and highest in individuals over 70. Newer data from the Centers for Disease Control and Prevention suggest that roughly 5% of patients die (Centers for Disease Control and Prevention 2019).

Data from the Centers for Disease Control and Prevention cases from the 2019 National Botulism Surveillance Summary included one death among roughly 41 wound botulism cases, three deaths among 21 foodborne cases, and none in suspected adult intestinal colonization. Death usually occurs from respiratory failure due to respiratory muscle weakness or from pneumonia.

Complications of all forms of botulism are those of prolonged weakness, including aspiration pneumonia and decubitus ulcers. Patients noted the following persistent symptoms up to a year: exercise intolerance (60%), shortness of breath (60%), general weakness (55%), and dry mouth (51%). In a 1978 outbreak of foodborne botulism in 34 individuals, initial pulmonary involvement was documented in 81%, ventilatory failure in 33%, aspiration pneumonia in 26%, and death in 6% (64; 88). Another series reported normal muscle strength. Patients seldom return to full-time work for months.

Clinical vignette

Vignette 1. A 35-year-old male presented with 2 days of blurred and double vision, ptosis, and difficulty swallowing (104). Initial testing including MRI brain was normal, and he was sent home. Over days, his symptoms worsened, and he re-presented with slurred speech and facial weakness. Skin examination revealed no abscesses or open wounds. He was admitted to the hospital for possible stroke and treated for laryngitis.

Past medical history included methamphetamine use over many years. He had recently injected methamphetamine, mixed with water from a glass (sitting for an unknown time), intravenously, 36 hours before his symptoms began. The recent intravenous drug use raised suspicion for botulism.

Heptavalent botulinum antitoxin was provided by the Centers for Disease Control and Prevention and administered to the patient within 24 hours of admission to the hospital. He did not require ventilatory support, and his symptoms of double vision, ptosis, difficulty swallowing, and facial weakness gradually improved until hospital discharge 5 days after antitoxin administration. Serum obtained before antitoxin administration tested positive for BoNT type B by the BoNT Endopep-MS assay.

Vignette 2. A 54-year-old woman with a history of Crohn disease on immunosuppressant medications presented to an outside hospital with sudden-onset right ptosis, dysarthria, and shortness of breath, followed over days by diplopia, soft voice, proximal arm weakness, and difficulty swallowing. Symptoms were preceded by abdominal pain and distension. She was intubated 8 days after symptom onset.

On examination, she communicated by writing. Facial muscles, tongue, and neck were moderately weak, with complete ptosis and no diplopia. She had severe proximal, greater than distal, weakness in the upper, more than lower, extremities. Reflexes were mildly reduced. Although antibodies for myasthenia gravis were negative, she was treated presumptively for myasthenia gravis with a 5-day course of intravenous immunoglobulin. Repeat nerve conduction study showed small-amplitude motor responses, and EMG showed small, short duration motor unit action potentials with early recruitment, consistent with a myopathic pattern. She was treated with antitoxin and penicillin G 2 weeks into her course. Botulinum neurotoxin type A, both in serum and stool, was detected by mouse bioassay. One month later, she was weaned from tracheostomy, and after 2 months she ambulated with a walker and her swallowing study normalized. Because she was treated late in the course, this may have limited her response to botulism treatment. She also had underlying medical conditions requiring ongoing treatment and has not returned to her baseline. She had a transient recurrence of symptoms, and concern was raised for a repeat occurrence of botulism. She was not retreated and gradually stabilized.

Biological basis

Etiology and pathogenesis

Botulism is caused by the release of botulinum neurotoxins, which are polypeptides that are naturally produced by the spore-forming, strictly anaerobic, gram-positive bacillus, Clostridium botulinum, as well as certain strains of Clostridium baratii (toxin type F) and butyricum (toxin type E). C botulinum is found ubiquitously in soil and aquatic sediments (96; 58).

Botulinum toxin is a family of serologically closely related neurotoxins: A, B, C, D, E, F, G, and a variably classified type H or FA (30; 37). On a milligram-per-kilogram basis, botulinum toxin is the most potent biological toxin known. It has been estimated that 6 picograms or 5,000,000 molecules of botulinum toxin are sufficient to kill a mouse (42). The estimated human dose (assuming 70 kg weight) of BoNT/A lethal to 50% of a population that is exposed (LD50) based on animal studies is approximately 0.09 to 0.15 μg by intravenous administration, 0.7 to 0.9 μg by inhalation, and 70 μg by oral administration (26). Other reports include BoNT LD50 is between 0.1 and 500 ng/kg (100).

In foodborne botulism, the individual eats the preformed toxin. In infant botulism and adult intestinal toxemia botulism, C botulinum spores germinate in and produce toxin directly in the gastrointestinal tract (105). In wound botulism, anaerobic wound infections that contain C botulinum produce the toxin that is then systemically absorbed.

Pathophysiology. Botulinum neurotoxins must travel via the circulation to reach their final target, the neuromuscular junction (95; 47). In foodborne botulism, this begins in the gut. BoNT is rapidly inactivated by the acid and proteolytic activity of stomach fluids. It is released from bacteria as part of a noncovalent multimeric complex containing auxiliary proteins. This complex is formed by BoNTs and nontoxic neurotoxin-associated proteins. The nontoxic non-haemagglutinin component (NTNHA) plays an important role in protecting BoNTs from the gastrointestinal tract, and other subunits enable binding to the surface of intestinal cells for subsequent transcytosis of the neurotoxin complex from the apical membrane to the basolateral membrane of intestinal epithelium (100; 65). Once released, the BoNT progenitor complex sequesters E-cadherin in its monomeric form, blocking E-cadherin dimer formation, which weakens the transepithelial barrier. This process leads to bulk entry of neurotoxin into the bloodstream (100). The toxin then reaches the lymph and blood circulations. Although BoNT does not cross the blood-brain barrier (97), it does cross the blood-nerve barrier. In wound botulism, spores of C botulinum contaminate a wound, germinate, and produce botulinum neurotoxin in the wound, which then enters the circulation.

All BoNTs are protoxins that target and enter motor nerve terminals at neuromuscular junctions. BoNTs are synthesized as single-chain polypeptides with a molecular weight of approximately 150 kDa. Subsequent enzymatic cleavage produces about 50 kDa light chain (LC) and 100 kDa heavy chain (HC) polypeptides that are linked via a single disulfide bond. A short linker region between the light chain and high chain needs to be cleaved by either bacterial or host proteases to convert the protoxin into its active form. The light chain incorporates the catalytic zinc-dependent protease activity, a zinc atom bound to HEXXH motif, sheltered within the core of the structure (85). The high chain contains two functional 50 kDa domains: a C-terminal domain, which delivers the light chain into the cytosol, and an N-terminal domain that recognizes specific cell-surface receptors. Although all BoNT serotypes inhibit acetylcholine release, their mechanisms of action, specific toxicities, and durations of persistence within the nerve cell differ. Mechanisms of action are via different intracellular protein targets (there are seven different cleavage sites within the three protein targets). After BoNT binds to high-affinity presynaptic receptors, it is transported into the nerve cell through a receptor-mediated endocytosis within approximately 30 minutes. Acidification of the endosome triggers conformational changes of the toxins that lead to the N-terminal HC domain enabling the catalytic domain (LC) to translocate across the endosomal membrane into the cytosol of the peripheral cholinergic nerve cell. Once inside the cytosol, the catalytic domain blocks neurotransmitter acetylcholine release by selectively targeting and cleaving a specific set of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins, which mediate fusion of synaptic vesicles to the presynaptic membrane in neurons. The SNARE proteins and their sensitivities to BoNT are synaptosomal-associated protein of 25 kDa (SNAP-25; cleaved by BoNT/A, BoNT/C, BoNT/E62); syntaxin 1 (cleaved by BoNT/C); and synaptobrevin (also known as vesicle-associated membrane protein [VAMP]) (cleaved by BoNT/B, BoNT/D, BoNT/F, BoNT/G) (95; 26; 28; 106).

Recovery from botulism occurs by sprouting of new axon terminals and by the nerve cell body producing and transporting new SNARE proteins down to the synapse via axoplasmic flow with the regenerating axon sprouting and forming contacts at the original synaptic sites. The time of recovery depends on the regeneration speed of nerve terminals and presynaptic membranes, weeks to months (63).

Epidemiology

BoNT toxin types A, B, E, and F are the main toxins that affect humans. C botulinum strains that produce toxins A, B, and F are usually found in soil of geographic areas having low rainfall and moderate temperatures. In the United States, prior reports suggested type A primarily west of the Mississippi river, and type B east of the Mississippi River. Type E toxin-producing bacteria are found in marine life and sediment around fresh water, particularly in Alaska and the Great Lakes region (44).

Epidemiology varies by location, reporting method, and year. Using defined criteria for botulism, Rao and colleagues collected cases from the Centers for Disease Control and Prevention for which botulism antitoxin was provided between 2002 to 2015 (79). Of 323 cases greater than 12 months of age, 70% were male. Toxin type A caused botulism in 82% of cases, (half wound, half foodborne), type E in 7%, type B in 6%, and type F in 3%.

In a systematic review of the literature on foodborne and wound botulism, the largest represented age group in 402 cases was 18 to 29 years, and the smallest group was older than 70 years (20). Eighty-six percent were foodborne, and 14% were wound botulism cases. Thirty-seven percent were type A, 17% type B, 20% type E, and 1% type F (with 25% unknown). Forty-six percent of patients were intubated, and 66% received antitoxin.

In the 2019 national botulism surveillance summary, the Centers for Disease Control and Prevention found 201 laboratory-confirmed cases: 71% infant, 19% wound, 10% foodborne, and <1% other (probable adult intestinal colonization). More information can be accessed at the following website:https://www.cdc.gov/botulism/surv/2019/index.html. These percentages are similar to those from 2018.

Foodborne botulism. One systematic review noted 197 outbreaks between 1920 and 2014 (median number of cases per outbreak was 3, range 2 to 97), and 55% were in the United States. Most of these outbreaks were fewer than 11 cases and due to a point source, whereas commercial food-related outbreaks were larger. Toxin types A, B, E, and F were identified as the causative agent in 34%, 16%, 17%, and 1% of outbreaks, respectively (32). In the United States in 2019, cases were of the following toxin types: 68% A, 26% E, and 5% F, with a median age of 58 years and 58% male. More information can be found:https://www.cdc.gov/botulism/surv/2019/index.html. Le Bouquin and colleagues described foodborne outbreaks in France 2008 to 2018, which have remained relatively stable at 0.02 cases per 100,000 (60). Of the 100 outbreaks of human botulism during the 2008 to 2018 period, 90% of cases were foodborne, 10% of cases were infant, and <1% of cases were wound botulism. More than 40% of foodborne botulism cases come from improperly home-canned vegetables and meats that fail to destroy C botulinum spores (32). Anaerobic conditions, low acidity (pH> 4.6), low salt and sugar concentrations, and temperatures higher than 39°F (4°C) promote germination of C botulinum spores and botulinum toxin production (14).

Reports have identified numerous sources: home-canned vegetables, meats and fish; carrot juice; stink heads; seal oil; commercially packaged vegetables and potatoes; salt cured or fermented seafood, tofu, or roach; potato salad; brewing of pruno (Centers for Disease Control and Prevention 2014; 15; Centers for Disease Control and Prevention 2019; 81); and home canned meats and fish in a series in Iran (252 cases) and Ukraine (8641 cases, mostly type B) (53; 90). Hendrickx and colleagues reported six cases involving salt cured dried roach in Germany and Spain, type E (43).

Wound botulism. Areas where skin popping is practiced, such as the southwest United States, experience higher numbers of botulism due to black tar heroin use (78).

In the United States in 2019, all cases reported to the CDC occurred in persons who injected drugs; 66% reported black tar heroin use (Centers for Disease for Control and Prevention 2019). Ninety percent were toxin type A, and the remainder were toxin type B. The median age was 44 years (range: 25 to 66), and 90% were male. No deaths were reported (104). More information can be accessed at the following site:https://www.cdc.gov/botulism/surv/2019/index.html.

In Europe, botulism cases are often sporadic, suggesting that C botulinum spores might be present in the environment and drug supply at varying levels of contamination (70).

Other, including adult intestinal toxemia and iatrogenic. Eighty-seven cases of iatrogenic botulism were reported in 2023 across Europe with the majority confirmed by endopeptidase assays, as the longest case was 11 days out from treatment. This followed intragastric botulinum neurotoxin injections for weight reduction, most of which were performed in a clinic in Istanbul (29). Guru and colleagues identified 35 cases of intestinal toxemia botulism in the literature (36). Three species of clostridia (botulinum, baratii, and butyricum) were incriminated as the cause in 51%, 31%, and 9%, respectively. The one probable adult intestinal colonization case reported by the Centers for Disease Control and Prevention in 2019 was toxin type F. More information can be found at the following website:https://www.cdc.gov/botulism/surv/2019/index.html.

Prevention

Foodborne botulism occurs when C botulinum spores are placed in an anaerobic environment and allowed to germinate and produce the toxin. C botulinum spores are capable of surviving 100°C for at least 6 hours, but they are killed at temperatures of 120°C for 5 minutes (08). Home canning with a pressure cooker will kill C botulinum spores.

For prevention of wound botulism, appropriate early debridement and cleaning of the wounds can help, and of course avoidance of injection drug use.

At present, no method is known to prevent colonization of the gastrointestinal tract in those with underlying gastrointestinal abnormality.

Those who have a high risk of toxin exposure, such as botulism laboratory workers or certain military personnel, may require vaccination. A bivalent recombinant vaccine against neurotoxin A and B may be protective against the A and B subtypes (54).

Studies have looked at genetically inactivated vaccines, directed at different portions of the botulism neurotoxin, some at the HC and others at the LC-HN domains, Although a vaccine against the full-length genetically-inactivated holotoxin has the most protection against BoNT challenge, vaccine against the LC-Hn construct had nearly equal protection. A vaccine against the HC domain is also important as it is expected to neutralize toxin cell entry, and vaccines against these domains are easily produced in E Coli, but further studies are needed (34). Research into a vaccine against a novel receptor binding domain (RBD) as opposed to standard light or heavy chain is underway, as the RBD had strong efficacy in an animal poison model (94).

The modified toxin (M-BoNT/A1) elicits a potent immune response and can be a vaccine strategy against botulism (77).

Initial studies in Egypt demonstrated that the use of Citrus limon leaf extract in fishery products suppressed the growth of C botulinum (38). The use of C sporogenes as a conservative nonpathogenic surrogate for C botulinum (having higher UV-C resistance) suggested UV-C as an effective treatment method to inactivate C botulinum spores in a suspending medium (02).

Differential diagnosis

Confusing conditions

The differential diagnosis for botulism includes Guillain-Barré syndromes, especially Miller Fisher variant and myasthenia gravis, predominantly. Lambert-Eaton syndrome, diphtheritic polyneuropathy, tick paralysis, brainstem stroke, curare poisoning, poliomyelitis, West Nile virus, organophosphate intoxication, and nerve agent poisoning are also possibilities. These diagnoses should be seriously considered if the weakness fluctuates during the day, if examination shows objective sensory loss or central nervous system involvement, or if the cerebrospinal fluid shows pleocytosis or elevated total protein and ataxia is prominent. Helpful distinguishing tests include the “Tensilon” (this brand name was withdrawn from the market in 2015) test with intravenous injection of edrophonium chloride, if available (for myasthenia gravis), CSF exam and nerve conduction studies (for Guillain-Barré syndrome and poliomyelitis and West Nile virus), and potentially measurement of cholinesterase blood levels (for organophosphate or nerve agent intoxication).

Repetitive nerve stimulation may not distinguish botulism from myasthenia gravis or Lambert-Eaton syndrome, as any of these neuromuscular junction disorders can show compound muscle action potential amplitude decrement at low rates on repetitive nerve stimulation (2 to 3 Hz). With high rates (30 to 50 Hz) of repetitive nerve stimulation or exercise, botulism and Lambert-Eaton syndrome, both affecting the presynaptic portion of the neuromuscular junction, often show an increment in compound muscle action potential amplitude. However, high rates of stimulation are not always tolerated, and exercising a severely weakened muscle may be difficult. Identification of slowed nerve conduction speed on nerve conduction study points to Guillain-Barré syndrome and its variants.

Associated or underlying disorders

Complications
Nosocomial infections
Deep venous thrombosis from prolonged immobilization

Diagnostic workup

Although the following tests aid in the confirmatory diagnosis, strong clinical suspicion is key to providing prompt treatment.

Basic laboratory and CSF evaluations are usually normal. Testing for toxin or bacterium needs to be performed prior to any antitoxin treatment.

The definitive diagnosis of botulism is made by demonstrating the presence of botulinum toxin in serum, stool, or food, or by culturing C botulinum, C butyricum, or C baratii from stool or a wound.

Intestinal colonization botulism is confirmed by detection of botulinum toxin in serum and/or stool, or isolation of neurotoxigenic clostridia from the stool. Clostridial spores may be identified in the stool, but as they are generally not harmful to healthy adults, may not be adequate to confirm the diagnosis. Toxin or isolation of clostridia from gastric aspirate has also been reported (96). A diagnostic clue is some abnormality of the gastrointestinal tract, such as prior surgery, achlorhydria, Crohn disease, or recent antibiotic treatment.

BoNT detection methods include in vivo simulation assays, such as the hemidiaphragm assay and local injection assays, and numerous in vitro assays using immunological detection methods, endopeptidase assays, or a combination of the two such as the endopeptidase-mass spectrometry assay and cell-based assays. The Centers for Disease Control and Prevention perform mouse bioassay, mass spectrometry, and polymerase chain reaction on samples.

The gold standard test for the presence of botulinum toxin is the mouse bioassay (MBA). It is the only FDA-approved method for laboratory confirmation of botulism. Although seldom performed at most hospitals, this test is usually available at state reference laboratories or the Centers for Disease Control and Prevention. Only a biologic assay such as the mouse bioassay, or an appropriate cell-based assay, can selectively detect biologically active BoNTs, as well as all subtypes and novel BoNTs (74). The mouse bioassay involves mouse intraperitoneal inoculation (with or without antitoxin) from the patient's serum, stool, or food extracts, to determine whether the mouse becomes weak and dies over 96 hours. Results might be available within 24 hours of receipt of the specimen if the botulinum neurotoxin level is high; however, low levels of toxin that are sufficient to produce human illness might not produce signs in mice. Injection of specimens intraperitoneally into the mice with antitoxin is performed to determine the BoNT serotype (46). Isolation of clostridial organisms from stool, food samples, or wound material also strongly argues for the diagnosis. These samples must be cultured in special anaerobic enrichment broth media for 4 or more days and then media are tested for the presence of botulinum toxin (41). If toxin is present, the bacteria can then be specifically isolated. The mouse bioassay remains essential in the characterization of the many BoNT types and is the only assay that can examine the pharmacodynamic properties of BoNTs and the pathological or pharmaceutical effects after intoxication (74). Polymerase chain reaction (PCR) detects clostridial DNA, not the actual proteinaceous toxin. Although PCR can identify the Clostridium species, another method must be used to confirm that a strain produces toxin. Specimen types, quantities, storage and shipping are detailed by Rao and colleagues (80).

The mass spectrometry method for detecting botulinum neurotoxin (endopep-MS) is highly sensitive and specific and can differentiate among botulinum neurotoxin serotypes A, B, E, and F within several hours. This method is only available at the Centers for Disease Control and Prevention and a limited number of other public health laboratories in the United States. This method successfully identified nine of 12 cases in a 2023 iatrogenic series up to two weeks after exposure, and was more sensitive than MBA or endopeptidase suspension immunoassay (Endopep-SIA) (29). Endopep-MS assays require the use of serotype-specific antibodies conjugated to beads. After incubation with clinical specimens, the resulting mixture is analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The mass spectrometry detects any whole substrate and any cleaved fragments that result from incubation with the active toxin (46). MALDI-TOF MS is also a promising device for the rapid detection and identification of pathogenic bacteria. In a study of 33 Clostridium-like isolates recovered from various baby food and surveillance samples, VITEK MS system (microbial testing system for the identification of Gram-positive bacteria, a fully automated throughput platform that can provide identification in less than 5 hours) identified 28 of 33 Clostridium-like isolates with a high confidence value (99.9%); nucleotide sequencing of the 16S rRNA region identified all 33 Clostridium-like isolates (98). These allow rapid detection and identification of pathogenic bacteria.

Neuronal cell-based assays can reliably and quantitatively detect BoNTs with greater sensitivity and less variation than the mouse bioassay. They have the potential to replace the mouse bioassay for the purpose of potency determination of specific pharmaceutical BoNT formulations. Neuronal cell-based assays can be easily standardized and detect only biologically active BoNTs if used at physiologically relevant doses, which is a requirement for potency determination of pharmaceutical BoNTs (74).

Immunopolymerase chain reaction (Immuno-PCR) is an ELISA-type immunological test that uses polymerase chain reaction to increase amplification of the ELISA signal. The detection methodology relies on forming complexes of antigen and antibody, which is then bound to known DNA molecules instead of the normally used enzyme format. Once the binding has occurred to form the complex, the amplification of the DNA fragments, which are bound to the BoNT unique antibody, is implemented using traditional or real-time quantitative polymerase chain reaction (qPCR). Immuno-PCR detects botulinum neurotoxin serotype A to sensitivity levels similar to those associated with the mouse bioassay and can determine active toxin levels (46).

Laboratory confirmation of botulism acquired by inhalation may be limited as individuals may not develop an antibody response due to the small quantity of toxin protein required to cause botulism symptoms. If only toxin is inhaled without spores, then bacterial cultures will be negative. Toxin may be detectable in the nares for up to 24 hours postexposure. An enzyme-linked immunosorbent assay or polymerase chain reaction of a nasal mucosal swab may be considered a diagnostic tool for inhalational exposure to botulinum toxin (detects contaminating bacterial DNA), but these tests have not been validated (26).

Electrodiagnostic studies provide additional support for the diagnosis of botulism before toxin or bacterial results become available. Nerve conduction studies show small-amplitude compound muscle action potentials (CMAPs) with normal distal latencies, conduction velocities, and sensory nerve action potentials. As a presynaptic neuromuscular junction disorder, botulism shares electrodiagnostic features with Lambert-Eaton syndrome. Repetitive nerve stimulation at low rates (2 to 3 Hz) may show decrement in CMAP amplitude (76; 92). Facilitation (or increment) of CMAP amplitude is often seen after exercise or high-frequency (30 to 50 Hz) repetitive nerve stimulation. This occurs due to significant increase in calcium influx into the presynaptic terminal contributing to an increase in acetylcholine release. Compared to Lambert-Eaton syndrome, only about 60% of adult patients with botulism exhibit significant postexercise facilitation, and the amount of postexercise facilitation is typically less (30% to 100% vs. > 100%), but the duration of facilitation is longer, usually over 2 minutes (51) and may be absent in severely affected muscles. Nonspecific EMG changes include low-amplitude and short-duration motor unit action potentials (a myopathic pattern) (66). One explanation of this apparently incongruent finding is that blocking of neuromuscular transmission causes a large percentage of individual muscle fibers of a motor unit to fail to generate action potentials. The resultant motor unit action potential is diminished in amplitude and duration. Later in the clinical course, the EMG often demonstrates fibrillation potentials, a sign of muscle membrane instability. Single fiber electromyography may be a useful adjunct as it has very high sensitivity for neuromuscular junction abnormalities, demonstrating increased jitter and blocking. However, it is not specific (76). Nerve conduction study/EMG usually distinguishes between other causes of weakness, such as Guillain-Barré syndrome (or the Miller Fisher variant) where there is slowed conduction; myasthenia gravis, which shows normal baseline CMAP amplitudes, CMAP amplitude decrement on low rates of repetitive stimulation especially following exercise; poliomyelitis where there is abundant active denervation; and tick paralysis, which may show reduced CMAP amplitudes and mildly prolonged motor and sensory latencies during the paralytic phase, and if prolonged weakness fibrillation potentials (92; 56). Normal electrodiagnostic studies do not exclude the diagnosis of botulism, particularly early in the disease.

Witoonpanich advocates for a post-10-second exercise single supramaximal stimulation to demonstrate post-exercise facilitation (using CMAP increment of at least 25% as confirmative), as seen in 95% of the 63 patients studied during an outbreak of foodborne botulism (107).

Following an outbreak of foodborne botulism, a single center evaluation of 18 males in acute, early post-acute, and late post-acute phases, respectively found reduced CMAP amplitudes in 100%, 20%, and 17% of patients; abnormal postexercise CMAP facilitation in 100%, 40%, and 0% of patients; and pathological incremental responses to high-frequency RNS in 80%, 50%, and 8% of patients (09). On needle EMG, small MUAPs were found in 100% of patients in the acute and early post-acute phases and in 50% of patients in the late post-acute phase.

Management

Treatment comprises supportive care, botulinum antitoxin, and potentially antibiotics and should begin as soon as the diagnosis is suspected (25; 86; 22). Patients with all forms of botulism require medical attention in an intensive care unit. They should be monitored carefully, particularly in the early stages of the disease because weakness may progress rapidly. The patient should undergo frequent respiratory monitoring and neurologic examinations for adequacy of gag and cough reflexes, ability to swallow and handle oropharyngeal secretions, and limb strength.

Monitoring for impending neuromuscular respiratory failure includes serial measurements with handheld bedside spirometry for forced vital capacity or negative inspiratory force (NIF). A normal vital capacity is approximately 65 mL/kg. A poor cough occurs at around 30 mL/kg. Typically, a vital capacity of less than 20 mL/kg ideal body weight , NIF less than -25 cm H20, or a rapidly declining vital capacity on serial measures are indications for elective endotracheal intubation (Chang 2019). Oxygen desaturation or hypoxemia with absolute or relative hypercarbia on arterial blood gas measurements does not develop until just before frank respiratory failure.

In the United States, the Centers for Disease Control and Prevention recommends administration of heptavalent (types A to G) botulinum antitoxin (HBAT) as soon as possible. Trivalent and tetravalent antitoxin therapies are available in other countries (84). Symptoms may often progress for up to 12 hours after antitoxin administration before an effect is observed. The equine polyclonal antibody fragments F(ab)2 of HBAT act against BoNT A through G. In circulation the polyclonal antibody fragments bind to free BoNT. This prevents the BoNT from interacting with anchorage sites and protein receptors on the cholinergic nerve endings. In turn, this prevents BoNT internalization into the target cells (U.S. Food & Drug Administration 2023). Heptavalent equine antitoxin is available through the Centers for Disease Control and Prevention, the Alaska Division of Public Health, and the California Department of Public Health. Antitoxin for infants is available from the California Department of Public Health (17). Fagan and coworkers found that toxin was detected in serum for up to 12 days after ingestion (31). This finding supports administration of an antitoxin up to 12 days after toxin ingestion yet does not indicate a time when toxin is clearly no longer present.

Each vial of heptavalent BAT contains varying units of antitoxin against each botulinum type and sufficiently neutralizes circulating toxin found in all forms of botulism. The half-life of the circulating antitoxin is 5 to 7 days. The Centers for Disease Control and Prevention recommend administration of one vial (01; 80). The antitoxin vial should be diluted 1:10 in 0.9% saline solution and administered by slow intravenous infusion 0.5 mL/min and gradually increased to a maximum 2 mL/min in adults. Administration of equine antitoxin may produce allergic reactions, and the antitoxin should not be given to individuals with known allergies to equine products. Diphenhydramine and epinephrine should be immediately available during the antitoxin administration for possible allergic reaction, which occurs in 1% to 2%.

To obtain the antitoxin, one should contact local and state health departments. In the United States, if local health departments are unavailable, the Centers for Disease Control and Prevention can be telephoned at (404) 639-2206 or at the emergency (during or after hours): 1(770) 488-7100.

As the use of antibiotics in botulism has not been established (13), antibiotics have no current role in foodborne, iatrogenic, or inhalational botulism except to treat secondary bacterial infections. Patients with wound botulism should have the wound surgically debrided and appropriate anaerobic cultures obtained. If antibiotics are required for secondary infections, they should be ideally given after administration of botulinum antitoxin is administered, as lysing C botulinum with the antibiotic will release more toxin. Additionally, individuals with botulism may worsen clinically if they receive aminoglycoside antibiotics, which can potentially increase presynaptic neuromuscular blockade (55).

Rarely, individuals have responded to cholinesterase inhibitors (22).

Attempting to remove unabsorbed botulinum toxin from the gut is controversial. Most studies have shown botulinum toxin absorption occurs mainly in the upper intestine, and it is unclear how much toxin is absorbed in the colon. If the food suspected of containing botulinum toxin was recently consumed and the patient does not have an ileus, use of cathartics (not containing magnesium, as it might worsen the neuromuscular blockade) or enemas can be considered as a way of removing non-absorbed toxin from the gut (93). Feeding should be done cautiously, usually through a nasogastric tube, when bowel sounds are present.

There are no guidelines for postexposure prophylaxis. Kodihalli and colleagues described the therapeutic efficacy of equine antitoxin in Rhesus monkeys as a postexposure prophylaxis treatment (57). The antitoxin delayed the progression of signs (muscular weakness, respiratory distress, oral/nasal discharge), and reduced the severity of the disease and subsequent death.

Outcomes

The clinical evidence for botulinum antitoxin efficacy in humans is based on retrospective analyses of small numbers of patients and animal studies. Guinea pigs intoxicated with serotypes A, B, C, D, E, F, or G treated with antitoxin had enhanced survival compared to placebo (p< 0.0001) and arrested or mitigated progression of clinical signs of botulism (06). Similar efficacy has been shown in Rhesus macaques (57) and, for some serotypes, in a rabbit spirometry model (27). In treated patients, the antitoxin was safe and provided clinical benefit. Improvements in any botulism sign or symptom were detected a median of 2.4 days, and in muscle strength a median of 4.8 days, after treatment (110). Systematic reviews showed timely administration of antitoxin reduces mortality (69) and resulted in 86% (n = 229/267) survival, compared with 53% (n = 72/135) who did not receive untreated (P < .0001) (20). Mortality rates vary according to mechanism and report. A French review noted 1% to 2% mortality between 1991 and 2016 with food-borne illness (83).

The time to full recovery varies by the type of toxin and the intensity of the exposure. Botulinum type A is the most toxic, whereas type E is the least. Patients requiring ventilator assistance often report that they continue to have marked fatigue up to a year or more (64). In experimental animal studies and human volunteers who were given IM injections of the toxin to paralyze specific muscles, recovery of normal muscle function required 12 months (52). A major factor contributing to the persistence of intoxication is the long half-life of the catalytic light chain, which remains enzymatically active months after entry into cells and evades breakdown by means of a deubiquitinating enzyme. The development of specific deubiquitinating inhibitors could be useful (21).

Incidence of hypersensitivity, serum sickness, and anaphylaxis occurred in approximately 3.5%, 0.1%, and 1.0%, respectively, of BAT product-exposed individuals in a 15-year systematic review (71). A joint task force report by the major infectious disease public health departments concluded that skin testing prior to administration of HBAT is not indicated in a mass botulinum toxin exposure or for individual cases (89). The IgG-Fc (receptor-binding fragment) is mostly responsible for hypersensitivity reactions and is commonly removed from heterogenous preparations.

Treatments on the horizon. Research studies for identification and use of non-antitoxin treatment strategies have led to other proposed treatments (not approved by the FDA), including guanidine hydrochloride, 3,4-diaminopyridine, and plasma exchange, but further clinical trials are needed to investigate their usefulness (18; 07). However, a systematic review and metaanalysis of case reports and treatments with antitoxins from 1923 to 2016 suggested that therapeutic agents other than antitoxin offered no clear benefit (69). Single-chain antibodies have been shown to be efficacious but are limited by their shorter lives in circulation. Huang and colleagues engineered mouse red cells to express these single-domain antibodies (VHHs) and described the protective properties to lethal doses of the toxin (49). Some relatively newer agents may offer therapeutic relevance, such as quinolinol inhibitors, which inhibit SNAP-25 cleavage, and picolinic acids, which inhibit the beta-exocite, a largely unexplored site on the LC that holds therapeutic relevance for botulism treatment (11; 10). The identification of acoric acid 1 as a potential scaffold against BoNT suggests that herbal treatment may play an adjunctive role (109). However, the relevance in a clinical setting remains to be seen.

New-generation therapies are based on a combination of humanized monoclonal antibodies (mAbs), which exhibit improved safety and pharmacokinetics. In a mouse model, homologous Fc increased the potency of the antibotox 1 mAbs and increased efficacy, arguing for development of human monoclonal antibodies as treatment (102). To develop recombinant antibodies that neutralize botulinum neurotoxins, targeting heavy and light chains (HC and LC) of the BoNT serotypes A, B, and E macaques were immunized, followed by the generation of immune phage-display libraries. Antibodies were selected and challenged with toxins from Clostridium strains. Protective antibody combinations against BoNT/A and BoNT/B were evident, and for BoNT/E, the anti-LC antibody alone was found highly protective (82) and may be suitable for further development. Significant progress has been made using mAbs development, but further progress is still required to address the BoNTs great variability (83). Two research teams have developed modified forms of botulinum toxin that join botulinum toxin endocytosis to motor nerve terminals and inactivate the toxin. One group made three genetic changes to botulinum toxin that prevented it from slicing SNARE proteins. The other team combined components of a disease-causing form with a related botulinum toxin that does not invade nerves. Both teams linked their engineered toxin to a nanobody, which can inactivate the toxin. Compared with full-size antibodies, nanobodies can be engineered to reach and keep their structure within specific targets in cells. This resulted in more rapid recovery, -hours in mice, better recovery, and survival to 10 days in monkeys, whereas those untreated died in 3.5 days (91).

Investigations into the use of antibiotic therapy have shown some promise. C botulinum type A and B strains and neurotoxigenic C baratii strains from California were highly susceptible to penicillin, cephalosporins, moxifloxacin, and trimethoprim-sulfamethoxazole (05). Yutani and colleagues investigated the effects of antibiotics on the viability of and toxin production by C botulinum group I strains in the stationary phase by measuring the minimal growth inhibitory concentrations (MICs) of these antibiotics against C botulinum cultivated from spores in anaerobic culture. Metronidazole was shown to rapidly reduce the viability of C botulinum strains, without enhancing BoNT production and may serve as a future alternative treatment (111).

Special considerations

Pregnancy

Limited animal studies suggest that botulinum toxin, type A or B, does not cross the placenta.

Treatment of pregnant females includes the same management protocol of using antitoxin with supportive management. A systematic review of botulism in pregnancy (with case reports of 16 patients) showed no neonatal losses or cases of congenital botulism, and there were no adverse maternal or neonatal effects with botulinum antitoxin therapy among 11 treated patients (03).

Anesthesia

Any agent that can cause paralysis should be used after careful consideration and with appropriate monitoring. Succinylcholine induces sustained depolarization of motor endplate at the myoneuronal junction. The nondepolarizing agents, such as rocuronium, vecuronium, and pancuronium, block acetylcholine from binding to motor endplate receptors (80).