Manage cats, manage toxoplasmosis
Managing a mature – but neutered – cat population on farm, alongside a suitable vaccination policy, is the best route for dealing with toxoplasmosis in sheep, according to David Buxton of the Moredun Research Institute.
Many people wrongly assumed cat control should involve getting rid of them all, as they were known hosts for the parasite that causes toxoplasmosis.
But removing existing cats simply created a vacancy that new strays or ferals would very quickly fill – and the health status of these animals will be unknown.
Older cats around farms tended to be safer and represented less of a risk, said Dr Buxton.
“Young juvenile cats which hunt for the first time will pick up infection from their prey and shed the infectious oocysts,” he said. “These microscopic eggs are capable of contaminating sheep feed, pasture and bedding.
“Old cats are often immune and are much less likely to spread infection, although new evidence from the USA suggests that some could re-excrete at six to seven years of age, so perhaps immunity is not for life.”
The cat was a definitive host for the protozoa responsible for causing toxoplasmosis in ewes – Toxoplasma gondii. Once inside the animal, the organism could produce its eggs (oocysts), which were then shed in the cat’s faeces.
When a cat was first infected as little as 1g of faeces could contain up to one million oocysts – and it only took around 200 to infect one ewe.
Vast numbers were shed for up to 14 days before the cat’s immune system kicked in, after which it would either stop shedding altogether or sheds reduced numbers of oocysts.
According to Maggie Rogers, head of veterinary services at the Cats Protection welfare organisation, the number of cats infected with toxoplasma ranged from 20-60 per cent of the population but only 1 per cent of those were shedding oocysts at any one time.
Research showed that trying to exterminate cats from farms often failed, and Ms Roberts said she believed farmers would benefit more from managing their own cat colony instead.
“Killing farm cats leaves an environment which is good for cats, as the process results in a new food source for strays and ferals, so they soon move in,” she said.
“This represents more of a risk for sheep than a stable, healthy, neutered and well-fed cat population.”
This was because cats were territorial and would naturally keep strays out. Making sure farm cats were neutered not only controlled numbers – one female cat could have 20,000 descendents within five years – but also prevented several litters of kittens each year becoming infected with toxoplasma and shedding oocysts. Plus, neutered cats were less likely to attract other cats for breeding.
Ms Rogers recommended feeding outdoor cats including ferals, as hunting was driven by instinct not hunger and if the cat was full it would kill its prey but not eat it, meaning effective rodent control without the cat becoming infected with toxoplasmosis.
But if flock owners were concerned about an over-large cat population on their farm, Cats Protection could offer a humane trapping service in some areas, staffed by local volunteers, she said. It could also provide vouchers to help with the cost of neutering.
Sheep, like cats, would also develop their own immunity to toxoplasmosis, particularly when infection occurred at any time other than during pregnancy.
The problem was that the visible effects were only seen in pregnant ewes – and the scale of damage to the ewe and her unborn lambs depended on the stage of pregnancy that infection takes place.
In the first 60 days, the foetus would be absorbed with the ewe appearing barren. From 60 days, the ewe would abort in late pregnancy producing mummified foetuses, stillbirths or weak/sickly lambs that frequently died soon after birth.
Rosemary Booth, large animal veterinary advisory for Intervet Animal Health, said it was a risky policy to hope infection would occur when ewes were not in-lamb, so the most reliable form of protection was through vaccination.
This required a one-off injection for each member of the whole flock, after which only replacement females would need to be vaccinated.
Intervet’s annual subsidised FlockCheck survey showed that around three quarters of UK flocks were being exposed to toxoplasmosis every year.
This could potentially lead to a significant increase in abortions and barrenness (an abortion storm) if ewes are not vaccinated, she said.
“Our 2007 FlockCheck results show exposure to toxoplasmosis was the highest yet at 82 per cent. Any flock with barren or abortion rates greater than 2 per cent should investigate the cause,” she said.
• FlockCheck is currently available to all farmers to analyse samples taken from aborted ewes. Results will show whether a flock has been exposed to the two main causes of abortion and barrenness – enzootic and/ or toxoplasmosis. Details available from your vet.