Quantitative real-time PCR for detection of neurotoxin genes of Clostridium botulinum types A, B and C in equine samples

Clostridial Diseases (Botulism and Tetanus)

Botulism and tetanus are the 2 primary manifestations of neurologic disease caused by clostridial toxins. Only a small dose of clostridial toxin is required to induce severe, and often fatal, disease. Consequently, definitive diagnosis of either disease is nearly impossible to achieve antemortem or postmortem; presumptive diagnosis is usually made based on physical and neurologic examination findings. Because the severity of clinical signs can worsen rapidly, prognosis worsens when therapeutic intervention is delayed. Highly effective vaccines are available against both botulism and tetanus and are critical in preventative approaches to control.

Public Health Risk Associated with Botulism as Foodborne Zoonoses

Botulism is a rare but severe neurological disease in man and animals that is caused by botulinum neurotoxins (BoNTs) produced by Clostridium botulinum and atypical strains from other Clostridium and non-Clostridium species. BoNTs are divided into more than seven toxinotypes based on neutralization with specific corresponding antisera, and each toxinotype is subdivided into subtypes according to amino acid sequence variations. Animal species show variable sensitivity to the different BoNT toxinotypes. Thereby, naturally acquired animal botulism is mainly due to BoNT/C, D and the mosaic variants CD and DC, BoNT/CD being more prevalent in birds and BoNT/DC in cattle, whereas human botulism is more frequently in the types A, B and E, and to a lower extent, F. Botulism is not a contagious disease, since there is no direct transmission from diseased animals or man to a healthy subject. Botulism occurs via the environment, notably from food contaminated with C. botulinum spores and preserved in conditions favorable for C. botulinum growth and toxin production. The high prevalence of botulism types C, D and variants DC and CD in farmed and wild birds, and to a lower extent in cattle, raises the risk of transmission to human beings. However, human botulism is much rarer than animal botulism, and botulism types C and D are exceptional in humans. Only 15 cases or suspected cases of botulism type C and one outbreak of botulism type D have been reported in humans to date. In contrast, animal healthy carriers of C. botulinum group II, such as C. botulinum type E in fish of the northern hemisphere, and C. botulinum B4 in pigs, represent a more prevalent risk of botulism transmission to human subjects. Less common botulism types in animals but at risk of transmission to humans, can sporadically be observed, such as botulism type E in farmed chickens in France (1998-2002), botulism type B in cattle in The Netherlands (1977-1979), botulism types A and B in horses, or botulism type A in dairy cows (Egypt, 1976). In most cases, human and animal botulisms have distinct origins, and cross transmissions between animals and human beings are rather rare, accidental events. But, due to the severity of this disease, human and animal botulism requires a careful surveillance.

Vaccines against Botulism

Botulinum neurotoxins (BoNT) cause the flaccid paralysis of botulism by inhibiting the release of acetylcholine from motor neurons. There are seven serotypes of BoNT (A-G), with limited therapies, and no FDA approved vaccine for botulism. An investigational formalin-inactivated penta-serotype-BoNT/A-E toxoid vaccine was used to vaccinate people who are at high risk of contracting botulism. However, this formalin-inactivated penta-serotype-BoNT/A-E toxoid vaccine was losing potency and was discontinued. This article reviews the different vaccines being developed to replace the discontinued toxoid vaccine. These vaccines include DNA-based, viral vector-based, and recombinant protein-based vaccines. DNA-based vaccines include plasmids or viral vectors containing the gene encoding one of the BoNT heavy chain receptor binding domains (HC). Viral vectors reviewed are adenovirus, influenza virus, rabies virus, Semliki Forest virus, and Venezuelan Equine Encephalitis virus. Among the potential recombinant protein vaccines reviewed are HC, light chain-heavy chain translocation domain, and chemically or genetically inactivated holotoxin.

Accuracy of a Mouse Bioassay for the Diagnosis of Botulism in Horses

Background

The laboratory diagnosis of botulism in horses traditionally has relied upon the mouse bioassay (MBA). The accuracy of this test for the diagnosis of botulism in horses is unknown.

Hypothesis/Objectives

Our goal was to determine the sensitivity, specificity, positive predictive value, and negative predictive value of the MBA on laboratory‐processed fecal and gastrointestinal samples for foals and adult horses.

Animals

Cases included all horses with a final clinical diagnosis of botulism that were admitted between 1986 and 2011 and had MBA testing performed. Controls included horses without botulism that were admitted during the same time period and had MBA testing performed.

Methods

Retrospective study. Horses suspected of having botulism had fecal or (less commonly) gastrointestinal content samples tested using MBA. For every hospitalized botulism suspect, control samples were obtained from ≥1 additional hospitalized horses not suspected to have botulism.

Results

One hundred and twenty‐nine adult horses and 253 adult controls were identified. Overall sensitivity of the MBA was only 32% but specificity was 97%. Forty‐three foal cases and 21 foal controls were evaluated; sensitivity of the MBA was 53% and specificity was 100%. Positive predictive value was substantially higher (100% for foals and 89% for adults) than negative predictive value (51% for foals and 67% for adults).

Conclusions and Clinical Importance

Mouse bioassay has low sensitivity but high specificity for the diagnosis of botulism in horses. Positive results are highly suggestive of botulism but negative results do not exclude the diagnosis. Unaffected horses and foals rarely shed C. botulinum in their feces.

Historical and current perspectives on Clostridium botulinum diversity

For nearly one hundred years, researchers have attempted to categorize botulinum neurotoxin-producing clostridia and the toxins that they produce according to biochemical characterizations, serological comparisons, and genetic analyses. Throughout this period the bacteria and their toxins have defied such attempts at categorization. Below is a description of both historic and current Clostridium botulinum strain and neurotoxin information that illustrates how each new finding has significantly added to the knowledge of the botulinum neurotoxin-containing clostridia and their diversity.

Outcome of adult horses with botulism treated at a veterinary hospital: 92 cases (1989-2013)

Background

There are no studies evaluating a large population of adult horses treated for botulism. Reported survival rates in outbreak situations are low; however, many horses in outbreaks do not receive treatment.

Hypothesis/Objectives

That adult horses treated at a veterinary hospital would have improved survival compared to outbreak situations. Additional aims included identification of predictors of nonsurvival.

Animals

All horses greater than 6 months of age with a final diagnosis of botulism admitted to a veterinary teaching hospital between 1989 and 2013 were included.

Methods

Retrospective study. Historical, admission, and hospitalization data were retrieved from medical records and associations between variables and nonsurvival were identified using logistic regression. Two multivariable models were developed pertaining to (1) information available at admission and (2) clinical findings during hospitalization.

Results

Ninety‐two records met inclusion criteria. Retained variables for the two models indicated that higher rectal temperature (OR, 1.94; CI, 1.19–3.17) and dysphagia (OR, 4.04; CI, 1.01–16.17) observed at admission increased the odds of survival, as did treatment with antitoxin (OR, 121.30; CI, 9.94–1,480.65). Horses with abnormal respiratory effort or inability to stand had decreased odds of survival. Overall survival was 48% but was significantly higher (67%, P = .011) for horses that arrived standing, and even higher (95%, P < .001) for horses that remained able to stand throughout hospitalization. Complications occurred in 62% of horses but were not associated with nonsurvival.

Conclusions and Clinical Importance

Horses that lose the ability to stand have a poor chance of survival. Complications are common in treated horses but do not reduce survival.