Neosporosis

(Mid-term abortions: Neospora caninum)

Neospora caninum is a microscopic, unicellular coccidian protozoan and a major infectious cause of weak calves, abortions and still-births in cattle around the world. It also produces disease in dogs, the definitive hosts of this opportunistic parasite that uses the predator / prey relationship between dog and cattle to complete its life cycle.

The disease manifests on-farm as either repeated spontaneous abortion of just a few cattle, or as abortion storms in which up to 30% of cows abort over a 1-2 month period, although some infected herds do not have any history of abortions.

Figure 1. Neospora caninum (arrow) encysted in a histologic preparation of a calf brain. Image courtesy of the National Centre for Veterinary Parasitology, ncvetp.org

Spontaneous abortion is thought to be due to reactivation of old infections during pregnancy that suppresses the immune response and allows infection to pass from the dam to the foetus.

By contrast, abortions storms are the result of sudden large-scale infection of previously uninfected cattle either through ingestion of contaminated water and feed, or due to a herd event such as exposure to another infectious agent, or perhaps toxins in mouldy feed that suppress herd immunity.

In dairy cows, N. caninum is the infectious agent responsible for between 10-30% of abortions especially of those reared under intensive systems.

Evidence to date indicates that eating raw or undercooked beef does not transmit Neospora infection to humans.

The economic impact of abortion storms was reported in 1997 to be in excess of $110M per year, with $85 million incurred by the dairy industry and $25 million by the beef industry.

It is thought that a sylvatic predator / prey cycle between dingoes and macropod species may also be operating in Australia.

Signs

Most infected cows are clinically normal (as are infected dogs) and the majority have normal pregnancies. Reduced weight gains in beef cattle and reduced milk production in dairy cattle are common.

A feature of this disease is that abortions occur in mid-gestation between 4-7 months whereas most abortions on-farm are late term.

In dogs, transmission from bitch to foetus can result in neurological signs such as progressive weakness and paralysis of one or both hind limbs. It is not known if infected pups produce oocysts or if a dog can be re-infected.

Life cycle

Neospora caninum can be transmitted to cattle by ingestion of oocysts (type of egg) in the faeces of an infected dog, but more typically, infection is transmitted by trans-placental infection from the dam to the foetus.

Oocysts passed in the faeces of dogs become infective to cattle in 1-3 days. Once ingested, the fast dividing stages released in the intestine move through the gut wall and into the body. During the acute phase of infection when clinical disease may be evident, these fast dividing stages may be found in virtually all host tissues including the placental and foetal tissues.

Once the host’s immune response is activated, these fast dividing stages transform into tissue cysts primarily in the central nervous system where they remain dormant until the host’s immunity is suppressed, for example, during pregnancy, and the infection is reactivated. These dormant cysts represent a persistent source of infection within the herd.

If a dog eats meat containing dormant muscle cysts, the parasites are released in the gut, multiply and produce oocysts into the faeces a few days later. The dog’s immune system will usually control the infection within a few weeks and shedding of oocysts will cease about 17 days after infection. Dog-to-dog infection occurs from ingestion of infectious oocysts as well as from bitch to foetus.

Oocysts are the resistant form of the parasite that can survive in the environment for many months. They do not appear to be infectious to humans.

Impact on cows

In a non-pregnant female, infection will result in a persistent asymptomatic infection but without any reduction in reproductive capacity.

In a pregnant female in early gestation, infection can result in early embryonic death which may not be noticed. Infection in the second trimester may result in abortion or birth of a weak or brain damaged live calf characterised by incoordination and ‘goggled eyed’ whereas infection in final trimester will usually result in a clinically normal but congenitally infected calf.

Once infected, a cow probably remains infected for life and gives birth to congenitally infected calves, which are usually clinically normal but sometimes may be affected. These calves in turn, are capable of transmitting the parasite to their multiple offspring. The dams remain asymptomatic.

Diagnosis

Neospora caninum is only one of many causes of abortion in cattle and needs to be differentiated from other causes of infectious abortions. It can be diagnosed by serology and/or histopathology. Tissue and blood samples are required from the aborting dam, together with the placenta and aborted foetal tissues particularly the brain and heart. Parasite DNA tests are used to detect infection on herd basis. Checking dog faeces for oocysts is unreliable as dogs with symptomatic infections usually only shed oocysts in faeces for a few days after ingesting infective tissue.

Treatment

Despite the importance of this disease, there is no vaccine or treatment currently available, and the only control measure is to reduce the impact of the disease on-farm through informed management.

Preventing dogs from contaminating food and water supplies of cattle, and practising high hygiene standards at calving is important. Farm dogs and wild dogs should not have access to placental membranes, or aborted, or dead calves.

As transmission from mother to foetus is considered by far the most important route of maintaining infection within the herd, it is often advised but hotly debated, to cull infected cattle. However, after culling the overall level of herd immunity drops and disastrous abortion storms have resulted when infection has been re-introduced to the herd.

It has been suggested that culling only the repeat aborters would be a better management strategy.