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
Options & trade-offs
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