There’s a saying in veterinary medicine ­– “When you hear hoof beats think horses, not zebras.” It means that patients are much more likely to have common diseases than rare ones. 


This is a good rule to live by, but it doesn’t mean that veterinarians can completely discount the disease equivalents of zebras. There are times when hoof beats may indicate an approaching zebra.


Foreign animal diseases are a good example. They are called “foreign” because they are not endemic to (or they have been eradicated from) our part of the world. By definition, therefore, these are diseases that your typical U.S. veterinarian has never diagnosed in one of his or her patients. We may have learned about them in veterinary school or in the process of our continuing education, but information gathered under those types of settings just doesn’t have the staying power of experience garnered in the trenches.


The National Veterinary Accreditation Program (NVAD) is working to keep awareness of foreign animal diseases high in the veterinary community. Why? Because if one of these diseases were to enter the United States and not be recognized quickly, the results could be catastrophic.


Here’s a real life example taken from the NVAD supplemental training module entitled “Foreign Animal Disease Detection in Category One Animals.”


On November 21, 1997 a Basset Hound was presented with lameness to a veterinarian in central Texas. The clients were a family returning from South America. The dog had been imported from South America three days before and a health certificate examination had been performed by a veterinarian six days before presentation.


The veterinarian discovered a wound on the dog’s right rear paw. Upon thorough examination of the wound under anesthesia, she removed six fly larvae, ranging in size from 8mm to 15mm. Due to the appearance and location of the larvae and the fact that the dog had just come from South America, the veterinarian suspected screwworm. 


The veterinarian immediately notified the State Animal Health Official, who in turn notified APHIS. State and federal veterinarians were dispatched to collect larvae samples and deliver them to the USDA’s screwworm laboratory in Mission, Texas for identification, resulting in a positive diagnosis for Cochliomyia hominivorax – New World Screwworm. The USDA staff was able to trace the patient’s route from South America to central Texas. They sprayed insecticide at all sites where the patient had made airport or kennel stops.


Screwworms are really nasty. They lay their eggs in damaged skin or body openings. When the larvae hatch they feed on living tissue, creating wounds that, at best, can severely damage livestock hides, and at worst, lead to terrible illness and even death. According the NVAD, “The USDA-APHIS estimates that if this pest were reintroduced into the United States, the livestock industry could suffer $750 million in production losses annually (USDA-APHIS Screwworm Fact Sheet, 2002).”


The take home message for owners is simple: If your pet is under treatment and is not responding as expected (or anything else about the case just seems “off” to you), ask your veterinarian if it’s time to start considering a “zebra” as a potential cause.



Dr. Jennifer Coates