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Toxoplasmosis
Introduction
- Cause: Toxoplasma gondii, a protozoan parasite of the cat; dogs, humans, etc are intermediate hosts.
- Infection via sporulated oocysts from cat feces, undercooked meat or transplacental.
- Signs: usually subclinical, occasional severe multisystemic or central nervous system (CNS) infection.
- Diagnosis: typically based on serology.
- Treatment: clindamycin, additional drugs.
- Prognosis: good to fair.
Presenting signs
- Chronic, subclinical infection.
- Severe multisystemic disease.
- Central or peripheral nervous system signs.
- Chronic hepatitis with acute terminal hepatic necrosis.
Acute presentation
Generalized infection
- Usually with concurrent canine distemper infection Canine distemper disease.
- Respiratory distress.
- Vomiting, diarrhea.
- Icterus.
- Progresses to death.
Geographic incidence
- Worldwide.
Age predisposition
- <1 year old.
- Neonate.
Public health considerations
- Whilst, oocysts are only formed within the feline GI tract, oocysts have been found in canine feces due to their non-selective eating habits, and therefore, canine feces can present human health risk.
- Predominate risk to humans is from consumption of undercooked meat, or soil/litter tray containing cat feces contaminated with sporulated oocysts (sporulation does not occur until 1-5 days after being passed).
- Primary infection of women during pregnancy can cause fetal malformation/miscarriage.
Pathogenesis
Etiology
- Toxoplasma gondii Toxoplasma gondii : coccidian protozoal parasite.
- Obligate intracellular parasite.
Predisposing factors
General
- Cat feces in soil.
- Undercooked meat.
- Scavenging/hunting.
- Feline litter tray if not cleaned daily.
Specific
- Canine distemper virus infection Canine distemper disease.
- Immunosuppression activates subclinical infection.
Pathophysiology
- Ingestion of sporulated oocysts or tissue cysts → organisms invade and multiply in tissues → intracellular pseudocysts → pseudocysts rupture → release numerous tachyzoites → spread throughout body → chronic phase.
- Bradyzoites divide slowly within parasitic membrane → tissue cyst, especially in muscles and brain.
- Once tissue cysts form, host has developed immunity → resistant to re-infection with the same strain.
- Tissue cysts persist for months/years.
Clinical infection
- This may occur in:
- Neonates/puppies.
- Immunosuppressed individuals (reactivation of chronic infection).
- Association with canine distemper virus infection Canine distemper disease.
- Release of organisms from tissue cysts → rapid multiplication of tachyzoites → respiratory/gastrointestinal disease, meningoencephalitis, necrotizing polymyositis, polyneuropathy, polyradiculoneuritis.
Timecourse
- Initial multiplication in tissues lasts 2-3 weeks.
- Tissue cysts may persist for months/years.
Epidemiology
- Definitive hosts are felidae.
- Intermediate hosts: mammals, amphibians, reptiles, fish.
- Seroprevalence: cats 40% (0-100%), dogs 30-40%, humans 30-60%.
- Cats ingest oocysts/tissue cysts → enteroepithelial cycle → shed oocysts within 3 days to 3 weeks → sporulate in environment in 1-5 days → ingested by intermediate host.
- Dogs infected via:
- Ingestion of sporulated oocysts from cat feces.
- Ingestion of meat containing pseudocysts or tissue cysts.
- Transplacental infection (dam develops parasitemia during pregnancy).
Diagnosis
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Treatment
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Prevention
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Outcomes
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Further Reading
Publications
Refereed papers
- Recent references from PubMed and VetMedResource.
- Wyrosdick H M, Schaefer J J (2015) Toxoplasma gondii: history and diagnostic test development. Anim Health Res Rev 16 (2), 150-162 PubMed.
- Dubey J P (2009) The evolution of the knowledge of cat and dog coccidia. Parastiology 136 (12), 1469-1475 PubMed.
- Cook A J, Gilbert R E, Buffolano W, Zufferey J et al (2000) Sources of toxoplasma infection in pregnant women - European multicenter case control study. BMJ 321 (7254), 142-147 PubMed.
- Thomas W B (1998) Inflammatory diseases of the central nervous system in dogs. Clin Tech Small Anim Pract 13 (3), 167-178 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.
- Dubey J P (1994) Toxoplasmosis. JAVMA 205 (11), 1593-8 PubMed.
Other sources of information
- Taboada J & Merchant S R (1995) Protozoal and miscellaneous infections. In:T extbook of Veterinary Internal Medicine. Eds: S J Ettinger & E C Feldman. 4th edn. Philadelphia: W B Saunders. pp 384. (Brief but comprehensive review.)