BSE epidemic had little effect on scrapie figures
BSE epidemic had little effect on scrapie figures
By Jeremy Hunt
A SURVEY of 11,000 UK sheep farmers suggests the rate of scrapie infection did not increase dramatically during the time of the BSE epidemic.
The Royal Society survey says these results indicate that BSE was not spread between cattle and sheep. Questionnaires were sent to over 11,000 flock owners running more than 30 breeding ewes. From an overall response rate of 61%, 838 farmers reported theyd had a case of scrapie.
The survey produced information on the proportion of farms experiencing their first case of scrapie in each year between 1962 and 1998. "We found no evidence of a large increase in the proportion of scrapie-infected flocks prior to, during or following the BSE epidemic in UK cattle," states the survey.
It adds that the current rate of infection is low at about 0.0045/farm a year and combined with a simple model of scrapie spread provides an estimate of the average duration of a scrapie outbreak on an individual farm.
Considering all farms, the average outbreak lasts for five years; but if only those farms that have cases in animals born on the farm are considered, it lasts for 15 years.
The survey showed that 2.7% of farms had a case of scrapie in 1998. The 5.3% of farms reporting having a case between 1993 and 1997 is consistent with the theory that the scrapie source of infection remained constant over this period.
Scientists involved in the survey, including Mike Gravenor of the Institute for Animal Health at Compton, Berks, say that control of scrapie has proved notoriously hard.
"Eradication programmes need a thorough understanding of the factors that determine the rate of spread between farms. These factors are poorly understood at present," says Dr Gravenor.
The report adds that in the early 1980s the BSE epidemic in cattle was caused by widespread exposure to infected protein supplements in animal feed.
"Sheep can be experimentally infected with BSE via the oral route leading to a disease with clinical signs similar to those of scrapie.
"Since British sheep were exposed to BSE-contaminated feed until a ban was introduced in 1988 – and reinforced during the 1990s – concerns have been raised as to whether BSE affected large numbers of sheep but went unnoticed due to misdiagnosis.
"The data collected is the only quantitative evidence that suggests a large epidemic of this type did not occur."
But the survey does draw some interesting conclusions regarding the estimated linear increase in scrapie infection.
"When analysed as a risk factor for having had a case of scrapie in the past 12 months, a three-fold increase in farm size is associated with a doubling in the odds ratio for having had scrapie.
"The roughly one-and-a-half-fold increase in flock size since 1970 is, therefore, likely to have contributed to the increase," adds Dr Gravenor.
When affected farms surveyed were grouped according to whether or not animals had been affected, the rate was three times higher for farms that only had scrapie in bought-in animals than for those with home-bred cases.
"In apparent contradiction, twice as many farms affected during 1998 were in the home-bred group."
Dr Gravenor says that despite the results of the survey the susceptibility of the national flock to scrapie as a direct result of being given feed containing meat and bonemeal was difficult to assess.
"Feed containing meat and bonemeal is not particularly palatable to sheep. But we do not know how much feed containing meat and bone meal was fed. It may be that only a little was fed and hence the level of scrapie did not increase. Alternatively, if a larger quantity of contaminated feed was fed to sheep we could deduce that sheep are less susceptible than cattle.
"From the information we have available, we are unable to draw a conclusion." *
Larger farms are more likely to have had scrapie in their sheep flocks, according to the Royal Society survey of 11,000 farms.
SCRAPIESURVEY
• No evidence of BSE link.
• Infection source constant.
• Flock size influences incidence.