CONDITION
Toxoplasmosis
Toxoplasmosis is an infection caused by a single-celled parasite called Toxoplasma gondii. Cats are the main host for this organism, and while many cats carry it without showing signs of illness, some—particularly those with weakened immune systems—can develop symptoms. Dogs can also become infected, though they do not shed the parasite in the same way and clinical illness is less common. Owners often arrive at this page because routine testing has detected exposure to the parasite, or because their pet is unwell and toxoplasmosis has been mentioned as one possibility. Signs, when they occur, can vary widely depending on which part of the body is affected—eyes, lungs, nervous system, or muscles. In many cases, the infection remains dormant and causes no trouble at all. This page explores what toxoplasmosis can look like in dogs and cats, what is happening in the body when infection occurs, how it is identified through testing, and the range of approaches that exist when treatment is considered. The goal is to help you understand the condition and the questions that shape decisions around it.
Why this matters now
Toxoplasmosis can affect dogs and cats at any age, but clinical illness tends to appear more often in younger animals or those with compromised immune systems. Cats may be exposed during hunting or by eating infected prey, while dogs typically acquire the parasite through ingesting contaminated soil, water, or undercooked meat. In many cases, infection occurs without the owner noticing, and the organism remains dormant for the animal's lifetime. Testing may reveal past exposure in animals that have never shown any signs of illness.
Most infections remain latent, with the parasite forming dormant cysts in muscle or brain tissue that cause no symptoms. If the immune system becomes weakened—through age, illness, or medication—the parasite can reactivate and begin multiplying again. The course of active disease varies widely depending on which organs are affected; some animals develop signs over days to weeks, whilst others show a more gradual onset over months. Not all animals with detectable antibodies will ever develop clinical illness.
Signals & patterns
Early signals
Reduced appetite or reluctance to eat
The animal may show less interest in meals or eat smaller portions than usual. This can be subtle at first and may come and go over several days.
Mild fever or lethargy
The pet may seem quieter than normal, spending more time resting or showing less enthusiasm for activities they usually enjoy. A slightly elevated temperature may be present but is often not obvious to the owner.
Mild eye changes
Some animals develop slight cloudiness or redness in one or both eyes, or the pupils may appear uneven in size. These changes can be easy to miss in the early stages.
Vague muscle stiffness or reluctance to move
The animal may move more carefully or seem reluctant to jump or climb. This can resemble the stiffness seen after heavy exercise and may not appear every day.
Later signals
Neurological changes
Some animals develop signs such as unsteady gait, circling, head tilt, tremors, or seizures. These reflect involvement of the brain or spinal cord and tend to progress over days to weeks.
Breathing difficulty or cough
If the lungs are affected, the animal may breathe more rapidly or with increased effort, sometimes accompanied by a cough. This pattern suggests inflammation within the lung tissue.
Vision loss or severe eye inflammation
Progressive inflammation inside the eye can lead to noticeable vision changes, bumping into objects, or visible swelling and discharge. This may affect one or both eyes.
Click to read about the biological mechanisms
How this is usually investigated
Investigation typically begins with a careful history—recent hunting behaviour, access to raw meat, any signs of illness in other household pets, and whether the animal has shown any gradual changes in behaviour or coordination. Physical examination may reveal fever, muscle pain, enlarged lymph nodes, or changes in the eyes. From there, testing is chosen based on which organs appear affected and how likely toxoplasmosis is compared with other conditions that can cause similar signs.
Serological testing (antibody measurement)
PCR testing
Ophthalmic examination
Imaging (radiography, ultrasound, or MRI)
Cerebrospinal fluid analysis
Options & trade-offs
Management decisions depend on whether the animal is showing signs of illness, which organs are affected, and how the infection is affecting quality of life. Some animals with positive antibody tests but no clinical signs are monitored without treatment, whilst others with active disease receive medication to reduce parasite replication and control inflammation. The combination of approaches varies widely between individuals, and what proves workable for one household may not suit another.
Antiprotozoal medication
Drugs such as clindamycin are used to interfere with the parasite's ability to reproduce inside cells. Treatment courses typically last several weeks, and the aim is to reduce the number of actively multiplying parasites and allow the immune system to regain control. The medication does not eliminate dormant cysts and cannot guarantee that reactivation will not occur later.
Trade-offs: Some animals experience gastrointestinal upset or loose stools during treatment, and compliance can be challenging over long courses. The parasite may not be fully cleared, and clinical signs can recur if the immune system becomes compromised again.
Anti-inflammatory or immunosuppressive drugs
Corticosteroids or other immunomodulating medications may be used to reduce the inflammation that contributes to tissue damage, particularly in the eyes or nervous system. These are often given alongside antiprotozoal treatment to manage the immune response whilst the parasite burden is being reduced. The balance between suppressing harmful inflammation and preserving the immune defence against the parasite is judged on an individual basis.
Trade-offs: Immunosuppressive drugs can increase the risk of the parasite reactivating or other infections developing, and long-term use carries its own set of side effects. Monitoring is often needed to adjust doses, and some animals tolerate these medications less well than others.
Supportive care and monitoring
For animals with mild signs or those in which active infection has not been confirmed, observation without specific anti-parasitic treatment may be considered. This might include nutritional support, management of secondary symptoms, and regular reassessment to detect any progression. In some cases, the immune system controls the infection without intervention.
Trade-offs: This approach depends on the animal's immune competence and the severity of signs. Delaying treatment may allow progression in some individuals, whilst others remain stable for months or years without active therapy.
Environmental and dietary modification
Preventing re-exposure involves measures such as feeding commercially prepared food rather than raw meat, limiting hunting behaviour, and managing contact with soil or litter that may harbour oocysts. These steps can reduce the risk of new infection or repeated challenge in animals that have been treated. They do not address existing infection but may play a role in long-term management.
Trade-offs: Behavioural changes can be difficult to enforce in cats that roam outdoors, and dietary shifts may not be acceptable to all animals or owners. These measures offer no direct treatment of active disease and are most relevant in preventing future exposure.
Common misconceptions
"A positive test for toxoplasmosis means my pet is currently ill with the infection."
Antibody tests indicate that the immune system has encountered the parasite at some point, but many animals carry antibodies for years without ever developing clinical signs. Active disease requires additional evidence, such as rising antibody levels, detection of parasite DNA, or clinical findings that fit the pattern of toxoplasmosis. A single positive result on its own does not confirm that current symptoms are caused by the parasite.
"If my cat has toxoplasmosis, it will always be shedding the parasite and pose a risk to people in the household."
Cats typically shed oocysts only during the initial infection, usually for one to two weeks, and most never shed again even if the parasite remains dormant in their tissues. The majority of seropositive cats are not actively shedding and do not pose an ongoing transmission risk. Routine hygiene measures and prompt removal of faeces from litter trays reduce exposure risk further, regardless of the cat's infection status.
"Once treated, toxoplasmosis is cured and will never return."
Antiprotozoal drugs reduce the number of actively multiplying parasites but do not eliminate dormant cysts that can persist in tissues for the animal's lifetime. If the immune system becomes weakened later—through age, illness, or medication—the parasite can reactivate. Some animals remain well after treatment, whilst others experience recurrence and may need repeated or prolonged therapy.
Related conditions
Leptospirosis
Feline immunodeficiency virus can weaken the immune system in ways that allow latent toxoplasmosis to reactivate, and cats with FIV may develop more severe signs when toxoplasma infection becomes active. Both conditions can affect similar organ systems, including the eyes and nervous system.
Feline Leukaemia Virus (FeLV)
Feline leukaemia virus, like FIV, may compromise immune function in ways that allow dormant toxoplasma organisms to multiply and cause illness. Cats with FeLV-related immunosuppression can develop more widespread or severe toxoplasmosis than those with intact immune systems.
Uveitis
Uveitis—inflammation inside the eye—can occur in toxoplasmosis when the parasite affects ocular tissues, though many other infectious, immune-mediated, and inflammatory conditions can produce similar changes. Distinguishing the underlying cause often requires additional testing.
Cryptococcosis
Cryptococcosis is another infection that can affect immunocompromised animals and may involve the eyes, lungs, or nervous system in patterns that overlap with toxoplasmosis. Both conditions may be considered when an animal presents with neurological signs or uveitis, particularly in cats.
Immune-Mediated Polyarthritis
Immune-mediated polyarthritis can occasionally develop in association with underlying infections, including toxoplasmosis, when immune complexes trigger joint inflammation. In some cases, toxoplasma infection has been identified in dogs presenting with signs of polyarthritis, though this is uncommon.
If your pet has been diagnosed with or tested for toxoplasmosis, you may find it useful to explore related content on immune system function and how infections can interact with other conditions that affect immunity. The broader Immune & Inflammatory Health section offers context on what shapes an animal's ability to control infections over time. Conversations around prevention, household risk, and long-term monitoring can help clarify what may be relevant in your particular situation.