Veterinary Researchers Identify Accomplice Of Calf-Killing Parasite

Working in the college's Center for Molecular Medicine and Infectious Disease, David Lindsay and colleagues have demonstrated that the dog is a "definitive host" for Neospora caninum, a single-celled parasitic organism which causes pregnant cows to abort their fetuses.

While the dog was suspected to be a definitive host (defined as an organism which can support all of the life cycles of a parasite) for Neospora caninum, it had remained unproved since it was discovered in 1988. The organism was identified by USDA microbiologist Jitender Dubey and subsequently isolated in cell culture by Lindsay, who was also working at the USDA.

In California alone, the nation's leading dairy producer, Neospora is believed to be the major cause of cattle abortions estimated to cause $35 million a year in economic loss. The parasite also causes central nervous system disorders in horses and dogs.

As a result of the discovery, the scientific team has recommended producers use fences and adopt cattle management strategies designed to keep pet and stray dogs from defecating in dairy feedlots or pastures. The work was published in the August 14, 1998 issue of the International Journal of Parasitology.

The parasite is passed through cattle populations as dogs or wildlife ingest placental tissues or aborted fetuses discovered in fields and shed oocysts contaminated feces. These feces are in turn eaten by other ruminants, which are subsequently victimized as the parasite continues its life cycle.

Lindsay and the other researchers have drawn their conclusions based upon experimental work in which the parasite was successfully passed from a group of Neospora infected mice, through a dog that passed oocysts, and into another group of experimental mice. Oocytes are protected by extremely durable capsules which are so tough they can withstand bleach, Lindsay says. The only thing that destroys them is high temperature and extreme dryness, he said.

While parasites are not all disease causing organisms per se, they all possess varying degrees of pathogenicity, depending upon how the various life cycles of the organism behave within the host.

"Biologically, it doesn't make sense for a parasite to destroy its host," said Lindsay, a parasitologist who specializes in studying the molecular interactions between parasites and hosts. "A parasite doesn't want to cause disease. It just wants to maintain itself. If the parasite kills the host, then it is threatening its own survival in the long run."

In the case of Neospora caninum, sporozoites, a life stage which follows oocysts, spread out throughout the body systems of the host, penetrating intestinal walls in some cases and central nervous system tissues in others.

Humans are not believed to be susceptible to Neospora, Lindsay says, although experimentally infected primates have demonstrated their suitability as host organisms.

Besides domestic and stray dogs, coyotes and foxes are also believed to harbor the organism, Lindsay said.

While the discovery provides sound scientific justification for the new cattle management recommendations, Lindsay says the true value of the work is the foundation it provides for future scientific research.

Those efforts might include work on the development of an immuno-parasitic approach to containing the parasitic disorder, where host animals are challenged to develop an immunological response to the parasites. Lindsay estimates such an approach could take five to ten years to develop.