Anthrax can affect horses
Grazing livestock, including horses, can contract naturally occurring anthrax from the soil. The disease is usually fatal; death often happens so quickly anthrax is not diagnosed until after the animal dies. Photo by Helena Lopes/Unsplash

Silent & Deadly: Protect Your Horse From Anthrax

Ask the average person about anthrax and they’ll probably bring up terrorism, remembering the envelopes filled with deadly white powder sent to high-profile individuals after the Sept. 11 attacks. They probably won’t mention dead horses or cows.

However, anthrax occurs naturally and has been blamed with killing livestock, in some cases horses, for many years. Extremely lethal, B. anthracis is a spore-forming bacterium found in the soil in many parts of the world, including several areas in the United States. It also can infect people, causing skin lesions or, in some cases, death. Although anthrax is not common in horses — it’s far more often seen in cattle — equines in some Western performance horse industry disciplines that involve cattle, or those in cattle-heavy states like Texas, could be more at risk to anthrax than those without bovine exposure.

“The unique thing about horses in Texas is that, as far as in the U.S., we’re one of the states that has the most amount of cattle and our horses are still associated with cattle,” said Dr. László Hunyadi, DVM, former medicine section chief of internal medicine at ESMS in Weatherford and now a professor at Texas Tech University. “So, many of our horses still are susceptible to it because of the high number of cattle.”

Because anthrax is so lethal to livestock — the first sign of a problem is often dead animals — and can cause illness in humans, prevention is key for horse owners whose equines live in areas where anthrax is known to occur, have interaction with cattle and experience the weather conditions that are favorable to infection.

Anthrax under a microscope.
Anthrax is caused by B. anthracis, a spore-forming bacterium that occurs naturally in the ground. Once inhaled or ingested by an animal, the bacteria multiplies and spreads toxins throughout the body. Photo by Janice Haney Carr, Courtesy of the CDC.

What is Anthrax?

Humans and animals have been living with anthrax for centuries. According to the U.S. Centers for Disease Control (CDC), anthrax is believed to have originated in ancient Egypt and Mesopotamia. Some scholars believe anthrax could be the deadly livestock disease, or one of the diseases, described in the book of Exodus as the fifth of the 10 plagues of Egypt:

“Behold, the hand of the LORD is upon thy cattle which is in the field, upon the horses, upon the asses, upon the camels, upon the oxen, and upon the sheep: there shall be a very grievous murrain.”

There is no visible evidence of the bacteria in the soil or in the vegetation and, once in the soil, it can remain there for years until disturbed and ingested or inhaled by grazing animals. Cattle seem to be the most susceptible, Hunyadi said, followed by sheep, horses, and then goats.

Once inside the animal, the bacteria travels to the nearest lymph nodes and begins to multiply. Soon, the animal has high levels of bacteria in its blood, which is circulated to other parts of the body and causes death. “Usually what anthrax causes is a high fever, hemorrhaging, and then sudden death,” said South Dakota State Veterinarian Dr. Dustin Oedekoven, whose state is one that often experiences at least some annual cases of anthrax. “Usually, sudden death is the first thing that’s seen.”

Other signs can include an elevated heart rate, anorexia or signs of colic. Hunyadi said they also can have edema, or exhibit staggering or convulsions. Most affected horses die within two to four days after the onset of clinical signs but often perish so quickly they are found dead and anthrax is diagnosed post-mortem.

Carcasses of animals killed by anthrax leak blood from their orifices, and the blood also fails to clot. Because humans can contract anthrax — through cuts on the skin, by inhaling the spores or eating affected meat — carcasses should not be disturbed and a veterinary professional should be immediately notified if anthrax is suspected.

Anthrax outbreaks are found in areas of drought followed by flooding, like the pastures in the picture.
Anthrax outbreaks are most often found in areas with alkaline soils that have undergone a series of extreme weather events, such as a drought followed by flooding. Photo by Kelsey Pecsek Hruska.

Where Outbreaks Occur

Most anthrax outbreaks in the United States happen where there’s a combination of alkaline soil and a series of extreme weather events, such as a prolonged dry spell or drought followed by heavy rain or flooding.

“Really alkaline-type of dirt; it has to be with the higher PH in the ground that would allow the bacteria to exist in both its vegetative form, as well as in its spore form,” Hunyadi said. “That’s why it’s so resilient as well, and this is why animals can both ingest it, as well as inhale it.” Texas, which in 2019 had its largest anthrax outbreak in a decade when 20 properties reported cases, is one of several states with the alkaline soil conditions where most outbreaks tend to occur in the U.S.

Anthrax cases in the Lone Star State tend to be reported in counties in an area around the Hill Country cities of Uvalde, Ozona and Eagle Pass, officials said. Outbreaks in other parts of the country are most often reported in parts of Nevada, Nebraska, Minnesota, South Dakota and North Dakota.

It’s also been reported in Mexico and the Canadian prairie provinces, and Hunyadi said outbreaks have happened in other parts of the U.S. when the right weather conditions happen.

“Typically, a wet winter precedes a long, dry spring followed by heavy rainfall, and these conditions play a factor in outbreaks,” Hunyadi said. “Farms with poor drainage and high amounts of organic content also serve as extreme risk factors.”

Cattle Connection

Cattle, who graze close to the ground, are the animal that veterinarians say is the most likely to get anthrax. In South Dakota, anthrax is typically only found in horses during severe outbreaks, said Oedekoven, who noted the last equine anthrax cases in the Rushmore State were in 2005 with horses grazing out on the range.

“Horses will sometimes get anthrax, although much less frequently than cattle,” he said. “I think the last time we had a case in horses was in 2005, a particularly bad year where we had 55 cattle and buffalo herds affected, mostly along the Missouri River, which runs right down the middle of our state.” During last year’s outbreak in Texas, six of the 20 properties with anthrax cases reported to the Texas Animal Health Commission involved horses. The commission did not know how many horses were affected on those properties.

Anthrax most often affects cattle.
Cattle are the domestic animal most affected by anthrax. Experts say cattle can inadvertently put horses in the area more at risk for contracting anthrax because the carcasses of dead cattle put the deadly bacteria back into the ground. Other animals, including horses, can consume the bacteria by grazing near the infected carcass. QHN File Photo.

Hunyadi believes the link between cattle and increased anthrax risk in horses is that when cattle or other animals die of anthrax, their carcasses put more anthrax bacteria back into the soil. That bacteria can then be consumed or inhaled by other animals, including horses, that graze near the carcass if it is not properly disposed, usually by incineration.

“They don’t get it from the cow, but from the soil where the cow died,” he explained. That’s why Hunyadi believes vaccinating livestock for anthrax can help lessen the chance of horses getting sick: the reasoning being that if cattle, which seem to be more susceptible to anthrax, aren’t getting infected, then they won’t die and put the bacteria back into the soil for other animals to pick it up.

“I won’t say all the time, but many times if a horse got it, you more than likely had previous cattle that same year that had gotten it, as well,” Hunyadi said.


There is an effective and readily available anthrax vaccine for horses, but veterinarians said horse owners shouldn’t go for the needle if they don’t need it. The vaccine is known to have side effects in horses, such as injection site swelling and, in some cases, pus discharge.

Though Hunyadi and Oedekoven said it makes the most sense to vaccinate in areas with known anthrax problems when weather conditions are favorable to outbreaks, they said owners should still talk to their veterinarians to determine if vaccination is the right course of action. They also said there are other nonvaccine prevention practices horse owners in anthrax-prone areas can take during an outbreak or during times of elevated risk due to weather conditions.

“You could move them [horses] from a grazing pasture to feeding hay in an off-ground hay feeder or move to other forms of feed so they’re not grazing where the soil could potentially be contaminated,” Oedekoven said.

If horses are being ridden through high-risk areas that cattle have grazed on or in areas of high risk that have mucky, stagnant water from a flood, Hunyadi said the animals should be washed off afterward.

“If you’re gathering cattle and you go out into particular fields to move cattle, push them to other pastures or anything like that and your horse is going through that type of debris, when you come back, really washing them well with soap and water, and getting that stuff off them is paramount,” he said.

*This story was originally published in the June 15, 2020, issue of Quarter Horse News.