Tracking Trends in Toxoplasmosis Transmission

When most people hear the word “toxoplasmosis” they often think of the parasitic disease that can be devastating to pregnant women. While exposure from infected cat feces and mother-to-child transmission are both methods of transmission, there are many others that allow ways that this zoonotic disease to move to humans.

The protozoan Toxoplasma gondii is an intracellular parasite existing all over the world, that mostly infects mammals and birds. As intermediate hosts for the parasite, warm-blooded animals do serve a vital role in the transmission chain; however, it is felids that are the only “definitive host” and shed oocysts that contaminate the environment. Investigators from Brazil recently explored the topic of Toxoplasmosis transmission in humans and the trends observed in outbreaks, in the latest issue of Emerging Infectious Diseases from the US Centers for Disease Control and Prevention (CDC).

Considered a leading cause of death related to foodborne illnesses, more than 40 million people in the United States alone carry the parasite. The CDC states that it is a neglected parasitic infection in the United States and is a target for public health efforts.

The authors note that on a global level, exposure is so high that serologic prevalence varies between 10% and 94% in the adult population. Infection is dependent upon factors like environmental conditions, eating habits, and prevalence of the parasite in the geographical area, among other factors. Transmission mostly occurs through the ingestion of oocyst-contaminated water or vegetables.

Reviewing publications on toxoplasmosis outbreaks since 1967, the study team excluded cases in nonhuman species. A total of 573 articles were further analysis was conducted on 33 articles covering 34 reported outbreaks. The highest concentration of outbreaks reported (73.5%) occurred in the Americas, with Brazil having the highest number of published outbreaks.

The authors note, “the incidence of cyst-related outbreaks from contaminated meat and its derivatives was 47.1% (16/34), and oocysts were implicated in 44.1% (15/34) of the outbreaks. Transmission through the intake of oocysts in water occurred with a frequency of 20.6% (7/34), through contact with sand and soil with a frequency of 17.6% (6/34), and through consumption of vegetables with a frequency of 5.9% (2/34).”

Nearly 9% of outbreaks were related to contaminated raw milk and 1416 people were affected across 15 outbreaks from oocysts. Ultimately, they found no statistically significant relationship between variables within the articles but did note that several outbreaks related to the ingestion of cysts in meat occurred in the 1960s and 1990s, while outbreaks in the 1980s were mostly related to milk contaminated with tachyzoites.

Interestingly, outbreaks in the 2000 were mostly related to oocysts in water and contact with feline feces, but since 2010, outbreaks related to oocysts and raw vegetable consumption has increased. There was not a dominant genotype of Toxoplasma gondii within the United States outbreaks and internationally, oocysts and cysts are the most frequent parasitic form of Toxoplasma gondii that is transmitted to humans.

This particular study found that dietary preferences, like the consumption of raw or undercooked meat, can facilitate the transmission of Toxoplasma gondii and milk consumption can also be a source of transmission. Additionally, the fecal-oral route is 1 of the main forms of transmission, with felines acting as the definitive host and environmental contamination facilitators. These findings should guide public health education and prevention efforts while also serving as a reminder for the public that cooking meat appropriately, not consuming raw milk, and taking proper precautions with feline feces, are critical to avoiding toxoplasmosis.