Toxoplasmosis is a widespread disease caused by the parasite Toxoplasma gondii (T. gondii). This single-celled organism is remarkably common, infecting almost all warm-blooded animals, including our beloved feline companions and humans. While cats are essential to the T. gondii lifecycle, they rarely suffer severe illness from it. Although toxoplasmosis often presents mild or no symptoms in healthy adult humans (with some exceptions), the Centers for Disease Control and Prevention (CDC) recognizes it as a significant neglected parasitic infection in people due to its high prevalence. It’s estimated that over 60 million individuals in the U.S. are infected.
Understanding the Toxoplasmosis Life Cycle
The T. gondii parasite has a complex life cycle involving two types of hosts:
- Definitive Hosts: These are hosts where the parasite sexually reproduces and forms oocysts (eggs). Wild and domestic cats are the only definitive hosts for T. gondii.
- Intermediate Hosts: In these hosts, the parasite reproduces asexually, creating clones of itself that cluster within tissue cysts. This category includes a wide range of animals, including humans, rodents, and other mammals and birds.
When a cat becomes infected, typically by consuming infected prey or raw meat, T. gondii is released from tissue cysts in the meat into the cat’s digestive system. Here, it undergoes sexual reproduction and produces oocysts. These oocysts are then shed in the cat’s feces in massive numbers.
Cats newly infected with T. gondii usually start shedding oocysts within 3 to 10 days after ingesting infected material and continue to do so for about 10 to 14 days. Oocysts are incredibly resilient and can survive in the environment for well over a year, posing a long-term infection risk.
Furthermore, some T. gondii parasites from ingested cysts can penetrate the cat’s intestinal wall and transform into tachyzoites, a rapidly multiplying form. Tachyzoites spread beyond the intestine to other parts of the cat’s body. Eventually, the cat’s immune system forces the parasite into a dormant stage, forming tissue cysts containing bradyzoites (slowly multiplying parasites) in muscles and brain tissue.
Other animals, including humans, become intermediate hosts by ingesting either tissue cysts or oocysts. Oocysts freshly shed in cat feces are not immediately infectious. They need to undergo sporulation, a process taking 1 to 5 days depending on environmental conditions, to become infective. Cat feces containing sporulated oocysts, whether in litter boxes, gardens, or sandboxes, become sources of infection.
Once an intermediate host ingests sporulated oocysts, infection leads to the formation of tissue cysts throughout the body. These tissue cysts persist for the life of the intermediate host and are infectious to cats, humans, and other intermediate hosts that consume cyst-containing tissue. In some instances, T. gondii tachyzoites can be excreted in the milk of infected cows and goats.
Figure 1: The complex life cycle of Toxoplasma gondii, highlighting the role of cats as definitive hosts and the various routes of transmission.
Clinical Signs of Toxoplasmosis in Cats
The majority of cats infected with T. gondii show no outward signs of illness. However, in some cases, clinical toxoplasmosis can develop, especially when a cat’s immune system is unable to control the spread of the tachyzoite form of the parasite. This is more likely to occur in cats with weakened immune systems, such as young kittens and cats with feline leukemia virus (FeLV) or feline immunodeficiency virus (FIV).
Common signs of toxoplasmosis in cats include:
- Fever
- Loss of appetite
- Lethargy
Depending on whether the infection is acute or chronic, and where the parasite localizes within the body, other symptoms can manifest.
- Pulmonary Issues: In the lungs, T. gondii can cause pneumonia, leading to progressively worsening breathing difficulties.
- Liver Involvement: Infection of the liver can result in jaundice, a yellowing of the skin and mucous membranes.
- Ocular and Neurological Signs: Toxoplasmosis can affect the eyes and central nervous system (CNS), leading to a range of symptoms:
- 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 response
- Blindness
- Lack of coordination
- Hypersensitivity to touch
- Personality changes
- Circling behavior
- Head pressing
- Ear twitching
- Difficulty chewing and swallowing
- Seizures
- Loss of urinary and fecal control
Diagnosing Toxoplasmosis in Cats
Diagnosing toxoplasmosis in cats typically involves considering the cat’s medical history, observed clinical signs, and laboratory test results. Blood tests to measure two types of antibodies to T. gondii – IgG and IgM – are valuable diagnostic tools.
- IgG Antibodies: High levels of IgG antibodies in a healthy cat suggest a previous infection and likely immunity. These cats are generally not shedding oocysts and pose minimal risk of infection to others.
- IgM Antibodies: Elevated IgM antibody levels often indicate an active, recent infection.
- Absence of Antibodies: If a healthy cat has no detectable T. gondii antibodies, it means the cat is susceptible to infection and could shed oocysts for a couple of weeks following initial exposure.
Detecting oocysts in feces is not a reliable diagnostic method for toxoplasmosis. Oocysts of T. gondii are difficult to distinguish from those of other parasites under a microscope. Furthermore, cats only shed oocysts for a short period, often not coinciding with the time they show clinical signs of disease. 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, administered alone or sometimes in combination with corticosteroids if there is significant inflammation, particularly in the eyes or CNS. Ideally, treatment should begin promptly after diagnosis and continue for several days after clinical signs resolve.
In cases of acute illness, treatment is often initiated based on high initial IgM antibody levels. If a cat does not show clinical improvement within two to three days of starting treatment, the diagnosis of toxoplasmosis may be reconsidered.
Prognosis for Cats with Toxoplasmosis
The prognosis for cats with toxoplasmosis varies depending on the affected organs or systems, 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 often have a favorable prognosis if they show improvement within the first 2-3 days of treatment. However, toxoplasmosis affecting the liver or lungs generally carries a poorer prognosis.
Prevention of Toxoplasmosis in Cats and Reducing Human Risk
Preventing toxoplasmosis in cats is crucial for their health and helps minimize the risk of human exposure. Key preventative measures include:
- Feeding Cats Cooked Food: Feed cats commercially prepared, cooked cat food. Heat effectively inactivates T. gondii cysts that may be present in meat.
- Avoid Raw Meat and Prey: Do not allow cats to eat raw meat or hunt and consume wild prey like rodents and birds.
- Restrict Outdoor Access: Limiting outdoor access reduces hunting opportunities and contact with potentially contaminated soil.
- Maintain Hygiene: Regularly clean litter boxes, ideally daily, as oocysts take 24-48 hours to become infectious after being shed. Wear gloves and wash hands thoroughly after handling cat litter.
While cats are part of the T. gondii lifecycle, the risk of human infection from pet cats is relatively low. Owning a cat does not automatically mean you will get toxoplasmosis. In the U.S., people are far more likely to become infected by consuming undercooked meat and unwashed produce or through gardening in contaminated soil than through contact with cats.
Specific Recommendations for High-Risk Groups:
Pregnant women and individuals with compromised immune systems are at higher risk of severe complications from toxoplasmosis. While past advice suggested avoiding cats, the CDC now recommends practical hygiene measures instead.
- Pregnant Women: Practice diligent handwashing after gardening or handling soil and after contact with raw meat. Have someone else clean the litter box daily if possible. If you must clean it yourself, wear gloves and wash hands thoroughly afterward.
- Immunocompromised Individuals: Follow the same hygiene precautions as pregnant women.
For detailed and up-to-date recommendations regarding toxoplasmosis and human health, consult the CDC website (https://www.cdc.gov/parasites/toxoplasmosis/index.html) or consult with your healthcare provider.
Currently, there is no vaccine available to prevent toxoplasmosis in either animals or humans.
Updated: Content aligned with original article’s update date of June 2018
