How the veterinary lab diagnoses anthrax in a beef herd
Professor, SDSU Extension Veterinarian, State Public Health Veterinarian
Written collaboratively by Russ Daly and Marlene Braun (Microbiologist, SD Animal Disease Research and Diagnostic Lab).
Anthrax is a serious disease of cattle that pops up somewhere almost every year in South Dakota. It’s caused by a bacteria that survives as a very tough spore form in the soil. Cattle encounter the bacteria (Bacillus anthracis) when they graze close to the ground or when spores have been washed up on grass from previous pasture flooding. Once the spores are eaten by the cow or bull, they activate into rapidly growing bacteria that have devastating effects on the animal’s whole system. The result is a rapid death, usually progressing so suddenly that no signs of illness are observed prior to death.
Knowing whether a death on pasture has been caused by anthrax is important for several reasons. The animals grazing the same area are usually at risk of contracting the disease as well. Prompt treatment with antibiotics and vaccine can help prevent further losses. Animals that have died from anthrax need to be disposed of by burning and burial within 36 hours of death so they don’t become a source of spores for their herdmates. And, importantly, anthrax can affect people too – with the potential to cause serious skin infections.
Detecting Bacillus anthracis in the dead animal is how diagnosticians at the South Dakota Animal Disease Research and Diagnostic Lab (ADRDL) confirm anthrax as the cause of death. Lab experts will initially make a smear of fluid from the sample onto a microscope slide, then stain it with a special Giemsa stain. The anthrax bacteria readily take up this stain, and the microbiologist can then view their very characteristic shape under the microscope (they look like a line of boxcars in a train). The Giemsa stain is considered a screening test – to get a quick idea of whether anthrax is present.
The ADRDL’s confirmatory test is a culture – growing the bacteria on a petri dish and identifying the growth as Bacillus anthracis via MALDI-TOF technology or other means. This usually takes overnight. And of course, all of this is done by the microbiologist under strict lab safety considerations. Other veterinary labs may use other methods but sample handling considerations are usually similar.
Because Bacillus anthracis is plentiful throughout an animal that has died from anthrax, it can be detected from many different sample types. However, taking a sample from an anthrax suspect is more complicated than sampling for other diseases. For one thing, if a veterinarian cuts opens the carcass of an animal dead from anthrax, the bacteria become exposed to oxygen. This causes them to quickly revert to their resistant spore form, creating new opportunities for other cattle to encounter anthrax. Opening the carcass also increases the risk of anthrax infection for the veterinarian through a cut or abrasion on the skin. For those reasons, veterinarians have long been cautioned not to open the carcass of an anthrax suspect.
How, then, is a veterinarian supposed to obtain a sample for the lab without opening the carcass? In most cases, the vet can obtain blood (or fluid containing blood) from the animal’s jugular or tail vein. That blood is then transferred to a red-top blood tube and sent to the ADRDL. This is the preferred sample. If this just isn’t possible, then a sample of organ or even an ear can be sent instead. Hot summer temperatures can quickly complicate sampling efforts due to their effect on the carcass; sometimes a proper sample just isn’t obtainable. Prompt detection of pasture deaths and sampling by a veterinarian becomes increasingly important as temperatures rise.
Along with the sample, the vet sends the appropriate test form that lists the animal ID, and the type and number of samples sent. Doing this removes any source of confusion that could delay lab testing. In the past, ADRDL personnel have been sent syringes, needles, unnecessary blood tubes, and even soil samples to be tested for anthrax. None of these items are appropriate for testing and present unnecessary hazards to the people working in the lab.
Anthrax testing is a critical part of dealing with this serious disease. Cattle producers should contact their veterinarian when anthrax is suspected, and veterinarians can always consult the ADRDL for sampling recommendations. Knowing what samples are best for your veterinarian to take for a proper diagnosis will help ensure timely and accurate answers that can mean the difference between life and death for your cattle.
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Outtagrass Cattle Co. cartoon by Jan Swan Wood for the Oct. 23, 2021, edition of Tri-State Livestock News