Bulk tank testing confirms leukosis prevalence on Ontario dairy farms

A project aims to provide producers with an accurate assessment of leukosis risk on their farm and provide assurance that the industry can respond quickly to a major disease outbreak. University of Guelph professor David Kelton provided an update on the ongoing program, which is funded by Dairy Farmers of Ontario.

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The $460,000 initiative, launched in 2020 with additional funding from the U of G and the Ontario Ministry of Agriculture, Food and Rural Affairs, will eventually see two samples from each of the province’s dairy farms collected and analyzed for a list of infectious diseases. The first batch of samples was collected by milk truck drivers in fall 2021. Dairy producers were provided with a letter outlining the results in September.

Why it matters: Having accurate disease prevalence assessments helps farmers manage disease in their herds.

The letters provided herd-wide ratings of low-risk, moderate risk or high risk in samples for Johne’s disease, leukosis (see Dairy Farmers of Canada fact sheet at bottom), salmonella Dublin and four prominent vectors for contagious mastitis. At the Nov. 2 Dairy at Guelph event, the DFO chair in dairy cattle health focused on the industry-wide results.

When Johne’s disease risk was assessed in 2013, Kelton noted, 47 per cent of Canadian herds were seen as high risk. This rose to 71 per cent in 2017. This time around, it was 46 per cent, with 40 per cent recorded as moderate risk and just six per cent as high risk.

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During Canada’s Johne’s control project between 2010-13, the dairy sector became recognized as one of the world leaders, Kelton said. But the focus on Johne’s has been reduced since 2013, while some other countries in Europe have kept up with the measures put in place in Canada at the time.

Kelton said the first-sample results from the bulk tank analysis support calls for an ongoing disease strategy.

“Our control strategies need to have more than that three-to-five-year timeline that we’ve gotten accustomed to if, in fact, we want to do something about these diseases,” he said.

Risk for salmonella Dublin, an emerging disease that some European countries are taking very seriously, was present in seven per cent of herds. Mastitis due to Staphylococcus Aureus was a risk in 47 per cent of herds, but that was down significantly from a 1992 study that pegged it at 95 per cent.

Risk from the other three mastitis-causing organisms was less than one per cent.

Risk for leukosis was evident in 82 per cent of herds. The blood-borne disease typically leads to clinical symptoms in only five per cent of positive animals but does have economic impacts on the beef side due to condemnation of carcasses with leukosis tumours.

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On the dairy side, a fact sheet provided by DFO says that “despite the low frequency of clinical signs, BLV causes significant losses for the North American dairy cattle industry. Test-positive herds produce less milk per cow than test-negative herds in general. There is also building evidence BLV can alter the proper functioning of the immune system, meaning infected animals will be more susceptible to other infections.”

Kelton said other countries have had success in dealing with leukosis by culling high-risk animals. Statistically, this probably isn’t the best strategy in Canada because the disease is so prevalent.

The project’s funding agreement called for bulk tank samples in two consecutive years. Kelton and his team are planning a second round of sampling this fall, with leukosis and salmonella Dublin explored again and other organisms added to the test list.

One audience member asked if the team had learned anything, given one of the goals of the project is to create a system for rapid collection of samples.

“We were certainly able to work out some kinks,” Kelton said. “I think we made some headway; we learned a few things.”