Infection and abortion is caused by a protozoan parasite Toxoplasma gondii.
Cats, particularly kittens will spread the organism in the form of millions of very resistant oocysts in their faeces. The oocysts contaminate pasture and supplementary feed. After ingestion of the contaminated feed the susceptible sheep are infected for life. Infection of the placenta and foetus only occurs at the time of the initial infection.
It has been suggested that it can also be transmitted to susceptible ewes, during mating through ram semen.
The effect on the pregnant ewe will depend on the time during pregnancy that she becomes infected.
An infection in very early pregnancy (<60 days) will lead to foetal resorption and the ewe is likely to return to oestrus or become a dry ewe.
An infection in mid pregnancy (60-120 days) will cause foetal death, mummification and abortion. The time from infection to abortion is about 40 days.
An infection in late pregnancy (>120 days) usually results in the birth of a normal lamb which could already be infected or develop immunity.
The best way to confirm a diagnosis of Toxoplasma abortion is by histology from an aborted foetus.
This is arguably the most common cause of sheep abortion in NZ and is caused by the bacteria Campylobacter fetus fetus.
Abortion usually occurs within the last 6 weeks of pregnancy, 7-25 days after infection.
Following abortion the ewes remain healthy and are immune to repeat infections.
Infection is spread during an outbreak by the ingestion of contaminated feed from aborted materials.
Infected carrier animals, which harbour the organism within the gut are believed to maintain the infection within a flock and spread it between flocks.
Infected farms will often see outbreaks every few years.
Diagnosis is usually confirmed by microscopic examination or culture of the stomach contents of an aborted lamb.
Although not regarded as a common cause of abortion, Leptospirosis (serovar Pomona) has been shown to cause foetal loss and abortion in Hoggets in New Zealand and abortion in both ewe and goat flocks in other countries.
The maintenance host for L. pomona is the pig. Hence infections are more commonly seen in areas where there is a high incidence of wild pigs.
Infection is spread via urine.
Leptospiral organisms can survive for several months in water. Which can lead to increases in the infection rate occurring after flooding.
Leptospirosis is a zoonotic disease and in New Zealand virtually 100% of sheep flocks show evidence of exposure to this organism.
SALMONELLA BRANDENBERG (Reproductive form)
Ewes are infected by direct contact with aborted material or from ingestion of contaminated feed.
On average about 3-4% of a flock may abort over a 30 day period (range 7-75 days). Although this can be higher in totally naive flocks. With no treatment about 30-50% of aborted ewes will die.
Ewes that do survive will become carriers for up to 6 months. Black backed gulls have been shown to carry the organism and it can also survive in the environment for at least 4 months.
Diagnosis can be made from culture of the foetal stomach contents, placenta or vaginal swabs. Dead ewes usually have a severely infected uterus and fetuses are very rotten with swollen livers.