How toxoplasma spreads... and spreads

The microscopic parasite Toxoplasma gondii is small enough to get inside the cells of the animals it infects. A single-celled organism known as a protozoan, it has a complicated life cycle involving rodents and cats. Toxoplasma can also infect humans and farm animals, but these species are accidental hosts.

The parasite can only reproduce sexually and form eggs within cats, which can shed millions of toxoplasma “oocysts” in their faeces. Cats become infected if they eat contaminated raw meat, or catch and eat infected mice or rats.

Cats can shed up to 10 million oocysts a day for up to 14 days after they initially become infected. Given that there are 8 million pet cats in Britain and 1 per cent of them are shedding toxoplasma oocysts at any one time, this means there are in the region of 800 billion toxoplasma eggs being released each day in Britain by domestic cats.

Toxoplasma oocysts in the soil remain viable for several years where they can be ingested by other animals which then become infected with tissue cysts in their vital organs, including brain and muscle tissue.

A survey of 51 cats on 22 sheep farms the South West of England found that nearly half the felines carried antibodies to toxoplasma, indicating past exposure to the parasite.

Lamb meat has been shown from limited testing to carry the greatest risk of toxoplasma based on past exposure to the parasite by various farm animals.

Fuller Torrey, director of the Stanley Medical Research Institute in Maryland, carried out studies of playground sandpits that are left uncovered at night and found that they are a favourite haunt of cats, which prefer to defecate in loose sand and soil.

“I’ve just been looking at the data on levels of oocysts secreted by cats and I’m frankly appalled at the level of contamination of areas where they go to the bathroom,” Dr Torrey said.

“I estimated in some of these public sandboxes [sandpits] there are about a million viable oocysts per square foot of sand. I would not let any children of mine play in a sandbox that had not been covered at all times when not in use, or in a sheltered place that cats could not enter,” he said.

Dr Torrey said that he would go as far as to recommend against having cats as pets if there are young children in a household.

“I would certainly advise families not to get a cat if they have small children. I gave this advice to my own daughter and granddaughter,” he said.

Toxoplama infection can result from the ingestion of oocysts or from tissues cysts that form in the muscle of infected farm animals. Most people show no symptoms but it is estimated that between 10 and 20 per cent of infected individuals develop flu-like symptoms in the early, acute stages of the infection.

The latent stage of toxoplasma is normally without obvious clinical symptoms but it can develop into serious illness when people become immune-compromised, for instance during Aids or when undergoing certain types of cancer treatment. One notable manifestation in immune-compromised patients is toxoplasmic encephalitis of the brain, which can be lethal.

If women become infected with toxoplasma while pregnant, the parasite can seriously damage the development of their baby in the womb, leading to miscarriage or congenital birth problems.

Toxoplasma is the second most common cause of abortion in sheep, resulting in the loss of more than 500,000 lambs a year at a cost of up to £24m. Sheep acquire the infection by eating oocysts on pasture grass or from concentrated feed contaminated with cat faeces.

A survey of 3,539 blood samples taken from breeding ewes in Britain revealed that 68.6 per cent are positive for toxoplasma antibodies, indicating past exposure to the parasite.