Toxoplasmosis: Infection Dangerous for Dogs and Humans

Toxoplasmosis is the most common parasitic infection worldwide. The organism that causes toxoplasmosis is Toxoplasma gondii that infects a wide range of warm-blooded vertebrates, including humans. Toxoplasmosis is responsible for considerable mortality in the United States. Toxoplasma infection is associated with a high incidence of traffic accidents and diseases such as epilepsy and schizophrenia.5 Evidence of infection is more frequently seen in black and Hispanic persons, and among persons who are foreign-born, have low educational levels or socioeconomic status, or have occupations involving exposure to soil. Evidence of Toxocara (dog or cat roundworm) infection, which is soil-borne, is more commonly seen among those with T. gondii parasite infection, suggesting common risk factors for acquiring the two infections. In the United States, studies examining data from the mid to late 2000s estimated toxoplasmosis to be the second leading cause of deaths attributable to foodborne illness (an estimated 327 deaths), the fourth leading cause of hospitalizations attributable to foodborne illness (an estimated 4,428 hospitalizations), and a leading contributor to loss of life years. Toxoplasma gondii infects an estimated 1.1 million persons each year in the United States.4

About fifty percent of stray dogs and cats carry T.gondii antibodies, which means that they have been infected and may transmit the parasite. Although rare in dogs, toxoplasmosis does occur in young dogs infected with distemper or other viral infection. In other cases, dogs develop disease without any other present infectious disease. Even though only cats shed the infective organisms, dogs may act as a mechanical factor in transmitting toxoplasmosis to humans by rolling in infective substances and by ingesting fecal material.

Many dogs get infected and become carriers of this parasite, but in most cases only puppies and young dogs develop the disease with clear clinical signs that vary depending on the site of the infection. Infection of the central nervous system and musculature may result in behavioral changes, limb shaking, blindness, circling, muscle pain, progressive paralysis, and death. In other cases, dogs may develop hepatitis, pneumonia, and ulcerative dermatitis. Because the disease resembles many other infections, diagnosis is difficult and requires several tests. Treatment may be effective, however, serious complications affecting nervous system and muscles often result.

To prevent toxoplasmosis:

  • Avoid handling free-roaming dogs or any that show signs of illness.
  • Protect your dogs from infections by avoiding feeding them raw meet or unpasteurized dairy products.
  • Pregnant women should avoid changing cat litter, if possible. If no one else is available to change the cat litter, use gloves, then wash hands thoroughly. Change the litter box daily because Toxoplasma oocysts require several days to become infectious. Pregnant women should be encouraged to keep their cats inside and not adopt or handle stray cats. Cats should be fed only canned or dried commercial food or well-cooked table food, not raw or undercooked meats.

Toxoplasmosis Pneumonia

Toxoplasmosis pneumonia is the inflammation of the lungs caused by Toxoplasma gondii; it is a hidden infection caused by invasion of the human body by pathogens, and the symptoms vary. T gondii mainly affects the eyes, brain, heart, liver, lymph nodes and other organs but rarely affects the lungs. The patients affected by toxoplasmosis pneumonia had not come into contact with any cats, dogs, pigs or other mammals. Most patients with acute onset experience headaches, muscle pain, dry cough and other symptoms, whereas other patients cough mucus bloody sputum, which are symptoms similar to those of upper respiratory tract infection.

List of Human Diseases Associated with Toxoplasmosis - The infection with intracellular protozoan Toxoplasma gondii, one of the world's most common parasites infecting most warm-blooded animals (more than 30 species of birds and 300 species of mammals). It is the most common infection in humans (estimated to be 30–50% of the world population), more than latent tuberculosis which infects about one-third of the human population.

Toxoplasmosis and Risk of Traffic Sccidents

It was found that the risk of an accident in individuals with latent toxoplasmosis was 2.65 times higher than the toxoplasmosis-negative individuals. Moreover, the value of the odds ratio significantly increased with level of anti-Toxoplasma antibody titer (P < 0.0001); the odds ratio of risk of an accident in subjects with low, moderate and high antibody titers were 1.86, 4.78 and 16.03 respectively. Studies documented the presence of Toxoplasma antibodies in 53% of victims of traffic accidents. People with latent toxoplasmosis have only bradyzoites which form cysts in nervous and muscle tissue, but do not produce symptoms.6

Recently, an exploratory study performed with a data mining technique on electronic records of 1.3 million patients of the University of Michigan Health System showed the existence of a strong association between dog and cat-bite injuries and the probability of being diagnosed with depression at some point in life. The association was stronger for cat bites than dog bites. While only 9 % of all patients of the data set were ever diagnosed with depression, this diagnosi was found in 41 % of those with cat bites and 28 % of those with dog bites.6


  1. Parasitic Disease Information
  2. Audra Charron, Sebastian Hakansson and Dana Mordue. Invasion and Intracellular Survival of Toxoplasma (In: Protozoans in Macrophages)
  3. Toxoplasma gondii Pneumonia in an Immunocompetent Individual
  4. Neglected Parasitic Infections in the United States: Toxoplasmosis
  5. Drug repurposing and human parasitic protozoan diseases
  6. Latent Toxoplasmosis and Human
  7. Cat scratches, not bites, are associated with unipolar depression - cross-sectional study. Jaroslav Flegrcorresponding author and Zdeněk Hodný



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