Drought conditions increase risk of anthrax

Cattle are one of most prominent species affected by anthrax. | Dr. Jason K. Blackburn photo

Producers are urged to take precautions this summer because of how dry it has been; vaccinations are recommended

Anthrax is one of the oldest killers of humans and livestock, mentioned in some of the earliest recorded history.

It has been called splenic fever, charbon, milztrand and woolsorter’s disease and is caused by a bacterium, Bacillus anthracis, which occurs sporadically in the United States and Canada.

The disease is seen worldwide, and associated with sudden death of cattle and sheep, though it can infect all warm-blooded animals.

This acutely infectious and deadly disease is generally not spread from live animal to live animal but transmitted by spores found in carcasses of dead animals. Most commonly it is found in soil that contains spores that form when an infected carcass breaks down. Scavengers consume the carcass and can spread the organism.

Anthrax organisms within an animal’s body or secretions can be destroyed by ordinary disinfectants or high heat. But once the animal dies and the carcass is opened — and bacteria is exposed to the air, they form spores. These spores are resistant to heat, cold, freezing, chemical disinfectants or drying and can survive in contaminated soil for years.

The carcass may be long gone — torn apart by predators and scattered, or decomposed and disappeared many years ago — but the spores are still viable in the soil.

Tasha Epp of the Western College of Veterinary Medicine in Saskatchewan was involved in a study of anthrax during an outbreak in Western Canada in 2006.

“My area of expertise is epidemiology, so I was looking at what happened and where it happened, and some of the factors associated with that outbreak,” she said.

“What we found was that areas with heavy rainfall and flooding in the spring and then dry conditions later in the summer were more likely to see cases of anthrax. When the flooded areas started to dry out in hot weather, we started seeing anthrax,” said Epp.

Anthrax generally appears in summer—May through October. The bulk of outbreaks occur in warmer months but a few occur in winter if there are spores in hay.

Changes in soil moisture (such as flooding or drought) or recent soil disturbance (such as excavation) can bring spores to the soil surface.

“Moisture draws spores from the ground and when the flooded areas dry out, the cows go back into those areas where there is lush grass, and ingest spores that end up on grass. That combination of wet, then hot and dry, tends to make a bad year for anthrax,” Epp said.

“This year, since much of the province is heading into drought conditions (with some quite severe), anthrax is likely to be a concern.”

Spores brought to the soil surface from earlier wet years can continue to be a problem during a drought, when cattle graze close to the ground.

For many years, the Canadian Food Inspection Agency kept track anthrax cases. In 2006 Saskatchewan had more than 800 confirmed cases.

“They (CFIA) have records back to 1912 for Saskatchewan, and this was the largest outbreak ever recorded. Every few years, and sometimes a couple years in a row, there are reports — either a single case of anthrax on a farm, and occasionally a few more, but never to the extent we had in 2006,” Epp said.

Today, the task of reporting cases falls to each province.

Each has its own policies on how to deal with anthrax cases and farmers and ranchers must be aware of their provincial legislation.

Most anthrax occurrences are in areas that have had anthrax in the past.

“One problem is that not all cases of anthrax in animals are diagnosed or found before the body begins to break down. We don’t have a good record of every place that has experienced anthrax events historically,” Epp said.

“Since the bacteria can remain viable in the soil for decades, predicting which animals are at risk is difficult. But since the 2006 outbreak, the database of areas where anthrax has occurred is better.

“People should know whether their farm was affected in 2006. Many of these farms were required to use vaccination, but livestock vaccinations have probably not been kept up since that time.”

Dr. Jason K. Blackburn of the University of Florida has studied anthrax for years. He said it occurs more frequently than most people realize.

“Sixty years ago, it was thought of as a disease that would only explode in outbreaks every 10 or 20 years. People didn’t worry much about it in between. But we know now, from the last 30 years of research, that it’s there all the time, whether or not we see cases,” he said.

Ranchers may not want to vaccinate against a disease forever, even though there are geographic areas where producers must continually vaccinate against certain diseases.

“I think it’s important to tell ranchers that if they are in an area with a history of anthrax, or a frequency of this disease, they have to continue vaccinating.”

He said many people get a false sense of security after not seeing the disease for a few years.

Epp said cattle seem to be one of the most prominent species in which it shows up, but bison were also hit hard in 2006.

“We hadn’t seen it very much in farmed bison in the past, perhaps because bison farming wasn’t so large in earlier years. Now, with more bison farms, we saw a lot of anthrax cases in bison in 2006. On many of those farms, we saw many more deaths than we would have seen with cattle,” said Epp.

She has also been involved with assessing anthrax outbreaks in the Northwest Territories with wild bison.

“The Mackenzie outbreak in 2013 resulted in devastating losses to the wild bison herd, reducing the population by half. What we have found is that immunity from past exposure to anthrax plays a role. Not all animals die from exposure to anthrax. Some develop antibodies, which can persist for a time, but we are unsure how long.

“It is assumed that as immunity wanes, potential for large outbreaks becomes possible, given the right environmental conditions,” she said.

“With that in mind, since it’s (been) 14 years since the last major outbreak and likely very few ranchers are now using the vaccine, livestock populations could have lost prior exposure immunity and may be open to anthrax events due to drought conditions.”

Anthrax probably affected bison in North America long before there were cattle here. Blackburn has looked at the potential distribution of anthrax spores in Mexico, the U.S. and Canada. Much of it seems to follow old cattle trails and bison migration routes.

“There were also some pigs lost to anthrax in 2006,” said Epp. “We also had anthrax in a couple of horses, and a few sheep and goats that year. Any warm-blooded animal that grazes can become exposed,” she said.

Most anthrax cases are not found in time to treat; the animal is usually found dead. Penicillin is effective, but has to be given in early stages.

“Most of the time, when an animal is diagnosed with anthrax, the CFIA takes charge of how things will then progress on that farm,” said Epp.

She said the vaccine is effective, but because cases seldom occur, most ranchers don’t vaccinate. However, this may be a year when ranchers want to be prepared.

“If there was a way to predict where and when anthrax would occur so people would know the risk — and know this is a year we should vaccinate — perhaps more people would do it,” she said.

It takes seven to 14 days to gain full immunity after vaccination. In some instances, a veterinarian may recommend a booster, especially in a bad year. An Australian research paper looked at whether vaccination would be quick enough to prevent disease in cattle once cases were seen in an area. Researchers thought it would, but it depends on whether there is regulatory ability to compel everyone in the area to vaccinate their animals.

“In most countries vaccination is a voluntary thing, especially if there’s not an outbreak currently. But if your neighbour is having a problem, the best way to protect your cattle would be to vaccinate them,” she said.

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