Issue 30, July 2007
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Parasite Lost

People rarely swing wrecking balls at their homes. After all, why would they? And with a few exceptions, parasites have a similar appreciation for maintaining their environments and therefore the lives of their hosts––at least until those lives no longer benefit the parasites.

But even a few exceptions can cause serious problems.

For example, parasites of agricultural animals can impair the ability of their hosts to thrive and grow, a situation that causes physical discomfort for the animals and economic distress for producers. Researchers with the Agricultural Research Service (ARS) have made significant strides toward addressing these problems.

Researching Resistance

Parasitic nematodes are abundant, highly adaptive worms that can be found in water and soil and within the bodies of their many host species. Most cattle become fairly resistant to gastrointestinal nematodes. But a few remain susceptible.

"Those few cattle are responsible for the bulk of parasite-shedding within a herd, and their susceptibility can eventually harm their health and that of the herd," says ARS researcher Louis C. Gasbarre.

Gasbarre is the research leader of the Bovine Functional Genomics Laboratory at the Henry A. Wallace Beltsville Agricultural Research Center (BARC) at Beltsville, Md. Researchers there have demonstrated that cattle's susceptibility to gastrointestinal nematodes is largely influenced by genetic heritage. Gasbarre and his colleagues have identified eight locations within the bovine genome that are related to resistance. Now they're trying to pinpoint exactly how genetic variations contribute to high or low resistance.

Their ultimate goal is to create a genetic test to identify which animals are most susceptible to these nematodes, Gasbarre says. This would enable producers to target highly susceptible animals for treatment, which would reduce unnecessary costs. It would also reduce the probability of nematodes developing resistance to treatment, a situation that has become increasingly prevalent and problematic significant in recent years.

Internal parasites caused serious problems for cattle producers before effective treatments were developed, Gasbarre says. Few drugs were available, and many of those on the market were fairly toxic. The invention of safer, more effective drugs enabled producers to reduce or eliminate the time-intensive management practices they'd relied on to control livestock parasites in the past.

Producers quickly realized that even healthy cattle grew better when treated with the new drugs, which led to more widespread administration. Unfortunately, their enthusiasm had a downside: increased drug-resistance in the parasites. Genetic research could identify which cattle require more intensive treatment, which could ultimately lead to more targeted drug administration and reduced resistance in the parasites.

Protecting Pregnancies from Protozoa

Another serious livestock pathogen is the protozoan parasite Neospora caninum, which infects many agricultural animals. N. caninum causes neosporosis, the most common cause of abortion in dairy cattle in the United States. The economic impact of neosporosis-induced abortions is estimated to be about $35 million annually in California alone.

For years, N. caninum had been confused with Toxoplasma gondii, a similar parasite that causes birth defects and abortion in mammals. Then in 1988, ARS microbiologist Jitender P. Dubey, at the BARC Animal Parasitic Diseases Laboratory (APDL), recognized that a disease causing paralysis in dogs was not caused by T. gondii, as suspected, but by the previously unidentified parasite, N. caninum. Nearly two decades of research have significantly expanded our understanding of these protozoa.

Dubey and his colleagues linked the newly discovered N. caninum to an outbreak of cattle abortion the following year, and discovered that the parasite was a significant cause of abortion in cattle in many countries. Infected animals don't always show symptoms, so the scientists developed tests to diagnose the parasite's presence.

With a research team led by Milton M. McAllister, an associate professor at the University of Illinois at Urbana-Champaign, Dubey found that dogs are an important N. caninum vector for cattle. Neosporosis can also be transmitted congenitally, resulting in abortion, birth defects, and infection. Even if they show no symptoms, calves that are born to infected mothers may pass the parasite on to their own offspring, says APDL animal scientist Wenbin Tuo. Unfortunately, no proven methods are currently available to prevent transmission of the parasite.

"Vaccine development for all protozoan parasites is highly challenging," Tuo says.

He and his colleagues are working to develop a vaccine against bovine neosporosis. They are using genetic and immunological technologies to identify and characterize potential vaccine candidates. Recent studies with mice and sheep revealed that an antigen-based vaccine can offer protection by preventing mothers from passing the parasite across the placenta. This ongoing research could eventually lead to vaccines that offer protection against neosporosis in cattle.

Bird of Parasite

Poultry research is another important aspect of APDL research.

"We're using fundamental research to help solve practical problems with avian coccidiosis and other economically important diseases," says immunologist Hyun S. Lillehoj. "Our work has helped the poultry industry not only by providing fundamental information about host-pathogen interaction, but also by developing important anti-parasite vaccines and alternative control strategies."

For more than two decades, Lillehoj and her colleagues have investigated the interaction between pathogens such as Coccidia––the tiny intestinal protozoan parasites that cause coccidiosis––and their hosts. This work has led to international research and development collaborations, new vaccines, and several patents on non-drug disease management.

This is particularly important because some avian parasites, including Eimeria species such as Coccidia, have developed resistance to the drugs used to treat them. So effective alternatives––such as administering recombinant vaccines to chicken embryos, boosting immunity through dietary supplements, or using antibodies to provide passive immunity––can give managers some essential support.

"Basically, we're trying to take an integrated approach, using immunology, genetics, and functional genomics, to develop alternative control strategies," Lillehoj says.

Her genetic research has identified two quantitative trait loci on the chicken genome that are related to avian coccidiosis. She and her colleagues are currently investigating single nucleotide polymorphisms that may explain why different chicken strains have different reactions to infection and vaccination.

Lillehoj's research has drawn the attention of scientists, companies, and industry members from around the world. Although the scientists focus on coccidiosis, genomics and immunological technology developed at APDL are helping to solve a variety of avian health problems, including influenza and clostridium and salmonella infections.

These programs offer a few examples of the many parasite research programs conducted through ARS National Program #103: Animal Health. For more information on any of these programs, please contact Jitender P. Dubey, Louis C. Gasbarre, Hyun S. Lillehoj, Wenbin Tuo or national program leader Cyril Gay.