Toxoplasmosis - Cat and Dog

Description

Toxoplasma gondii is an obligate, intracellular protozoa that is capable of infecting most mammals. Cats and other felids are the definitive host for T. gondii, and all other mammals, including dogs, are intermediate hosts.

Felids are the only definitive hosts of T gondii ; both wild and domestic cats therefore serve as the main reservoir of infection. There are 3 infectious stages of T gondii ; tachyzoites (rapidly multiplying form), bradyzoites (tissue cyst form), and sporozoites (in oocysts). T gondii is transmitted by consumption of infectious oocysts in cat feces, consumption of tissue cysts in infected meat, and by transplacental transfer of tachyzoites from mother to fetus. T gondii initiates enteroepithelial replication in unexposed cats after ingestion of uncooked meat containing tissue cysts. Bradyzoites are released from tissue cysts by digestion in the stomach and small intestine, invade intestinal epithelium, and undergo sexual replication, culminating in the release of oocysts (10 µm diameter) in the feces. Oocysts are first seen in the feces at 3 days after infection and may be released for up to 20 days. Oocysts sporulate (become infectious) outside the cat within 1-5 days, depending on aeration and temperature, and remain viable in the environment for several months. Cats generally develop immunity to T gondii after the initial infection and therefore shed oocysts only once in their lifetime. Following consumption of uncooked meat containing tissue cysts (carnivores) or feed or drink contaminated with cat feces containing oocysts (all warm-blooded animals), T gondii initiates extraintestinal replication. Bradyzoites and sporozoites, respectively, are released and infect intestinal epithelium. After several rounds of epithelial replication, tachyzoites emerge and disseminate via the bloodstream and lymph. Tachyzoites infect tissues throughout the body and replicate intracellularly until the cells burst, causing tissue necrosis. Tachyzoites measure 4-6 × 2-4 µm in diameter and stain with Giemsa. Young and immunocompromised animals may succumb to generalized toxoplasmosis at this stage. Older animals mount a powerful cell-mediated immune response to the tachyzoites (mediated by cytokines) and control infection, driving the tachyzoites into the tissue cyst or bradyzoite stage. Tissue cysts are usually seen in neurons but also occur in other tissues. Individual cysts are microscopic, up to 70 m in diameter, and may enclose hundreds of bradyzoites in a thin, resilient cyst wall. Tissue cysts in the host remain viable for many years, and possibly for the life of the host.

Diagnosis

Clinical Signs

The tachyzoite is the stage responsible for tissue damage; therefore, clinical signs depend on the number of tachyzoites released, the ability of the host immune system to limit tachyzoite spread, and the organs damaged by the tachyzoites. Because adult immunocompetent animals control tachyzoite spread efficiently, toxoplasmosis is usually a subclinical illness. However, in young animals, particularly puppies, kittens, and piglets, tachyzoites spread systemically and cause interstitial pneumonia, myocarditis, hepatic necrosis, meningoencephalomyelitis, chorioretinitis, lymphadenopathy, and myositis. The corresponding clinical signs include fever, diarrhea, cough, dyspnea, icterus, seizures, and death. T gondii is also an important cause of abortion and stillbirth in sheep and goats and sometimes in pigs. After infection of a pregnant ewe, tachyzoites spread via the bloodstream to placental cotyledons, causing necrosis. Tachyzoites may also spread to the fetus, causing necrosis in multiple organs. Finally, immunocompromised adult animals (eg, cats infected with feline immunodeficiency virus) are extremely susceptible to developing acute generalized toxoplasmosis.

Laboratory Tests

• Serology
  • Sabin-Feldman Dye test (old method)
  • ELISA
  • Mouse inoculation for confirmation
  • 30-80% test seropositive
  • Each cat sheds oocysts for 1-2 weeks of its life

Treatment

Prevention

• Cat
  • Impossible if cat is allowed outdoors due to hunting
  • If kept indoors, only canned food should be fed and vermin controlled
  • ELISA to check if seropositive

Links

• Cornell College of Veterinary Medicine Toxoplasmosis Factsheet
• Feline Advisory Bureau: Toxoplasmosis in cats and man
• The Merck Veterinary Manual - Toxoplasmosis

dogs

• Caused by Toxoplasma gondii
• Cats are definitive hosts but other species may become intermediate hosts if they ingest the oocysts
• Usually induces antibody response but remains silent clinically
• Often show clinical signs when immunosuppressed
• Involves many different tissues
• Multifocal necrotising interstitial pneumonia
• Proliferation of type II pneumocytes
• Macrophage and neutrophil infiltration