An ineffective screening test could be responsible for an outbreak of anaplasmosis that has spread to eight cow-calf operations in eastern Manitoba.
Since January, the Canadian Food Inspection Agency has been testing herds around Lac du Bonnet. As of May 6, 305 animals in eight herds had tested positive for the blood disease.
“They (the farms) are all geographically linked,” said Lynn Bates, veterinary program officer for the CFIA.
Investigators continue to test herds adjacent to the eight cow-calf operations in an effort to contain the spread of anaplasmosis, a red blood cell disease that is fatal 29 to 49 percent of the time for cattle older than two.
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For cattle younger than two, the disease is serious but rarely fatal. Symptoms are mild in cattle younger than one.
The source of the outbreak is still unknown, Bates said. But an unidentified breeding animal, brought into eastern Manitoba from the United States, is likely the cause.
“Perhaps it could have come from an imported animal from the United States,” said Bates. “In certain parts of the United States anaplasmosis is endemic.”
Canada has import restrictions requiring pre-entry testing for anaplasmosis on U.S. breeding cattle, but Bates speculated that a blood test called complement fixation that was used before 2007 might have failed to detect the disease.
In 2006 a CFIA consultation paper on anaplasmosis found that the complement fixation test had a sensitivity of 20 percent, meaning that it produced a high number of results in which it failed to detect anaplasmosis in animals that have the disease.
The CFIA has since switched to a blood test called CELISA, which has a sensitivity of 95.6 percent.
“The CELISA test is a test that is more sensitive and more specific… it’s a much better test,” said Bates.
Although Bates suspects that a breeding cow from the U.S. is the source of the outbreak, Rob McNabb, general manager of operations for the Canadian Cattlemen’s Association, is not so sure.
“I’m not aware that we (Canadian producers) have imported any breeding cattle, probably for at least the last five or six years,” he said.
Nonetheless, determining how anaplasmosis arrived in Manitoba is not the priority, Bates said. The objective at the moment is containing the spread of the disease, which has the biggest impact on cow-calf operations.
Anaplasmosis is a production limiting disease in adult animals, reducing calving rates in beef cows and milk production in dairy cattle.
The disease is typically spread by ticks or biting flies, which transfer blood from one animal to another.
Re-use of dehorning and castrating tools can also spread it.
The disease is common in the southern U.S., but less prevalent in the northern plains, because of the shorter tick and fly season, Bates said.
Until this outbreak in Manitoba, and a positive test from a herd in southern Saskatchewan last fall, Canada was considered to be anaplasmosis free, according to the CFIA.
