Tissue cyst-forming coccidia

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

• 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

Toxoplasma can cause acute interstitial pancreatitis in systemic toxoplasmosis

• T. gondii in pneumonia
• Toxoplasma gondii, Sarcocystis and Neospora caninum in myositis

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

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