AAEP 2002: Infectious Diseases Subdued, Not Eradicated

In discussing anthrax, screwworms, and piroplasmosis, Irby said that the three disease are related in that each "can affect equids, has the potential to devastate domestic livestock, is present in many foreign countries, and may first be detected by veterinarians in private practice."

Irby re-iterated that it is imperative that private practice veterinarians realize that they are the sentinels for any unusual disease conditions or exotic pests affecting animals in the United States.

"I was fortunate to be part of the first group of U.S. veterinarians detailed to the United Kingdom in 2001 to assist with the foot and mouth disease outbreak. I witnessed just how easily a contagious disease can be spread through routine marketing channels and how devastating it can be to the livestock industry of an entire nation. Even more impressive was the realization that a non-zoonotic disease involving food animals can also impact businesses seemingly unrelated to agriculture. Tourism, probably the number one industry in the United Kingdom, suffered terrible economic losses during the foot and mouth disease outbreak. Tourism is important to the U.S. economy. Our livestock are marketed on a much larger scale and subjected to less movement restrictions compared with those in the United Kingdom. We must never lose sight of our part in providing a strong national defense."

Anthrax is an acute, usually fatal infectious disease caused by B. anthracis. Irby explained that while anthrax is primarily a disease of herbivores (such as cattle, horses, deer, sheep, etc.), it can affect all warm-blooded animals, including humans (of which we became acutely aware during the anthrax terrorism attacks last year).

"Although anthrax spores have been found naturally in soil samples from around the world, the organisms cannot be regularly cultivated from soils where there is an absence of endemic anthrax," noted Irby. "In the United States, there are recognized endemic areas of infection in South Dakota, Nebraska, Arkansas, Texas, Louisiana, Mississippi, and California." He said even in these endemic areas, disease occurs irregularly, often with many years between outbreaks.

Irby reminded veterinarians not to open the carcass of an animal suspected of dying from anthrax, and not to salvage hides, horns, or antlers from those animals since the bacilli from the anthrax are known to be in bloody discharges. These bacilli can naturally seep from the dead animal through body openings, and they form spores that are resistant to extremes of temperature, chemical disinfectants, and desiccation (drying out). "For this reason, necropsy of an animal that died suddenly in an endemic area and exhibits bloody discharges from body openings is not recommended," noted Irby.

He said spores can live in the soil for years, and that outbreaks often are associated with drought conditions. All carcasses and contaminated materials should be burned or covered with quicklime (calcium oxide) and buried at least two meters deep.

Millions of dollars were spent eradicating screwworms (Cochliomyia hominivorax) from the United States by 1966, and from Mexico by 1991. However, Irby noted, the pest still flourishes in Panama, some Caribbean islands, and most of South America. Eradication was mainly through using sterile male adult flies.

The screwworm fly lays eggs on the edges of skin wounds, and around the prepuce of stallions and geldings. Larvae hatch quickly, in 12-21 hours later, and migrate into the wound. After feeding for five to seven days, the larvae exit the wound, fall to the ground, and burrow into the soil, where they complete their life cycle. Wounds can become greatly enlarged because of multiple infestations, and could result in death of the animal if not treated.

Equine Piroplasmosis
This infectious disease is caused by a protozoan parasite (as is equine protozoal myeloencephalitis, or EPM). The two species of protozoa known to affect horses are Babesia equi and Babesia caballi. Only Canada, Australia, Japan, England, Ireland, and the United States are not considered endemic areas, noted Irby. Piroplasmosis has been proved to be transmitted by ticks no longer widespread in this country, such as Dermacentor nitens and Boophilus sp. It has been suggested that domestic ticks, such as Dermacentor albipictus and Dermacentor viriabilus, could prove capable of transmission.

"Regulatory control of equine piroplasmosis relies on ensuring that any equine presented for entry from endemic countries are thoroughly checked and found free of ticks and on infected equine testing positive to the compliment fixation (CF) test at a USDA import station," explained Irby. "It has been suspected that, before being presented for importation, infected horses are being treated with a babesicidal drug that could produce false-negative results on the CF test. There are increasing concerns that this practice could allow infected horses to enter the United States. USDA is now considering other types of pre-entry tests, such as the competitive ELISA (C-ELISA), the indirect fluorescent assay (IFA), and the polymerase chain reaction (PCR), as alternatives to the CF test."

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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