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
- 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

Toxoplasma can cause acute interstitial pancreatitis in systemic toxoplasmosis

Toxoplasma gondii causes myositis

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

Sarcocystis in myositis

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

Neospora caninum in myositis

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