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Toxoplasmosis in Cats and Humans

Toxoplasmosis, a Zoonotic Disease Affecting Cats and People


Image of Toxoplasma gondii in mouse ascitic fluid

Toxoplasma gondii in mouse ascitic fluid

Photo Credit: © CDC

Toxoplasmosis, caused by the toxoplasma gondii protozoa, is a zoonotic disease. Most people who care for cats are aware of the potential dangers of toxoplasmosis, especially women who are pregnant or plan to be. The other high-risk humans are those who are immunocompromised, such as HIV+ patients, heart transplant patients, and those undergoing chemotherapy or radiation treatments.

The truth is that with or without cats, many of us may have been exposed to toxoplasma gondii. According to the CDC,(Centers for Disease Control and Prevention), "More than 60 million men, women, and children in the U.S. carry the Toxoplasma parasite, but very few have symptoms because the immune system usually keeps the parasite from causing illness."

How Do Cats Get Toxoplasmosis?

Cats are usually infected with toxoplasma gondii by eating intermediate hosts, such as birds and rodents which contain tissue cysts of the T.gondii parasite. The cat then sheds fecal oocysts in his feces, which find their way into soil and water, and are then ingested by the intermediate hosts, creating a vicious circle of contamination.

Another way cats become infected is by the consumption of the raw or undercooked flesh of infected animals, including lamb, pork, and venison.

Toxoplasmosis and Sea Otters

As long ago as 2003, scientists in California discovered that sea otters in the Monterey Bay were sickening and dying from Toxoplasma gondii. David A. Jessup, senior wildlife veterinarian for the California Department of Fish and Game, said that the infected live otters were suffering seizures and showed obvious damage to their brains. Three possible sources for the T. gondii were:
  • Fecal waste from a colony of feral cats living under and near a wharf at nearby Moss Landing
  • Watershed from sewage treatment plants
    The resultant "safe" effluent is discharged into streams and rivers flowing to the Pacific Ocean. Unfortunately, although the sewage treatment process kills bacteria, the T gondii survives in the effluent, thanks to "flushable" cat litter. Since that discovery, the State of California now requires warnings on cat litter containers not to flush the used litter down the toilet. After my Joey had radioactive iodine therapy for hyperthyroidism, I devised a means of complying with both that law and the law regulating disposal of radioactive cat waste.
  • Storm drain runoff
    Since outdoor cats bury their feces, deep watering or heavy rains can float the oocysts out to a bay. Another sound reason to keep our cats indoors.

How Do Humans Get Toxoplasmosis?

According to the CDC, "The only known definitive hosts for Toxoplasma gondii are members of family Felidae (domestic cats and their relatives)." Since Cats shed the oocysts in their fecal matter, we ingest the Toxoplasma gondii in several ways:
  • From Contaminated Litter Box Waste
  • From Eating Undercooked Contaminated Meat
  • Ingesting Vegetables Grown in Contaminated Soil
  • Drinking Contaminated Water
From a PDF file the CDC provides to cat owners:
After a cat has been infected, it can shed the parasite for up to two weeks. The parasite becomes infective one to five days after it is passed in the feces of the cat. The parasite can live in the environment for many months and contaminate soil, water, fruits and vegetables, sandboxes, grass where animals graze for food, litter boxes, or any place where an infected cat may have defecated
Most of these can be avoided by wearing gloves when gardening or cleaning litter boxes, scrubbing or peeling fruits and vegetables before eating them, and scrupulously cleaning our hands with soap and warm water after gardening, and before eating. Also, meat should be cooked thoroughly, including poultry. See the CDC Guidelines for cooking meat and other food safety tips.

How Serious is Toxoplasmosis?

Remember the "vicious cycle" described above, between cats and rodents? This is an important point, as a British scientist postulates that the T gondii organism, in its drive to multiply, manipulates the brains of rats to slow their reaction times and cause an "almost suicidal attraction to cats," said Joanne Webster, a British scientist who led the research, according to a 2007 article by Paul Voosen in the Prague Post.

Recently, the long-term studies of a Czech scientist, Jaroslav Flegr, hypothesizing a link between schizophrenia and T. gondii, have acquired worldwide attention. Although the schizophrenia connection has not yet been proven, Flegr's studies connecting traffic accidents with T gondii seemed to show a more decisive cause and effect. According to the study, affected subjects were 2.65 times more likely to be the cause of accidents.

Even more recently, on July 2, 2012, ABC News reported of a new study linking T gondii with suicide risks among women in Denmark. According to the article:

The study, authored by University of Maryland School of Medicine psychiatrist and suicide neuroimmunology expert Dr. Teodor T. Postolache, was published online today in the Archives of General Psychiatry.

The study found that women infected with T. gondii were one and a half times more likely to attempt suicide than those who were not infected. As the level of antibodies in the blood rose, so did the suicide risk. The relative risk was even higher for violent suicide attempts.

The CDC takes Toxoplasmosis seriously enough to include it as one of five Neglected Parasitic Infections in the United States. The definition of that category is "five parasitic diseases that have been targeted by CDC as priorities for public health action, based on the:

  • Number of people infected
  • Severity of the illnesses
  • Ability to prevent and treat them

I consider Toxoplasmosis seriously enough to follow these studies as more information unfolds.


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