Anthrax is one of the oldest killers of humans and livestock and was described in some our earliest recorded history—several thousand years ago. It is seen worldwide and called by many names, including splenic fever, charbon, milztrand and woolsorter’s disease. Anthrax is caused by a bacterium, Bacillus anthracis, and occurs sporadically in North America. In the U.S. it is associated with sudden death of cattle, sheep, goats, horses, bison and hoofed wildlife, though it can infect all warm-blooded animals.

This acutely contagious and deadly disease is usually not spread from live animal to live animal (except occasionally by flies that bite a sick animal and then bite a healthy one), but is typically transmitted via spores found in and around the carcasses of animals that died of the disease. Horses most commonly become infected by ingesting spores from the soil. These spores can blow around with dust or be spread by floodwaters and then contaminate pastures and hayfields, ending up on the grass or in the hay. 

The anthrax organisms within an animal’s body or bodily secretions can be readily destroyed by ordinary disinfectants or pasteurization/high heat. But once the animal dies and the carcass is opened and these bacteria are exposed to air, they form spores. These spores are resistant to heat, cold, freezing, chemical disinfectants or drying, and can survive in contaminated soil for decades. The original carcass may be long gone - torn apart by predators and scattered, or decomposed many years ago - but some spores are still viable in the surrounding soil. These can be spread to other areas by flooding or other situations that disturb the soil.

Dr. Charles Stoltenow, Extension Veterinarian, North Dakota State University, says anthrax often occurs along old cattle drive trails where bison migrated many years ago. “The spores last at least 100 years (some researchers say 250 years) and lie dormant in the soil. Grazing animals pick up spores when grass is contaminated. Hay that contains spores may cause anthrax outbreaks during winter. Animals and humans can also be infected if spores are inhaled or get into a wound,” he says. Once they get into the body, they germinate and spread rapidly.

Jason K. Blackburn, PhD (Associate Professor, Spatial Epidemiology and Ecology Research Laboratory Director, Department of Geography and the Emerging Pathogens Institute, University of Florida) has been studying anthrax for many years. His PhD is in Medical Geography. “I am a spatial epidemiologist or medical geographer. I am jointly appointed to the Department of Geography and as a principal investigator with the Emerging Pathogens Institute. I run the Spatial Epidemiology and Ecology Research (SEER) Laboratory. Our lab is focused on the ecology of zoonotic diseases (those which can affect many species of animals, including humans), particularly the bacterial diseases like anthrax, brucellosis, plague, tularemia, etc.,” he says.

Blackburn has studied anthrax in many countries around the world and has spent a lot of time working with anthrax cases in wildlife and livestock in the U.S., particularly in Montana and Texas. “We have seen cases of anthrax in horses on some ranches in Texas. In my own experience we generally see it in association with other cases, either livestock or wildlife.  We might see these cases on the same ranches, or on neighboring properties, or within the outbreak region,” he says.

Horses are vulnerable to this disease, and the vaccine is labeled for horses as well as cattle. “We recommend that ranchers in a known anthrax zone vaccinate their horses early each year. If they are vaccinating cattle, they should not forget their horses. A couple of the wildlife ranches (that have farmed deer, etc.) that we work with have horses or mules and those ranchers always vaccinate those animals, especially if they can’t vaccinate their wildlife. In any area that has a history of anthrax, we recommend vaccination of horses.”

Local veterinarians would be aware of what might be going on in their area, that year. “The states that have persistent anthrax situations - like Texas, the Dakotas, Montana, Minnesota, etc. - also have well-informed public veterinary services. The university agricultural extension offices in those states often send out pre-season or springtime letters reminding livestock owners to vaccinate,” Blackburn says. Horse owners should also pay attention to these recommendations and contact their local agricultural extension offices. This is a good way to keep in touch with what’s going on with anthrax in any given year.

“Currently there is no single formula, or national pre-season reporting, regarding which years might be worse for anthrax. So, we advise ranchers and horse owners to look to their regional extension offices and state veterinary offices’ websites.  This is a place to start for information,” he says.

This disease is reportable; every anthrax case must be reported to federal authorities. The disease is of national interest, but most of the educational information that an animal owner will get must come from a state source. “It might be a state veterinary office or state public health or animal extension office,” Blackburn says.

Animals with anthrax can die quickly. Cattle that are not being observed closely may not be noticed as being ill, and they are generally found dead. Symptoms may be noticed with a horse, just because the owner sees the horse daily or several times a day.   

Signs Of Anthrax In Horses 

The signs of anthrax depend on how the animal was infected. Horses most commonly are infected by ingestion of anthrax spores (on vegetation, in hay or any feed that may have been contaminated) or through the skin from an insect bite.

This disease progresses so fast that you might not see a sick animal; you just find it dead - especially if spores entered via mouth or nostrils. When infection enters through the skin via injury or insect bites, it stays localized awhile. The affected area becomes hot and swollen, then cold and insensitive. Later, the infection may spread throughout the body, according to Stoltenow.

If you find the animal alive, symptoms include high fever, trembling, staggering, difficult breathing, and swelling over most of the body. There may be frothy, blood-tinged discharge from the mouth. “As the animal dies, blood oozes from body openings. This bloody discharge is heavily laden with anthrax bacteria,” he says.

When spores have been ingested,
the primary signs are depression, fever, chills, colic, severe bloody diarrhea/enteritis and death unless the animal is treated very early in the course of the disease. If the infection is from an insect bite, the first signs are enlarged lymph nodes, swelling in the throat, difficulty breathing, and a large, painful, swollen area at site of bite.

Diagnosis

To diagnose anthrax, your veterinarian may take a blood sample from your horse. In most cases, anthrax is diagnosed after the horse’s death. If anthrax is the suspected cause of death, the carcass should not be moved or opened up until the proper authorities have been notified. Anthrax should be reported to the state veterinarian. If anthrax is diagnosed on your farm, the farm will be placed under quarantine and the remaining animals may be vaccinated against it. 

Treatment

Anthrax is treatable with the proper antibiotic therapy, but many cases of anthrax are undiagnosed until after death of the horse. These bacteria are usually susceptible to antibiotics such as penicillin and oxytetracyline, although the disease must be caught early and treated vigorously for treatment to be successful. Best results are seen when long-acting penicillin can be given very early in the disease. “Some ranchers give penicillin to all their cattle if they lose one from anthrax,” says Stiltenow.

“We’ve dealt with anthrax in our region nearly every year. It’s important to investigate every unexplained death. There’s a good vaccine against anthrax, but many people don’t utilize it unless they are suddenly faced with a problem. It’s always better to give it in the spring, however,” he says.

Risk And Transmission

In the northern hemisphere, anthrax generally appears in warmer months - May through October. “The really bad years seem to be associated with wet spring weather followed by a hot, dry early summer, then sudden thunderstorms or showers and a hot, dry period after that,” explains Blackburn.

Moisture may help percolate spores up to the ground surface. “Some studies suggest that flood plains and alluvial soils are associated with anthrax. Flooding that moves soils around may expose spores,” he says.  

“These bacteria may be able to germinate and live in the rhizosphere of some grasses, like fescue. This may be why grazing animals pick up spores after a wet period followed by green-up of grasses.”    

Blackburn has also studied transmission via flies. “When animals die of anthrax and their carcasses are torn open, blowflies start feeding and laying eggs and can pick up bacteria from tissue they feed on,” he says.

“We know from laboratory studies that go back more than 100 years that anthrax bacteria can survive through the gut of adult blowflies. We find viable anthrax bacteria in material regurgitated by the fly, and in its feces. We also know from old laboratory studies that bacteria can survive in maggots feeding on a carcass - all the way through the pupae stage and adults. We confirmed this in Texas in 2005 during a large outbreak in deer. We captured blowflies feeding on a carcass and isolated bacteria from them,” explains Blackburn.

“In 2010, while mapping outbreaks in west Texas, we collected samples from carcasses, maggots and adult flies, and collected leaves from vegetation surrounding the carcasses. We found bacteria everywhere - on the carcass, in the carcass, in the maggots, the adult flies on the carcass and flying around it, and on leaves (in fly-spot droplets) where the flies landed,” he says.

Any animal eating these leaves could become infected. This transmission route was discovered in the early 1990’s when an anthrax expert from Kruger National Park described this phenomenon in anthrax outbreaks in kudu. Flies were a major factor.  Some of the plants the flies contaminated were part of the kudu diet (not eaten by other herbivores). Kudu were the only animals ingesting bacteria on these leaves.

“In our early studies we found that if we look at leaves around a carcass that’s been there for several days (because the rancher was unable to dispose of it), we find a lot of fly droplets. Around a deer carcass, we find leaves with fly droplets up to 15-plus feet away and up to 7 to 10 feet high. We found a larger area of fly droplets around a bison carcass in 2010 in Texas,” he says.

This is why it’s important to find carcasses quickly and dispose of them, to interrupt the contamination cycle by reducing flies and their activity. Spraying the carcass with bleach or formaldehyde can also help to deter flies.  

“When it’s not safe to burn, some ranchers use a bulldozer, backhoe or grader to knock down brush around the carcass,” says Blackburn. This clears the area to make it safer to burn, and provides fuel for the fire. The researchers speculate that this might also interrupt transmission of anthrax, using the brush around the carcass that has been contaminated by flies and burning it for fuel. 

Another way flies play a role in transmission is that some of them - like horse flies and deer flies - serve as mechanical vectors. They land on a sick animal (with bacteria in the bloodstream), bite it, and while sucking blood they get bacteria on their body parts. They move to another animal, and when they bite that animal they may transmit bacteria. The fly slices the skin, and creates an opening bacteria can enter.

“Studies in the 1990’s on bison outbreaks in Canada suggested that outbreak years were associated with a bad fly season,” says Blackburn.  After a wet spring, there’s usually a larger hatch of tabanids (horse flies, deer flies).

“On the Texas ranch where I did the blowfly study, I also radio-tracked a small group of whitetail deer and ran tabanid flytraps throughout the 2005 summer. I found a direct correlation - where we find high numbers of biting flies, we see more anthrax cases,” says Blackburn. Flies also play a role by increasing stress on animals or making them anemic.  Flies may reduce immunity, as well as move the disease around. 

People wonder how many bacteria or spores must enter the animal before it gets the disease. In an early study, researchers tried to find out how many spores it takes to create the disease. “It’s a relatively high number for cattle and bison, and we thought it would be similar for other species. But, there’s new evidence that looks at serological data to see how many animals were exposed after an outbreak season. We need to know how many are exposed, compared with how many die,” he explains.

“What we don’t understand yet is whether animals that get a smaller dose (not enough to get sick and die) get short-term immunity/protection against the pathogen. Many species can be affected by anthrax - horses, swine, all domestic livestock, and most hoofed wildlife in North America. From our work in Texas, we wonder if deer may be more sensitive than cattle or bison,” he says.

“The next question is exposure. This is a spillover disease. We see it in one species and then it spills over into another. In Canada, a study during outbreaks in the 1990’s suggested it started in bison and spilled over to moose. In the big outbreak in 2008 in Montana, the first case found by the ranch staff was a bison. They lost dozens of bison early in the season, and it wasn’t until they started losing bison more rapidly that they started flying the area to check on them - since it’s a large ranch, more than 150 square miles. While checking bison, they started finding dead elk. By the time they found the elk, however, some had been long dead.”  

Did anthrax start in elk and spill over into bison? Or were there two separate outbreaks in elk and bison at the same time? Perhaps they would have each gotten the disease, whether the other species was there or not. The more we can learn about anthrax, the more chance we can prevent outbreaks.  

“Anthrax occurs more frequently than people realize. Fifty years ago it was thought of as a disease that only exploded in outbreaks every 10 or 20 years. People didn’t worry much about it. But we know now, from the last 20 years of research, that it’s there all the time. Ranchers in an area with a history of anthrax should continue vaccinating,” he explains.

Prevention

There is a vaccine approved for use in livestock (including horses) for anthrax, but the AAEP recommendations suggest horse owners should not use this vaccine unless their animals are located in an endemic area. Anthrax is mainly encountered in certain geographic areas where alkaline soil conditions favor survival of the organism.  

With this disease, vaccine is very cheap insurance. “We are not talking about a vaccine that’s expensive or difficult to administer to livestock or horses,” says Blackburn. “With valuable animals, like horses, it is foolish to gamble and not vaccinate,” he says. The vaccine can be obtained from a veterinarian, who can also answer any questions a horse owner might have about this disease.

Anthrax was among the first animal diseases for which a vaccine was developed. In 1881, Louis Pasteur developed a vaccine consisting of two different preparations given two weeks apart. This procedure was only slightly modified over the next 50 years. In the late 1930’s, a vaccine using a weakened, non-disease causing strain was developed (the Sterne strain) and it is still the strain used in today’s vaccines.

The only vaccine currently licensed for use in horses is a live Sterne strain, non-encapsulated spore-form. This vaccine is effective, but vaccination of pregnant mares is not recommended. Adverse reactions have been reported in young horses and miniature horses. Local swelling may occur at the injection site, which resolves within a few days.

The vaccine is given subcutaneously in the neck, and immunity develops within 5 to 10 days. This vaccine is labeled for use in all farm animals at a dose of 1cc. It has proven very safe, but horses are at higher risk for painful swellings. The vaccine manufacturer advises that administering the vaccine in the brisket (breast/pectoral) area may minimize adverse reactions in horses. Consultation with a veterinarian is recommended before using anthrax vaccine on any animals for the first time.  

In heavily contaminated areas, a booster should be given 2 to 3 weeks after the first dose. Ideally, animals should be vaccinated about 4 weeks before anthrax usually appears. Immunity wanes after about 6 months, so annual vaccination is required. Adult horses that have not been previously vaccinated should receive a primary series of two subcutaneous doses of vaccine with a 2 to 3-week interval between doses, and should then be vaccinated annually thereafter. There are no guidelines for vaccinating foals.

Appropriate caution should be used during storage, handling and administration of this live product. Consult a physician immediately if human exposure to the vaccine occurs through accidental injection, ingestion, or contact with eyes or broken skin.

Antimicrobial drugs should not be given to horses at the same time as the vaccination, as this may interfere with adequate response to the vaccine.

Carcass Disposal

Don’t necropsy the dead animal; a veterinarian can take a blood sample for testing first, if anthrax might be a possibility, according to Dr. Dustin Oedekoven (South Dakota State Veterinarian and Executive Secretary, South Dakota Animal Industry Board). “Today the lab can do a rapid test to determine if it is anthrax or not. We don’t want the carcass opened because that will release more bacteria into the environment and allow more contamination and spread of the disease,” says Oedekoven.

“We want to dispose of a carcass quickly so there won’t be flies landing on it, and so there’s no chance for wildlife, dogs, etc., to drag the carcass around. In South Dakota, we have a state law that requires burning and burying carcasses that are known or suspected to have died from anthrax,” he says.

“A few years ago, a study in Canada showed we should stop using quick-lime on carcasses because it adds calcium to the soil around the carcass - which aids spore survival,” says Blackburn. “We still have some veterinarians who recommend covering a carcass in lime rather than burning or burying.”

Stiltenow reminds people to be careful when handling and disposing of carcasses. “Avoid skin contact with the carcass or contaminated soil. Spores may enter through a cut. The resultant infection causes itching, followed by swelling and discoloration (turning black). Without treatment, this could become fatal. Wear rubber gloves, rubber apron and rubber boots that can be disposed of (burned and buried) or thoroughly disinfected afterward with hot water and detergent,” he says.

When handling carcasses, take appropriate safety precautions and wear safety gear, including gloves, mask and goggles to protect the face and eyes.

How Did Anthrax get Here?  

Scientists are still discussing whether this disease came to North America with European settlers’ livestock, or whether it was already here in wildlife (such as bison), or arrived by both routes. “Work by molecular biologists suggest that anthrax made its way across the Bering land bridge with one of the species of bison that is no longer in existence. There are an equal number of studies that suggest it came through European cattle settlement and trade (the Spanish via Mexico, and the French through Louisiana),” says Blackburn.

“We know there was more than one introduction. We don’t understand exactly how it got here, but it got here more than once,” he says. Records from the Hudson Bay Company in the 1820’s reported anthrax in bison, long before cattle were brought to the northern plains.

Anthrax also arrived more recently from other countries via hides and wool. “Dr. Martin Hugh-Jones (Louisiana State University) said the so-called industrial strains of anthrax were introduced with the tanning and bone meal industries - imported animal products from India, Pakistan, etc. We know that there’s a genetic diversity associated with the industrial outbreaks and wool handlers.  What was called woolsorter’s disease was often a different genetic group than the anthrax established in the western U.S. today,” says Blackburn.

“Texas is another interesting example.  The Ames strain of anthrax - which became famous in 2001 because it was associated with the Amerithrax outbreak - belongs to the A3b group. It has very limited distribution in the U.S. So far, it has only been documented in southern and west Texas, and has not been found outside of Texas. This is a very different type of anthrax than what circulates in the Dakotas, and was probably introduced just a few hundred years ago,” says Blackburn.