Vertical and horizontal disease transmission - Toxoplasma gondii and Neospora caninum Flashcards
What are the key aims of reproductive performance in cows?
One calf per cow per year
- minimal cost in achieving this aim
- Production of replacement heifers and occasionally bulls
- cows calving with minimal stress and interruption
How is reproductive performance measured?
o % of cows pregnant by 100 days after calving (100 day in-calf rate)
o % of cows not pregnant by 200 days after calving (200 day not-in-calf rate)
o % of cows receiving at least one insemination by 80 days after calving (80 day submission rate)
o % of first inseminations resulting in pregnancy by pregnancy test (conception rate)
What are the usual bacterial, parasitic and viral suspects for infection in cows?
o Bacterial
Brucellosis
Vibriosis (Campylobacter fetus)
Leptospirosis
Salmonellosis
o Parasitic
Trichomonosis
Neosporosis
o Viral
BVDV (pestivirus)
What is neospora caninum?
- Single cell protozoal parasite (Apicomplexa) causing neosporosis
- Cows are the intermediate host; dogs are the definitive host, but they can serve as an intermediate host as well
- Diagnostically challenging, so both vets as well as animal owners struggle with the understanding of the disease (antibodies do not mean disease)
Distribution
- 1/5 farms in NSW have N. caninum
How is neospora caninum transmitted?
Can be transmitted horizontally or vertically
What is the difference between horizontal and vertical transmission?
- Horizontal transmission: Transmission of an infection between hosts that are not in a parent-child relationship
- Vertical transmission (mother-to-child transmission): Transmission of an infection from mother to child during the perinatal period
What is the lifecycle of neospora caninum?
- Indirect lifecycle -> intermediate host is a cow, dog is the definitive host BUT can also serve as an intermediate host as well
- Sexual cycle (definitive host)
o Canine host eats material (infected placenta, raw infected carcasses or aborted foetus) containing N. caninum tissue cysts
o Sexual multiplication takes place in the intestine of the definitive host (dog, dingo) and unsporulated oocysts are produced (~10-20 days post-infection) - Asexual cycle (intermediate and definitive host)
o Cattle ingest N. caninum oocysts while grazing on pasture or eating food contaminated by infected dog faeces
o Sporozoites then enter different tissues but are mostly found in the reticulo-endothelial system and differentiate into tachyzoites
o Tachyzoites reproduce rapidly asexually and invade different tissues and in pregnant animals the foetus via the placenta
o With the onset of the bovine immune response, tachyzoites revert to dormant bradyzoites within tissue cysts (found mainly in neural tissue)
o During future pregnancies, in infected animals bradyzoites are reactivated and differentiate into tachyzoites that invade the foetus via the placenta
What are the usual outcomes of infection with Neospora caninum? Does it make a difference if there is maternal immunity?
- Embryonic death
- Abortion
- Persistent infection in the calf
- Normal seronegative calf
- Calf born with neurological symptoms
Maternal immunity
- o During pregnancy maternal immunity is biased toward a Th type 2 response, however during a parasitic infection host mounts a Th type 1 response
o The balance between Th type 1 and type 2 cytokines will determine the outcome of both the pregnancy and the infection
o Type 2 cytokines will support the development of the foetus, while allowing the infection to continue uninhibited
o Type 1 cytokines will counteract the infection but will not support the development of the foetus
Is an epidemic abortion (abortion storm) horizontal or vertical transmission?
Horizontal -> Caused by a point source of exogenous infection by N. caninum oocysts e.g. contamination of feed with dog faeces
Is an endemic abortion horizontal or vertical transmission?
- Vertical transmission -> abortion rate of <5% per year
- Endogenous transmission
- self-perpetuating
Is neospora a problem if you have absence of abortion in a neospora seropositive herd?
Yes -> seropositive healthy calves will likely abort again (= vicious cycle)
- Seropositive congenitally infected heifers have a 7.4 times increased risk of abortion in their first pregnancy
- o The abortion incidence decreases with subsequent pregnancies, suggesting that maternal immunity matures after the first pregnancy and influences the outcome of subsequent pregnancies. However, a small percentage of cows may have repeated abortions
- no clinical signs in cows that abort
Should neospora seropositive animals be culled?
No, the cheapest option is to do nothing
Does the definitive host host sustain the neospora infection?
Yes, the dog (definitive host) sustains the infection
Is neospora a public health risk?
No, there are no documented human infection of neosporosis
What does Toxoplasma gondii cause?
Toxoplasmosis
What is the lifecycle of toxoplasma gondii?
- The lifecycle of toxoplasma gondii is nearly identical to N. caninum
- The only definitive host is a cat (sexual reproduction in the intestine)
- Oocysts shed by the cat are environmentally resistant
- Intermediate hosts include all types of mammals and some birds
- Antibodies found in 30% of human population >18 years
o Antibody-mediated lysis of tachyzoites (not cysts) clears infection
o Recovery yields complete immunity in humans (unusual for protozoan parasite)
o Immunity can be boosted by tissue cyst breakdown (premunition)
What is the epidemiology of T. gondii?
- Domestic and wild cats main transmitters
- Oocysts must sporulate before becoming infectious
- Oocysts in cat faeces most likely source e.g. sandpits that become litterboxes (=children, <5 yrs)
o Sporulation in litterboxes/sandpits takes 305 days
o Sporulated oocysts remain infectious for up to 2 years in suitable environments - Raw or undercooked meat (=adults, >18 yrs) but infectious cysts in meat can be killed by freezing
How is T. gondii transmitted in a wild environment?
How is T. gondii transmitted in a home environment?
o Previously unexposed (non-immune) woman acquires toxoplasmosis during pregnancy (congenital infection)
o 40% chance that the foetus will acquire the infection, and in around 10% of these cases, severe neurological or ocular disease is present at birth
o Owning a cat or having direct contact with a cat does not constitute an increased risk of acquiring toxoplasmosis
o Ingestion of infected meat appears to be the major source followed by poor hygiene
What is the pathogenesis of T. gondii?
- Acute infection
o Often asymptomatic in cats (may kill kittens?)
o Humans have flu-like symptoms, usually full immunity develops
o Sheep and goats – abortion!!! (tachyzoites in milk) - Subacute infection
o Hosts humoral immunity develops slowly
o Tachyzoites produce lesions in lung, liver, heart, brain, and eyes - Chronic infections
o Ab immune response depresses tachyzoites but cysts form from bradyzoites
o Causes intense inflammation
o Encephalitis
o Retinal degeneration
o Myocarditis - Congenital toxoplasmosis
o Infection during pregnancy can infect foetus
o Can cause still birth, abortion, birth defects
o Hydrocephaly, microcephaly, blindness
