Some infections are specific for a particular animal while others may infect many different species. Anthrax infects many animals including man. Horses also develop potentially fatal infections when infected with this bacterium.
Bacillus anthracis, the causative organism of anthrax, is a gram-positive aerobic bacillus. An important factor in the disease caused by B. anthracis is the bacterium’s ability to produce spores. Spores of B. anthracis resist environmental pressures remaining viable in the soils for many years. Animals grazing on pasture contaminated by anthrax ingest these spores and develop the illness. The disinfection of pastures contaminated by B. anthracis spores is difficult to achieve. It involves the use of formaldehyde and frequently the removal and replacement of contaminated topsoil.
Signs of anthrax infection in horses include fever, chills, severe colic, anorexia, depression, weakness and bloody diarrhea Some horses develop a swellings on the neck, sternum, abdomen and genitals. Death usually occurs within 2-3 days after onset of these signs. Between the ingestion of the spores and the appearance of these signs, there is an incubation period. Usually this incubation period is 3-7 days long but it may range from 1−14 days.
Diagnosis of anthrax requires a qualified veterinarian. A number of conditions produce similar signs to anthrax infection. In horses, acute infectious anemia, purpura, colic, lead poisoning, lightning strike or sunstroke may all produce similar signs to anthrax. To diagnose anthrax in animals a veterinarian collects clinical samples for laboratory testing by culture or polymerase chain reaction (PCR).
Treatment with the antibiotic penicillin or oxytetracycline is treatment of choice. To be effective treatment must start as soon as signs of infection occur. Even with prompt treatment fatalities still occur. Other antibiotics that may be of use in combating anthrax include amoxicillin, chloramphenicol, ciprofloxacin, doxycycline, erythromycin, gentamicin, streptomycin and sulfonamides.
Horses not showing signs of infection require preventative treatment. Moving the animals to fresh pasture may prevent infections developing. All these horses require treatment with a long-term antibiotics and vaccination 7-10 days later.
The administration of antibiotics and vaccines concurrently is contraindicated. Animal anthrax vaccine contains live bacteria of the non-capsulated Sterne strain of anthrax. Antibiotics would kill this vaccine strain before it could stimulate an immune response in an animal. Annual vaccination with the Sterne strain vaccine helps prevent anthrax in animals living in endemic areas
Control of infection measures form an important in managing anthrax infections, the Merck Veterinary Manual lists these as
1) Notification of the appropriate regulatory officials
2) Rigid enforcement of quarantine (after vaccination, 2 weeks before movement off the farm, 6 weeks if going to slaughter)
3) Prompt disposal of dead animals, manure, bedding or other contaminated material by cremation (preferable) or deep burial
4) Isolation of sick animals and removal of well animals from the contaminated areas
5) Cleaning and disinfection of stables, pens, milking barns and equipment used on livestock
6) Use of insect repellents
7) Control of scavengers that feed on animals dead from the disease
8) Observation of general sanitary procedures by people who handle diseased animals, both for their own safety and to prevent spread of the disease.
If you suspect your horse has anthrax contact your veterinarian immediately. Delay in starting treatment may cost the animal its life.