Botulism in Animals

Botulism is a neuroparalytic condition caused by botulinum neurotoxins (BoNTs) produced by C botulinum. This spore-forming, anaerobic, gram-positive bacterium proliferates in decomposing animal tissue and sometimes in plant material.

In most cases, botulism results from the ingestion of preformed botulinum toxin in contaminated feed. Less commonly, botulism develops when botulinum toxin is elaborated in vivo by C botulinum, resulting in toxemia, as occurs in wound botulism or shaker foal syndrome.

Seven serotypes of neuroparalytic botulinum toxins (A–G) are recognized. In addition, several novel BoNT molecules have been described (1, 2, 3).

BoNTs result in flaccid paralysis by blocking acetylcholine release at neuromuscular junctions. Types A, B, E, and, rarely, F are the most important in humans. Types C and D are the most common culprits of animal botulism; however, A, B, and E have also been reported in animals.

The strains of C botulinum that cause disease in animals can also produce mosaic or mixed toxin types C/D and D/C, meaning that the genes that encode the toxins are carried by bacteriophages on mobile genetic elements. In addition to the ingestion of toxin, another possible route of exposure is in vivo production of BoNTs within the GI tract secondary to the ingestion of botulinum spores—referred to as toxicoinfectious botulism.

Both means of exposure to botulin toxins are likely to occur in botulism outbreaks in poultry. In botulism outbreaks in cattle, a biphasic distribution of deaths is typical—the first phase corresponding to cases due to the ingestion of preformed toxin, the second phase due to in vivo production via the toxicoinfectious pathway a few weeks later.

In shaker foal syndrome, young horses develop botulism after ingesting C botulinum spores from soil that then germinate in the immature digestive tract and release BoNTs that spread hematogenously. Toxicoinfection is also suggested as a cause of equine grass sickness (equine dysautonomia); however, this has not yet been confirmed.

Wound botulism in animals can occur when C botulinum infects anaerobic wounds, releasing BoNTs. Wound botulism has been linked to puncture wounds, castration sites, injection site abscesses, and, in foals, umbilical/urachal abscesses.

Humans and dogs show relative resistance to botulism, cattle are more susceptible, and horses might be up to 10,000 times more susceptible to the botulinum toxin than are mice, making the mouse bioassay a poor choice for laboratory confirmation in this species.

BoNT type C/D causes disease in birds in many parts of the world. Although birds seem to be quite resistant to BoNT type D, some D or D/C outbreaks have been reported in turkey flocks. Botulism outbreaks due to BoNT type E have also been reported in birds (and fish). Waterbirds and fish-eating birds have been impacted by botulism type C in many parts of the world, sometimes with massive mortality rates reported (4, 5, 6).

Botulism can occur in a wide range of poultry production systems, including those for turkeys, broiler chickens, pheasants, layer hens, ducks, geese, and guinea fowl. In contrast, avian scavengers seem resistant to the effects of BoNTs.

Botulism in cattle occurs worldwide. Previous reports implicated BoNT type D; however, more recent reports show predominantly D/C detections (7), raising suspicions that this discrepancy might be due to improved capabilities for distinguishing the toxin types with newer molecular tools rather than to a difference in the predominant BoNT. Type B has also been reported in cattle in the Netherlands and in the US, and an outbreak of botulism type A has also been diagnosed in dairy cattle in the US (8, 9).

Horses are very susceptible to BoNTs, to the extent that they are likely affected by less than the minimum lethal dose for a single mouse.

Predominant BoNT types vary by region:

• Type B toxin is responsible for the majority of equine botulism cases diagnosed in the US and is endemic in the mid-Atlantic region and Kentucky, type A predominates in equine cases west of the Mississippi River, and type C is uncommon.

• In Europe, equine botulism cases are typically caused by types C and D; in the Netherlands and Italy, however, type B is more common.

• Type D predominates in other areas, such as South America and South Africa.

To the author's knowledge, there is only one case report of laboratory-confirmed botulism in a donkey (type B, in Italy) (10), indicating that donkeys likely are more resistant to BoNTs or are less frequently exposed because of differences in their feeding behavior, or that donkey botulism cases are less likely to be reported in the literature.

BoNT type C is the most common cause of botulism outbreaks in mink, foxes, and ferrets raised for their fur; however, types A and E have been reported in rare instances. Large botulism outbreaks have been reported on fox breeding farms in Finland (7).

Although C botulinum type E spores are ingested by healthy fish without any ill effect, BoNT type E is the cause of botulism described in some fish populations, including coho salmon and catfish (11). The data are scarce, so the full impact of BoNT type E on wild fish populations and its prevalence in the fish industry is not well known. Fish-eating waterfowl are also susceptible to botulism from type E.

Pigs and cats seem relatively resistant to botulism, and although dogs are susceptible, very few reports of dog cases are available (7). Botulism can occur in sheep and goats, as in cattle.

The usual source of botulinum toxin is decaying carcasses or vegetable materials, such as decaying grass, hay, or grain, or spoiled silage. Healthy pigs can act as reservoirs for C botulinum; therefore, cross-contamination between pigs and susceptible species such as poultry, ruminants, and horses should be avoided. Some healthy birds can act as carriers of C botulinum spores(but this is not documented in broiler chicken flocks). Some reptiles and fish can also act as healthy carriers.

Botulism outbreaks occur more commonly in summer and autumn, when temperatures favor C botulinum growth. In addition, spore germination is more likely to occur in water when the pH is between 7.5 and 9.0 and in feed when the pH is > 4.5.

Clinical signs of botulism are caused by flaccid muscle paralysis and include progressive motor paralysis, disturbed vision, difficulty with chewing and swallowing, and generalized progressive paresis. Death is usually due to respiratory or cardiac paralysis.

Botulinum toxin prevents the release of acetylcholine at the neuromuscular junction, resulting in muscle paralysis. No characteristic gross and histological lesions develop, and pathological changes can be ascribed to the general paralytic action of the toxin, particularly in muscles of the respiratory system, rather than to the specific effect of toxin on any particular organ.

Botulism in birds results in loss of motor control and flight, with wing muscles typically affected first. Affected birds often lose the ability to lift their heads—a clinical sign called "limber neck." An exotoxin called C2 can cause GI necrosis.

Cattle with botulism can initially present with decreased feed intake and diminished milk production. Later, constipation or colic and hind end weakness, followed by recumbency and dysphagia, might be noted. Tongue paralysis and hypersalivation might also occur. Abdominal effort might be observed during respiration, and death from diaphragm paralysis can occur. Skin sensation is usually normal, and withdrawal reflexes of the limbs are weak. Initially, clinical signs resemble second-stage parturient paresis; however, affected cows do not respond to parenteral administration of calcium.

Reported clinical signs of botulism in adult equids are very similar to those in cattle; however, the initial clinical sign might be mild abdominal discomfort, followed by progressive muscle paresis, recumbency, dysphagia, and decreased muscle tone (tail, tongue, jaw), respiratory distress, and death. Type C botulism might be more likely to result in mydriasis; types A and B seem more likely to cause dysphagia.

Foals with shaker foal syndrome are usually < 4 weeks old. Affected foals can be found dead without premonitory clinical signs; most often, they exhibit signs of progressive symmetrical motor paralysis. Stilted gait, muscular tremors, and an inability to stand for > 4–5 minutes are salient features. Other clinical signs include dysphagia, constipation, mydriasis, and frequent urination. As the disease progresses, dyspnea with extension of the head and neck, tachycardia, and respiratory arrest occur. Death due to respiratory failure occurs usually 24–72 hours after the onset of clinical signs.

Minks and ferrets are highly susceptible to botulinum toxins and typically experience a rapid progression of disease. Signs of hind end paralysis might be evident, quickly progressing to general paralysis and recumbency, with death often occurring within hours after initial clinical signs were detected.

In fish, increased mortality rates are often the first sign of botulism. Some fish demonstrate fin paralysis progressing to tail paralysis, an inability to swim, and, finally, death.

Gross and microscopic lesions are often not evident in animals with botulism, with the exception of catfish, which can experience a visceral toxicosis. Catfish can have clear fluid in the coelom, intestinal intussusception, a reticular pattern in the liver, splenic congestion, and eversion of the stomach into the oral cavity. Equine cases due to BoNT types A and C can show evidence of edema of the nuchal ligament.

• Clinical evaluation

• Toxin identification

• Mouse bioassay

Botulism cases are often suspected when the characteristic motor paralysis is evident and laboratory diagnostic testing has ruled out other neuroparalytic diseases or toxicoses. However, diagnostic confirmation requires detection and identification of the neurotoxin type.

Laboratories can detect the neurotoxin or the toxin-producing bacteria that cause botulism. BoNT-producing clostridia are classified as tier 1 select agents because of their potential for bioterrorism; therefore, these organisms may be cultured only in secure facilities. In the US, such laboratories must be registered with the Federal Select Agent Program.

The samples required for culture include serum, rumen or stomach contents, feces (or colon contents), and tissues such as the liver. These samples should be collected as soon as possible after the onset of the clinical signs of botulism or immediately after death. Samples of feed that is suspected to be contaminated can also be tested.

Isolation of BoNT-producing clostridia can be challenging because the clinical specimens can contain high numbers of other organisms; therefore, spore selection using heat or ethanol might be needed, and selective media can also be used. BoNTs are heat labile, so samples should be delivered to the testing laboratory on ice within 24 hours after they are collected.

BoNTs can be detected using the mouse bioassay, historically considered the gold standard, in which mice receive intraperitoneal injections of the suspected sample (serum, GI contents, feces, or feed) that has been processed into a solution. They are then monitored for signs of botulism.

The mouse bioassay can identify the type of BoNT by using mice that have neutralizing antibodies against various BoNTs. Mice with protection against the offending BoNT survive; those that lack this protection die.

The mouse bioassay can produce false-negative results in species that might be more susceptible to BoNTs than mice are, such as horses, cattle, and some birds. It can also yield false-negative results because of toxin degradation in the samples or because toxin is no longer circulating in serum at the time of sample collection.

The mouse bioassay also cannot differentiate mosaic types of botulism, because of cross-reactivity, and it is labor-intensive, taking up to 96 hours to yield results. In addition, it is expensive and is offered at only a few laboratories worldwide.

Immunoassays to detect BoNTs have been developed. ELISA to detect the botulinum toxin makes it feasible to test large numbers of samples in very little time (6–24 hours), increasing the chances of diagnostic confirmation. However, these tests are not widely available, because of the difficulty of obtaining antibodies for the assay.

ELISAs cannot differentiate between active and inactivated toxins, resulting in some false-positive detections. The sensitivity of ELISA might be decreased because of genetic variation in the toxin serotypes.

Experimentally, lateral flow assays for detecting BoNT type A have shown promising analytical performance with human serum samples and contaminated food samples. However, before these assays can be considered for routine diagnostic use, further development is needed to detect additional BoNT serotypes and validate use on additional sample types.

PCR assay has been used to detect some BoNT serotypes. PCR assays can also be used to identify C botulinum, targeting the neurotoxin gene. PCR assay can be applied to feed, feces, and body fluids, and it is much more sensitive than the mouse bioassay; however, its specificity might be slightly lower because of the inability of PCR to differentiate between formed and inactivated toxins.

In addition, the detection of toxin genes in C botulinum bacteria via PCR assay does not necessarily mean that the toxin was produced. Therefore, this testing approach must be paired closely with clinical signs and history for appropriate interpretation of the results. The best strategy to confirm a botulism diagnosis is to use this method in conjunction with BoNT detection techniques.

Endopeptidase assays are another promising alternative diagnostic approach for BoNT detection; they are much more sensitive than the mouse bioassay. These assays detect only biologically active neurotoxin, with high specificity. The downside to endopeptidase assays is the requirement for specialized equipment (this modality relies on matrix-assisted laser desorption/ionization time-of-flight mass spectrometry [MALDI-TOF MS]).

Endopeptidase assays are capable of detecting active BoNTs in various specimen types, with limits of detection at or below that of the mouse bioassay. The endopeptidase assay is used on serum and food samples for human botulism diagnosis.

• Ensuring high-quality feed and water sources free of carcass contamination

• Botulinum antitoxin treatment

• Vaccination with region-specific type toxoid available for some species

• Supportive care

Prevention of botulism involves ensuring that feed is safe and of high quality (free of carcass contamination, silage properly fermented) and that it is stored appropriately. It also requires water sources that are free of carcass contamination. Minimizing organic inputs into wetlands can decrease the amount of decaying matter that can serve as a substrate for BoNT-producing clostridia, thus decreasing waterfowl exposure.

When poultry litter is applied to fields, it must be inspected to ensure that it does not contain birds or dead animals, and it should not be used as bedding material for other animals.

Vaccination is an additional botulism prevention measure that is available for select species. Toxoid botulinum commercial vaccines are available for mink (type C), cattle (types C and D), sheep (types C and D), and horses (type B).

Vaccines can be administered to foals as young as 2 weeks old (3 doses required, at intervals of 10–12 days), or to their dams during pregnancy (4–6 weeks before foaling) to confer protection through colostral antibodies for the first 8–12 weeks of life (12). Annual boosters are required. Of note, only a type B toxoid is available for horses, and it confers no cross-protection against type A botulism.

Vaccines may be administered to cattle and horses even in the midst of a botulism outbreak.

Polyvalent and monovalent botulinum antitoxins have been used to treat animals with botulism with varying degrees of success, depending on the type of toxin involved and the timing of administration. To be effective, the antitoxin must be administered when the toxin is still circulating, before it binds to the neuromuscular junction. In addition, antitoxin is not available in all countries and can be expensive. For equine cases, a single dose is recommended: for foals, 200 mL (30,000 IU), IV; for adult horses, 500 mL (70,000 IU), IV (12).

In cases of botulism due to toxicoinfection or to wounds, treatment with potassium penicillin (44,000 IU/kg, IV, followed by 22,000 IU/kg, IV, every 6 hours, or maintained at 44,000 IU/kg, IV, every 6 hours throughout the course of therapy) can be effective in horses (12). Aminoglycosides, procaine penicillin, and tetracyclines should be avoided because they can further potentiate neuromuscular weakness (13).

Oral penicillin is contraindicated for treating botulism in foals because it can result in increased release of toxin in the GI tract secondary to death of the bacteria. In poultry affected by the toxicoinfectious form of the disease, ampicillin (30 mg/kg) administered in drinking water has been used with some success (14).

Additional supportive care for animals with botulism involves fluid therapy, correcting electrolyte disturbances, providing nutritional support, wound care, and other protective measures needed for botulism cases characterized by prolonged recumbency. Affected waterfowl should receive antitoxin and have free access to shade and fresh water, with minimal stress or disturbances.

Euthanasia is often required in botulism cases.

• Anniballi F, Fiore A, Löfström C, et al. Management of animal botulism outbreaks: from clinical suspicion to practical countermeasures to prevent or minimize outbreaks. Biosecur Bioterror. 2013;11(S1):S191-S199.

• Johnson AL, McAdams-Gallagher SC, Aceto H. Accuracy of a mouse bioassay for the diagnosis of botulism in horses. J Vet Intern Med. 2016;30(4):1293-1299.

• Skarin H, Tevell Åberg A, Woudstra C, et al. The workshop on animal botulism in Europe. Biosecur Bioterror. 2013;11(suppl 1):S183-S190.

• Centurioni DA, Egan CT, Perry MJ. Current developments in diagnostic assays for laboratory confirmation and investigation of botulism. J Clin Microbiol. 2022;60:e00139-20.

• Uzal FA, McClane BA, Gohari IM, Navarro MA, Popoff MR, eds. Clostridial Diseases of Animals. 2nd ed. John Wiley & Sons; 2026.

• Also see pet owner content regarding botulism in dogs and horses.

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2. Zhang S, Masuyer G, Zhang J, et al. Identification and characterization of a novel botulinum neurotoxin. Nat Commun. 2017;8:14130. doi:10.1038/ncomms14130

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7. Uzal FA, McClane BA, Gohari IM, Navarro MA, Popoff MR, eds. Clostridial Diseases of Animals. 2nd ed. John Wiley & Sons; 2026.

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9. Frye EA, Egan C, Perry MJ, Crouch EE, Burbank KE, Kelly KM. Outbreak of botulism type A in dairy cows detected by MALDI-TOF mass spectrometry. J Vet Diagn Invest. 2020;32(5):722-726. doi:10.1177/1040638720943127

10. Lanci A, Rinnovati R, Anniballi F, et al. The first case of botulism in a donkey. Vet Sci. 2019;6(2):43. doi:10.3390/vetsci6020043

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14. Bano L, Drigo I, Tonon E, Agnoletti F, Giovanardi D, Morandini E. Rice hulls as a possible source of Clostridium botulinum type C spores for poultry. Vet Rec. 2013;173(17): 427-428. doi:10.1136/vr.f6521