What is toxoplasmosis?
Toxoplasmosis is a disease caused by infection with the organism called Toxoplasma gondii (T. gondii). This microscopic, single-celled organism is related to coccidia.
All warm-blooded animals, including people, can be infected with this organism. It is an extremely well-adapted parasite and rarely causes significant disease in the individuals it infects.
How common is Toxoplasma in cats?
Toxoplasma occurs worldwide, and infection in cats is widespread. Many more cats are infected than show symptoms.
In some geographical regions, up to 43% of adult cats have Toxoplasma antibodies in their blood, which indicates they were exposed to the infection at some point in their life. Infection rates are higher in free-roaming and stray cats, and uncommon in pet cats that eat commercial cat foods and do little or no hunting. Very few cats show significant clinical signs, despite the high number of cats infected.
How is Toxoplasma transmitted?
Cats are usually infected by ingesting the organism in the flesh of an infected animal, known as an “intermediate host”. The intermediate host is usually a rodent. After infection, the Toxoplasma organism reproduces in the cat’s intestinal tract, then sheds oocysts (a form of eggs) in the feces. Cats are the only definitive hosts for T. gondii, which means that the organism can only produce oocysts when infecting a cat.
Oocysts are a hardy form of T. gondii that can survive in the external environment for many months or even years. Other animals can become infected by ingesting these oocysts, but the disease occurs only if they ingest large numbers of oocysts. In some cats, particularly if their immune defenses are compromised, Toxoplasma can invade beyond the intestine and spread into other internal organs. In the internal organs, the parasite may cause enough damage to cause signs of disease, or it may become dormant in a tissue cyst.
If Toxoplasma continues to replicate, spread, and cause damage to tissues, various clinical problems can develop, including eye disease, respiratory disease, diarrhea, liver disease, and neurological signs, depending on which tissues are affected.
Such disease may be acute (rapid onset) or more chronic, with periods of illness interspersed with periods of some recovery. It is important to remember that T. gondii does not usually cause disease in cats. Consult your veterinarian if you are concerned about any clinical signs your cat is exhibiting.
How is toxoplasmosis diagnosed and treated?
Toxoplasmosis is difficult to diagnose in cats because the signs can vary. Blood tests determine whether a cat has been exposed to the organism. These tests do not necessarily mean that Toxoplasma is the cause of any disease, since most exposed cats do not develop disease; however, high levels of certain antibodies can suggest current infection. When toxoplasmosis is suspected in a cat, it is usually treated with a course of antibiotics.
How do humans get toxoplasmosis?
Humans usually get toxoplasmosis by eating contaminated food. Sheep, cattle, and pigs that graze on contaminated pastures or eat oocyst-contaminated food can develop the encysted form of Toxoplasma in their body tissues. Infected meat (typically beef, pork, or lamb) can cause infection in humans if it is not properly cooked, or if proper hygiene precautions are not followed when the uncooked meat is handled. Humans can ingest oocysts from infected cats, for example when gardening in contaminated soil, but this is a much less common source of human infection.
How concerning is Toxoplasma in humans?
Most people infected with Toxoplasma do not develop clinical disease, or they may show mild and transient flu-like signs. However, there are some cases in which significant disease occurs.
In particular, if Toxoplasma infection occurs during pregnancy, the infection may be transmitted to the fetus and can sometimes cause severe damage. This risk occurs only if the infection is acquired during pregnancy. If someone is exposed to the organism and becomes pregnant later, there is no risk of transmitting it to the fetus.
Are cat owners more likely to become infected?
Although cats are essential to the T. gondii life cycle, people who own cats have no higher risk of infection than the general population.
There are several reasons for this:
- Many pet cats will never be exposed to Toxoplasma and so cannot pass the infection on to humans.
- If a cat becomes infected with Toxoplasma, it will only shed oocysts in its feces for about 10 days after initial exposure. After that, there is no further, significant oocyst shedding and no additional risk to humans.
- While humans may become infected through exposure to oocysts in the environment, infected meat is a more common source of infection.
How can humans avoid infection?
You can significantly reduce the risk of human infection by following a few sensible environmental and meat hygiene measures:
- Cook all meat thoroughly, to a consistent temperature of at least 160°F to 180°F (70°C to 82°C).
- Wash hands, utensils, and surfaces thoroughly after handling raw meat.
- Wash all vegetables carefully.
- Wear gloves when gardening in soil potentially contaminated by cat feces.
- Empty cat litter boxes daily, dispose of litter carefully, and disinfect litter boxes with boiling water. Oocysts passed in the feces take more than 24 hours to develop into the infective stage, so if you sanitize the litter box daily, the oocysts are gone before they can cause infection.
- Discourage pet cats from hunting and avoid feeding them raw or undercooked meat.
- Cover children’s sandboxes to prevent cats from using them as litter boxes.