SOME AMERICANS EAT free-range animal meat for perceived ethical and health reasons. However, they should be aware of some practical concerns about possible subsequent infections, such as toxoplasmosis, that may occur from consuming meat from animals that have access to plants from contaminated soil, contaminated water in the environment, or other infected animals.1
Caused by a parasite called Toxoplasma gondii, toxoplasmosis has infected over 22% of adults in the United States.2 In some parts of the world, including South America, the Middle East, and Africa, infection rates are over 90%.3,4 Most healthy people who are infected develop an asymptomatic chronic infection that can later develop into a significantly symptomatic one if immunosuppression occurs.2
Toxoplasmosis is a common opportunistic infection in patients with HIV/AIDS; therefore, at-risk immunocompromised patients require prophylaxis.5 Pregnant women are also particularly vulnerable to infection, which can be transmitted to the fetus. Congenital transmission accounts for between 500 and 5,000 cases of toxoplasmosis in the United States each year.6
How infection occurs
T. gondii is an intracellular protozoan parasite that can live in birds or mammals. But in order to replicate, the parasite needs to complete its sexual life cycle in the intestinal tract of cats.7,8 Cats become infected by eating infected birds, mice, and other small animals. Cat feces containing parasitic oocysts can contaminate a litter box, garden soil, drinking water, or anything else with which contaminated feces come in contact.3 Cattle and other farm animals that eat where cats defecate are at risk for exposure.
Most humans contract the infection through accidental consumption, which can occur from eating uncooked or undercooked contaminated meat, consuming uncooked or unwashed fruits and vegetables contaminated in soil, or drinking contaminated water. The infection can also be contracted indirectly from hands, counters, or cooking utensils that weren't washed adequately after coming in contact with contaminated food or another source of infection, such as cat feces.
T. gondii isn't usually passed person-to-person, apart from the transmission of the infection during pregnancy to an unborn child. In rare cases, infections occur from organ transplantation and blood transfusion.3T. gondii DNA has been isolated in male semen, and some studies indicate that it can be sexually transmitted from males to females.9
Cultural practices, sanitation, water cleanliness standards, climate, and host factors affect infection rates.6,9 Oocysts can remain infective for longer periods in warm, wet climates.6
Signs and symptoms
Most people infected with T. gondii remain asymptomatic, though some may experience mild flulike signs and symptoms, including lymphadenopathy, fever, lethargy, fatigue, pharyngitis, headache, and myalgia.2 Though initially thought to be a largely self-limiting infection in healthy, nonpregnant adults, emerging evidence links the presence of T. gondii antibodies with the development of schizophrenia, suicidal ideation, dementia, addiction, and autism.1,6,10,11
Even asymptomatic patients are at risk for developing ocular disease, particularly retinochoroiditis.2 Infected patients who are immunocompromised may develop signs and symptoms of encephalitis, including fever, confusion, loss of coordination, headache, and seizures.2 In these patients, toxoplasmosis can be fatal.12
In immunocompromised patients, signs and symptoms of toxoplasmosis include acute onset of fever and lymphadenopathy. Serologic testing for T. gondii typically confirms the diagnosis. The CDC recommends Toxoplasma-specific immunoglobulin G (IgG) testing first. If the patient is IgG positive, he or she has been infected at some point.13 IgM levels can also be evaluated if determining the timing of the infection is important, as for pregnant patients.
Negative IgG and IgM results generally rule out recent infection, but repeat testing in 2 to 3 weeks is recommended in patients who present within a week of symptom onset.8,13,14 IgG and IgM levels that are slightly elevated may indicate an early acute infection or an infection that occurred in the past 18 months.13 It may be possible to directly observe the parasite in serum, bronchoalveolar lavage material, lymph node biopsy, tissue, or cerebrospinal fluid.2,13
Due to the low prevalence and high rate of false-positive IgM screening tests, routine serologic screening isn't recommended.3,14,15 All infants born in the United States are included in newborn screening programs, but not all states include T. gondii infection in these screening panels. Although some state health departments require reporting of T. gondii infection, it's not a nationally reportable disease.16,17
Immunosuppressed patients should be tested routinely.5 Immunosuppressed women should be tested during pregnancy to facilitate early detection and treatment of primary maternal infection.6 Advise women who aren't pregnant and experience an acute infection with T. gondii to avoid pregnancy for 6 months after the primary infection.15
Immunocompetent nonpregnant adults infected with T. gondii will usually mount an appropriate immune response, making treatment unnecessary. However, with severe infections, pyrimethamine, sulfadiazine, and leucovorin may be administered for 2 to 4 weeks.2,8,18 Because treatment doesn't destroy tissue-based parasitic cysts, patients should be advised that even after treatment they'll be chronic carriers and illness may recur, particularly if they become immunocompromised.5 Previously infected patients with recurring signs and symptoms should notify their healthcare provider. Because the parasite isn't spread through routine contact or airborne mechanisms, standard precautions are appropriate when caring for an infected patient in a healthcare setting.3
Patients who are HIV-positive and who test positive for T. gondii antibodies should be treated for the infection and maintained on a prophylactic regimen even if they don't have an active infection.5 Without prophylactic treatment, the risk of conversion to active infection status can be as high as 30% among HIV-infected patients, particularly when the patient's CD4 count is below 200 cells/mcL.5,12
Teach patients the importance of thoroughly cooking all meat before consuming it. Inform them that microwaving (which heats food unevenly) and ordinary refrigeration aren't effective, but freezing meat to below 0° F (-17.8° C) for several days will kill the parasite.2,15,19 Advise them to meticulously clean hands, counters, and all cooking utensils that have come in contact with raw meat such as cutting boards, knives, and bowls, and to wash all fresh fruits and vegetables thoroughly before eating.3
Advise immunosuppressed and pregnant patients not to clean or change a cat's litter box.2 Cats should be kept indoors and litter boxes cleaned regularly to decrease the risk of infection, because the Toxoplasma parasite doesn't become infectious until 1 to 5 days after being shed in a cat's feces.19 All pets should be fed commercially available pet food or well-cooked meat.3,5 Parents should keep children's sandboxes covered to prevent cats from getting into them.3 Advise patients, especially pregnant women, to wear gloves when gardening and to wash hands thoroughly when done. For more disease prevention tips, see What's cooking from the CDC.
What's cooking from the CDC
To prevent food-borne infection with T. gondii, the CDC recommends using a food thermometer to ensure that meat is thoroughly cooked. Warn patients not to sample meat until it's thoroughly cooked to a safe temperature, as follows:
- For whole cuts of meat (excluding poultry): Cook to at least 145° F (63° C) as measured with a food thermometer placed in the thickest part of the meat. Allow the meat to rest* for 3 minutes before carving or consuming.
- For ground meat (excluding poultry): Cook to at least 160° F (71° C); ground meats don't require a rest time.
- For all poultry: Cook to at least 165° F (74° C). Whole poultry should be allowed to rest for 3 minutes before carving or consuming.
In addition, remind patients to peel or thoroughly wash fruits and vegetables before eating them, and to wash cutting boards, dishes, counters, utensils, and hands with hot soapy water after contact with raw meat, poultry, seafood, or unwashed fruits or vegetables. Freezing meat for several days at temperatures below 0° F (−17.8° C) also reduces the risk of infection, according to the CDC.
*According to the United States Department of Agriculture, a rest time is “the amount of time the product remains at the final temperature, after it has been removed from a grill, oven, or other heat source. During the three minutes after meat is removed from the heat source, its temperature remains constant or continues to rise, which destroys pathogens.”
Source: Centers for Disease Control and Prevention. Reduce risk from food. www.cdc.gov/parasites/toxoplasmosis/prevent.html.
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Minnesota Department of Health. Reporting toxoplasmosis. http://www.health.state.mn.us/divs/idepc/dtopics/reportable/toxoplasmosis.html.
Commonwealth of Massachusetts Department of Public Health. Communicable and other infectious diseases reportable in Massachusetts by healthcare providers. 2017. http://www.mass.gov/eohhs/docs/dph/cdc/reporting/rprtbldiseases-hcp.pdf. http://www.uptodate.com
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