For most of us, the immune system never reaches a state where toxoplasmosis can get enough of a foothold to cause symptoms. However, a few healthy people suffer mild symptoms from toxoplasmosis infection.
About 80% to 90% of people show no symptoms when infected with toxoplasmosis. About 10% to 20% of people will develop swollen glands and some of these people will develop symptoms that are similar to the flu or infectious mononucleosis (e.g., low-grade fever, muscle aches, sore throat, an enlarged spleen and liver, and sometimes mild diarrhea). There may also be a mild anemia. These symptoms can last for weeks or longer, but will go away without treatment.
Of people with AIDS, about 30% to 40% develop disease from toxoplasmosis, usually because of the reactivation of an old infection. Most of these infections involve the central nervous system. Symptoms can include:
- loss of vision or other senses
- partial paralysis
- difficulty speaking
For the majority of people with AIDS who have toxoplasmosis, the onset of symptoms is quite slow and subtle. There may be changes in emotions, behaviour, or cleanliness. Later, there may be numbness or weakness in the arm or leg.
Rarely, areas outside the central nervous system are affected and can lead to inflammation of the brain (encephalitis), its lining (meningitis), the heart (myocarditis), the lungs (pneumonitis), and various other organs. Other symptoms such as high fever, chills, sweating, and rash can occur with these infections.
Women who already have toxoplasmosis infection and become pregnant have little to worry about. But if a woman becomes infected while pregnant, there's a risk the fetus will be infected. The risk is about 6% in the first trimester. Fetuses infected at this stage often miscarry. The risk of infection of the fetus rises to 30% if infection occurs in the second trimester and rises up to 60 – 81% if infection occurs in the third trimester. Without treatment, there is a 20 – 30% risk of passing the toxoplasmosis infection from mother-to-fetus during pregnancy.
Babies infected late in pregnancy rarely miscarry but instead have symptoms such as:
- brain damage
- enlarged spleen and liver
- inflammation of the eye
- jaundice (yellowing of the skin and whites of eyes)
- poor motor coordination
- unusually small head
- easy bruising
Less severe infections may not be obvious at birth but can show up months or years later. The baby often grows up into a healthy young adult. But around age 20 or 30, the eyesight starts to degenerate as toxoplasma attacks the retina, the light-sensitive membrane at the back of the eye. Ocular toxoplasmosis can cause pain, blurred vision, and permanent damage, including blindness, and can occasionally occur in adults.