While walking through the cow shed, Jimmy sees a couple of cows grouped together in the corner. He instinctively knows that something is wrong. As he gets closer, he realises that an aborted calf lies in the corner. He scratches his head as he wonders what happened. Did the cow get injured? Was it salmonellosis, leptospirosis, BVD (bovine viral diarrhoea) or neosporosis? About 1-3% levels of abortion are considered to be normal in a dairy herd, but when levels get higher, something is wrong and it can be difficult to pinpoint the cause. It is estimated that an aborted calf costs the farmer about €710 in lost revenue. Scale that up when abortion rates increase due to a problem within the herd and the cost of the losses can quickly run into thousands. All abortions should be reported to the Department of Agriculture and blood samples and foetuses can and should be sent for testing, but apparently the diagnosis rate is very low at only 30%.
Neospora caninum, (its official name) is a parasite that is of economic interest as it can cause spontaneous abortion in cows. Oocysts (parasite eggs) are passed in the faeces of the definitive host (dog) and are ingested by an intermediate host, such as cattle. The cow becomes indefinitely infected with the parasite, and cysts begin to form in the tissues of the cow. When the cow becomes pregnant, the cysts become activated, which sometimes causes spontaneous abortion. If a definitive host (dog) then scavenges on the aborted foetus and afterbirth they cause subsequent infection, which completes the cycle. Calves of infected mothers that were not aborted carry antigens for the parasite, and when the female calf goes on to breed herself, she too can pass on the parasite, thus maintaining the parasite within the herd and leading to further spontaneous abortions and economic losses to the farmer. See an illustration here.
The parasite is also found naturally in wild animals and there has been some suggestion that animals such as foxes might be involved in cross infection between the wild and dairy herds. Although, it is not known for sure if foxes and other wild carnivores are definitive or intermediary hosts. To investigate if wildlife carry this parasite, a study entitled “Investigating the role of wild carnivores in the epidemiology of bovine neosporosis” has just been published in the journal, Parasitology. Stuart and co authors from the Finnish Forestry Research, University College Dublin and the National University of Ireland, Galway investigated the role of wild carnivores including foxes, badgers, pine marten, otters, stoats, mink and feral ferrets in the epidemiology of bovine neosporosis. Peter Stuart, the lead author in the study found that a small portion of foxes, mink, and otters tested positive for the parasite, but that only brain tissue was found to be infected and wild scats (faeces) did not test genetically positive for the parasite, indicating that these animals are intermediary hosts and not definitive hosts for the parasite.
To test for presence of the parasite, Stuart and colleagues collected carcasses of the wildlife species mentioned above throughout Ireland. Some of the animals were roadkill individuals, while others were supplied by hunters. Three types of samples were taken from the animals including blood, brain and faecal samples. Stuart used a commercially available antibody test to check for the presence of Neospora antibodies from the blood samples. Samples that tested positive were also tested for Toxoplasma gondii, a parasite with a similar life history to Neospora.
DNA was extracted from the brain tissues and tested for the presence of Neospora using primers developed to identify the species. DNA extracted from aborted calves, known to be positive for Neospora, was used as a positive control. DNA was also extracted from faecal samples and tested in the same way, following microscopic screening to firstly detect oocysts (eggs), the final transmission stage of the parasite.
Using the antibody test, less than 1% of tested foxes and mink tested positive and when further examined, both were positive for T. gondii. Using DNA extracted from brain samples, less than 6% of fox tissues, 3% of mink brain tissues and one otter sample tested positive for Neospora. The faecal DNA samples were all negative for Neospora, but oocysts resembling Neospora were detected in 28 scats originating from fox, badger and pine marten. In this situation, I would think that the oocysts originated from a different parasite, or that the DNA quantities were too low to detect using the molecular techniques that Stuart used in the study. It may also be possible that the DNA extraction techniques were not extracting enough DNA from the oocysts. I noticed that the DNA extraction technique used for the oocysts was developed for human infection of Cryptosporidium, and that success rate varied between ~60-90% depending on the number of oocysts present in the starting material material. Newer commercial DNA extraction kits combined with more sensitive PCR techniques such as Real-time PCR could help improve the detection rate in the study and also more accurately state whether wild animals are definitive hosts for the parasite.
Stuart also noted that brain tissue provided the highest levels of detection of Neospora. However, the authors noted that two areas within the brain needed to be tested to avoid false negatives or under representing infection levels. Stuart noted that it was unusual to detect Neospora in an otter, but the sample size was small and this was the only coastal otter sampled in the study. The otter was a road killed individual from Dungarvan, Co. Waterford. Stuart suggests that habitat and food availability may play an important role in the transmission of the parasite, but the area needs further investigation.
Stuart was less surprised by the presence of Neospora in the fox as previous studies had shown that the fox predates on small mammals and rabbits, all of which have previously been shown to carry Neospora. Foxes may also scavenge the afterbirth from farmyards, and could be an important source of cross infection in the fox population. Other studies also showed that ungulates may be more efficient hosts for the parasite than smaller animals, thus increasing the parasite load. Stuart also found that the antibody test was less successful at identifying the parasite and recommended that DNA methods be used in future for detection of the parasite. The role of feral cats was not investigated in this study, but could of course also play an important role in farm yards.
The advice to farmers seems to be relatively simple. Don’t leave afterbirth lying around for dogs, cats and foxes to feed on and ensure that aborted calves are carefully disposed of. Another report advises farmers to make sure that their dogs do not defecate near silage pits or on feed stuff as cattle can become infected if they eat contaminated food. Regularly test the herd and remove cows that have positive antigens for Neospora and be cautious of new animals entering the herd. It is recommended that all new animals entering the herd are tested for antigens and should be culled if found to be carriers.
STUART, P., ZINTL, A., WAAL, T., MULCAHY, G., HAWKINS, C., & LAWTON, C. (2012). Investigating the role of wild carnivores in the epidemiology of bovine neosporosis Parasitology, 140 (03), 296-302 DOI: 10.1017/S0031182012001588
