New Study: "Remarkable" Deterioration in Memory Functions of Seniors Infected by Common Parasite Found in Free-roaming Cats

(Washington, D.C., February 20, 2014) A new study just published in the scientific journal Brain, Behavior, and Immunity documents what scientists describe as “remarkable” working memory performance reductions in seniors 65 and older that test positive for infection by the parasite Toxoplasma gondii. The parasite is believed to infect about one-third of the world's population.

The peer-reviewed study was authored by Patrick D. Gajewski, Michael Falkenstein, Jan G. Hengstler, and Klaus Golka of the Leibniz Research Centre for Working Environment and Human Factors, World Health Organization (WHO) Collaborating Centre for Occupational Health, Dortmund, Germany.

According to the study, infected “seniors showed lower performance in the verbal memory test, both regarding immediate recall and delayed recognition. … Substantial differences were also obtained from long-term semantic memory…The size of the effect is remarkable.”

The authors report “for the first time that T. gondi infection significantly deteriorates functions of the episodic and working memory. … Considering the high prevalence and the relatively strong association with cognitive functions, the impact of T. gondi infection on public health may be higher than hitherto expected.” Because of “an increasing population of older adults, this finding is of high relevance for public health.”

Free-roaming cats a key transmission vector

Although infection by the T. gondii parasite is sometimes associated with improperly cooked meat, a key transmission vector in the U.S. that has been growing to staggering levels is free-roaming cats. According to Dr. George Fenwick, a Johns Hopkins-educated pathobiologist who is also President of American Bird Conservancy, “The number of domestic cats in the U.S.—both owned and un-owned—has increased to as many as 188 million. Studies have shown that up to 74 percent of those cats will test positive for this parasite during their lifetime.”

Recognizing that this is an issue that elicits strong public response, some have been reluctant to admit that the growing number of free-roaming cats—both feral and owned—is having a negative impact on society. “Our civic leaders are not only ignoring but embracing practices that foster the burgeoning populations of feral cats. The consequences will be felt for decades, if not longer, by both the suffering cats and people,” he said. “Those officials continue to accept the emotionally laden arguments of cat advocates, suggesting that feral cats pose no harm to the environment or to their communities—in contrast to a mountain of peer-reviewed science saying just the opposite.” Recent studies ringing alarms on the feral cat issue have been authored by scientists from the Centers for Disease Control and Prevention, the Smithsonian Migratory Bird Center, and the U.S. Fish and Wildlife Service.

The 82 participants in the current study (42 T. gondi positive and 42 T. gondi negative) underwent extensive assessment to document their demographic, socioeconomic, neuropsychological, and psychiatric status. Participants were 65 or older, physically and mentally fit, living independently, and possessed sufficient or corrected visual and auditory acuity. All tests were performed according to published standard procedures.

The link among cats, T. gondii, and humans is documented in numerous studies. According to one, up to 74 percent of cats in the United States will become infected with T. gondii during their lifetime, depending on the type of feeding and whether cats are kept indoors or allowed to roam outdoors.

Additional recent studies have also chronicled the public health consequences of outdoor cats. A study from scientists at the Stanley Medical Research Institute and Johns Hopkins University said that “because cats are now so ubiquitous in the environment, one may become infected [with toxoplasmosis] by neighboring cats which defecate in one's garden or play area, or by playing in public areas such as parks or school grounds.” The authors went on to say that prevention should include public education, keeping cats indoors, and minimizing the population of feral cats.

While a T. gondii infection may not show symptoms, dormant cysts in the human brain or elsewhere may still cause lasting, negative effects. T. gondii infections are potentially fatal to immuno-compromised people and may cause spontaneous abortion in pregnant women or cause birth defects.  Some preliminary studies suggest that chronic infection may be linked to schizophrenia or suicidal behavior.

Connection between toxoplasmosis and cats unquestionable

The connection with cats—the sole, definitive host of this parasite—is unquestionable. One study found that almost 80 percent of mothers of congenitally infected infants in the sample contracted their infections as a result of environmental contamination from cat feces.

The spread of toxoplasmosis to other wildlife was also documented in a study by 11 British scientists who examined 271 Eurasian otter cadavers across England and found that 108 (almost 40 percent) of those animals tested positive for toxoplasmosis, which is described in the study as a “globally important [disease] with potentially devastating health impacts both for humans and a range of domestic and wild species.” The authors said, “The relatively high prevalence of [the toxoplasmosis parasite] in a predominantly [fish-eating] freshwater mammal suggests widespread faecal contamination of freshwater ecosystems.”

Another new study just published found for the first time an infectious form of Toxoplasma gondii in western Arctic Beluga whales, prompting a call for caution for the Inuit people who eat whale meat.

This most recent study, published in Brain, Behavior, and Immunity, concluded by saying: “The relatively large effects of T. gondii on memory functions observed here correspond to the mean decrease in performance usually observed between ages 60 and 70, which may have considerable socioeconomic consequences.”