More than 40 million men, women, and children in the U.S. carry T. gondii, CDC reports, but very few exhibit symptoms, courtesy of the immune system usually keeping the parasite from causing illness. Nonetheless, toxoplasmosis, the disease caused by T. gondii, is considered to be a leading cause of death attributed to foodborne illness in the U.S., according to CDC.
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Explore this issueOctober/November 2018
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Estimates suggest that 23 percent of adolescents and adults are infected with T. gondii, and CDC says the parasite accounts for 24 percent of deaths due to foodborne illness in the U.S.
Cats and other felids are the only hosts in which the parasite can complete its entire life cycle, and the only animals that excrete, in their feces, the environmentally resistant and infectious stage called the oocyst, which is microscopic. This knowledge came to light in 1970, thanks to landmark research led by Jitender Dubey, DVM, PhD, a microbiologist with the USDA ARS Animal Parasitic Diseases Laboratory (APDL), Beltsville, Maryland.
Since then, Dr. Dubey has been involved with myriad T. gondii research projects documented in peer-reviewed journals. In his 2008 “History of Toxoplasma gondii—The First 100 Years,” he calls the featured organism “one of the most well-studied parasites because of its medical and veterinary importance, and its suitability as a model for cell biology and molecular studies with a unicellular organism.”
Kittens and cats can shed millions of oocysts in their feces for as long as three weeks after infection, CDC says, adding that mature cats are less likely to shed Toxoplasma if they have been previously infected. A Toxoplasma-infected cat that is shedding the parasite in its feces contaminates the litter box. Outside, cats can contaminate the soil or water as well.
“Compounding this environmental contamination issue, oocysts in soil can be mechanically transmitted by invertebrates such as flies, cockroaches, dung beetles, and earthworms, which can spread oocysts into human food and animal feeds,” Dr. Dubey says.
The tissue form of T. gondii (a microscopic cyst consisting of bradyzoites, the asexual third stage of the parasite) can be transmitted to humans by food. CDC explains that people become infected by eating undercooked, contaminated meat, especially pork, lamb, and venison; accidental ingestion of undercooked, contaminated meat after handling it and not washing hands thoroughly; or eating food that has been contaminated by knives, utensils, cutting boards, or other foods that had contact with raw, contaminated meat. (Of note, per CDC: Toxoplasma cannot be absorbed through intact skin.)
Moreover, people can accidentally swallow the oocyst form of the parasite. CDC points out that this can occur after cleaning a cat’s litter box when the cat has shed Toxoplasma in its feces; after touching or ingesting anything that has come into contact with a cat’s feces that contain Toxoplasma; by ingestion of oocysts in contaminated soil, such as by not washing hands after gardening, or eating unwashed fruits or vegetables from a garden; or drinking water contaminated with T. gondii.
A serological test that can distinguish oocyst transmitted toxoplasmosis from tissue cyst transmitted toxoplasmosis in humans has been developed by ARS APDL. “Thousands of serum samples representing Toxoplasma infected people from the U.S. and other countries were tested,” says Dolores Hill, PhD, an ARS APDL research parasitologist. “Results of this investigation indicate that more than 70 percent of infected humans acquired their infection by exposure to oocysts,” she relates.
“Given recent evidence that many people become infected by ingesting oocysts, we must ascertain which foods confer greatest risk, and devise ways to remove or inactivate those oocysts without eroding such foods’ nutritional quality, flavor, or palatability,” notes Benjamin Rosenthal, SD, also an ARS APDL research parasitologist.
Pregnant women are advised not to clean litter boxes. A woman who is newly infected with Toxoplasma during pregnancy can pass the infection to her unborn child (congenital infection). The woman may not have symptoms, but her unborn child can be stricken with diseases of the nervous system and eyes, CDC warns.
At present, there is no vaccine to prevent human infection with Toxoplasma, which has just the one species, gondii.
Food Safety Issues and Attention
In a peer-reviewed article titled “Toxoplasma gondii as a Parasite in Food: Analysis and Control,” published in the March 2016 issue of Microbiology Spectrum, Drs. Hill and Dubey provide a comprehensive literature review of statistics and studies of the parasite conducted by scientists throughout the world.
Despite being recognized as a significant foodborne pathogen, the knowledge related to food safety and public health significance T. gondii is typically limited, these scientists point out. “In particular, information on the wide range of food animals that can harbor this protozoan pathogen and thus serve as potential reservoirs for human infections is needed,” they mention.
T. gondii infects food animals and many game animal species, Drs. Hill and Dubey note. “Infected animals harbor tissue cysts, and human consumers can be infected by ingestion of these cysts in raw or undercooked meat,” they elaborate, adding that just one (potentially infective) tissue cyst may be present in 100 grams of meat. “A single T. gondii–infected pig can be a source of infection for many people since one market-weight hog (220 pounds or more) can yield over 600 individual servings of meat. Virtually all edible portions of an animal can harbor viable T. gondii tissue cysts, and tissue cysts can survive in live food animals for years.”
Dr. Hill collaborates with H. Ray Gamble, PhD, director of the National Academy of Science’s Fellowships Office, in authoring a Pork Safety Fact Sheet sponsored by the National Pork Board.
Drs. Hill and Gamble point out that human toxoplasmosis in the U.S. is estimated to cost $5.26 billion annually in medical costs, losses in personal productivity, and costs of special education and residential care. “An additional $100 million is attributed to medical costs of toxoplasmic encephalitis in AIDS cases,” these scientists say.
Drs. Hill and Gamble emphasize that animals exposed to this parasite rarely show signs of infection. “Animals are infected by ingestion of oocysts from the environment; or by predation of infected animals such as mice, birds, and other wildlife; by consumption of undercooked meat scraps; and in some species, through in utero transmission,” they relate. “Sheep and goats are important hosts of Toxoplasma in some countries and pose a major risk for human exposure, but are minor species in the U.S. Similarly, free-range chickens are known to be infected, often at high rates, but have not been indicated as a source of human infection in the U.S.”
Currently, no regulations require that meat be inspected for T. gondii and no further processing is mandated to inactivate the parasite, Drs. Hill and Gamble mention. However, they explain that many of the methods that have been followed for processing pork for inactivation of Trichinella spiralis (trichinae) are also effective for the inactivation of T. gondii. “For that reason, processed pork products should be safe for human consumption without further treatment,” Drs. Hill and Gamble say.
Inactivating T. Gondii
Thermal death curves for the interaction of temperatures and times required to kill T. gondii in meat have been generated in closely controlled scientific studies. “From these data, we know that T. gondii is killed in 336 seconds at 120 degrees Fahrenheit, in 44 seconds at 131 degrees Fahrenheit, and in six seconds at 142 degrees Fahrenheit,” Drs. Hill and Gamble note. “These times and temperatures apply only when the product reaches and maintains temperatures evenly distributed throughout the meat. The use of microwaves is not effective in killing T. gondii, probably because of uneven heating throughout the meat.”
Thermal death curves have also been developed to establish the effect of cold treatment on the viability of T. gondii in meat, Drs. Hill and Gamble continue. “Although tissue cysts remain viable at temperatures slightly below freezing, 11.2 days at 20 degrees Fahrenheit and 25 days at 30 degrees Fahrenheit, parasites are inactivated almost instantaneously at temperatures of 15 degrees F. and lower,” they say.
According to Drs. Hill and Gamble, scientific knowledge of the effect of various curing processes on T. gondii is limited; currently no ready-to-eat meat preparation process for salami, pepperoni, dry-cured ham, summer sausage, etc. exists that can be correlated with inactivation of T. gondii.
Recent studies in Dr. Hill’s lab tested the effect of five variables, salt/brine concentration, water activity, pH, temperature, and time on inactivation of T. gondii bradyzoites in pork during preparation of dry-cured pork sausage. “Results indicated that encysted T. gondii bradyzoites do not survive the early stages of the dry curing process, resulting in rapid inactivation of bradyzoites, rendering these products safe from risk with respect to T. gondii transmission to human consumers,” Dr. Hill reports. “Previous studies have shown that pumping of pork products with salt solutions containing 2 percent sodium chloride, or greater than or equal to 1.4 percent potassium or sodium lactate, has been shown to inactivate T. gondii tissue cysts in pork.”
T. gondii tissue cysts were rendered non-infectious by treatment with 40 to 50 krad of cesium-137, indicating that irradiation is a suitable method for eliminating the risk of this parasite in meat and meat products, they add.
The USDA advises that consumers cook any meat or poultry product to an internal temperature of 145 degrees Fahrenheit, followed by a 3-minute rest, which is effective for killing T. gondii.
“While meat products have been identified as an important source of T. gondii infections in humans, overall, the prevalence of viable T. gondii in U.S. retail meat, including beef, pork, and chicken, is very low, according to research,” Drs. Hill and Gamble emphasize.
Drs. Hill and Gamble concur that, despite the widespread distribution of T. gondii in wildlife and the opportunity for cats to contaminate the environment with the resistant oocyst stage, it is possible to raise pigs free from T. gondii infection, as evidenced by many negative pork production sites found in recent prevalence studies. “Prevention of infection in pigs is accomplished through good production practices on the farm, which include: 1) adopting an effective rodent control program to minimize mouse and rat populations, 2) creating a level of biosecurity which reduces or eliminates exposure of pigs to wildlife, 3) eliminating feral cats and securing feed and pork production areas from access by cats, 4) prompt removal of dead pigs, and 5) changing or thoroughly washing boots before entering barns to avoid tracking in oocysts,” they recommend.
Toxoplasma’s Impacts on Human Behavior
If you’re feeling more entrepreneurial than usual, or more interested in pursuing business-related activities or studies, you may be infected with Toxoplasma gondii (T. gondii).
Implausible as this may seem, a recent study conducted by researchers at the University of Colorado (CU), Boulder demonstrates that T. gondii-positive individuals are more likely to major in business and more likely to pursue a management and entrepreneurship emphasis than those not infected with the organism, according to Stefanie Johnson, PhD, associate professor of management in the CU Leeds School of Business, the lead author of the study.
Dr. Johnson and her colleagues point out that the pesky protozoan parasite that infects an estimated 2 billion people worldwide has been linked to behavioral alterations in humans, as well as other vertebrates. Studies have linked suicide and schizophrenia to Toxoplasma infection, for example.
“While disciplines such as business and economics often rely on the assumption of rationality when explaining complex human behaviors, growing evidence suggests that behavior may concurrently be influenced by infectious microorganisms,” Dr. Johnson relates. “The goal of our study was to investigate how infection by a globally distributed parasite, through its potential influence on individual human behavior, is associated with local to large-scale cultural and business-related outcomes, specifically entrepreneurship.
“Using a saliva-based assay, we found that, of the 1,495 CU undergraduate students who participated, the 22 percent that tested IgG positive for T. gondii exposure were 1.4 times more likely to major in business and 1.7 times more likely to have an emphasis in ‘management and entrepreneurship’ over other business-related emphases, when compared with the students who tested negative,” Dr. Johnson elaborates.
To understand patterns of infection among professional entrepreneurs, Dr. Johnson’s team collected data from 197 individuals attending entrepreneurship events. Among those 197 participants, the T. gondii-positive individuals, also determined by saliva tests, were 1.8 times more likely to have started their own business compared with other attendees.
As an additional endeavor, the CU researchers evaluated global patterns of toxoplasmosis and entrepreneurship.
“We compiled national statistics from 42 countries spanning the last 25 years and found the infection prevalence of T. gondii, which range from 9 percent in Norway to 60 percent in Brazil,” Dr. Johnson relates. “We combined those statistics with the Global Entrepreneurship Monitor of entrepreneurial activity, and the results proved to be a consistent, positive predictor of entrepreneurial activity at the national scale, even when we controlled for relative national wealth and opportunity factors. We believe all of our findings emphasize the hidden role of parasites as potential drivers of complex human behavior and economic outcomes.”—L.L.L.