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The hunt for the foal killer

By BRUCE KAPLAN, D.V.M.

© St. Petersburg Times, published June 2, 2001


Question: The news reports from Kentucky talk about a mystery illness killing horse foals. The reporters usually talk at length to horse breeders, trainers and owners but rarely veterinarians.

I suspect these people are parroting what the veterinarians say about the illness possibly resulting from a fungus on the Bluegrass, but I would like to hear directly from the veterinarians. What could a fungus in food do to female horses to cause problems?

Answer: There is a long list of mycotoxicoses that affect domestic animals (equine, cattle, sheep, swine, poultry and dogs). Some fungus agents in feedstuffs ingested by horses, including Aspergillus, Mucor and Candida, may produce mycotoxins, causing an inflammation of the placenta and resulting in abortion.

The placenta is the organ in the uterus of mammals that joins the mother and the offspring (fetus) during pregnancy. It provides an exchange of many blood-borne substances, such as endocrine secretions and chemicals.

Horses exposed to contaminated feeds from pastures, hay, straw, cereal grains or any other fodder can develop an acute or chronic infection, called equine mycotoxic disease.

Mycotoxins ingested from feeds may have different properties, resulting in a wide variety of signs and lesions. Some can suppress the horse's immune system, allowing a secondary disease to develop from bacteria, parasites or viruses in the mare and her foal.

Mycotoxic disease from feedstuffs can be difficult to detect. Veterinarians must take samples and submit them for laboratory analysis.

However, confirmation that a mycotoxin caused the disease usually requires a combination of information including pathologic studies of tissues from organs involved along with blood tests.

Outbreaks are generally seasonal, depending upon climatic conditions favoring fungal growth and mycotoxin production. Not all funguses found in feeds are toxin producers.

A team of scientists that included veterinarians, equine epidemiologists and agronomists assembled at the University of Kentucky's Maxwell H. Gluck Equine Research Center and Livestock Disease Research Center are investigating mare reproductive loss syndrome, the designated name for the mystery illness.

These investigations employ epidemiological methods. Epidemiology is the basic science of public health for humans and animals.

The answer to this mystery probably lies within this field of study. The causative agent was initially believed to be either a mycotoxin produced from a mold (not identified) found in pastures or a fescue (type of grass) endophyte toxicity. Endophytes are parasitic plant organisms (for example, funguses or algae) that live and grow within a host plant.

It was thought that a fungus growing inside the fescue grass might be producing alkaloid toxins that can cause abortions, foal deaths and other reproductive disorders in horses. However, the current theory is that the disorders are caused by mares ingesting the feces of eastern tent caterpillars, which contain cyanide. The caterpillars feed on the leaves of black cherry trees, which naturally contain cyanide.

Dr. Douglas T. Byars, a prominent equine veterinarian associated with the acclaimed Lexington, Ky.-based Hagyard-Davidson-McGee equine clinic said recently, "This cyanide/black cherry/caterpillar etiology theory is probably 90 percent accurate. More work must be done to prove this conclusively and clarify the entire picture.

"A common post-mortem finding in these foals is pericarditis (an inflammation of the membranous sac that surrounds the heart). As much as 2 to 3 gallons of fluid may accumulate around the heart. This excess fluid sometimes compresses and chokes the heart."

This disease has caused many broodmares in Kentucky to abort and deliver stillborn or sick foals, some of which have died. About 10,000 foals were expected to be born in Kentucky this year. Some veterinarians estimate that 30 to 40 percent of this year's foal crop is already lost.

A surveillance questionnaire is being circulated among practicing veterinarians to determine the accuracy of these estimates by the team of epidemiology investigators. -- Robert A. Fritz, D.V.M., Distance Learning Program instructor-in-charge, veterinary technology, St. Petersburg Junior College

* * *

Dr. Bruce Kaplan is a veterinarian editor/writer. Please send questions to Ask a Veterinarian, Pinellas Animal Foundation, P.O. Box 47771, St. Petersburg, FL 33743-7771. Because of the volume of mail, personal replies are not possible. Questions of general interest will be answered in the column.

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