Neospora caninum is an obligate intracellular protozoan that infects a wide variety of mammals and causes the disease neosporosis. There are several developmental stages of the parasite which differ in size and distribution. The rapidly dividing tachyzoite stage (pictured here) is found within many different cells of the host and is about 6 x 2 microns in size. Tissue cysts are round to oval, about 100 microns in diameter, and found primarily in nervous tissue. The oocyst stage is round, about 10 microns in diameter, and found in feces excreted from definitive hosts of the parasite. Under the light microscope, tachyzoites, tissue cysts, and oocysts are similar in many respects to other protozoa including Toxoplasma gondii.
The parasite appears to be spread by two routes, although only one has been proven so far to occur in nature. This route is congenital transfer of the rapidly multiplying developmental stage of the parasite, called tachyzoites, from the mother across the placenta, to the fetus. Although abortion can occur after tachyzoites infect the fetus, many calves are born with no clinical signs of neosporosis. Even still, these calves are infected and are capable of transmitting the parasite to their offspring. The other route of infection, which has been shown only under experimental conditions, is infection of cows with oocyst stage of the parasite. Neospora caninum oocysts, similar to Toxoplasma gondii oocysts, are very resistant to harsh environmental conditions, and can remain infections for many months in the environment. Oocysts are shed by animals that are termed "definitive hosts". For Neospora caninum, the dog can serve as a definitive host, but it is possible that other canids (for example, foxes) can also produce oocysts. It is believed that, similar to the cat in toxoplasmosis, canids eat tissues of animals that have muscle stages (called tissue cysts) of Neospora caninum. Once in the gut digestive system, the parasites are activated and burst out of the tissue cysts to start the life cycle which leads to production of oocysts in the intestine and excretion of these stages in feces. It is believed that cattle ingest oocysts that contaminate feed or pastures and then become infected.
Neosporosis is an infectious disease primarily of dogs and cattle. Due to the similarity of Neospora caninum to Toxoplasma gondii, neosporosis for many years was misdiagnosed as toxoplasmosis. Neosporosis was first described in dogs in Norway in the mid-1980's as causing neuromuscular degeneration leading to hind limb paralysis. Neosporosis now appears to a major cause of abortion in dairy cattle worldwide. The disease has been found in many countries around the world, but appears to be an important cause of reproductive failure in cattle in the U.S., Netherlands and New Zealand.
Due to the lack of knowledge concerning both the exact life cycle of Neospora caninum and immunity to the parasite, no effective preventive regimen can be recommended. Research at the ARS has shown that vaccination with Neospora tachyzoite antigens can protect against congenital transmission of the parasite in mice. One preventive treatment that may become available is vaccination of cows with parasite antigens to induce protective immunity against oocyst infection or activation of tissue cyst stages. Research at the ARS is also directed at more complete understanding of the life cycle of Neospora caninum so that a link in the life cycle may be broken.
The parasite causes an immune response in animals that it infects. One immune response is production of antibodies which bind to the parasite. Blood can be drawn from animals and tested by different methods, such as immunofluorescence (IFA), enzyme-linked immunosorbent assay (ELISA), or an agglutination test, for antibodies to Neospora caninum. The parasite also elicits cellular immune responses in hosts that it infects. These immune cells, in particular, T lymphocytes, release cytokines that are involved in the response of the host against the parasite.
Sensitive methods to detect parasite DNA, such as the polymerase chain reaction (PCR), are also being employed to detect oocysts in the environment and to diagnose infection in dogs, cattle, and small rodents. PCR uses small pieces of DNA that recognizes and bind to specific sites on genes, in this case, of Neospora caninum. Parasite DNA, hence the parasite itself, can be detected in DNA extracted from nervous tissue using enzymes that produce large quantities of the gene of interest. This procedure is being used to identify oocysts in environmental samples taken from dairy farms that may have experienced an outbreak of neosporosis-associated abortion.
Neosporosis has been found in both dairy and beef cattle, although most abortion outbreaks have been reported in dairy cows. The causative organism, Neospora caninum appears to have a world-wide distribution having been reported in North and South America, Europe, Scandanavia, Africa, the Middle East, Asia, Australia and New Zealand. In some cases, all of the cows in a herd appear to have been exposed to the parasite. Neospora caninum is transmitted from infected cows to offspring by congenital infection. It is thought that Neospora caninum can be transferred this way during pregnancy due to 'reactivation' of the cyst form of the parasite, followed by entry into the bloodstream, and transport across the placenta to infect the fetus. Although abortion is often observed, most calves are born infected with the parasite and yet show no clinical signs of disease. These calves can then transmit the parasite to their own offspring during gestation. Epidemiological studies of neosporosis may provide clues on how Neospora caninum enters a dairy herd and what animals are likely sources of the oocyst stage of parasite.