Cat-Borne Parasite Implicated in Frailty of the Older

Vertical Pupils cat snake

Toxoplasma gondii, a parasite primarily transmitted by cats and implicated in various mental disorders, is also suggested to be associated with the incidence of frailty (exhaustion, loss of muscle mass, etc.) in older adults, according to a new study. While the infection with the parasite is generally asymptomatic, the results suggest that it could have significant adverse effects later in life, potentially leading to frailty.


Cats (as well as other felines) serve as the definitive hosts for T. gondii, while warm-blooded animals such as birds and rodents act as its intermediate hosts. When cats become infected by consuming these animals, the parasite establishes itself and multiplies in their intestines, with eggs being excreted in their feces. Human infection occurs through poorly cleaned cat litter, contaminated water and food, or undercooked meat.

By analyzing the evolutionary strategies favoring the life cycle of T. gondii, investigations have shown that infected rodents tend to lose their fear of felines, making it easier for the cats to hunt them and ingest the parasite in turn. Additionally, infected chimpanzees are oddly attracted to the smell of leopard urine, their natural prey.

In healthy individuals, toxoplasmosis (infection with T. gondii) is typically latent and may only manifest over a decade after the initial infection or in cases of immunosuppression. Most people are unaware of their infection, and only around 10% experience brief flu-like symptoms at the onset.

The parasite lodges in the form of cysts in skeletal muscles and brain tissues. Its presence in the amygdala could disrupt emotion processing and is likely involved in the behavioral disorders observed in some infected individuals. Previous studies have revealed that these individuals tend to be more impulsive and are more prone to schizophrenia, suicidal tendencies, and various cognitive disorders.

Life cycle of T. gondii.
Life cycle of T. gondii.

On the other hand, the new international study, published in the Journals of Gerontology Series A, suggests that the presence of the parasite could have impacts on health later in life, even in initially asymptomatic individuals. “We often think of T. gondii infection as relatively asymptomatic, but this study highlights that for some people it may have significant health consequences later on,” explains co-author Christopher Lowry from the University of Colorado at Boulder.

High Antibody Levels Associated With Frailty

The new study focuses on a group of 601 Spanish and Portuguese adults aged 65 and older. All participants underwent assessments related to a common geriatric syndrome called “frailty.” Symptoms include involuntary weight loss, chronic fatigue, cognitive decline, chronic inflammation, etc. Among these individuals, 67% were infected with T. gondii and showed markers of latent infection. This observation led researchers to hypothesize that the infection was likely directly involved in the frailty of the elderly.

Indeed, research has shown that toxoplasmosis activates the inflammatory cytokine-controlled enzyme indoleamine 2,3-dioxygenase 1 (IDO-1). This enzyme is one of the two key enzymes responsible for the metabolism of tryptophan (TRP) into kynurenine (KYN), which is involved in certain immune processes. Furthermore, the KYN/TRP ratio is notably higher in animal models infected with T. gondii and showing signs of frailty compared to those considered “non-frail.” The team of the new study thus assumed that frailty is positively associated with the presence of specific immunoglobulin G antibodies against the parasite.

Although researchers ultimately did not find a direct link between parasite infection and frailty, they observed that individuals with high serointensity (meaning a high level of antibodies) are more likely to be frail. This could possibly indicate a more virulent infection, multiple infections, or a recent reactivation of a latent infection, aligning with the presence of associated markers. These individuals also exhibited higher levels of certain inflammatory markers, suggesting that the parasite might exacerbate the chronic inflammation inherent in aging.

This paper is important because it provides, for the first time, evidence of the existence of a link between frailty in older adults and intensity of the response to T. gondii infection,” suggests the co-author of the research, Blanca Laffon, from the University of La Coruña. Moreover, given that the parasite resides in skeletal muscles, experts suspect its involvement in accelerating age-related sarcopenia (a significant decrease in muscle mass and strength).

In light of these findings, researchers advocate for the adoption of preventive measures primarily based on hygiene practices (cleaning of litter boxes, washing of fruits and vegetables, etc.). These measures should apply not only to immunocompromised individuals and pregnant women but also to healthy individuals whose immune functions will decline with age.