Neospora

Neospora was one of one of those hot topic health issues for discussion during our 2021-member forum meetings. Here, Gerard te Lintelo MRCVS explains the parasite-based disease’s life cycle, what to look out for, testing, treatment and prevention. Whilst testing remains optional, he reminds that annual testing of all female breeding animals as per the CHeCS scheme should be routine; herds require three consecutive clear annual herd screens to obtain risk level 1 status.

Introduction

Neosporosis was first discovered in cows with abortions in the UK in 1987. This disease is nowadays recognized as a major cause of reproductive problems and abortion in cows and it can have a significant economic impact. Neosporosis is caused by Neospora caninum (N. caninum), a tiny parasitic single-cell protozoal organism about half the size of a red blood cell that can invade and live inside animal cells.

Transmission

N. caninum has a unique life cycle (fig.1). Cows are intermediate hosts and dogs, and other canines like wolves, are the definitive hosts. A definitive host is an animal which harbours the adult parasite and where the parasite reproduces sexually and sheds eggs. An intermediate host is an animal which harbours the larval stage(s) or the asexual forms of the parasite. N. caninum is spread either vertically from one generation to the next generation within one species (cow to calf) or horizontally from one host to the other - dog to cow or cow to dog. Vertical transmission is considered to be the principle route of N. caninum infection in cattle and this vertical transmission maintains the infection within a herd.

Horizontal transmission from definitive host to cows

This occurs mainly when a cow becomes infected from eating dog faeces through contaminated feed and water supplies. After infection N. caninum will spread from the cow’s digestive system to other tissues in the cow. This can then cause

Fig. 1 Neospora life cycle

acute infection - abortion, or N. caninum can change into a dormant state by hiding in tissues. The incidence of horizontal transmission in cows is generally low. However, an abortion storm can be seen when multiple animals within a herd have been exposed to the same contaminated feed and or water. Dogs usually acquire N. caninum by eating infected placentae, foetuses or dead calves.

Vertical transmission in cows

The main route of infection in cattle is trans-placental from dam to calf and the same cow can pass the infection on to multiple offspring. N. caninum infection is thought to be maintained by vertical infection on most farms. This is practically often seen when seropositive animals can be traced through maternal family lines within a herd. The probability of a seropositive dam producing a calf that is seropositive prior to consumption of colostrum is around 95%. Note that these antibody-positive calves can appear clinically normal at birth.

Clinical manifestations

N. caninum can cause encephalomyelitis - inflammation of the brain and spinal cord, in calves infected during pregnancy. This can have the following consequences; early embryonic death, abortion, stillbirth or birth of a feeble abnormal calf or birth of a normal calf with no obvious effect of N. caninum infection. The following scenarios can be seen on farms.

Aborted foetus

Infection of a naïve nonpregnant cow

If a naïve - not previously infected, cow is not pregnant when infected horizontally, the infection usually produces no clinical signs, but seroconversion - becoming antibody positive, occurs. This can lead to persistent infection with N. caninum within tissue cysts in the brain and spinal cord of a cow. However, infection of calves in subsequent pregnancies is less likely to occur when a non-pregnant naïve animal has been infected via the horizontal route compared to cows first infected during pregnancy.

Infection of a naïve pregnant cow

If a naïve cow is pregnant and in early pregnancy (two to three months) when horizontally infected, the infection may lead to early embryonic death due to suspected uterine-placental inflammation. If a naïve cow is pregnant and in mid-pregnancy (three to seven months) when infected, the infection may lead to either abortion or birth of a weak, abnormal calf, depending on the month of gestation.

At this stage of gestation, the foetus has an immature immune system and is unable to fully fight off the infection with subsequent invasion of the placenta and calf. This leads to extensive tissue damage and the abortion of an autolysed foetus or a weak abnormal calf that is born with neurological symptoms and low weight at birth. If the naïve cow is in late pregnancy when infected, the infection leads to the birth of a weak or normal calf that is seropositive for N. caninum. During this stage of pregnancy, the immune system of the foetus is more mature than that of a younger foetus and it is therefore more able to control the infection, leading to limited or no clinical signs in the new-born calf.

Infection in persistently infected cows

Cows can become persistently infected after horizontal transmission or after vertical transmission as a calf. During pregnancy, the N. caninum larval stages which were lying dormant become reactivated and will cause similar symptoms as mentioned above; early pregnancy  embryonic death, middle pregnancy  abortion or birth of a weak or brain damaged live calf and late pregnancy  a seropositive weak or brain damaged calf or a normal calf. It is a very important feature of the epidemiology of this disease that apparently healthy calves can be born to apparently healthy, but infected dams.

Diagnostics tests

There are two situations where diagnostic testing for N. caninum is indicated.

• testing is indicated in individual clinical cases of abortion, general fertility problems and weak or dead born calves where N. caninum might play a roll. Practically this entails histopathology and PCR testing on the foetus and placenta. Blood samples for antibody testing should be taken from the dam.

• screening for N. caninum on herd level (including health schemes) via anti-body testing. All individual antibody results for N. caninum should, however, be interpreted with caution because the immune system is not static and antibody levels fluctuate as this parasite forms cysts that wall themselves off from the host’s immune system. A single serum sample from an individual cow may not reflect her infection status accurately, particularly on farms without a history of N. caninum abortions.

Treatment and prevention

There is no treatment for N. Caninum infection in cattle. Infected cows and all their offspring should not be retained as breeding animals. Remember that >90% of infected cows will pass infection on to their calves and that some infected cows can sometimes test negative.

Prevention of exposure of cows to feed and water potentially contaminated with faeces from dogs, including farm dogs, that may contain N. caninum eggs is the main measure farmers can take to prevent horizontal transmission from dog to cows. Farm dogs should not have access to potentially N. caninum infected placentae, dead foetuses or dead calves to prevent horizontal transmission from cows to dogs. A Neospora-infected animal is of no direct risk to the rest of the herd - there is no horizontal transmission of infection occurs between cows, but as mentioned above, infected material, if eaten by the definitive or intermediate hosts can pose a risk. It is important to remember that Neospora eggs can survive in the environment for many months.