Cats may be increasing their owners' risk of Alzheimer's.
A new report warns a parasite commonly found in cat faeces alters the chemistry of the human brain, making it fertile ground for the disease to take hold.
It builds on previous studies that linked the parasite - Toxoplasma gondii - to increased risk of brain cancer, anxiety, and schizophrenia.
Though pregnant women are already advised to avoid kitty litter, experts say the new study shows even more people should be cautious.
About one-third of people worldwide are suspected of having a T. gondii infection, and many more are at risk.
This includes cat owners, who don't wash their hands thoroughly after handling contaminated litter, and pregnant women, who can pass the infection to a child in the womb.
Once infection occurs, the parasite moves to the brain.
Symptoms can vary widely, with some people revealing no outward signs of infection and others becoming acutely ill and showing behavioural and body-chemical changes that mimic schizophrenia.
However, because this chronic infection targets the brain, researchers have been investigating whether it could actually be a cause of Alzheimer's, schizophrenia, or mood disorders as opposed to simply mimicking the signs of such illnesses.
The study published in the Journal of Parasitology focused on mice, some that were infected with the parasite and others that were altered to create an Alzheimer's disease model.
The mice were then tested for behavioural and molecular changes.
The authors, from Lorestan University of Medical Sciences in Iran, found that after infection with the parasite, mice had impaired learning and memory function, just as they would with Alzheimer's disease.
The infection also altered their brain chemistry compared to uninfected mice.
In one group, the infection increased the likelihood of Alzheimer's disease.
Michael Sukhdeo, editor of the Journal of Parasitology says that it is highly plausible that T. gondii may be linked to Alzheimer's disease, "given that the parasite likes to live in the brain".
He added: "It is also of great public health concern with pregnant women, who are warned against cleaning cat litter for fear of infecting the foetus."
The authors found that the parasite can affect the progression of Alzheimer's disease in the brain of a mouse both directly and indirectly.
If these findings were to hold true for human patients, the parasite may be a public health concern for newborns as well as adults.
What is this cat faeces parasite?
T.gondii is considered one of the most prevalent pathogens in the world.
The commonly-found parasite inhabits 50 per cent of living organisms.
It lives inside the intestines of warm-blooded animals, but particularly thrives in cats.
Humans can ingest the parasite by eating contaminated meat or water, or coming into contact with cat faeces while cleaning out the kitty litter.
It can lead to toxoplasmosis, which has devastating effects in people with weakened immune systems.
Infected healthy people can experience flu-like symptoms, though many experience no symptoms at all.