Anthrax Affects Everyone

Anthrax is a rapidly developing, acute or sub-acute, fever-producing, infectious disease of all warm-blooded species, including humans. It is a dangerous, reportable disease that when suspected in an animal should be brought immediately to the attention of a veterinarian. Anthrax outbreaks are commonly found in regions with alkaline (basic) soil, such as the Great Plains states (North Dakota to Texas) and the Intermountain Basin states (Nevada and Utah). Field outbreaks of anthrax in these areas are more prevalent than in other geographical parts of the United States.

Its infectious nature was first demonstrated in 1836, and the causative bacteria, Bacillus anthracis, was first cultivated by Robert Koch in 1876 and Louis Pasteur in 1877. The pioneering work of Pasteur showed that by changing the bacteria Bacillus anthracis, a vaccine could be produced to protect susceptible animals.

Anthrax in the horse is characterized by a high fever (up to 107°F) and a quick death. Rigor mortis is absent or incomplete in an anthrax carcass. Horses are mildly resistant, therefore cases might be seen before they die. In cattle and sheep, outbreaks declare themselves with dead animals. Horses frequently have ventral edema (swelling), are fevered, and are obviously “sick.” Horse carcasses often have dark blood oozing from the mouth, nostrils, and anus. Diagnosis can be based on these clinical signs, but laboratory confirmation is based on finding Bacillus anthracis in a blood sample or blood culture.

Time is of essence in diagnosing a horse herd with anthrax--the "quick killer." Although blood cultures take longer for results, they are the final confirmatory measures for the disease and can be important if possible human exposure has occurred or if litigation should follow an anthrax episode.

How It's Spread

Transmission of anthrax usually occurs through the interrupted feeding of horse flies (Tabanidae) or through the horse's natural grazing activity. A unique feature of anthrax is that the bacteria has two principal forms, the first of which is the resting form or "spore form." Spores have been known to survive for 100 years in bison bones. During times of summer flooding, the spores can be consumed by horses or other animals that graze on floodplain pastures. When this happens, the second form of anthrax bacterium develops, called the "vegetative form." This form rapidly multiplies in the animal's bloodstream, resulting in subsequent blood poisoning (septicemia). Accompanied by the formation of toxins, this sequence can lead to death unless promptly treated.

Since humans can get anthrax, special precautions should be taken. Don't handle suspected anthrax cases without protective rubber gloves. The less you move or transport any carcass, the better. Remember, the spores can live in the soil at least 100 years—don't "scatter" potential sources of spores in the environment.

Handling Anthrax

In dealing with an outbreak of anthrax in a horse herd, the following things should occur:

  • Your veterinarian should promptly establish a field diagnosis followed by a laboratory confirmation. If anthrax is suspected, initiate treatment immediately. Animals respond quickly to long- acting antibiotic treatment.
  • Body temperatures should be taken and recorded on every horse. Any animal with a temperature more than two degrees above normal should be treated with penicillin or a penicillin derivative. Early antibiotic administration might save some animals.
  • The incubation period for anthrax is three to seven days. Temperatures should be recorded for the next 10 days to be sure that anthrax has been overcome in the herd.
  • Vaccinate non-febrile herd members (those without fever). There is no vaccine licensed for use in horses, but the Sterne’s strain, non-encapsulated live spore vaccine licensed for use in cattle has been used to vaccinate horses. The initial doses should be administered two to three weeks apart, followed by annual revaccination. The horse might show edema around the injection site and in surrounding dependent tissues.

Anthrax is considered a potential military germ warfare agent, so take precautions to protect yourself at all times when anthrax is diagnosed or suspected in your herd. If you think you've been exposed to anthrax, consult your physician immediately. Sores or carbuncles on the skin are the most common form of anthrax in humans, which can be effectively treated with antibiotics.

About the Author

Ralph C. Knowles, DVM

Ralph C. Knowles, DVM, is a distinguished life member of the American Association of Equine Practitioners. He is also an international consultant for equine piroplasmosis and electronic identification of animals.

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