Tissue cyst-forming coccidia

Neospora

Sarcocystis

Toxoplasma

Toxoplasma gondii - Ke Hu and John Murray

Toxoplasma Sporulated Oocyst - Wikimedia Commons

Toxoplasma Tacchyzoites - Wikimedia Commons

Toxoplasma Life Cycle Diagram - Dennis Jacobs & Mark Fox RVC

Toxoplasma gondii - Courtesy of the Laboratory of Parasitology, University of Pennsylvania School of Veterinary Medicine

• Major pathogenic species called Toxoplasma gondii

• Causes disease in a wide range of animal species including humans

• Important cause of abortion in sheep

• Zoonotic
  • Can cause abortion
  • Can cause congenitally aquired defects

• Forms a sporulated oocyst which is only 10μm
  • Contains 2 sporocysts with 4 sporozoites

• Transmission through ingesting the intermediate host or via the faecal-oral route

Life Cycle

• Complex

• Usually indirect
  • Referred to as facultatively heteroxenous
  • Intermediate host is not essential for completion of the life cycle

• Gametogony (sexual stage) is host specific for felids

• Any warm blooded animal can act as a facultative intermediate host
  • Asexual reproduction occurs in the intermediate host forming tissue cysts
  • Intermediate host swallows sporulated oocysts or tissue cysts
  • Can be transferred between intermediate hosts by carnivorism

• Cats
  • Sporulation occurs in 2-3 days
  • Cats either swallow infective (sporulated) oocysts where Toxoplasma gondii has a prepatent period of 3 weeks
  • Or eat the tissues of an infected intermediate host where Toxoplasma gondii has a prepatent period of 3-10 days
  • Self-limiting infection
  • Oocysts are shed for 1-2 weeks
    • Shedding can occur later if immunity wanes or cat is immuno-compromised

• Intermediate host
  • 3 sources of infection
    • Oocysts from environment contaminated by cat faeces
    • Eating cysts in tissues of other infected hosts through carnivorism or undercooked meat
    • Transplacental transmission in some host species during the acute phase of infection

• Acute phase of infection
  • After infection of the intermediate host the organism undergoes a phase of rapid division and dissemination throughout the body
    • Parasite enters cell and asexual reproduction occurs by endodyogeny (budding) producing 8-16 tachyzoites
    • Tachyzoites are released when host cell bursts
    • Haematogenous spread as more cells are infected
    • Infection continues until the animal develops immunity (around 2 weeks) at which point the infection enters the chronic phase

• Chronic phase of infection
  • Occurs once the host's immune response has become effective
    • Groups of slow growing intracellular bradyzoites become walled off forming infective cysts
    • Bradyzoites inside cysts are protected from the host immune response whereas extracellular tachyzoites are killed
    • Cysts remain viable for months to years and are particularly numerous in muscle and nervous tissue
    • If immunity is suppressed the infection can revert to the acute form

Pathogenesis

• Cat
  • In the intestinal phase of infection only the superficial cells at the tips of the villi are affected
  • Little significant pathogenicity

• Sheep
  • Mostly asymptomatic
  • However, if a non-immune ewe is infected during pregnancy the consequences will be serious
    • Infection during the first trimester leads to resorption
  • Infection during the second trimester leads to foetal death and mummification
  • Infection during the last trimester leads to a weak or stillborn lamb
  • Aborted ewes show focal necrotic placentitis with white lesions in the cotyledons and foetal tissue
  • Diagnosis is confirmed by Giemsa and serology of the ewe's blood

• Clinical outbreaks of toxoplasmosis are sporadic
  • Immunity is acquired before tupping
  • Significant ill-effects are unlikely if immune ewes are infected during pregnancy
  • Not shed from sheep to sheep so predicting outbreaks is difficult

• Humans
  • Mostly asymptomatic
  • Virulent strains cause flu-like symptoms, malaise and/or lymphadenopathy
  • In immunodeficient patients, disease can even be caused by avirulent strains
  • If a non-immune women is infected during pregnancy, abortion or the birth of a congenitally infected child can result
    • E.g. Hydrocephalus, opthalmitis, mental retardation

• Dogs
  • Complication of canine distemper
  • Causes pneumonia and encephalitis

• Cattle and horses
  • Sometimes infectious causing opthalmitis

• Toxoplasma can cause acute interstitial pancreatitis in systemic toxoplasmosis

• Toxoplasma gondii causes myositis

Epidemiology

• Serology
  • Sabin-Feldman Dye test (old method)
  • ELISA
  • Mouse inoculation for confirmation

• Cat
  • 30-80% test seropositive
  • Each cat sheds oocysts for 1-2 weeks of its life

• Human
  • 30% seropositive in UK, 70% seropositive in France

• Meat animals
  • Significant proportion of cattle, sheep, pigs and rabbits can tissue cysts

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

• Human
  • Avoid oocyst ingestion
    • Wash potentially contaminated raw food thoroughly
    • Wash hands after gardening or handling cats and especially before eating
    • Clean out cat litter trays every day before oocysts sporulate
  • Avoid ingestion of tissue cysts
    • Do not eat undercooked meat
    • Wash hands after handling raw meat
    • Take care when lambing or dealing with sheep abortions and stillbirths
    • Pregnant women should avoid lambing altogether

• Sheep
  • Toxovax vaccine
    • Live, avirulent strain of Toxoplasma
    • Does not form bradyzoites or tissue cysts
    • Killed by host immune system
    • Single dose given 6 weeks before tupping
    • Protects for 2 years
    • Immunity boosted by natural challenge
  • Medicated feed can be given daily during the main risk period
    • 14 weeks before lambing
  • The best method of protection is to prevent cats from contaminating the pasture, lambing sheds and feed stores