If you have cats and are raising meat chickens, be aware of the potential dangers of toxoplasmosis in humans.
Anyone who has had chickens for more than a few minutes becomes aware of the devastating protozoal disease coccidiosis, which happens wherever chickens are. It is caused by one of the best-known and most widely studied coccidial parasite, Eimeria, which comes in many species that, as a group, can infect nearly every kind of livestock. But each Eimeria species is highly host specific, meaning the species that parasitize chickens do not affect any other animal, including humans.
A lesser-known coccidial protozoa that is rapidly gaining notoriety is Toxoplasma gondii. Cats are the natural host for this protozoal parasite, which, in contrast to Eimeria, can infect any warm-blooded animal, including chickens and humans. Much the same as the disease coccidiosis is referred to as cocci, toxoplasmosis is typically shortened to toxo.
The Centers for Disease Control (CDC) estimates that in the United States, 22.5 percent of the human population 12 years and older is infected with toxo. Yet toxoplasmosis in humans is not well understood, and in chickens is even less well understood, primarily because the condition rarely produces readily observable symptoms of toxoplasmosis in humans or signs in chickens.
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The CDC has reported that, “Chicken, other fowl, and eggs almost never contain this parasite.” Yet their toxoplasmosis prevention page (cdc.gov/parasites/toxoplasmosis/prevent.html) includes guidelines for safely cooking chicken meat. Why the discrepancy?
Well, the first statement refers to commercial poultry products sold in most retail grocery stores, which are largely produced in isolated indoor environments that exclude cats, wild birds and rodents. The prevalence of toxo in free range chickens, on the other hand, is estimated to be “very high” according to microbiologist J. P. Dubey, PhD, an expert on toxoplasmosis in both animals and humans.
Protozoa are single-cell animal-like creatures that live in damp habitats. They are the simplest members of the animal kingdom and also the smallest, too small to see without a microscope. Many species of protozoa are harmless, but others — such as those causing cocci and toxo — can be serious parasites.
The complex life cycle of T. gondii begins when a member of the cat family — which includes bobcats and cougars, as well as domestic cats — ingests a rodent or other animal that has been infected with T. gondii and therefore harbors the parasite within tissue cysts. Enzymes in the cat’s stomach and small intestine dissolve the cyst and release the infectious protozoa. The parasites lodge in the cat’s intestinal lining, where they rapidly multiply through an orgy of sexual reproduction.
The cat is the only known animal in which this parasite infects the intestinal lining and multiplies through sexual reproduction. However, rarely does a cat become visibly ill. It wouldn’t do for this parasite to endanger the lives of cats, because cats are the natural hosts that give life to its future generations.
Fertilized T. gondii eggs are encased in egg cysts, called oocysts, which burst from the cat’s intestine wall and are excreted in the cat’s feces. An infected cat sheds these oocysts for only about two weeks, but during that time can release millions of them. Meanwhile, the cat’s immune system is producing antibodies that eventually encase the organisms in cysts and cause them to go dormant. The encysted parasites, however, remain in muscle and nerve tissue throughout the cat’s body for the rest of the cat’s life. At this point, the cat usually stops shedding oocysts and becomes immune to future infections.
The shed oocysts are spread around the environment through the soil and are carried on the feet of rodents, wild birds, flies, cockroaches, dung beetles and similar creatures. Or they might be blown by wind. After leaving the cat, oocysts in the environment multiply by subdividing (asexual reproduction) and become infective to any animal that inhales them from contaminated dust or ingests them from contaminated soil, plants, water, feed, or by eating a creature carrying an infective oocyst on or in its body.
To show you how sneaky this parasite is, it can cause an infected rodent to lose its fear of cats. In fact, it can cause a rodent to become attracted to a cat’s odor. The cat is rewarded with a ready meal, and if it isn’t already infected by T. gondii, it soon will be.
In the environment, infective oocysts can survive for 18 months in water or warm, damp soil but they do not survive well in extreme cold or in hot, dry conditions. Toxo is therefore more common in northeastern states than in western states, and more common yet in tropical climates.
Contamination of soil by these microscopic parasites is difficult to determine by direct examination of the soil. Chickens, it turns out, are one of the best indicators of soil contamination, because they feed from the ground. The presence of this parasite in suspect soil is confirmed if the chickens test positive for Toxplasma antibodies.
Toxoplasmosis in Chickens
Aside from cats, infected animals — including chickens and humans — are secondary hosts in which the parasites do not achieve sexual maturity. Inside a secondary host, the parasites multiply through asexual reproduction and travel through the host’s lymph and blood to invade any part of its body.
Within about three weeks, multiplication slows, the parasites become encased in cysts within muscle and nerve tissue, and they go dormant. Each tissue cyst contains as many as 2,000 organisms, which remain dormant for the life of the infected chicken. But they will reactivate if the infected chicken is eaten by an animal that hasn’t previously been infected.
The Toxoplasma’s life cycle begins again when a cat that isn’t already immune becomes infected. A kitten may be born infected if its mother becomes infected while pregnant. A cat can become infected by eating an infective oocyst from the environment (such as while chewing on a blade of grass); eating an infected bird, rodent, or other intermediate host; or being fed raw or undercooked meat from an infected animal, such as a chicken. A chicken can become infected not only by ingesting egg cysts from the environment, but by picking at the flesh of an infected animal, including some other hapless chicken.
While most chickens show no outward sick chicken symptoms, a chick under the age of about 8 weeks, with its undeveloped immune system, or an older bird that’s experiencing unusual stress, can develop an acute infection. Such an infection may affect the central nervous system, giving the appearance of Marek’s disease or any similar disease that involves the nerves. Signs in acutely affected chickens include a drop in egg production and feed consumption, emaciation, white diarrhea, neck spasms, twisted neck, paralysis, blindness and sudden death. No effective cure is known.
Control of toxoplasmosis in chickens includes keeping cats out of the chicken yard, not feeding raw or undercooked chicken meat (or any kind of undercooked meat) to cats, and controlling filth flies, cockroaches, rodents and wild birds. Since T. gondii thrives in moist environments, avoid puddles in the poultry yard and maintain dry litter inside the coop.
Toxoplasmosis in Humans
As more people keep backyard chickens along with house cats, more people have the potential to become infected with toxoplasmosis. Although humans can acquire this parasite in many different ways — most commonly by eating either unwashed fruits and vegetables or undercooked infected meat, particularly pork or lamb — the ways that do involve chickens include eating undercooked meat from an infected chicken, and failing to use proper sanitation (such as washing hands, cutting boards, and other utensils) when handling the meat of an infected chicken. Although a cat is most likely to get toxo by eating infected rodents or wild birds, the cat (like a human) can become infected after being fed the raw or undercooked meat of an infected chicken.
Most healthy humans — like healthy chickens, cats, and other animals — show no signs of being infected. Some humans, however, may have temporary, mild flu-like symptoms that include sore throat, headache, muscle pain, low fever, fatigue, diarrhea and swollen lymph nodes that may last from two to four weeks. The most serious effects occur in people with immunodeficiency disorders and in unborn children whose mothers are infected while pregnant. A woman infected during pregnancy can pass the infection to her unborn child.
Most people who have been infected by this parasite develop antibodies against future infection. The disease is self-limiting, and any symptoms that develop typically go away on their own. The parasite, however, remains in an infected person’s body for the person’s life and can cause such things as eye disease and reduced reaction time, resulting in an increased incidence of traffic accidents. The parasite has also been implicated in such behavioral disorders as anxiety, schizophrenia and a tendency toward suicide.
Toxoplasmas are common, spread fairly easily, and are not readily killed by the usual disinfectants. Although drugs may be used to treat an overt infection, no drug or vaccine has been developed to prevent or treat the dormant, or chronic, stage of this disease.
The easiest way to avoid getting this parasite from your homegrown chickens raised for meat is to wash your hands, cutting boards, countertops and utensils after butchering chickens or handling their meat. Freezing the meat at 12°F for at least 24 hours destroys any parasites that might be present, as does cooking the chicken. According to Dr. Dubey, cooking meat to an internal temperature of 150°F kills T. gondii.
Gail Damerow is the author of the new revised and completely updated The Chicken Health Handbook, 2nd edition, which includes a wealth of information on how to protect yourself from illnesses that may be acquired from chickens. The Chicken Health Handbook is available from our bookstore at www.countrysidenetwork.com/shop.
OBSERVATIONS ON TOXO IN CHICKENS
In his report “Toxoplasma gondii Infection in Chickens” for the journal Zoonoses and Public Health, microbiologist J. P. Dubey, PhD, observed that:
• Chickens are resistant to clinical toxoplasmosis (meaning they rarely show signs when infected), and worldwide only a few reports have confirmed clinical disease in chickens, including two reports from the United States.}
• A high prevalence of the parasite was found in chickens raised in backyards (up to 100 percent) and free-range organic establishments (30 to 50 percent).
• Free range chickens play an important role in the occurrence, distribution, and control of T. gondii in the rural environment, perhaps more than rodents, because they are clinically resistant to the parasite and they live longer than rodents.
• Raw chicken eggs are unlikely to be a source of toxoplasmosis in humans (although Dr. Dubey cautions against eating raw eggs to avoid the possibility of infection from either toxo or Salmonella.)
• The prevalence of viable T. gondii in chickens from commercial indoor farms was low and the ingestion of meat from these chickens is considered a low risk of transmission to humans.