A young cat can shed up to 100 million oocysts – little egg-like structures – in its feces. All it takes is one oocyst to cause an infection of Toxoplasma gondii.
Largely, the parasite is asymptomatic in humans, but it can sometimes cause problems for infants born to infected mothers – including hearing loss, mental disability and blindness. People with compromised immune systems, especially those who have HIV/AIDS, may also develop serious complications.
Researchers are trying to understand why marine mammals in the Pacific Northwest started dying of protozoal diseases starting in 2000; before then, there weren’t any documented cases, but samples from the Pacific Northwest have found a rate of about 4% of protozoal disease among stranded animals, says Michael Grigg, investigator at the National Institutes of Health.