Scrapie eradication possible through selective breeding

Scrapie, which was first documented in Spanish Merino sheep in 1732, is the most ancient and widespread of all the diseases classified as transmissible spongi-form encephalopathies.

It is similar to BSE and chronic wasting disease, affecting the brains and spinal cords of sheep and goats. An infectious prion protein is suspected to be the cause.

As with similar diseases, there is a long period between infection and onset of clinical disease.

A wide variety of clinical signs can occur. Affected sheep display abnormal behaviour, including aggression, weakness, trouble walking and trembling.

Scrapie is named for its tendency to make sheep itchy, which prompts them to scrape out their wool. This itchiness seems to be more of a problem in Europe than in Canada.

Death follows a few weeks to six months after clinical signs begin.

Signs of scrapie can be subtle, so testing all poor-doing and dead animals older than one year is recommended.

Affected flocks have five percent scrapie-associated mortality, but up to 20 percent annual mortality has been reported in exceptional circumstances.

Scrapie is transmitted from infected ewes and nannies to their young and herd mates through contaminated birth fluids and placentas. Environmental contamination is an important route of infection.

Infected replacement females can be introduced unwittingly into a group and spread the disease when they give birth.

Microscopic slides of brain, lymph nodes and spleen are used to diagnose scrapie.

Lymphoid biopsies, which are taken from the third eyelid of live animals, can confirm scrapie in an individual, but a negative result cannot determine with any certainty that the animal is not infected.

Scrapie is a reportable disease under the federal Health of Animals Act, and all cases must be reported to the Canadian Food Inspection Agency. Cases are also reported to the World Organization for Animal Health.

Eradication efforts in Canada are in line with current efforts in the United Kingdom and United States, which is important to maintain unrestricted trade.

The Canadian eradication program has been in place since 2005. It is a partnership between sheep and goat producer organizations and the CFIA.

Scrapie was identified in 11 Canadian sheep flocks last year, but no goat herds. Half were in Ontario, four in Quebec and one in Alberta. Only seven sheep flocks were affected in 2011.

No treatment or vaccine is available for scrapie, so infected flocks are quarantined and all individual animals humanely euthanized to prevent disease spread. Compensation is available in Canada.

Unlike BSE, there is no scientific evidence to suggest scrapie can cause disease in people.

The fairly recent discovery of genetic resistance in sheep has stim-ulated interest in exploring selective breeding to minimize the effects of the disease.

A blood test can determine the relative susceptibility of an individual to scrapie, and breeding choices can be based on increasing the herd’s overall resistance to the disease.

Selective breeding for scrapie resistance has not produced significant negative effects in terms of growth, meat quality and fertility.

Canada’s Voluntary Scrapie Flock Certification Program requires annual testing for the disease.

In addition, participating herds must be either closed or limit new animal acquisitions to those from flocks with equal or higher status to minimize the risk of introducing the disease into their flocks.

The CFIA recommends sheep and goat producers implement good management and biosecurity practices such as keeping records, animal identification, removing sick and birthing animals from the main group, pen and equipment hygiene and single-use needles.

Persistent efforts to eliminate this disease in individual herds and ongoing measures to prevent reintroduction should lead to successful eradication.

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