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. Many people wrongly assume that cat control should involve full despatch (extermination), because they are known hosts for the parasite which causes toxoplasmosis.

But removing existing cats simply creates a vacancy that new strays or ferals will very quickly fill – and the health status of these animals will be unknown. Older cats around farms tend to be safer and represent less of a risk, says Dr David Buxton from the Moredun Research Institute. “Young (juvenile) cats which hunt for the first time will pick up infection from their prey and shed the infectious oocysts. 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 6-7 years of age, so perhaps immunity is not for life,” he says.

The cat is a definitive host for the protozoa (Toxoplasma gondii) responsible for causing toxoplasmosis in ewes. Once inside the animal, the organism can produce its eggs (the oocysts) which are then shed in the cat’s faeces. When a cat is first infected, as little as 1g of its faeces can contain up to one million oocysts – and it only takes around 200 to infect one ewe. Vast numbers are shed for up to 14 days before the cat’s immune system kicks in, after which it either stops shedding altogether, or sheds reduced numbers of oocysts.

According to Maggie Rogers, head of veterinary services at Cats Protection (the UK’s oldest and largest cat welfare organisation) the number of cats infected with toxoplasma ranges from 20% to 60% of the population, but only 1% of these are shedding oocysts at any one time. Research shows that trying to exterminate cats from farms often fails, and Ms Roberts believes 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. This represents more of a risk for sheep than a stable, healthy, neutered and well-fed cat population,” explains Ms Roberts.

Cats are territorial and will naturally keep strays out. Making sure farm cats are neutered not only controls numbers – one female cat can have 20,000 descendents within five years – but also prevents several litters of kittens each year becoming infected with toxoplasma and shedding oocysts. Plus, neutered cats are less likely to attract other cats for breeding.

However, for effective rodent control (and reducing the risk of them eating prey), outdoor cats including ferals must be fed, as hunting is driven by instinct not hunger, says Ms Rogers. For flock owners concerned about a cat population on their farm, Cats Protection can offer a humane trapping service in some areas, staffed by local volunteers. It can also provide vouchers to help with the cost of neutering.

Sheep, like cats, will also develop their own immunity to toxoplasmosis, particularly when infection occurs at any time other than during pregnancy. The problem is that the visible effects are only seen in pregnant ewes, and the scale of damage to the ewe and her unborn lambs depends on the stage of pregnancy that infection takes place. In the first 60 days, the foetus will be absorbed and the ewe appears barren. From 60 days, there is abortion in late pregnancy with mummified foetuses, stillbirths or weak and sickly lambs that frequently die soon after birth.

It’s a risky policy to hope that infection occurs when ewes are not in-lamb, so the most reliable form of protection is through vaccination with a product like Toxovax®. This requires a one-off injection for each member of the whole flock, after which only replacement females will need to be vaccinated. Intervet’s annual subsidised FlockCheck survey shows that around three quarters of UK flocks are 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, warns the company’s large animal veterinary adviser, Rosemary Booth.

“Our 2007 FlockCheck results show exposure to toxoplasmosis was the highest yet at 82%. Any flock with barren or abortion rates greater than 2% should investigate the cause,” she says.