Anthrax is a rapidly developing, acute or sub acute, fever producing, infectious disease of all warm-blooded species including humans. Anthrax is a serious, dangerous, reportable disease that when suspected should be brought immediately to the attention of your veterinarian. Anthrax outbreaks are most commonly found in regions with alkaline soil such as the Great Plains states (North Dakota South to Texas) and the Intermountain Basin states (Nevada and Utah). The frequency of field outbreaks of anthrax in these areas is more prevalent than in the other geographical parts of the United States.
Its infectious nature was first demonstrated in 1836. The causative bacteria Bacillus anthracis was first cultivated by Koch in 1876 and Pasteur in 1877. In fact the pioneering and classical work of Pasteur showed that by changing the bacteria Bacillus anthracis, a vaccine could be produced to protect susceptible animals, against infectious diseases.
Anthrax in the horse is characterized by a high fever (body temperature may reach 107 degrees) and a rapidly fatal course. 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, 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; however, it is routine to obtain a laboratory confirmation by demonstrating Bacillus anthracis from a blood sample and/ or a 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 disease confirmatory measure, and can be important if possible human exposure occurs 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 principle forms: First, 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 flood plain pastures. When this happens, the second form of anthrax bacterium develops called the vegetative form. This form rapidly multiplies in the animal's blood stream, resulting in subsequent blood poisoning (septicemia). Accompanied by the formation of toxins, this sequence can lead to death unless promptly treated to stop the progression of the blood poisoning. Since humans can contract anthrax, special precautionary measures need to be taken. Do not handle cases of suspected anthrax unless you wear protective rubber gloves. The less you move or transport a horse carcass the better. Remember the spores can live in the soil or bison bones at least 100 years; don't "scatter" potential sources of spores in the environment.
In dealing with an outbreak of anthrax in a horse herd the following needs to occur: 1) 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. 2) Body temperatures should be taken and recorded on every horse herd member. Any animal in the herd that reveals a temperature in excess of two degrees above normal (99.5 F) needs to be treated systemically with penicillin or a penicillin derivative. Antibiotics administered early in the disease might save some animals. 3) The incubation period for anthrax is three to seven days. Temperatures need to be taken and recorded for the next ten days to be assured that anthrax has been successfully overcome in the herd. 4) Vaccination of the non-febrile herd members (those without fever) should be completed. There is no vaccine licensed for use in horses, but the Sterne's strain, nonencapsulated live spore vaccine that is licensed for use in cattle has been used to vaccinate horses. The initial doses of the vaccine should be administered 2-3 weeks apart followed by annual revaccination. The horse might show edema around the injection site and in surrounding tissues.
Anthrax is considered a potential military germ warfare agent, so precautions to protect yourself at all times when anthrax is diagnosed or suspected in your horse herd. If you think that you have been exposed to anthrax, consult your physician immediately. Sores or carbuncles on the skin are the most common form of anthrax in humans, but can be effectively treated with antibiotics.
About the Author:
Dr. Knowles 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.