Tissue cyst-forming coccidia



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PARASITES
PROTOZOA



Toxoplasma

Toxoplasma Life Cycle Diagram - Dennis Jacobs & Mark Fox RVC
  • 1 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
    • Reffered to as facultatively heterxenous
    • Intermediate host is not essential for completion of the life cycle
  • Gametogeny (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 as a prepatent period of 3 weeks
    • Or eat the tissues of an infective intermediate host where Toxoplasma gondii as a prepatent period of 3-10 days
    • Self-limiting infection
    • Oocysts shed for 1-2 weeks
      • Shedding can occur later if immunity wanes or cat is immunocompromised
  • Intermediate host
    • 3 sources of infection
      • Oocysts from environment contaminated by cat faeces
      • Eating cysts in tissues or 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 endogeny (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 an immunity in 2 weeks when 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 particulary 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
    • If a non-immune ewe is infected during pregnancy the consequences will be serious
      • Infection during the first trimester leads to resorbtion
    • Infection during the second trimester leads to fetal 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 aquired 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 be caused by even 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

Epidemiology

  • Serology
    • Sabin-Feldman Dye test (old method)
    • ELISA
    • Mouse innoculation 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 (will hunt!)
    • 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 eating raw meat
      • Take care when lambing or dealing with sheep abortions and stillbirths
      • Pregnant women should avoid lambing altogether when pregnant
  • 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

Sarcocystis

  • Most infections are asymptomatic
  • Heavy infections are causes of chronic wasting in large animals, hide sondemnation and downgrading of carcasses
  • Sarcocystis should be differentiated from other tissue-cyst forming coccidia
  • There are many species of Sarcocystis
  • Sporulated oocyst has 2 sporocysts containing 4 sporozoites
    • Naked oocyst usually seen in faeces as the oocyst wall is very delicate
  • Indirect life cycle
  • Life cycle alternates between the final and the obligatory intermediate host
  • Only one final and one intermediate host

Pathogenesis

Neospora

  • 2 main species
    • Neospora caninum in the dog
    • Neospora hughesi in the horse
  • Sporulated oocysts measuring just 10μm
  • Oocyst contains 2 sporocysts with 4 sporozoites
  • Route of transmission not fully understood
  • Often misdiagnosed as Toxoplasma gondii
    • Sarcocystis cysts have thicker walls
  • Infection diagnosed by IFAT, ELISA or PCR
    • Identification of lesions and organisms in tissue using immunohistochemical staining
    • Eliminate other causes of abortion first

Life cycle

  • Life cycle similar to Toxoplasma gondii
  • Limited range of warm-blooded intermediate hosts
    • Asexual reproduction occurs in intermediate host forming tissue cysts
  • Host range of sexual stage is unknown for N.caninum
    • Intermediate host for N.hughesi is the horse, but the definitive host is unknown
  • Final host
    • Dogs pass oocysts
    • Role not fully understood in pathogenesis
    • 5 day prepatent period
    • Other wild canids may also act as final definitive hosts
  • Intermediate host
    • Mostly cattle
    • Natural infection has been documented in other herbivores
  • Transmission
    • Transplacental infection occurs in all intermediate hosts and in the canine final host
      • Transplacental can occur in successive pregnancies
    • In cattle, vertical transmission occurs
      • Post-natal infection occurs but is less common

Pathogenesis

  • Dogs
    • Occurs mainly in puppies
    • Causes ascending paralysis, especially of hind limbs, with muscle wasting
    • Causes sudden collapse due to myocarditis
    • More than puppy in a litter may be affected, although this may not occur simultaneously
    • Successive litters affected
  • Cattle
    • Commenest cause of infectious abortion in dairy cattle
    • Congenitally infected calves can have encephalomyelitis and paresis
    • Abortion usually occurs between 5-7 months of gestation but can occur as early as 3 months
    • No other clinical signs in the cow
    • Repeat abortions possible in same cow (persistently infected)
  • Horses
    • Myeloencephalitis
    • Transplacental infection occurs
    • Disease only diagnosed in USA

Prevention and Control

  • Do not allow dogs access to calving cows, placental membranes and aborted or dead calves
  • Do not allow dogs to defecate in cattle feeding areas
  • Identify and cull seropositive cattle, or do not breed from them or their progeny
  • Select seronegative cattle for breeding
  • Vaccinate
    • Only in the USA
    • Neoguard or Intervet
    • Killed protozoal vaccine for healthy, preganant cows
    • Dosed in first 3 weeks of pregnancy and then every 3-4 weeks during gestation
    • Revaccination with 2 doses during each subsequent pregnancy