New Test To Identify Screwworm Larvae

The same technology that brought pregnancy test kits home is now being used to help identify larvae of the screwworm, Cochliomyia hominivorax, in the field. The test can differentiate them from those of a related screwworm, C. macellaria, which is not harmful to cattle.

A new test developed by ARS scientists at the Midwest Livestock Insects Research Laboratory in Lincoln, Nebraska, makes quick work of a complex identification process to distinguish screwworm larvae from close relatives in the fly family.

The test is expected to be used at airport terminals and other ports of entry to identify suspicious insects found on dogs, horses, and other animals which can carry screwworm larvae. Currently, specimens must be viewed in a lab under microscopes by trained entomologists.

Called an ELISA, for enzyme-linked immunosorbant assay, the test will make it easier for scientists and agricultural officials in developing countries to identify and track screwworm infestations and accidental re-introductions and to positively identify laboratory populations quickly and easily.

Little or no training is required to use the kit. Just crush a suspect larva in a small container and add a few drops of the kit's enzyme. If the sample turns a distinctive blue color, the larva was a screwworm.

"This test is important because the screwworm larva and its close relatives can be easily confused in the primary larval stages," says ARS entomologist Steve R. Skoda. "This test allows anyone to make a positive identification so steps can be taken to prevent an accidental infestation."

Before its eradication in the 1970s, screwworm was one of the most serious insect pests of the domestic livestock industry. It devastated livestock populations across the southern United States and in Mexico, costing cattle producers hundreds of millions of dollars each year.

The adult female lays her eggs in living tissue, and the larvae hatch and feed on the wound, creating an opportunity for bacterial infections. Other females lay more eggs in the infected wounds and continue the cycle. Many screwworm-infested animals die.

The screwworm has been eradicated from the United States, Mexico, and part of Central America to Nicaragua. It still persists in Costa Rica and Panama.--By Dawn Lyons-Johnson, Agricultural Research Service Information Staff.

Steve R. Skoda is at the USDA-ARS Midwest Livestock Insects Research Laboratory, University of Nebraska, 305 Plant Industry, P.O. Box 830938, Lincoln, NE 68583-0938; phone (402) 437-5267, fax (402) 437-5260.

"New Test To Identify Screwworm Larvae" was published in the September 1998 issue of Agricultural Research magazine. Click here to see this issue's table of contents.