By Russ Daly
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.
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