Neosporosis is a relatively newly recognised coccidian parasite. It was first recognised in dogs in Norway in 1984. Since then, neosporosis has emerged as a serious disease of cattle worldwide, and is one of the major causes of abortion.
Neosporosis is caused by the coccidian parasite Neospora caninum. N. caninum is a parasite with a wide host range, with disease primarily being seen in cattle. Dogs are thought to act as carriers of the disease. The N. caninum agent is similar to the agent Toxoplasma gondii, which causes abortion in sheep but with cats as the carrier of the disease.
It is currently thought that the path of transmission of N. caninum is that dogs initially become infected by eating food containing N. caninum cysts. These cysts colonise the gut, replicate and are excreted in the faeces. The excreted N. caninum oocysts are ingested by cattle, which then become infected. Once infected with N. caninum, the cow remains infected for life. Although N. caninum does not cause illness in the cow, she has the potential to transmit N. caninum across the placenta to her unborn calf. This can trigger abortion. Any heifer calves carried to term will also be infected, and when pregnant may also either abort or give birth to carrier calves.
However, the theory of spread via infected dog faeces has not been conclusively proven, and N. caninum oocysts have only been identified in a few dogs worldwide and with only a few reports of N. caninum shedding by naturally infected dogs. Consumption of infected bovine foetuses seems to be an unimportant source of infection, but ingestion of naturally infected placentas may be more important. It is also not yet clear how N. caninum is transmitted between dogs. Transmission across the uterus from a mother to her pups only occurs rarely.
Transmission in cattle has been more conclusively proven, and N. caninum is one of the most efficiently transplacentally transmitted parasites among all known microbes in cattle. In certain herds, it has been proven that virtually all calves born alive are born infected but without symptoms of infection. Transmission from infected bulls is thought not to occur.
Risk factors associated with Neospora abortion
When assessing risk factors for Neospora abortion, the abortion risk may be increased by either the number of Neospora oocysts ingested by the cow and the stage of pregnancy, or in a chronically ill cow it is postulated that the immune status of the cow may play a role.
Risk is also thought to increase with the number of dogs on farms. Defaecation by farm dogs on feeding alleys and on conserved forage has been reported more often on farms where it is thought that the cows have becomeinfected during pregnancy. There is also a higher reported incidence where new dogs are introduced to a farm compared to when dogs have been resident for a long time. This is possibly because these dogs are more likely to shed Neospora oocysts than dogs which have been exposed to infection for a long period of time. there is also evidence to suggest that young dogs shed a higher number of Neospora oocysts than older dogs.
Another risk factor is the water source. There is evidence that water is drunk from ponds on a farm poses a higher risk than water from a mains supply.
There is no proven link between the feeding of colostrum from infected cows to calves born to non-infected cows.
It has been proved that seropositive cows are more likely to abort as a result of Neospora than seronegative cows. Also, the abortion risk seems to increase with the level of Neospora specific antibodies. A high antibody level could indicate a recent high infection dose, and or/ an efficient multiplication of the parasite within the host cow. Occasionally in infected cows there is recrudescence of latent infection during pregnancy, and so the abortion risk is increased.
Feeding of poor quality fodder has been associated with an increased risk of abortion. There are two reasons for this. Firstly, if mouldy fodder is fed, it is thought that fungal toxins may have a negative effect on the animal's immune system. Secondly, remnant fodder may contain a higher level of contaminants including the possibility of fecal contaminants.
In certain countries there seems to be an increase in abortion rate associated with certain seasons. Generally this seems to be in more mild and humid times. It is thought that in mild humid weather Neospora oocysts survive better, and also fungi grow better in these conditions hence there being a higher level of fungal toxins. It has been seen that in two Spanish herds an increase in rainfall seemed to correlate with an increased incidence of abortion. This is possibly because this caused a higher stress level to the cow in terms of temperature control, hygiene and fodder quality, thus suppressing the animals' immune system.
Infections with other agents may also cause stress or immune suppression in animals latently infected, increasing the risk or recrudescence of latent infection. Herds with concurrent infections such as BVD, leptospirosis and salmonella are more likely to be associated with a higher risk of abortion in infected cows. Therefore, it is important to control these other diseases.
It has also been shown that infected cows that have already aborted have a higher chance of aborting than congenitally infected cows which have never aborted.
There has been a lower incidence of abortion seen in infected dairy cows inseminated with beef bull semen compared to using dairy bull semen to inseminate infected dairy cows. This is possibly as a result of an increase in pregnancy associated glycoproteins.
Prevention and Control
Where control plans are instigated they should be done using a cost benefit analysis comparing the costs of testing and control measures with the benefit of reduced economic losses due to N. caninum infection or abortion.
Clearly, the main cost of Neospora is the direct cost associated with foetal losses and reproductive failure. however, there are also indirect costs such as laboratory fees in determining a diagnosis, rebreeding, loss of milk yield and replacement costs if cows or heifers which have aborted are culled,
Where Neospora has not previously been isolated in a herd, there are several measures which can be taken to reduce the risk of the disease entering the herd. These include:
Quarantining and testing of all replacements before entry to the herd to ensure freedom from infection;
Prevention and transmission from dogs. This generally involves keeping dogs away from foodstuffs, and ensuring that dogs have no access to either placentas or aborted foetuses;
Prevention or waterborne transmission by using a mains water supply and avoiding cattle drinking from stagnant water such as ponds;
Good rodent control. Some studies have implicated rodents in the spread of disease;
Prevention of feeding or mouldy fodder.
In herds where Neospora is present, there are further methods which can be used to reduce the risk of animals aborting:
The use of beef semen on cows which are known to have aborted previously;
Testing and culling. N. caninum-infected cows must be considered a reservoir of infection with the potential to transmit the infection to other cows. Ths can either be through giving birth to live infected offspring, or through environmental contamination. Although there is no proven direct cow to cow spread, it is postulated that these infected cows act as a source of infection to the intermediate host the dog, which then becomes infected, replicates the Neospora, and then deposits it back into the environment that is infectious to cows. Although this method of control is effective, it is not always economically realistic. It can be applied as:
test and cull either seropositive or seropositive aborting cows;
test and inseminate seropositive cows with beef semen only; or
test and exclude the progeny of seropositive cows from breeding.
If testing of cows is carried out and cattle culled on the basis of these results, it should be ensured that the main route of infection in the herd is through vertical transmission, i.e. from mother to calf and not from picking up the infection from the environment.
Currently, there are no treatments available for the control of bovine neosporosis that has been shown to be safe and effective. However, studies have been carried out looking at the efficacy of the drug toltrazuril and one of its derivatives ponazuril. In calves treated with ponazuril, the organism was no longer detectable in the brain and other organs. In experimentally infected mice, toltrazuril was shown to experimentally block transplacental transmission of the infection. Toltrazuril is currently available for threatment of coccidiosis in calves, and is the active ingredient in Baycox Bovis. Unfortunately though Bayer are refusing to comment on the effectiveness of this drug in actively eliminating Neospora infection in replacement heifer calves.
At present, work is being undertaken to develop a vaccine against Neospora abortion. Desirable vaccine properties are to protect the cow against aborting and prevent any vertical transmission from the dam to her calf. The vaccine also needs to be effective when given to cows which have been infected from birth, and cows that are at risk of picking the infection up from the environment.
A vaccine is available in several countries called Neoguard. However, this vaccine had had very mixed results. In some cases there has been a positive effect on a reduction in abortion, but in many herds there has been no significant reduction in abortion rate.