2 August 2002

Toxoplasmosis warning

By Jessica Buss

EVERY flock is at high risk of toxoplasmosis and recent research shows how vaccination is cost-effective even in flocks which believe they have been free of the disease.

A post-graduate vet studying at the Royal Vet College in Herts set out to assess the costs of losses caused through toxoplasmosis. To find out the differences in performance between at-risk animals and resistant animals, he vaccinated half of 14 south Wales flocks, totalling about 4000 ewes, with Toxovax.

Flocks were not selected because they had suffered from abortions or infertility problems, says Alasdair King, a vet with Intervet which manufactures Toxovax.

Only three of the flocks owners believed their sheep had suffered from toxoplasmosis in recent years. But blood testing showed antibodies in all flocks. Almost half the sheep tested had been exposed to toxoplasmosis at some time, but ewes which had been in flocks less than one year showed little exposure.

Following vaccination before tupping, ewes produced 1.36 viable lambs each, whereas unvaccinated ewes produced 1.3 lambs. This difference is worth 6.4 lambs/100 ewes tupped, says Mr King.

Independent sheep consultant Lesley Stubbings believes to break even on the cost of vaccination, these flocks needed to produce an extra three lambs/100 ewes. In more prolific flocks, the break-even point would be even lower, she adds.

"The increase in lamb survival was due to more viable lambs born. Clients using toxoplasmosis vaccine also report a decrease in weak lambs. The benefits of that are difficult to calculate, but todays shepherds are stretched to their limit, with more ewes to look after, so it helps to have fewer weak lambs to deal with," says Ms Stubbings.

"Normal lambs shouldnt need any help at lambing, they should clear their own membranes and go," adds Mr King.

The other benefit was fewer empty ewes. Toxoplasmosis is a possible cause of abortion, but, unlike other abortion causes, foetuses can be lost at any stage in pregnancy, he says.

This can occur before scanning, as the number of ewes not conceiving is usually small, or at anytime during pregnancy, according to Ms Stubbings.

"Thats where toxoplasmosis is insidious, it keeps adding to the number of unproductive ewes all the way down the line, rather than causing an abortion storm. Producers should be looking for reasons for barreners above 1-2%.

"As barren ewe numbers increase, unit costs/kg of lamb goes up. So, the higher your abortion rate the quicker you could recover the investment in vaccine."

Mr King believes many producers accept a 5% or 10% abortion rate as normal, but he also warns that there could be many reasons for those abortions. Vet Lab Agency testing shows about half investigated cases are enzootic abortion and a quarter toxoplasmosis.

Parasite oocysts

But unlike other abortion causes, it is impossible to keep toxoplasmosis out of a flock with biosecurity, says Ms Stubbings. "Cats are the main vector. Parasite oocysts are shed in young cats faeces. One cat mess in a yard can infect thousands of sheep. But if you get rid of cats from a farm, you are more likely to attract young male cats in."

Mr King adds that young male cats can have a 10-mile territory and it is not unusual to have toxoplasmosis on a farm that has never seen a cat.

Every flock is susceptible to toxoplasmosis, adds Ms Stubbings, who points out that there are also risks to pregnant women.

"Vaccine only needs to be given once in a lifetime. If a vaccinated animal comes across toxoplasmosis it will resist the challenge and in doing so reinforce its immunity."

Following vaccination, some flocks she works with have reduced abortions to 2%. But to achieve that, flock management and nutrition must also be good. There is research to show ewes at condition score three are at less risk of abortion than those at a score of one or less and four or more, she says.

There are two options for introducing vaccination into a flock: Either whole flock vaccination then replacements each year or just vaccinating each years replacements, as younger animals are less likely to have any natural immunity.

The Royal Vet College research calculated that over five years, only vaccinating replacements each year was worth an extra margin of £59/100 ewes each year compared with non-vaccination. But starting with whole flock vaccination brought about quicker benefits, increasing the margin by £117/100 ewes compared with non-vaccination. &#42

Even farms on which cats are never seen can suffer foetus losses throughout pregnancy due to toxoplasmosis, says Alasdair King (above).

&#8226 It is everywhere.

&#8226 All ewes at risk.

&#8226 Reduces flock profit.