Introduction

Classification

Taxonomy

• Phylum: Apicomplexa.
• Class: Sporozoea.
• Subclass: Coccidia.
• Order: Eucoccidiidae.
• Suborder: Eimeriina.
• Family: Sarcocystidae.
• Genus: Toxoplasma; single species -T oxoplasma gondii.

Etymology

• Gr: toxon - bow; plasma - mold, image, formation.

Active Forms

Clinical Effects

Epidemiology

Habitat

• Definitive hosts are all species of felids.
• Intermediate hosts include any mammal or bird.
• Oocysts are found in the environment.

Lifecycle

• Cats (definitive hosts) become infected by ingesting animals, usually rodents, infected with T. gondii.
• Parasites multiply in cat's intestinal epithelium to produce oocysts in 3-10 days, which are shed in the feces.
• Sporulated oocysts are ingested from the environment by the intermediate host.
• Tachyzoites spread by the hematogenous route and multiply asexually in cells (the acute phase).
• Antibody limits the invasiveness of tachyzoites and results in formation of slow-growing cysts containing bradyzoites, usually in muscle cells.

Transmission

• By ingestion of bradyzoites, tachyzoites or oocysts.
• Transplacental transmission of tachyzoites (sheep and humans).

Pathological effects

• Immunosuppression is probably responsible for asymptomatic chronic infection in man.
• T. gondii has the ability to survive in macrophages which would otherwise kill extracellular organisms. It induces phagocytosis and blocks the delivery of liposomal contents to the vacuole in which it is contained.

• In the acute form of the disease the organisms enter via the gastrointestinal tract and are disseminated via the lymphatics and portal blood. Multiplication occurs in the tachyzoite form and areas of necrosis occur. Organisms may appear in secretions and excretions and death may occur at this stage. Animal to animal spread does not occur in the acute phase.
• In the subacute form of the disease the blood and tissues are quickly cleared of tachyzoites by antibodies. The liver, spleen and lungs are also cleared quickly but the brain and heart are cleared late.
• Persistence of bradyzoites in cysts occurs in chronic infections. They have been found in pigeons, rats and mice up to 3 years after infection.
• Some strains are more virulent than others.
• Prenatally acquired toxoplasmosis more severe than post-natally acquired.

Other Host Effects

• Most infections with T. gondii are probably asymptomatic.

Control

Control via chemotherapies

• Pyrimethamine Pyrimethamine with trimethprim/sulfonamides drugs Trimethoprim or clindamycins Clindamycin.

Vaccination

• Vaccines available for sheep.

Other countermeasures

• Feed cats only dried, canned or cooked food.

Diagnosis

Further Reading

Publications

Refereed papers

• Recent references from PubMed and VetMedResource.
• Cook A J, Gilbert R E, Buffolano W et al (2000) Sources of toxoplasma infection in pregnant women: Europeanmulticentre case-control study. European Research Network on Congenital Toxoplasmosis.​ BMJ 321 (7254), 142-147 PubMed.
• Lin D S (1998) Seroprevalence to Toxoplasma gondii in privately-owned dogs in Taiwan. Preventive Vet Med 35 (1), 21-27 PubMed.
• Stiles J, Prade R & Greene C (1996) Detection of Toxoplasma gondii in feline and canine biological samples by use of the polymerase chain reaction. Am J Vet Res 57 (3), 264-267 PubMed.
• Pimenta A L, Piza E T, Cardoso Júnior R B et al (1993) Visceral toxoplasmosis in dogs from Brazil. Vet Parasitol 45 (3-4), 323-326 PubMed.