A foal in California has become the first documented case of a horse surviving type A botulism.
Researchers from the University of California, Davis, have chronicled the survival of the foal, who arrived at the institution’s veterinary teaching hospital when she was just 10 days old.
Botulism is an often fatal flaccid paralytic disorder caused by the neurotoxins produced by Clostridium botulinum. Forms of botulism in horses include toxicoinfectious, food-borne, and wound botulism.
The toxicoinfectious form is more commonly seen in foals less than six months of age, and occurs when spores are swallowed and germinate in the intestinal tract.
Dr Gary Magdesian and his colleagues, writing in the Journal of Veterinary Internal Medicine, said the 42kg pony/paint-horse cross was admitted with a low head and neck carriage, originally suspected to be caused by a fractured neck vertebrae. However, no neck abnormalities were found in x-rays taken before her hospital admission.
The foal worsened and was unable to stand unassisted. Her owners had helped her to rise, and supported her head in order to nurse every two hours while en route to the William R. Pritchard Veterinary Medical Teaching Hospital.
“No abnormalities were noted on physical examination other than weakness and an inability to maintain the head and neck in a normal position and posture,” the study team reported on her admission.
They said the cranial nerve examination was within normal limits. She was able to walk, but her gait was short and stiff, and she tired quickly. There was no evidence found of neck pain on movement, but specialists noted a decrease in tone that allowed over-extension of the nuchal ligament and neck.
The foal was able to suckle well provided she was supported, and had normal tongue tone.
Repeat neck x-rays and blood gas analysis revealed no issues.
During the first three days of hospitalization, clinical signs progressed and she became unable to stand. Her tongue tone also worsened.
Because of her progressive weakness, and without other clinical or blood abnormalities, toxicoinfectious botulism was suspected. Botulism types A and C were considered the most likely serotypes in the foal, having been born on the West coast of the United States.
A dung sample collected at the time of admission was submitted for detection of C. botulinum toxin genes to the National Botulism Reference Laboratory at the University of Pennsylvania. The results were available on day four of hospitalization and confirmed the presence of type A botulism spores, consistent with toxicoinfectious botulism.
The foal was given plasma containing antibodies to C. botulinum type B and C toxins.
When it became available 24 hours later, plasma with antibodies to types A, B, and C2 were given.
Other therapies included polyionic fluids, potassium penicillin, selenium, omeprazole and vitamin E7.
She was helped to stand, or she was turned over, every two hours. She was fed 12% of her body weight daily as a mixture of mare’s milk and a commercial milk replacer.
The foal was weaned on to the commercial milk replacer, and the amount fed increased to 25% of her body weight as she became more active. The transition to milk replacer was required because of the unavailability of mare’s milk.
To treat constipation – a frequent complication of botulism – the foal was given enemas as well as mineral oil once.
Clinical improvement, including a very gradual return of motor function and strength, was noted on day four and continued throughout hospitalization, which spanned 30 days.
By day six, tongue tone had increased, allowing for the removal of the nasogastric tube, which allowed the foal to be fed from a bowl. By day 10 she could stand on her own with assistance to rise, and by day 15 she was able to right herself into sternal recumbency. On day 22, the foal was able to rise without assistance.
Additional therapy included provision of nutrition, maintenance of hydration, ensuring ability to urinate and defecate, and management of decubital ulcers.
The foal was treated with potassium penicillin intravenously for the first 11 days and then switched to ceftiofur for an additional 10 days.
Repeat testing for botulism toxin on day 14 was negative and antibiotic therapy was discontinued when the ultrasound examination showed normal umbilical structures at day 20.
At discharge, on day 30, the foal was still weak and tired easily, but was able to stand unassisted, walk with a shortened stride, drink from a pan, and eat hay and pellets. Her head and neck position were normal.
Her owners were instructed to gradually increase turn-out time as her strength increased.
“In order to avoid re-exposure to soil borne botulism spores, the clients were instructed to remove the surface soil from the filly’s stall and paddock, and disinfect the area with 1:10 dilution of sodium hypochlorite.
“New dirt was brought in to fill the paddock, and rubber mats were placed in the stall in order to reduce exposure.
“Four months after discharge, the foal reportedly had returned to normal strength, with no persistent neuromuscular deficits or weakness.”
“This,” the study team reported, “is the first documented case of survival following type A botulism in a horse.”
Botulinum neurotoxin type A is often associated with more severe disease, longer recovery, and a higher case fatality rate than the other types of botulism in both humans and horses.
Survival of type B botulism was most recently recorded as 48% in adult horses. The median hospitalization period for foals with type B botulism was 14 days if not ventilated or 22 days if ventilated.
In the only study evaluating type A botulism in horses, there were no confirmed survivors and clinical signs progressed rapidly and were more pronounced than those reported for other types. In that report, the three cases in horses under 1 month of age proved fatal. The other 54 horses were all older than 11 months, with 49 of them dead at the time their samples were submitted for testing.
“The remaining five horses were alive at the time of sample submission, but were lost to follow up with an unknown outcome, resulting in a mortality rate of somewhere between 90 and 100%.
“There have been no reports of survival of type A cases in horses,” they said.
“Given the age of onset of this foal, presence of type A spores in feces, and lack of obvious wounds, it is likely she was exposed to type A spores in the soil of the premises, although infection of the umbilicus cannot be ruled out.
“Timely therapeutic management appears critical for the successful treatment of Type A botulism.
“Therapies used in this case included plasma containing antibodies against C. botulinum neurotoxins, antimicrobial therapy, and supportive care.
“It is recommended that the trivalent form of plasma containing antibodies against C. botulinum be used when there is a suspicion for type A botulism or the case is from west of the Rocky Mountains in the United States.”
Head and neck weakness can be an early indicator of botulism, they said.
Prutton, J.S.W., Magdesian, K.G., Plummer, A., Williams, D.C. and Aleman, M. (2016), Survival of a Foal with Type A Botulism. Journal of Veterinary Internal Medicine, 30: 675–678. doi: 10.1111/jvim.13840