Disease ABCs: Anthrax, Botulism, Corynebacterium

Although the causative bacteria are naturally found in the soil and exposure is an everyday occurrence, most owners will never need to treat their horses for pigeon fever, botulism, or anthrax. These infections are not common, but they can have serious consequences. In some cases, an owner’s awareness of the signs of illness might save a horse’s life.

Anthrax is caused by Bacillus anthracis, which can infect the horse through ingestion of grass or water heavily contaminated with the bacteria. Insects feeding on carcasses can spread the disease to nearby animals. Weather changes may increase the risk: a rainy period followed by several weeks of warm, dry weather may encourage bacterial growth in the soil, and a dry spell followed by storms may spread waterborne bacteria into ponds or grazing areas. Sporadic outbreaks are reported worldwide, most often in regions with a warm climate and slightly alkaline soil. In recent years, a few cases have been reported in Australia, Canada, Minnesota, North and South Dakota, Nebraska, and Texas. Equines are not as susceptible to anthrax as ruminants such as sheep or cattle. Affected horses show a high fever, diarrhea, and loss of appetite. A discharge of black blood from the mouth, nostrils, and anus occurs as the disease progresses, and survival is rare for seriously ill animals.

Protection is available by administering anthrax vaccine. Two initial injections four weeks apart are followed by an annual booster. Because anthrax is not common in horses, most veterinarians do not routinely give this vaccination unless there are outbreaks in a particular area. Recently publicized human deaths from anthrax infection have increased awareness of this disease, but it is not considered to be on the rise in the equine population.

Botulism occurs in horses affected by the extremely potent neurotoxin produced by Clostridium botulinum, an anaerobic microbe widely distributed in the environment. There are at least eight distinct strains, with 85% of U.S. cases caused by type B toxin found in decaying vegetation, spoiled silage, or forage contaminated by toxin leaching out of dead animals. Type C, rare in the U.S., has been reported in Europe and was linked to the deaths of 33 yearlings in Australia several years ago. Signs of disease include muscular weakness and trembling. There may be drooping of the eyelids, tongue, and tail, or feed and water spilling from the mouth. Dilated pupils are often noticed. As the disease advances, the horse stops breathing and collapses.

The disease may progress slowly over several days, or death may occur soon after the first signs are manifested. Horses are extremely sensitive to the toxin, and a very small amount may be fatal. Often toxin cannot be recovered from the gastrointestinal tract of the affected animal and the diagnosis is reached only after elimination of other possibilities. Wound botulism may follow castration, hernia repair, or other surgery. Puncture wounds and abscesses from injections may also provide an environment conducive to growth of the bacteria. Type B botulism affecting foals up to about two months of age is known as shaker foal syndrome. Managers may notice milk spilling from the mouth as the foal tries to nurse. Further signs are stiff gait, weakness, and trembling followed by collapse. Mortality is about 90%. In areas where this disease has been reported, mares can be vaccinated during the last trimester of pregnancy to protect against shaker foal syndrome. Toxoid has been developed to prevent type B botulism, but most horses are not routinely vaccinated because the disease is not regularly encountered. A vaccine for type A was developed for mink and has been used in horses, but is not labeled for this application.

Antitoxins for treatment of affected animals must be used early in the disease process for the best results. Horses that survive need up to six months to recover full use of nerves and muscles. Measures to prevent botulism include storing hay in dry areas and disposing of any forage showing moist spoilage. Hay bales should be inspected for rotted or molded grass or the inclusion of dead mice or other small animals. The transmission of botulism in prepared feeds is uncommon but has been reported in pelleted feed contaminated with dead animals. Any feed or grain having an unusual odor or appearance should not be offered to horses.

Dryland distemper, Colorado strangles, and pigeon fever are three descriptive names for the same bacterial disease. Corynebacterium pseudotuberculosis, an organism living in the soil, invades the horse’s body through an insect bite or another break in the skin or mucous membrane. The disease is found worldwide, often in young horses living in warm, dry climates. In some individuals the infection manifests as abscesses throughout the body and requires aggressive antibiotic therapy, while other horses develop ulcerative lymphangitis characterized by lameness. In the third and most common disease form, abscesses occur in the muscles of the chest and along the horse’s midline. Fever, lethargy, and weight loss may be noticed weeks or months before the large, deep abscesses swell enough to give the horse a pigeon-breasted appearance.

After a veterinarian opens and drains the pockets of infection, chances for a full recovery are good, although a number of horses continue to form new abscesses for several months. Because of the slow development of external signs of illness, the potential exists for infected but symptom-free horses to be shipped to an area where the disease is less often encountered. When abscesses and swelling are noticed in these cases, the local veterinarian may not immediately consider a diagnosis of pigeon fever. Once treatment begins, the horse may take two to three months to heal, so prompt identification of the problem is important in returning the horse to use and health. Several strains of the C. pseudotuberculosis bacterium cause local variations in disease incidence. For example, pigeon fever is seen in the United States but not in Australia, where the organism occasionally causes ulcerative lymphangitis in horses but is recovered most frequently from sheep and goats.