Researchers search for solutions to drug-resistant parasites
Posted Jan. 1, 2004
A serious health problem in goats has researchers scrambling to come up with solutions. A high percentage of gastrointestinal parasites that reside in these animals have become resistant to multiple anthelmintics, and the situation is rapidly getting worse.
"Resistance is getting worse and worse, so we need to take a long-term view approach," says Dr. Ray Kaplan, a parasitologist at the University of Georgia and one of the researchers looking into the problem.
During the past 35 years, numerous case reports and studies on the prevalence of anthelmintic resistance in goats in various countries have been published. A study in the Aug. 15, 2003, JAVMA (page 495) highlighted the problem of highly prevalent resistance in the southern United States.
To curtail the problem, researchers from the southern United States and Puerto Rico have banded together to develop educational programs and pursue research into novel methods of parasite control. They have also been fortunate to gain the help of several international colleagues. This group includes researchers from the University of Georgia, Fort Valley State University (Fort Valley, Ga.), Louisiana State University, U.S. Department of Agriculture offices in Florida and Arkansas, University of Puerto Rico, Danish Center for Experimental Parasitology, and University of Pretoria and Onderstepoort Veterinary Institute in South Africa.
One tactic is promoting "smart drenching" techniques, such as withholding feed for 24 hours before administering a drug. "By withholding the feed, you slow down the flow of digestive material, and by slowing this movement, you increase the amount of time the drug has contact with the parasite," Dr. Kaplan says.
Methods include using proper dose and quarantine-treat-test protocols for new additions. Improving pasture management and reducing stocking rates, which can minimize the concentration of infective larvae on the pasture and decrease the spread of the disease, have also proved helpful.
In the Aug. 15 JAVMA study, scientists recommended that practitioners deworm only animals that are clinically affected. This works because parasitic infections are always aggregated within herds, with 20 percent to 30 percent of the animals harboring 70 percent to 80 percent of the worms. Treating only a percentage of the animal population reduces the number of drug-resistant parasite eggs shed in a pasture.
Recently, researchers have started using a tool in the field that is proving effective in identifying infected goats. The tool, called FAMACHA, helps identify animals clinically affected by Haemonchus contortus, which is, by far, the most pathogenic worm species in goats and sheep in the United States. It accounts for roughly 80 percent to 90 percent of the parasite eggs shed in sheep or goat feces in the southern states and slightly less in cooler parts of the country, according to Dr. Kaplan.
The tool is a simple chart that allows individuals to determine the degree of anemia by comparing the color of an animal's ocular mucous membranes to colors of eyes on a chart. The level of anemia is a telltale sign of the degree of parasitic infection with H contortus.
Developed and used by scientists in South Africa, this tool is now also being used in the United States, Uruguay, Paraguay, Brazil, Mexico, Ethiopia, Namibia, and Malaysia, and training is being planned for Uganda, according to Dr. Jan van Wyk, a veterinarian at the University of Pretoria's Faculty of Veterinary Science. He says the system has proved to be effective in sheep and has only recently been tested in goats. While more studies are needed, Dr. van Wyk says, "Results of testing in goats also holds much promise, even though it is somewhat more difficult to apply in goats than in sheep."
Dr. van Wyk's research group provides the initial training to individuals in other countries, and they train others how to use the chart. Dr. Kaplan and several of his colleagues, for example, are currently teaching others in the United States. The tool is available at a minimal price that covers manufacturer's costs. For more information, email firstname.lastname@example.org.
Controlling parasites without drugs
In addition to smart drenching techniques, researchers are looking into several other novel methods of battling the drug-resistant gastrointestinal parasites in goats. They are testing a fungus found in soils throughout the world that, when fed to goats in large amounts, can help to control gastrointestinal parasites.
"If you feed this to animals, it will accumulate in their feces and kill the parasite larvae in the feces, so they can never get out onto the pasture," Dr. Kaplan says. "In the laboratory, we can get a reduction of 90 percent in the number of larvae."
While the technique has also proved to be environmentally safe, field studies of the technique's efficacy in sheep and goats have not yielded such spectacular results. "It could be the weather, being that it is hot in the southern U.S. and the fungus prefers a cooler environment," Dr. Kaplan says.
Feeding bioactive forages is another possibility. Many types of forage contain condensed tannins, and some studies have shown that these can reduce worm numbers in the animals. Others have shown that tannins reduce the number of eggs voided in a pasture.
In addition to these feed methods, researchers are examining the efficacy of using copper wire particle boluses to control the parasites, with the hopes that copper particles will be toxic to the Haemonchus worms, Dr. Kaplan says.
Pursuing new methods of controlling gastrointestinal parasites in goats is imperative, because it is unlikely that the pharmaceutical industry is going to invest in developing new drugs for goats. Industry doesn't perceive the market as being large enough, Dr. Kaplan says. This is understandable, says Dr. van Wyk, since it costs an estimated $230 million to discover and bring a wide-acting new anthelmintic drug to the market.
"We are in a situation where we have severe drug resistance and no new drugs coming down the pipeline. There have been recent reports of total anthelmintic failure in some areas of the world. The situation here in the southern U.S. is not much better," Dr. Kaplan says. "We have only one or two drugs that still work on many farms, and it is critical that we maintain their efficacy. The only way we can do that is by using the drugs differently—with the view that they are highly valuable and limited resources that must be preserved."
He also holds hope for some of the new techniques currently being studied.