Anthrax: A concern for herbivores

As we enter summer months, livestock producers and outdoor enthusiasts across the state, especially those in South Texas, have more to think about than rising temperatures. 

Anthrax, sometimes incorrectly referred to as bovine anthrax, is a bacterial disease caused by Bacillus anthracis, which occurs naturally in soil. The Texas A&M Veterinary Medical Diagnostic Laboratory, TVMDL, historically sees a rise in positive cases in June, July and August, followed by a slow decrease through September and October.

How concerned should you be about anthrax? Two Texas A&M AgriLife experts weigh in and explain what anthrax is and what you should do if you suspect a positive case in your area.

Terry Hensley, DVM, is the TVMDL assistant agency director and a Texas A&M AgriLife Extension Service veterinarian. He has broad knowledge of veterinary medical topics and has been a valuable resource for the Texas Animal Health Commission, TAHC, Texas Parks and Wildlife Department and the public during previous outbreaks.

Narayan Paul, Ph.D., DVM, Diplomate ACVM, is a board certified veterinary clinical microbiologist and the bacteriology section head for TVMDL, where anthrax testing takes place.

How do animals contract anthrax?

Hensley: Anthrax is not contagious like the flu. The most common route of infection is by ingestion of the bacterial spores from contaminated forage, soil or water. Infection by inhalation or through skin wounds is also possible.

The disease can infect all animals with some being more susceptible than others. Herbivores such as cattle, sheep, goats and horses, as well as wild herbivores like deer, are more susceptible than swine and carnivores such as dogs.

Can humans get anthrax from animals?

Hensley: Humans are considered relatively resistant to anthrax; more resistant than cattle but less resistant than dogs. Cases in humans are uncommon in the U.S. In rare circumstances, infection could be acquired from the inappropriate handling of the carcass of an animal that died of the disease. The source of infection would most likely be by ingestion, inhalation or wound contamination by anthrax spores.

Anthrax poses little risk to the average person. Veterinarians, ranchers and hunters who work, live and recreate in areas of the state where anthrax is known to occur should be aware of the disease and take precautions when finding a carcass with the classic signs of anthrax. Normal day-to-day activities pose little threat of acquiring infection.

In Texas, the “anthrax triangle” is an area created by connecting Uvalde, Ozona and Eagle Pass. Encompassing parts of Crockett, Edwards, Kinney, Maverick, Sutton, Uvalde and Val Verde counties, this is where most positive cases originate.

What are the signs or symptoms of anthrax?

Hensley: The bacteria produce two toxins that attack various organ systems resulting in fever, depression and organ system failure. In highly susceptible animals such as cattle, sheep, deer and other herbivores, death may occur suddenly, so no signs of illness are noticed beforehand. Horses may exhibit signs of subcutaneous edema and a more protracted course of disease than cattle. Animals that die from the disease will often have bloody discharge from the orifices, absence of rigor mortis and lack blood clotting. These can all be signs of an anthrax-related death, but are not absolute without lab confirmation.

Treatment success is unlikely in animals that are severely ill when discovered. If illness is detected in the febrile stage but before other clinical signs, treatment with appropriate antibiotics may be successful.

There is an anthrax vaccine available for use in livestock species. However, the vaccine is not approved for use in dogs.

How many positive cases of anthrax does TVMDL generally see in a year?

Paul: We see three to four positive cases per year. However, during 2019, TVMDL confirmed 23 positive cases in several species including cattle, white-tailed deer, goats, horses and exotic antelope.

What should people do if they suspect an animal of having anthrax?

Paul: If you see the sudden death of an animal with leakage of uncoagulated blood from natural openings, contact your local veterinarian, the TAHC or TVMDL.

Hensley: People should not open a carcass suspected of anthrax. If you must handle the carcass, wear disposable gloves, a mask, long pants and a long sleeve shirt or coveralls. Launder your clothes in hot water. In anthrax endemic areas, do not pick up bones or antlers since you have no way of knowing whether the animal died from the disease or not. The spores can remain in the environment for decades.

How should samples be taken for submission to TVMDL?

Paul: The preferred specimen is blood collected from a superficial vein, such as the jugular. The sample should be submitted in a sterile red top or purple top anticoagulant blood tube. Swabs of bloody fluid oozing from body orifices may also be submitted in a sterile red top tube or commercial transport medium. Although not recommended, if the carcass has been opened, the spleen, lung and liver may be submitted for bacterial culture. Ears should not be submitted for anthrax culture tests as they are relatively avascular and are likely to yield inaccurate results.

It is important to collect samples from animals at the point of death or freshly dead as Bacillus anthracis can die in an anaerobic carcass after a period and would yield a false negative bacterial culture.  

How does testing work?

Paul: Upon arrival of anthrax-suspect samples in the lab, they are streaked onto different bacterial culture media that contain essential growth factors/nutrients for the growth of the bacterium.

The cultured media are then incubated at 98.6 degrees in aerobic condition for 48 hours. The culture media plates are examined at 24 hours and 48 hours to see if any suspected anthrax bacteria appear in the form of ground-glass, dry colonies and non-hemolytic colonies on blood agar plates.

Two confirmatory tests, bacteriophage lysis and penicillin antibiotic tests, are performed on any suspected bacteria. Anthrax bacteria are positive for bacteriophage lysis and susceptible to penicillin antibiotic.

More information

Additional information on anthrax can be found at the Texas Department of State Health Services website.

Those interested in anthrax testing should call one of TVMDL’s full-service laboratories in College Station or Canyon. For more information on TVMDL’s testing services, visit tvmdl.tamu.edu.