Toxoplasmosis

By Nancy Otten, Animal Health Technologist

What is toxoplasmosis?

Toxoplasmosis is a disease caused by the protozoan Toxoplasma gondii.  It can affect mammals, birds and reptiles.  It has a very complicated life cycle and although it can affect many species, it can only complete its life cycle within a cat (the cat is its definitive host.)

How is the disease transmitted?

There are three routes of transmission.  The first is the ingestion of infected animal tissues.  This is the most common route of infection for cats and is probably the most common route of infection for people too.  Cats become infected by eating infected birds and mice, and people become infected by eating undercooked meat (usually pork or mutton.)  The second route of infection is oocysts shed in cat feces.  Oocysts are the resistant stage of the organism’s life cycle, similar to hibernation.  They can remain in this inert state for a long period of time and only come out of it when they are consumed by the appropriate organism.  This is a minor source of infection for cats because they generally avoid each other’s feces.  You might think that people avoid cat feces too. However when you consider who cleans out litter boxes, gardens where cats are defecating, plays in sandboxes where cats are defecating, etc., you might think differently.  Also, feces can easily be spread around by the cat as it exits the litter box and then jumps on the bed or table.  The third route of infection is transplacental.  This is when the baby becomes infected through its mother’s placenta.     

How does the disease affect people?

In order to understand how the disease affects us, we need to know a little bit about Toxoplasma gondii’s life cycle.  When a cat consumes the protozoan, it multiplies within its digestive tract.  Some of these protozoans become oocysts and are then shed in the cat’s feces.  The rest of them become tachyzoites which migrate through the intestinal wall and enter the blood stream and the lymphatic system.  They can then travel to anywhere in the body and infect cells.  They rapidly multiply within these cells until the cells eventually rupture.  The protozoans can then infect adjacent cells.  These little infections cause areas of necrosis and inflammation that the body normally fights and controls.  A small percentage of the time, these lesions become extensive enough to cause clinical disease.  Most of the time, the body’s immune system encysts these areas and they become dormant.  These dormant tachyzoites are then called bradyzoites.  The protozoans can stay in these cysts for the entire life of the host or until the host is eaten by something else. 

When we become infected, either by consuming oocysts or by eating infected meat, many different organs of the body can be affected causing many different clinical signs.  Most commonly, our respiratory system is involved causing mild flu-like symptoms lasting for a few days.  If our immune system is intact, we successfully fight these protozoan invaders and develop immunity.  If our immune system is compromised in some way (we are young or old or have an immune disorder like HIV,) we can’t fight the protozoan at the tachyzoite stage and they multiply unchecked until they cause serious damage.  Infected babies can develop chorioretinitis (a disease of the eye,) intracranial calcification and hydrocephalus (enlargement of the cranium caused by an abnormal accumulation of cerebrospinal fluid.)  Immunocompromised adults often develop encephalitis (inflammation of the brain.)

How does the disease affect cats?

It affects cats the same way it affects us.  A cat with a healthy immune system may show some short-lived respiratory symptoms.  Sometimes they have no symptoms at all.  Kittens and older cats are at a greater risk of developing toxoplasmosis, as are cats that are immunocompromised in some way (have the feline leukemia virus or the feline immunodeficiency virus.)  Unlike humans, unborn kittens aren’t at risk of developing toxoplasmosis through their mother’s placenta.

How common is toxoplasmosis?

It is estimated that 50% of cats and 30% of people have the antibodies for Toxoplasma gondii.  That doesn’t mean that they have developed the disease toxoplasmosis.  It just means that they were exposed to the protozoan at some point in their lives and have developed immunity to it.  Toxoplasmosis is one of the most common infections seen in people with advanced HIV.  They become susceptible to new infections from oocysts and undercooked infected meat but they are also susceptible to old infections.  If they were infected while they still had a strong immune system, their bodies would have forced the tachyzoite stage into the dormant bradyzoite stage.  Without a proper immune system to keep them in the bradyzoite stage, they can return to the tachyzoite stage to form areas of necrosis and inflammation again.  Toxoplasmosis is less common in babies; approximately 0.028% of them develop it.

If we are pregnant, or if our immune system is compromised in some way, how can we prevent exposure to Toxoplasma gondii?

People at risk should make sure that the meat they eat is cooked thoroughly.  They should even avoid the handling of raw meat.  They should not clean out the litter box at all.  This should be someone else’s responsibility.  The person cleaning out the litter box should be very diligent about it.  Feces should be scooped out daily and the box itself should be bleached weekly.  The reason for this is that the oocysts are not infective until they’ve had a period of 1 to 5 days to sporulate.  If the litter box is cleaned out daily, the sporulation will occur in the garbage can rather than the litter box.  There is less chance of it getting spread around this way.  Don’t let the cat sleep in your bed or go up on the table or counter (this is a hard one.)  Gardening should be done with gloves on to prevent contact with cat feces.  All garden vegetables should be washed thoroughly before eating them.  Children’s sandboxes should be covered when not in use.  These are deluxe toilets for neighbourhood cats. 

Women considering having a baby can be tested for antibodies to Toxoplama gondii.  If they’ve already been exposed to the protozoan, their baby will be protected because immunity has been developed.  If they are tested after they’ve already become pregnant, it is a bit more complicated.  A positive result can either mean that they were exposed previously or that they are presently infected.  A second blood sample will need to be taken approximately 2 to 4 weeks later.  If their antibody titre is the same in both blood samples, they weren’t recently exposed.  If the antibody titre is higher in the second sample, then exposure was recent.  The cat can be tested in a similar way.  Positive results require retesting in 2 to 4 weeks.  It should be noted that a cat will only shed oocysts upon primary exposure, usually for a period of about 10 days.  So the older your cat gets, the chance of him shedding oocysts is less; it’s so common that he was probably exposed to it as a young cat so his contagious stage is long gone.  If your cat tested negative for antibodies, you will need to keep your cat indoors to prevent him from eating mice and birds.  Don’t feed him any raw meat either.