This document discusses botulism in horses. It begins by providing background on Clostridium botulinum, the bacteria that causes botulism. It then discusses the history and epidemiology of botulism, noting it is an intoxication acquired by ingesting pre-formed toxin. The document outlines the different types of botulism (forage, carrion-associated, wound, toxicoinfectious), hosts affected, risk factors, pathogenesis, clinical signs, and treatment options.

1. BOTULISM IN
HORSES
NEERAJA E.S

2. Pathogenic Clostridium species

3. BOTULISM
A serious, potentially fatal intoxication usually
acquired by ingestion of pre-formed toxin

4. HISTORY
• 1793- Justinus Kerner (Kerner’s disease)
• Published first accurate and complete
descriptions of the symptoms of food-borne
botulism in humans
• ‘Botulus’ (Latin)= Sausage
• ‘Sausage poisoning’ , ‘ Fat poisoning’
Justinus Kerner

5. HISTORY...
• 1895- Emile von Ermengem
• Isolated organism during Belgium outbreak from
piece of ham
• Modern botulinum toxin treatment was
pioneered by Alan B. Scott and Edward J. Schantz
Emile von
Ermengem

6. BOTULISM
• Clostridium botulinum
• Obligate anaerobe
• Gram - positive rod
• Produces oval, subterminal endospores
• Spores survive in the environment for over 30 years
• Produces neurotoxins during vegetative growth
• Toxin capable of surviving for long periods, particularly in
bones

7. Characteristic morphology of Clostridium botulinum with the presence of endospores
(Phase contrast visual micrograph)

8. Types C & D- common in animals
• Nine types are recognized on the basis of toxins which they
produce
(A, B, Cα, Cβ, D, E, F, G, H)
• Type G renamed C. argentinense
• Farm animal disease is produced primarily by types B, C, D, and
occasionally type A
• Type A, B, E, and F toxins - human botulism
SEROTYPES

10. • Divided into groups I to IV depending on their physiologic
properties
▫ Group I: proteolytic C. botulinum type A, B and F degrade protein
such as milk, serum, meat, and chicken protein
▫ Group II: nonproteolytic C. botulinum, includes nonprotelytic
type B and F and all type E
▫ Group III: C. botulinum type C and D
▫ Group IV: C. botulinum type G

11. Geographic distribution of serotypes
• The geographic distribution of types varies considerably
• Types C and D are more common in warm climates
• Type B is also common in soils in the UK and in Europe
• In a study in the United States
▫ Type A found in neutral or alkaline soils in the west
▫ Types B and E were in damp or wet soil all over, except that B was not
found in the south
▫ Type C was found in acid soils in the Gulf coast
▫ Type D in alkaline soils in the west

12. HOST AFFFECTED
• Occur most commonly in waterfowl, cattle, horses, sheep,
mink, poultry and farmed fish
• Pigs and dogs are relatively resistant to the neurotoxins
• Botulism is rare in domestic cats

13. • The organism is present in the alimentary tract of animals
that have recently ingested contaminated material
• Introduced into new areas by birds and blowflies
• In healthy animals with normal intestinal fauna and motility
C. botulinum does not multiply in the gastrointestinal tract

14. EPIDEMIOLOGY
• No geographic limitations
• Isolated cases and sporadic outbreaks occurring in most
countries
• Exposure to toxin and the risk for disease differ between
regions because of differences
▫ In food storage
▫ Feeding
▫ Management practices
OCCURENCE

15. OCCURENCE.....
• Outbreaks associated with ingestion of toxin in
 Conserved feeds- common in the northern states of the United
States and in Europe
 Animals on pasture are reported from South Africa, Australia and
the gulf coast area of the United States
• Occurs in a number of animals at one time
• High case–fatality rate

16. SOURCE OF INFECTION
Forage
botulism
Carrion
associated
botulism
Wound
botulism
Toxico-
infectious
botulism

17. 1. Forage botulism
• Occurs when pH, moisture, and anaerobic conditions in the
feedstuff allow the vegetative growth of C. Botulinum and the
production of toxin
• Occur in a number of spoiled stored forages
▫ If the forage is very succulent or is wet by rain when it is made
• Cereal silages carry a risk in the united states

18. Forage botulism...
• Big bale silage- particular risk
• Forage used has insufficient water soluble CHO- less lactic
acid- high pH
• Higher DM content in forages- higher pH
pH < 4.5 Clostridial multiplication inhibited

19. Forage botulism...
• Organism proliferate in decaying vegetable material
• Decaying grass at the base of old tussocks and in trampled
stubble are known to be suitable sites for growth of
C. botulinum
• Most non-carrion associated botulism is caused by type B
strains, and horses appear to be especially susceptible

20. Forage botulism...
• Disease reported in horses
▫ Fed on spoiled vegetables and potatoes contaminated by C. botulinum
▫ Alfalfa haylage packed in airtight aluminum foil envelopes
▫ Grass clippings allowed to accumulate and decay in a pile
▫ Round bale hay that spoiled after rain
▫ With brewers grains, and high-moisture grain has the potential for
toxicity

21. 2. Carrion-Associated Botulism
• The cause of botulism in animals on pasture
• A common cause in animals on conserved feeds
• Carrion includes domestic and wild animals and birds
• In endemic area- carcasses of dead animals are invaded by
C. botulinum
▫ High concentrations of toxin are produced
▫ Very small amounts of flesh or bone have lethal concentrations

22. Carrion-Associated Botulism....
• Most outbreaks associated with type C and D strains
• These strains produce much higher concentrations of toxin in
carrion than type A and B strains
• Toxin can persist in carrion for at least 1 year
• Carcasses of rodents, cats, and birds contaminate hay or
silage
• Toxin can leach out and contaminate surrounding hay or other
feeds
• Eg., Incident 427 of 444 dairy cattle died after ingesting feed
contaminated with BoTN type C from a cat carcass

23. Carrion-Associated Botulism....
• Direct carrion ingestion can occur (pica)
• Drinking of water contaminated by carcasses of dead animals
• Not uncommon occurrence is livestock drinking lake water
contaminated by the carcasses of ducks and other waterfowl
that have died of botulism

24. 3.Wound Botulism
• Toxicoinfectious form of botulism
• Toxin is produced in wounds infected by C. botulinum
• Recorded in horses following
▫ Castration
▫ With omphalophlebitis
▫ Umbilical hernias treated with clamps
▫ With an infected wound
▫ In association with an injection abscess

25. 4.Toxicoinfectious Botulism
• When toxin is produced by C. botulinum present in the
intestine
• Two conditions in horses- potential forms of toxicoinfectious
botulism
▫ Shaker foal syndrome
▫ Equine grass sickness

26. Shaker foal syndrome
• Foals with toxicoinfectious botulism
• Young foal up to 8 month of age
• Highest prevalence in foals 3-8 weeks of age
• Common signs noted initially being
▫ Nasal return of milk
▫ Difficulty keeping up with the mare
▫ Significant muscle tremor- hence the term ‘shaker foal
syndrome’ (bernard 1997)

27. Shaker foal syndrome video

28. RISK FACTORS
Animal Risk Factors
• Most common in birds, particularly- domestic chicken and
wild waterfowl
• Cattle, sheep, and horses are susceptible
• Pigs, dogs, and cats appear to be resistant
• Cattle and sheep are usually affected by types C and D
Horse susceptible to type B- toxin

29. RISK FACTORS...
Environment risk factors
• In range animals has a seasonal distribution
• Outbreaks are most likely to occur during drought periods
▫ When feed is sparse
▫ Phosphorus intake is low
▫ Carrion is plentiful
• Silage-associated botulism is also seasonal with the feeding of
silage

30. PATHOGENESIS
• Toxins are neurotoxic
• They are absorbed from the intestinal tract/wound and
carried via the bloodstream to peripheral cholinergic nerve
terminals
 Including neuromuscular junctions
 Postganglionic parasympathetic nerve endings
 Peripheral ganglia
• Produce functional paralysis without the development of
histologic lesions

31. Neurotoxins of C. botulinum are the most potent
biological toxins known
One minimum lethal dose, which is sufficient to kill a mouse, is
equivalent to 10 pg of toxin for botulinum toxin type A
(AOAC International, 2001 )

32. Toxin
• Synthesized as a single polypeptide chain (low potency)
• Nicked by a bacterial protease/Gastric proteases to produce
two chains
▫ Light chain (A fragment)
▫ Heavy chain (B fragment)
• A fragment of nicked toxin become most
potent toxin

33. Toxin.....
• The heavy chain of the toxin is responsible for binding to the
receptors and translocation into the cell
• Light chain of the toxin for resultant blockade of the release
of acetylcholine at the neuromuscular junction
• Flaccid paralysis develops and the animal may die of
respiratory paralysis

34. Mechanism of action of toxin
• Acts primarily presynaptically at the peripheral cholinergic
neuromuscular junction
• Block the release of the neurotransmitter acetylcholine (ACh)
• Neuromuscular blockade occurs in 3 stages:
 Rapid irreversible binding of the toxin to receptors on the
presynaptic nerve terminal
Internalisation involving receptor-mediated endocytosis of toxin
Final blocking step preventing release of ACh from the vesicle
resulting in flaccid paralysis

35. Mechanism of action of toxin....
• Regeneration of new end plate receptor proteins is required for
improved neuromuscular function
• Common delay of 4–10 days is observed even after provision of
antitoxin before clinical improvement begins (Whitlock and Buckley
1997)
• Cranial nerves and nerves supplying the extensor muscles are
affected earliest
• Neurotoxins do not affect the CNS or sensory nerves, facilitating
differentiation from many other neurological diseases.

38. CLINICAL FINDINGS
• Signs appear 3-17 days after access to the toxic material but
occasionally as soon as day 1
• Incubation period is shorter as the amount of toxin available
is increased
• Disease is not accompanied by fever
• Characteristic clinical picture- progressive symmetric
muscular paralysis affecting particularly the
▫ Limb muscles
▫ The muscles of the jaw, tongue, and throat

39. CLINICAL FINDINGS....
• Muscle weakness and paralysis commence in the
hindquarters and progress
• Hindquarters Forequarters Head Neck
• Onset is marked by muscle tremor and fasciculation sufficient
to make the whole limb tremble
• Colic may be an initial sign

40. CLINICAL FINDINGS....
Per-acute cases
• Die without prior signs of illness
• A few fail to take water or food for a day before hand
• In most cases the disease is subacute
▫ Restlessness
▫ Incoordination
▫ Stumbling
▫ Knuckling
▫ Ataxia are followed by inability to rise or to lift the head

41. CLINICAL FINDINGS....
• Mydriasis and ptosis - early in the clinical course
• Mydriasis can be prominent in type C botulism in the horse
• Skin sensation is retained
• Affected animals lie
▫ In sternal recumbency
▫ The head on the ground or turned into the flank, not unlike the
posture of a cow with parturient paresis

42. Mydriasis
Reduced anal tone. Note that
this degree of anal dilatation is
unusual

43. Recumbency. Note the disturbed bedding due to frequent
failed attempts to stand

44. • Tongue tone is reduced, as is the strength of
tongue retraction
• Some cases- tongue becomes paralyzed
▫ Hangs from the mouth
▫ Unable to chew or swallow
▫ Drools saliva
• In others there is no impairment of
swallowing or mastication and the animal
continues to eat until the end

45. CLINICAL FINDINGS....
• Variation in signs is a characteristic of an outbreak; either all
the cases have tongue paralysis or all of them do not have it
• Defecation and urination are usually unaffected, may be
constipated
• Paralysis of the chest muscles- terminal abdominal type
respiration
• Sensation and consciousness are retained until the end, which
usually occurs quietly, and with the animal in lateral
recumbency, 1 to 4 days after the commencement of illness

46. CLINICAL FINDINGS....
• Chronic cases
▫ Restlessness
▫ Respiratory distress
▫ Followed by knuckling
▫ Stumbling
▫ Disinclination to rise
• Anorexia and adipsia are important early signs but are
often not observed in pastured animals

47. CLINICAL FINDINGS....
• In some there is a pronounced roaring sound with each
respiration
• The roaring persists for up to 3 months
• In some animals there is difficulty in prehending hay but
concentrate and ensilage may be taken
• This disability may persist for 3 weeks

48. • Muscle tremor is often a prominent early sign
• If the foal can walk, the gait is stiff and stilted and the toes are
dragged
• If the foal sucks, milk drools from the mouth
• If it attempts to eat hay some of the material is regurgitated
through the nostrils
• Constipation occurs consistently
• There is a rapid progression to severe muscular weakness and
prostration, with the foal going down and being unable to rise
Toxicoinfectious botulism in foals
(Shaker foal syndrome)

49. Shaker foal syndrome....
• If it is held up, there is a gross muscle tremor, which is not evident
when the foal is lying down
• Prostrate foals are bright and alert, have normal mentation and
pain perception, and have dilatation of the pupils with a sluggish
pupillary light reflex
• During the latter period of the illness there is a complete cessation
of peristalsis
• The temperature varies from being slightly elevated to slightly
depressed
• Death occurs about 72 hours after the onset of signs and is caused
by respiratory failure

50. CLINICAL PATHOLOGY
• There are no changes in hematologic values or serum
biochemistry that are specific to botulism
• In many cases under field conditions the diagnosis is solely
based on clinical presentation and by ruling out potential
differential diagnoses
• Laboratory diagnosis of botulism in the live or dead animal is
difficult because of the lack of sensitive confirmatory
laboratory tests

51. DIAGNOSIS
▫ Laboratory confirmation is attempted by the following
 Detection of preformed toxin in serum, intestinal tract
contents, or feed- Mouse bioassay
 Demonstration of spores of C. botulinum in the feed or
gastrointestinal contents
 Detection of antibody in recovering or clinically normal
at-risk animals

52. Mouse bioassay (MBA)
• Most sensitive, although less acceptable
• Intraperitoneal inoculation of 2 mice with plasma/serum or other
sample (intestinal contents, wound exudate etc.) from a suspect
case
• If signs of neuroparalysis arise, 4 further mice are inoculated with
the sample, with 2 also receiving multivalent Antitoxin
• If protection is demonstrated using antitoxin, botulism is
confirmed, and the serotype may be determined
• Some laboratories will also perform the MBA on enriched samples
• Enrichment allows any spores present in the original sample to
germinate and produce neurotoxin.

53. Mouse bioassay (MBA)...
• Rate of positivity in clinical cases particularly when testing
serum is low, which has been explained by the much higher
sensitivity to BoNt of cattle and horse compared with mice
and the rapid binding of BoNt in the neuromuscular junctions,
leaving low to no amounts of free BoNt in blood
• Currently gastrointestinal content or faecal material is
preferred over faecal material for the detection of bont

54. • Quantititative real-time polymerase chain reaction for
detection of the neurotoxin gene of C. botulinum type B
• Type C and D botulinum toxins can also be detected via ELISA
although both the specificity and sensitivity of this test are
lower than the MBA
• Demonstration of spores in gastrointestinal contents from
affected horses is highly supportive, as botulism spores are
rarely found in healthy horses (whitlock and buckley 1997)
• Needle electromyography appears to have been of some
diagnostic use in foals (Aleman et al. 2011) but less so in
mature horses (Mayhew 2008)
DIAGNOSIS....

55. DIAGNOSIS....
• To get around the problem of lack of sensitivity with the
mouse test, suspect feed has been fed to experimental cattle
• Alternatively, one can make an infusion of the feed sample
and use this as the sole drinking water supply for
experimental animals
• The problem with all feeding experiments is that the BoTN is
likely to be very patchy in its distribution in the feed
• Failure to produce the disease in animals vaccinated against
botulism, when deaths are occurring in the unvaccinated
controls, has also been used as a diagnostic procedure

56. DIAGNOSIS....
• The detection of antibody in chronically affected animals and
at-risk herd mates or as retrospective diagnosis by an ELISA
test has been used to support a diagnosis in outbreaks of type
C and type D botulism
• Increased antibody prevalence over time or increased
antibody prevalence in an affected group compared with a
similar group nearby was reported by some authors

57. ‘Tongue pull/retraction’
• Gently pull the tongue away from the commissure of the lips
• Most normal horses will retract the tongue rapidly
• In botulism cases the tongue is held easily, without significant
resistance and is very slowly or incompletely retracted
Reduced tongue tone: tongue easily pulled
away from commissures of the lips

58. ‘Grain test
• Feed the horse 250 ml of grain and closely observe
• Normal horses should consume this volume in less than 2 min
• Horses with botulism will take much longer, often with
accompanying quiet chewing sounds, excessive salivation and
evidence of grain falling from the mouth

59. Evidence of dysphagia
Dropping haylage from
mouth
Difficulty in
Coping with fluids
Bilateral nasal return of
saliva and ingesta

60. NECROPSY FINDINGS
• There are no specific changes detectable at necropsy
• Presence of suspicious feedstuffs in the stomach may be suggestive
• There may be nonspecific sub endocardial and sub epicardial
haemorrhages
• Congestion of the intestines
• Microscopic changes in the brain are also nonspecific -perivascular
hemorrhages in the corpus striatum, cerebellum, and cerebrum,
• The brain should be examined histologically to eliminate other
causes of neurologic disease

61. Samples for Confirmation of Diagnosis
• Bacteriology: suspected contaminated feed material, feces,
rumen and intestinal contents, serum from clinically affected
herd mates (bioassay, anaerobic CULT, ELISA)
• Histology: formalin-fixed brain

62. DIFFERENTIAL DIAGNOSIS
• Equine encephalomyelitis
• Equine herpesvirus-1 myeloencephalopathy
• Atypical myopathy of unknown etiology; the condition that presents
frequently fatal myopathy can be differentiated by the characteristic
increase in serum creatine kinase activity and the presence of
hemoglobinuria
• Equine motor neuron disease
• Hyperkalemic periodic paralysis
• Hepatic encephalopathy
• Paralytic rabies
• Ionophore toxicity
• Myasthenia gravis

64. TREATMENT
• The initial aim of therapy is early neutralisation of circulating toxin
prior to neuronal binding and internalisation
• This is attempted by early intravenous administration of toxin-
specific antitoxin- where it is commercially available
• Antitoxin to the various BoTN types is not available universally
• Specific or polyvalent antiserum if administered early in the course
at a dose of 30,000 IU for a foal and 70,000 IU for adult horses, can
improve the likelihood of survival
• A single dose is sufficient, but it is expensive

65. TREATMENT.......
• Antimicrobial drugs may be required if secondary aspiration
pneumonia is suspected
• Aminoglycoside, tetracycline and procaine-based antimicrobial
drugs may exacerbate neuromuscular blockade, thereby worsening
clinical signs and probably should be avoided (Mayhew and MacKay
1982)
• Gastrointestinal cathartics or laxatives such as mineral oil may aid
removal of any toxin still present in the GI tract, and should help
prevent impaction secondary to reduced GI motility and reduced
water intake

66. Supportive care
• Animals should be confined to a stall with supportive fluid
therapy and enteral feeding
• Supply of deep, soft bedding, bandaging of distal limbs and
frequent turning of recumbent cases are required to prevent
decubital ulcers and muscle necrosis
• Ocular lubricants should be applied regularly to prevent
exposure keratitis secondary to the reduced eyelid tone and
the effects of recumbency

67. Supportive care....
• Attempting to maintain cases in sternal recumbency should improve
respiratory dynamics
• Mechanical ventilation can be performed in severely affected cases with
respiratory paralysis, but this is clearly much easier to implement in foals
than in mature horses
• Regular urinary catheterisation of recumbent males may be required to
help prevent secondary cystitis
• In cases with prominent dysphagia, additional enteral (high protein slurry
via nasogastric tube) or parenteral nutrition may be required

68. Supportive care...
• Enteral feeding should be provided whilst the horse is either
standing or is positioned in sternal recumbency to aid gastric
emptying and prevent secondary aspiration
• Muzzling may be required to prevent aspiration pneumonia
• Mature horses may be supported with a sling, dependent upon
patient acceptance and on availability of this resource, as well as
experience in its use. This approach may be most indicated when
antitoxin is available to provide support until clinical benefits are
observed.

69. Affected horse supported by sling

70. • A rapid progression of signs suggests a poor prognosis
• treatment should only be undertaken in subacute cases in
which signs develop slowly and there is some chance of
recovery
• Where groups of animals have had the same exposure factor,
the remainder of the animals in the group should be
vaccinated immediately
• Vaccination with either type-specific or combined BoNT
toxoid in clinically affected animals is ineffective because
binding of BoNT to neuromuscular junctions is irreversible

71. • Recent studies report a survival rate in foals of 96% which was
achieved by the early administration of antitoxin (before
complete recumbency) coupled with a high quality of
intensive care fluid therapy, enteral or parenteral feeding,
nasal insufflation with oxygen, and mechanical ventilation if
required.
• Duration of hospitalization was approximately 2 weeks.

72. PROGNOSIS
• Associated with the severity of clinical signs, the rapidity of their
onset and progression, or not, to recumbency (Whitlock 1990)
• The prognosis for standing horses receiving antitoxin is good
(Kinde et al. 1991; Whitlock 1996)
• But is significantly reduced without provision of antitoxin (Mayhew
1996)
• some minimally affected horses may survive without specific
treatment
(Whitlock 1996)
• The prognosis for recovery after over 24 h recumbency is extremely
guarded (Whitlock and Buckley 1997)
• Recovery is dependent upon synthesis of new motor end plates, but is
considered to be complete in survivors (Kinde et al. 1991)

73. CONTROL
• In range animals, correction of dietary deficiencies by
supplementation with phosphorus or protein should be
implemented if conditions permit
• Hygienic disposal of carcasses is advisable to prevent further
pasture contamination but may not be practicable under
range conditions.
• Vaccination with type-specific or combined (bivalent C and D)
toxoid is practiced in enzootic areas in Australia and southern
Africa

74. Avoid feeding your horse moist or musty
food
Check your horse’s water supply daily
for dead animals

75. Treating woundKeep soil dry and away from the horse's feed

76. VACCINATION
• Type-specific or combined (bivalent C and D) toxoid is practiced in
enzootic areas in Australia and southern Africa
• Type B and C vaccines would be more appropriate for prevention of
disease in North America and Europe
• The immunity engendered by vaccination is type specific
• Vaccination of the mare may not prevent the occurrence of
botulism in foals
• If disease appears to have resulted from feeding contaminated
silage, hay, or other feed the stock should be vigorously vaccinated
with a toxoid on three occasions at 2-week intervals and then
feeding of the same material can be recommenced.

77. Neogen®Vet BotVax® B
2 mL IM

78. Botuvax- alum-precipitated C. botulinum types C 1 + 2 and D toxoids

79. Vaccination- AAEP Guidelines
Foals % Weanlings < 12 months of age
• Mares vaccinated in the prepartum period
▫ 1st dose: 2-3 months of age
▫ 2nd dose: 4 weeks after 1st dose
▫ 3rd dose: 4 weeks after 2nd dose
• Unvaccinated mare or lacking vaccination history
▫ 1st dose: 1-3 months of age
▫ 2nd dose: 4 weeks after 1st dose
▫ 3rd dose: 4 weeks after 2nd dose

80. Vaccination- AAEP Guidelines
Adult horses
• Broodmares
▫ Previously vaccinated- Annual, 4-6 weeks postpartum
▫ unvaccinated/lacking vaccination history
 1st dose: 8 months of gestation
 2nd dose: 4 weeks after 1st dose
 3rd dose: 4 weeks after 2nd dose
• >1 year of age previously vaccinated- annual
• >1 year of age unvaccinated/lacking vaccination history
▫ 3 dose series
▫ Annual revaccination

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