Anthrax

Also known as splenic fever, charbon, milztrand and woolsorter’s disease

Description

Anthrax is one of the oldest known diseases of humans and livestock, described in some the earliest recorded history—several thousand years ago. This highly fatal disease is caused by a bacterium, Bacillus anthracis, and occurs worldwide, causing sudden death of cattle, sheep, goats, bison and hoofed wildlife. It can infect all warm-blooded animals.

The active anthrax organisms within an animal’s body or bodily secretions can be readily destroyed by ordinary disinfectants or high heat. But once the animal dies and the carcass is opened—and the bacteria are exposed to the air--they form spores and go dormant.

These spores are resistant to adverse conditions (such as heat, cold, freezing, chemical disinfectants or drying) and can survive in the soil a very long time. The 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.

Anthrax spores can survive for many years, causing disease again when conditions are right. Anthrax often occurs after flooding or drought. Pastures may be contaminated by water from an area where there were infected carcasses at an earlier time. Low-lying ground or marshes are readily contaminated by flooding; stagnant water holes as the area dries again may serve as a source of infection.

This disease may have come to North America with European settlers’ livestock, but was already here in wildlife (such as bison). 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. Some strains of these bacteria were introduced with imported animal products from India, Pakistan, and other countries.  What was called woolsorter’s disease was a different genetic group than the anthrax established in the western U.S. today.

Anthrax in North America circulates routinely in wildlife. In parts of west Texas, for instance, the disease is seen every year. Some years there’s only a case or two, but in explosive years (epizootic years), there might be hundreds or thousands of cases in wildlife, which can put domestic livestock at risk. Livestock can be protected by vaccination, however.

In 2008 there was a massive bison and elk outbreak in western Montana, the first confirmed report of anthrax in that part of Montana in a very long time. During that epizootic about 300 bison and 50-plus elk died. In 2010 two more cases occurred in that same bison herd. In some years it really gets going and some years it doesn’t. People often get a false sense of security after not seeing the disease for a few years, but in regions where anthrax is known to have occurred, it’s wise to keep vaccinating.

On ranches with mixed species (domestic livestock and wildlife), people should be alert and looking for wildlife cases when they find livestock cases. Because of the risk for humans, livestock deaths must be reported, and carcasses properly disposed of.

Not all animals exposed to anthrax die. It depends on how many spores enter their body. It’s a relatively high number, for cattle and bison. Animals that get a smaller dose (not enough to get sick and die) may get short-term immunity/protection against the pathogen.

Many species can be affected by anthrax, including horses, all domestic livestock, and most hoofed wildlife in North America. Deer may be more sensitive than cattle or bison. Ruminants seem to be most susceptible to this disease. The rumen seems to be a perfect environment (moisture, temperature, pH, etc.) for anthrax bacteria to incubate and start releasing toxins. Anthrax is sometimes seen in deer, elk and other wild ruminants in certain regions; it probably existed long ago in the bison herds that grazed these areas.

This is a spillover disease. It crops out it in one species and then spills over into another. A study in Canada, during outbreaks in the 1990’s showed 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. Researchers are still not sure if bison become more easily infected than cattle and then it spills over into cattle. In Texas an outbreak in deer may spill over into cattle, or vice versa.

Signs

Causes

This acutely contagious and deadly disease is usually not spread from live animal to live animal (except by flies that bite a sick animal and then a healthy one) but typically transmitted by spores found in and around the carcasses. As the animal dies, blood oozes from body openings. This bloody discharge is heavily laden with anthrax bacteria which can infect other animals quickly, or years later if they pick up the dormant spores.

Anthrax cases often occur along old cattle drive trails, and where bison migrated many years ago. The spores last at least 100 years (some researchers say 250 years) and lie dormant in the soil.

Spores usually enter through the mouth. Other animals may contract the disease by licking the carcasses of animals that died of it, or eating contaminated feed. Outbreaks have occurred because of contaminated feed, such as bone meal, meat scraps or other animal protein products, but current regulations for manufacture and importation of these products eliminates these as a source of infection in the U.S.

Grazing animals pick up spores when grass is contaminated. Hay that contains spores may cause outbreaks during winter. In the northern hemisphere, anthrax generally appears in warmer months—May through October. The really bad epizootic years seem to be associated with wet spring weather followed by a hot, dry early summer, then a sudden thunderstorm/shower and hot, dry period after that.

Moisture may help percolate spores up to the ground surface. Flood plains and alluvial soils are often associated with anthrax. Flooding that moves soils around may expose spores. These bacteria may be able to germinate and live in the rhizosphere of some grasses (the region of soil in the vicinity of plant roots where the soil microbiology is influenced by the root growth and nutrient exchange). This may be why grazing animals pick up spores after a wet period followed by green-up of grasses.

Anthrax can also be spread through skin wounds caused by blood-sucking insects, dehorning, or castration. Flies feeding on carcasses that died from anthrax can pick up blood and pass it on to the next animal they feed on. Horse flies and deer flies (tabanids) serve as mechanical vectors. They land on a sick animal (with bacteria in the bloodstream), bite it, and while sucking blood 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 big outbreaks were associated with a bad fly season. After a wet spring there’s usually a larger hatch of tabanids. When there are high numbers of biting flies there are more anthrax cases. Flies also play a role by increasing stress on animals, exhausting them or making them anemic. Flies may reduce immunity, as well as move the disease around.

When animals die and their carcasses are torn open, blowflies start feeding and laying eggs and can pick up bacteria from tissue they are feeding on. Laboratory studies that go back more than 100 years show that anthrax bacteria can survive through the gut of adult blowflies. Viable anthrax bacteria can be found in the material regurgitated by the fly, and in its feces. These bacteria can survive in maggots feeding on a carcass—all the way through the pupae stage and adults.

While mapping outbreaks in west Texas, researchers collected samples from carcasses, maggots and adult flies, and collected leaves from vegetation surrounding the carcasses. They found the 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. Any animal eating these leaves could become infected. Leaves around a carcass that’s been there for several days will have a lot of fly droplets—more than 15 feet away, and 10 feet high or more in the surrounding trees.

Animals and humans can also be infected if spores are inhaled or get into a wound. When spores enter the body, they come to life and start swiftly reproducing, releasing toxins. The result is system-wide bleeding and rapid death.

Most animals are not found in time to treat. This disease progresses so fast that you generally don’t 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.

Prevention

It’s important to investigate every unexplained death of livestock. There’s a good vaccine against anthrax, but many people don’t utilize it unless they are suddenly faced with dying animals. It’s better to vaccinate in the spring, however, rather than have to gather cattle in hot weather, to vaccinate after anthrax shows up.

Anthrax vaccine is effective for producing adequate immunity in livestock, and very inexpensive insurance if you live in an area that’s ever had cases of anthrax. The Sterne vaccine is labeled for use in all domestic species, though the disease is less common in horses and swine than in ruminants.

The vaccine is given subcutaneously in the neck, and immunity develops within 5 to 10 days. In heavily contaminated areas, a booster should be given 2 to 3 weeks after the first dose. Ideally, cattle should be vaccinated about 4 weeks before anthrax usually appears. Immunity wanes after about 6 months, so annual vaccination is required.

If anthrax is found in a herd, the animals should be promptly moved to different pasture, away from the carcass, to prevent additional infections due to eating contaminated grass, or to curiosity. Other animals often nuzzle and smell the dead one, and may lick contaminated blood and fluids.

The living anthrax organisms within an animal’s body or bodily secretions can be readily destroyed by ordinary disinfectants or high heat. But once the animal dies and the carcass is opened, exposing bacteria to the air, they form spores that are less easily killed.

If an animal dies unexpectedly, consult a veterinarian—especially if multiple animals died without an obvious cause. Don’t necropsy the animal; a veterinarian can take a blood sample for testing first, if anthrax might be a possibility. Today labs can do a rapid test to determine if it is anthrax. Don’t open the carcass because that would release more bacteria into the environment and allow more contamination and spread of the disease.

Dispose of a carcass quickly so there won’t be flies on it, and so there’s no chance for wildlife, dogs, etc. to drag open it or drag the carcass around. Many states require burning and burying carcasses that are known or suspected to have died from anthrax.

Always be careful when handling and disposing of carcasses.  Avoid skin contact with the carcass or contaminated soil. Spores may enter through broken skin. The resultant infection causes itching, followed by swelling and discoloration; it turns black. Without treatment, this could eventually 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.

Keep track of wildlife deaths. When searching for missing cows, also look for other carcasses. The best way to slow down an outbreak is to burn carcasses. Where burning is not possible, bury them. You can also spray the carcass (to reduce spread of spores until you can burn or bury it) with formaldehyde or bleach to deter flies and kill some of the surface spores. Cover the carcass to keep predators or scavengers away from it.

The old practice of using quick-lime on carcasses that can’t be burned or buried is no longer recommended, because it enriches the calcium in the soil around the carcass—which aids spore survival.

It’s important to find carcasses as quickly as possible and dispose of them, to interrupt the contamination cycle by reducing flies and their activity. When it’s not safe to burn, some ranchers use a bulldozer, backhoe or grader to knock down brush around the carcass--to clear the area to make it safer to burn, and provide fuel for the fire—and get rid of the brush around the carcass that has been contaminated by flies.

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 in between. But now we know, from the last 25 years of research, that it’s there all the time. Ranchers in an area with a history of anthrax should continue vaccinating.

Treatment

Since anthrax is quickly fatal, it’s difficult to find animals in time to treat them. 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. When faced with an outbreak, administering antibiotic and vaccine at the same time has been effective.