A protozoan parasite that infects nearly all mammals

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Disease Caused by Toxoplasma, a Type of Protozoa




  • “Toxoplasmosis” is a disease caused by Toxoplasma gondii

  • Toxoplasma gondii—a protozoan parasite that infects nearly all mammals; cats are the definitive hosts (meaning that reproduction of Toxoplasma gondii occurs in cats, with the release of one form of the parasite [known as an “oocyst”] that sporulates and becomes infective); all other warm-blooded animals are intermediate hosts (they maintain cysts in various tissues, such as muscle; infection occurs when another animal or person eats the cyst-containing tissue, such as raw or under-cooked meat)

Signalment/Description of Pet


Mean Age and Range

  • In one study, mean age was 4 years; range, 2 weeks–16 years

Predominant Sex

  • Male cats—more common

Signs/Observed Changes in the Pet

  • Determined mainly by site and extent of organ damage

  • Sudden (acute) disease—at the time of initial infection

  • Long-term (chronic) disease—reactivation of infection (encysted Toxoplasma organisms); caused by decreased ability to produce a normal immune response (known as “immunosuppression”), which allows the cyst to rupture and for Toxoplasma organisms to infect new cells

  • Non-specific signs of sluggishness (lethargy), depression, and lack of appetite (known as “anorexia”)

  • Weight loss

  • Fever

  • Discharge from the eyes, avoidance of light (known as “photophobia”), constricted or narrowed pupils (known as “miotic pupils”) in cats

  • Breathing distress

  • Nervous system signs—wobbly, incoordinated, or “drunken”-appearing gait or movement (known as “ataxia”); seizures; tremors; weakness (known as “paresis”) or paralysis; cranial nerve deficits (the “cranial nerves” are nerves that originate in the brain and go to various structures of the head [such as the eye, face, and tongue])

  • Digestive tract signs—vomiting; diarrhea; abdominal pain; yellowish discoloration to the gums and other tissues of the body (known as “jaundice” or “icterus”)


  • Most severe in kittens infected across the placenta; kittens may be stillborn or die before weaning

  • Surviving kittens—lack of appetite (anorexia); sluggishness (lethargy); high fever unresponsive to antibiotics; inflammation of lungs leading to difficulty breathing (known as “dyspnea”) or increased noises while breathing; abnormalities of the liver, leading to yellowish discoloration to the gums and other tissues of the body (jaundice or icterus) and possible abdominal enlargement from fluid buildup (known as “ascites”); and central nervous system signs, if the infection involves the brain

  • Respiratory and gastrointestinal disease following birth—most common; lack of appetite (anorexia); sluggishness (lethargy); high fever unresponsive to antibiotics; difficulty breathing (dyspnea); weight loss; yellowish discoloration to the gums and other tissues of the body (jaundice or icterus); vomiting; diarrhea; buildup of fluid in the abdomen (ascites)

  • Nervous system disease following birth—seen in less than 10% of affected pets; blindness; stupor; incoordination; circling; contraction of the neck muscles, pulling the head to one side (known as “torticollis”); unequal size of the pupils (known as “anisocoria”); seizures

  • Signs involving the eyes—common; inflammation of the iris (pigmented part of the eye) and other areas in the front part of the eye (known as “uveitis”); blood in the anterior chamber of the eye (the front part of the eye, between the cornea and the iris; accumulation of blood known as “hyphema”); dilated pupils (known as “mydriasis”); inflammation of the iris (known as “iritis”); separation of the back part of the eye (retina) from the underlying, vascular part of the eyeball (known as the “choroid”; condition known as “retinal detachment”); aggregates of inflammatory cells adhering to various areas of the inner lining of the cornea (known as “corneal endothelium”; condition known as “keratic precipitates”)—the cornea is the clear outer layer of the front of the eye

  • Rapid course of disease—suddenly (acutely) affected pet with central nervous system and/or respiratory involvement

  • Slow course of disease—pets with reactivation of long-term (chronic) infection


  • Young dogs—usually generalized infection; fever; weight loss; lack of appetite (anorexia); inflammation of the tonsils (known as “tonsillitis”); difficulty breathing (dyspnea); diarrhea; vomiting

  • Old dogs—tend to have localized infections; mainly associated with the muscles and nervous system

  • Nervous system disease—signs are quite variable; usually reflect widespread (diffuse) nervous system inflammation; seizures; tremors; wobbly, incoordinated or “drunken” appearing gait or movement (ataxia); weakness (paresis); paralysis; muscle weakness

  • Signs involving the eyes—rare; similar to those found in cats

  • Heart involvement—occurs; usually not clinically apparent


Risk Factors

  • Inability to develop a normal immune response (immunosuppression)—may increase likelihood of infection or reactivation of infection—feline leukemia virus (FeLV), feline immunodeficiency virus (FIV), feline infectious peritonitis (FIP), Mycoplasma, canine distemper virus, and administration of steroids or chemotherapy drugs, or following a kidney transplant

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