Swamp fever on the rise

Sask. numbers high | 102 horses tested positive in 2011

Swamp fever continues to turn up in Canadian horse herds, particularly in Saskatchewan.

Dr. Betty Althouse, a Canadian Food Inspection Agency veterinary disease control specialist, told the recent Saskatchewan Horse Federation conference that nearly 60 new cases were found in the first three months of this year.

In 2011, 102 horses on 15 premises tested positive in Saskatchewan, compared to 62 in British Columbia on two farms, 10 in Quebec on three farms and four in Alberta on four farms.

Althouse said eight cases were found on two new premises in January, followed by 48 on 13 premises in February and two cases on two farms in early March.

All types of horses have been affected, including show, bucking, draft and pleasure horses.

North of Saskatoon is the hot spot for swamp fever, but Althouse cautioned that doesn’t mean there are no cases in the south. It could be a matter of looking for it.

Swamp fever, or equine infectious anemia, can present in three ways: acute, chronic or inapparent.

“You can’t tell by looking at them,” Althouse said.

Horse flies, stable flies and deer flies spread the virus by biting an infected horse and then an uninfected horse. The threat of spread can depend on the level of infection.

If a fly bites a horse that is acutely infected, just one millilitre of blood has enough virus to infect a million horses, Althouse said. So, when that fly bites another horse, it is extremely likely to transfer the disease.

If that fly bit a chronically infected horse, which displays symptoms on and off for years, and the blood is taken during a feverish episode, it could infect 10,000 horses.

Althouse said just one horse fly out of six million is likely to pick up and transmit the virus from a horse that is an inapparent carrier.

She cautioned that even using the same needle on different horses could transmit the virus. Breeding infected stallions to mares can cause the disease, and mares can pass the virus to their foals.

Testing is critical.

Dr. Byrnne Rothwell, a long-time veterinarian with a special interest in horses, said March, April and May are the best times to test, followed by another period after the first killing frost when insects disappear.

Insect season was fairly long last year because of the warm weather, and horses that tested negative at the end of October were positive six weeks later.

He said swamp fever is a federally reportable disease, but the current control program is voluntary.

“The program needs modernization,” Althouse said.

Some horse events are already implementing mandatory testing to get control of the disease.

About 3,500 horses were tested in Saskatchewan last year. Typically, that number is 2,500.

As many as 9,000 Saskatchewan horses were tested when testing was mandatory under the federal control program.

Rothwell said 1.8 million head were tested and 14,000 reactors confirmed in Canada between 1972 and 1993. The rate of infection at that time dropped from 2.9 percent to .39 percent.

Between 1994 and 1998, when there was less CFIA involvement, the infection rate climbed to .66 percent.

“In 1999, the pony chuck wagon (industry) realized the problem and requested and were granted a testing program from CFIA,” he said.

Eight thousand horses were tested and a lot of the positives were removed from herds.

No positive cases were reported in Canada between 2004 and 2009 until “Typhoid Mary” appeared in the Edmonton-Calgary corridor. The mare was an inapparent carrier, and horses that tested negative in spring were positive by fall.

“Regular testing is the only way out,” Rothwell said. “Only if you test can you say you are not part of the problem.”

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