TOXOPLASMOSIS

TOXOPLASMOSIS


Cats have gotten a bad rap about how they transmit toxoplasmosis to humans and how they cause spontaneous abortion in women and blindness and mental retardation in newborns and older children. While it is true that cats are the only definitive host for the infective stage of the Toxoplasma organism, they are not the most common source of human infection. The most efficient way to get toxoplasmosis from a cat is to eat the cat undercooked!



MORBIDITY: +++




MORTALITY: +




ETIOLOGY: PARASITIC


Toxoplasmosis is caused by Toxoplasma gondii, a microscopic, intracellular protozoan. Its life cycle has a sexual phase and an asexual phase. The sexual phase takes place only in the walls of the small intestines of wild and domestic cats, resulting in oocysts that are passed in the cats’ feces. A cat will shed millions of oocysts per day, for about 2 to 3 weeks, and is then done shedding oocysts—for life.


The asexual phase occurs when the oocysts become infective, or sporulate, 1 to 5 days after they are passed in the feces. These oocysts are resistant to environmental conditions and can remain infective for over a year. When the sporulated oocysts are ingested by an animal, they go to the animal’s small intestine, enter the tachyzoite stage, penetrate the intestinal wall, and travel to other parts of the body in lymph and blood. During this stage, tachyzoites rapidly multiply in the cytoplasms of monocytes and macrophages. The tachyzoite (asexual) stage, which is called the active or acute phase, lasts until the host’s immune system produces some immunity—about 2 weeks. Once the immunity develops, the tachyzoites slow their multiplication rate and become bradyzoites, which accumulate in the cytoplasm of tissue cells and form cysts. The cysts can be found anywhere in the body, but are most commonly seen in skeletal muscle, myocardium, and brain tissue. The bradyzoite (asexual) stage is the inactive phase. Cysts can remain in the host for the rest of its life. If the host’s immunity is somehow suppressed, the bradyzoites can become rapidly multiplying tachyzoites again, resulting in a latent or chronic infection (Figure 46).


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FIGURE 46 Toxoplasmosis.



HOSTS


Cats are the only definitive host for T. gondii. This means cats are the only animals that harbor the organism during the organism’s sexual, adult phase. Other animals, including people, that become infected with T. gondii are considered intermediate hosts. Most species of warm-blooded animals and birds are susceptible to T. gondii infection.



TRANSMISSION


Transmission from cats to animals occurs when animals come in contact with sporulated Toxoplasma oocysts. The oocysts can be found where cats defecate, such as moist soil, litter boxes, sand boxes, flower beds, and gardens. Rodents, birds, and other small animals can become infected and infect the cats that prey on them. This perpetuates the life cycle in cats. Cats also can become infected by coming in direct contact with the feces from other infected cats.


Animals on pasture, such as sheep, pigs, goats, and cattle, can ingest the oocysts while they are grazing. Their meat (skeletal muscle) will then become infected with T. gondii cysts that can be transmitted to people or other carnivores and scavengers, including cats, that eat the meat raw or undercooked. Undercooked meat, especially lamb, beef, or pork, is the most common source of T. gondii infection in people.


Once ingested by any susceptible animal other than a cat, the Toxoplasma organisms are released in the intestines, develop to the tachyzoite stage, and go through the migration into body tissues as previously described.


If a cat eats the infected tissues, some of the released Toxoplasma organisms stay in the cat’s intestines to go through the sexual phase, releasing oocysts in the feces. Other released Toxoplasma organisms will develop to the tachyzoite stage, penetrate the intestinal wall, and eventually become bradyzoites encysted in body tissues.


Rarely people can become infected directly from cats if they ingest or breathe in the sporulated oocysts from an infected cat. This can happen when hands become contaminated while changing a litter box or if the litter is dusty enough to allow the oocysts to ride on the dust and be inhaled. Working in soil without gloves and playing in a sandbox are other methods of direct infection from cats. Infection can also occur after eating unwashed vegetables from gardens or after drinking contaminated water. Oocysts can be carried on the wind or in water to distant places.


Transmission can also occur transplacentally in people if a mother is infected near the time she becomes pregnant or soon after. This results in congenital toxoplasmosis, which is the least common but most severe form of toxoplasmosis in people. Congenital toxoplasmosis is also seen in goats and sheep.


Transmission among people can occur through blood transfusions and organ donation.

Related posts:

  1. ABOUT THE DISEASES
  2. CAMPYLOBACTERIOSIS (VIBRIOSIS)
  3. MYCOBACTERIUM INFECTIONS (TUBERCULOSIS)
  4. PSITTACOSIS

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Oct 1, 2016 | Posted by in EXOTIC, WILD, ZOO | Comments Off on TOXOPLASMOSIS

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