Toxoplasmosis In Cats and Dogs

Toxoplasmosis is the most common parasitic infection worldwide. It is estimated to affect several billion people. Toxoplasmosis is becoming a global health hazard as it infects 30–50% of the world human population. The disease is caused by a single-celled organism Toxoplasma gondii. Toxoplasmosis presents a serious health risk for people. Infection is especially dangerous for people with supressed immune system and pregnant women. The cat is the only animal in which sexual reproduction of the organism occurs, so cats are the only domestic animals that have the potential to shed the infected eggs. It is the most common food-borne parasitic infection requiring hospital treatment, and the third most common cause of hospitalization due to food-borne infection. Both competent and immunocompromised persons can develop the disease, especially retinochoroiditis (ocular toxoplasmosis). In non-pregnant immunocompetent adults, acute disease may also lead to impaired eye sight. For example, in the United States, one million new infections occur each year, which result in approximately 20,000 cases of retinal pathology.2

Human Diseases Associated With Toxoplasmosis

  • Encephalitis
  • Diabetes mellitus type 1 and 2
  • Chronic heart failure; myocarditis; arrhythmia
  • Tumors, cancers
  • Abdominal hernia
  • Brain tumors (meningioma; ependymoma; glioma).
  • Diverse abnormalities in aggregate personality; including aggressive behavior in animals and humans
  • Psychosis; schizophrenia; bipolar disorder
  • Mood disorders; suicide; depression
  • Obsessive-compulsive disorder
  • Attention/concentration deficit hyperactivity disorder
  • Anorexia
  • Autism spectrum disorders
  • Down syndrome
  • Alzheimer's disease
  • Parkinson's disease
  • Migraine, other headaches
  • Idiopathic intracranial hypertension
  • Aseptic meningitis
  • Mollaret meningitis
  • Epilepsy
  • Aphasia and epilepsy
  • Facial nerve palsy
  • Hearing loss
  • Hypothalamo-pituitary dysfunction; panhypopituitarism
  • Non-Hodgkin's lymphoma
  • Melanoma
  • Breast cancer
  • Carcinoma of female genitalia, including cervical tissue
  • Inflammatory bowel disease
  • Ulcerative colitis
  • Crohn's disease
  • Celiac disease
  • Hepatitis, including HCV infection
  • Granulomatous liver disease
  • Liver cirrhosis; granulomatous liver disease; impaired liver function
  • Liver cirrhosis; granulomatous liver disease; impaired liver function
  • Primary biliary cirrhosis; biliary atresia; cholestatic disorders
  • Goitre; iodine deficiency
  • Hashimoto's thyroiditis
  • Graves' disease; thyroid adenoma
  • Rheumatoid arthritis; Still's disease
  • Polymyositis
  • Systemic sclerosis
  • Systemic lupus erythematosus
  • Wegener's granulomatosis; other vasculitides
  • Anti-phospholipid syndrome
  • Cryoglobulinemia
  • Ocular toxoplasmosis (retinochorioiditis; uveitis; blurred vision; floaters; macular scars; nystagmus; strabismus; reduced visual acuity; blindness; scleritis; papillitis; retinal necrosis; vasculitis; retinal detachment; vitritis; congenital cataract; neuroretinitis; atrophic optic papilla; retinitis pigmentosa)
  • Glaucoma
  • Ovarian dysfunction
  • Uterine atrophy
  • Impaired reproductive function (T. gondii was present in testicles, epididymis, seminal vesicles, prostate gland in rams, and caused abnormalities in sperm motility, viability and concentration rates, weight of epididymis in rats, orchitis)
  • Nephrotic syndrome; lipoid nephrosis
  • Schönlein-Henoch purpura
  • Glomerulonephritis (various forms; including these with development of fibrosis); impaired kidney function
  • Atherosclerosis; obesity; cardiovascular deaths; all-cause mortality:2

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. 4

Toxoplasmosis affects cats that ingest raw meat or prey that contains the parasite Toxoplasma gondii. Kittens can become infected in the womb and die before birth. Although infection is very common in cats, most cats do not get sick. Kittens are at the highest risk.


The signs of toxoplasmosis often include fever, lack of appetite, lethargy, diarrhea, and weight loss. Pneumonia, liver disease, and infection of the central nervous system are more devastating. Many afflicted cats do not survive.

Toxoplasmosis in dogs and cats can cause chorioretinitis (a form of bacterial uveitis which can lead to loss of vision), anterior uveitis (inflammation of the iris), or both. Eye lesions are a common manifestation of systemic toxoplasmosis. A diagnosis is made through blood tests. Infections involving only eyes can be treated successfully.

Toxoplasma gondii oocysts
Image source: CDC

Role of Dogs in Transmitting Toxoplasmosis

Dogs may act as a mecanical factor in transmitting toxoplasmosis to people by rolling in infected feces and by ingesting fecal material. It is estimated that 50% of stray dogs and cats carry T.gondii antibodies, which means that they have been infected and may transmit the parasite to people. Reports show that dogs in shelters, dogs living in close contact with wild birds and rodents in rural areas, and dogs fed raw meat are at mich higher risk for being infected.

Protect Your Cat And Yourself

  • Do not eat raw or undercooked meat. The most common source of infection to people in the United States is through undercooked or raw meat, especially pork and rabbit meat. Proper cooking or freezing will kill the parasite.
  • Avoid eating unpasteurized dairy products.
  • Avoidraw oysters, clams, or mussels.
  • Avoid handling free-roaming cats or any that show signs of illness.
  • Protect your cats from infections by keeping them indoors, and do not feed them raw meat or unpasteurized dairy products.
  • Change your cat's litter every day. If infected, cats can release infected eggs in their feces for up to one to two weeks. Once infected, cats cease shedding infected eggs. It is extremely rare for them to shed eggs at any other time in their lives. An egg must be in the environment for 1 to 5 days before it can cause an infection.

T. gondii oocysts have been found on fresh fruits and vegetables from shops and gardens, suggesting environmental contamination in Poland. The risk of acquiring T. gondii infection from environmental sources versus meat was measured and the investigators detected sporozoite-specific protein (SSP) antibodies in 43% of recently infected pregnant women in Chile, implying the significant risk of the contaminated environment, which was almost equal to the hazard of meat containing the parasite cysts. In Japan, parasite antibodies were found to be 10.3% in pregnant women, while another report showed 5.4% IgG anti-T. gondii antibodies in HIV patients. Moreover, ocular toxoplasmosis was diagnosed through detection of T. gondii DNA in ocular fluid taken from patients.1

Cats that will live in households with pregnant women should be tested for the presence of toxoplasma antibodies. A positive-testing cat is probably immune to infection. A negative-testing cat is susceptible to infection, and, if exposed, might shed the organism in the feces for a week or two afterward. In either cases, be sure to reduce your cat's chance of exposure by following the list of recommendations.


  1. Seroprevalences of Toxoplasma gondii and Neospora caninum in Pet Rabbits in Japan
  2. Toxoplasmosis – A Global Threat. Correlation of Latent Toxoplasmosis with Specific Disease Burden in a Set of 88 Countries
  3. The relationship between owning a cat and the risk of developing a brain cancer in a prospective study of UK women: comment on Thomas et al.
  4. Cat scratches, not bites, are associated with unipolar depression - cross-sectional study. Jaroslav Flegrcorresponding author and Zdeněk Hodný

Home Contact RSS