Causes
The most common way people get toxoplasmosis is by ingesting toxoplasma
eggs (oocysts). Cats are the primary source of these eggs. While
Toxoplasma gondii can reproduce asexually in a variety of animals, only
in the stomachs of cats does it undergo sexual reproduction to generate eggs,
which are then passed out in the cat's feces. The eggs that are passed by the
cat can remain viable in moist soil for up to 18 months. This means it is possible
to ingest these eggs by directly contacting the cat or its litter box or by
ingesting food (vegetables) from contaminated soil.
Another way in which people get toxoplasmosis is by eating undercooked meat.
Animals such as cows, pigs, and sheep may ingest soil contaminated with
the eggs shed by the cats. In these animals the eggs hatch, and the Toxoplasma
penetrate through the intestines and form tiny cysts within their tissues. If
we eat meat from these animals that is not thoroughly cooked, the cysts break
apart in our stomach and the Toxoplasma inside the cysts are then released
to invade our tissues. Pork and lamb are much more likely than beef to contain
these cysts.
Only rarely can toxoplasmosis be transmitted by a blood transfusion. Toxoplasmosis
is not spread from human to human.
Toxoplasmosis is everywhere, and many of us are infected without knowing
it. The good news is that most infections occur without causing any symptoms
and eventually the parasites lie dormant in their cyst form in humans, just
as they do in cows and sheep. They do not usually cause harm unless the immune
system is not functioning properly.