Toxoplasmosis, caused by the parasite Toxoplasma gondii (T. gondii), is a prevalent parasitic disease affecting virtually all warm-blooded animals, including our beloved feline companions and humans. While cats play a crucial role in the T. gondii lifecycle, it’s important to understand that clinical disease from this parasite is uncommon in cats. Despite often being asymptomatic in healthy adults, toxoplasmosis is recognized by the Centers for Disease Control and Prevention (CDC) as a significant neglected parasitic infection in people due to its widespread occurrence, with over 60 million individuals estimated to be infected in the U.S.
What is Toxoplasmosis?
Toxoplasma gondii is a single-celled parasite with a complex life cycle that involves two types of hosts: definitive hosts, where the parasite sexually reproduces and forms oocysts (eggs), and intermediate hosts, where it asexually reproduces, creating clusters of parasites within tissue cysts. Remarkably, wild and domestic cats are the only known definitive hosts for T. gondii.
The Lifecycle of Toxoplasma gondii
The Toxoplasma gondii lifecycle begins when a cat consumes infected prey or raw meat. Within the cat’s digestive system, the parasite is released from tissue cysts and initiates sexual reproduction, resulting in the production of oocysts. These oocysts are then shed in the cat’s feces, often in massive numbers. Cats newly infected with T. gondii typically start shedding oocysts around 3 to 10 days post-infection and continue for approximately 10 to 14 days. Oocysts are incredibly resilient and can persist in the environment for well over a year, posing a long-term infection risk.
Furthermore, some T. gondii parasites released from cysts in ingested meat can penetrate the cat’s intestinal wall and multiply in a rapidly dividing form called tachyzoites. These tachyzoites spread beyond the intestine to other parts of the cat’s body. Eventually, the cat’s immune system responds by forcing the parasite into a dormant, slow-growing stage, forming tissue cysts, primarily in muscle and brain tissue. These cysts contain bradyzoites, another form of Toxoplasma organism.
Other animals, including humans, act as intermediate hosts and can become infected by ingesting either tissue cysts or oocysts. Oocysts freshly passed in cat feces are not immediately infectious. They require a process called sporulation, taking 1 to 5 days depending on environmental factors, to become infective. However, once sporulated, oocysts in cat feces, whether in litter boxes, gardens, or outdoor sandboxes, become sources of infection. When an intermediate host ingests sporulated oocysts, tissue cysts develop throughout the body, persisting for the host’s lifetime and remaining infectious to cats, humans, and other intermediate hosts if the cyst-containing tissue is consumed. In some instances, T. gondii tachyzoites can be excreted in the milk of infected cows and goats.
Figure 1: The life cycle of Toxoplasma gondii.
Clinical Signs of Cat Toxoplasmosis
The vast majority of cats infected with T. gondii will exhibit no outward signs of illness. However, in some cases, clinical toxoplasmosis can develop, particularly when a cat’s immune system is unable to control the spread of tachyzoites. This is more likely in cats with weakened immune systems, such as young kittens or cats with feline leukemia virus (FeLV) or feline immunodeficiency virus (FIV).
Common symptoms of toxoplasmosis in cats include:
- Fever
- Loss of appetite
- Lethargy
Additional symptoms can vary depending on whether the infection is acute or chronic and where the parasite localizes in the body. In the lungs, T. gondii infection can cause pneumonia, leading to progressively worsening breathing difficulties. Liver involvement may result in jaundice, characterized by a yellowish discoloration of the skin and mucous membranes.
Toxoplasmosis can also affect the eyes and central nervous system (CNS), leading to a range of neurological and ocular signs, including:
- Uveitis (inflammation of the pigmented part of the eye)
- Retinitis (inflammation of the retina)
- Anterior chamber inflammation (space between the lens and cornea)
- Abnormal pupil size and light responsiveness
- Blindness
- Lack of coordination
- Increased sensitivity to touch
- Personality changes
- Circling behavior
- Head pressing
- Ear twitching
- Difficulty chewing or swallowing
- Seizures
- Loss of urinary and fecal control
Diagnosing Toxoplasmosis in Cats
Diagnosing toxoplasmosis typically involves evaluating a cat’s medical history, observing clinical signs, and conducting laboratory tests. Blood tests to measure two types of antibodies against T. gondii – IgG and IgM – are valuable diagnostic tools.
High levels of IgG antibodies in a healthy cat generally indicate a previous T. gondii infection, suggesting immunity and a low likelihood of oocyst shedding. These cats are unlikely to be a source of infection. Conversely, elevated IgM antibody levels often suggest a recent or active infection. The absence of T. gondii antibodies in a healthy cat indicates susceptibility to infection and potential oocyst shedding for up to two weeks following initial infection.
Detecting oocysts in fecal samples is often unreliable for diagnosis because they resemble oocysts of other parasites. Furthermore, cats typically shed oocysts for only a brief period and might not be shedding when showing clinical signs. A definitive diagnosis often requires microscopic examination of tissue samples to identify characteristic tissue changes and the presence of tachyzoites.
Treatment Options for Feline Toxoplasmosis
Treatment for toxoplasmosis in cats usually involves a course of the antibiotic clindamycin, which may be used alone or in combination with corticosteroids if significant inflammation is present, particularly in the eyes or CNS. Ideally, treatment should commence promptly after diagnosis and continue for several days after symptoms resolve. In acute cases, treatment might be initiated based on high initial IgM antibody levels. If clinical improvement is not observed within 2 to 3 days of starting treatment, reassessing the toxoplasmosis diagnosis may be necessary.
Prognosis for Cats with Toxoplasmosis
The prognosis for cats with toxoplasmosis varies depending on the affected organs, the time between infection and treatment initiation, and the cat’s response to therapy. Cats with CNS and eye involvement tend to respond more slowly to treatment, but their prognosis remains favorable if clinical signs improve within 2-3 days of starting therapy. However, the prognosis for cats with toxoplasmosis affecting the liver or lungs is generally less optimistic.
Preventing Toxoplasmosis in Cats and Reducing Human Risk
Reducing toxoplasmosis incidence in cats involves strategies to minimize exposure to infective oocysts and limit oocyst shedding into the environment. Key preventive measures for cat owners include:
- Feeding cats commercially prepared, cooked cat food. Heat treatment effectively inactivates any T. gondii cysts in food.
- Preventing cats from consuming raw meat or hunting prey like rodents.
- Limiting cat access to areas housing livestock and food storage facilities.
While cats are definitive hosts, the risk of human exposure to T. gondii from pet cats is relatively low. Owning a cat does not automatically mean you will contract toxoplasmosis. Since T. gondii oocysts require at least 24 hours to sporulate and become infectious, daily removal of feces from the litter box, while wearing gloves and washing hands afterward, significantly reduces infection risk. Direct contact with an infected cat is unlikely to transmit the parasite as cats typically do not carry it on their fur. Transmission through cat bites or scratches is also improbable. Indoor cats that do not hunt or eat raw meat are at a minimal risk of T. gondii infection.
In the U.S., humans are more commonly infected through consuming undercooked meat and unwashed fruits and vegetables than through contact with cat feces. Gardening in soil contaminated with cat feces also poses a potential infection risk, which can be mitigated by wearing gloves and washing hands after gardening.
Pregnant women and individuals with compromised immune systems are at higher risk of health complications from T. gondii exposure. Congenital toxoplasmosis, infection of a fetus during pregnancy, is a significant concern. A substantial proportion of infants born to mothers who acquire Toxoplasma infection during pregnancy become infected themselves. While most infected mothers are asymptomatic and many infected infants show no symptoms at birth, these children may develop problems later in life, including vision and hearing loss, intellectual disability, and, in severe cases, death.
For immunocompromised individuals, including those undergoing immunosuppressive therapy or living with AIDS, toxoplasmosis can cause severe health issues affecting lymph nodes, eyes, CNS, respiratory system, and heart. Relapses are common in these patients, particularly those with AIDS, and mortality rates can be high.
While past recommendations advised pregnant women and immunocompromised individuals to avoid cats, the CDC now advises that this is generally unnecessary. For the most current guidelines regarding toxoplasmosis risk to humans, please refer to the CDC’s website.
For specific health concerns related to toxoplasmosis infection in humans, consulting a healthcare professional is recommended. Currently, there is no vaccine available to prevent toxoplasmosis in either animals or humans.
Updated June 2018
