Screwworms all but eliminated in Panama

by Eric Jackson

As the spread of foot-and-mouth disease captures world headlines, another major agricultural plague has been almost entirely eliminated from Panama and the Central American isthmus, with little public notice. An effort led by the US Department of Agriculture (USDA) and the Panama Ministry of Agriculture and Livestock Development (MIDA) has eliminated the screwworm fly from all of Panama except for a few shrinking spots in the Darien, and it is expected that soon we won't have to deal with that pest anymore.

The Ministry of Agricultural and Livestock Development (MIDA) has taken over part of Curundu Heights as its headquarters, and the building from which its efforts against plagues like screwworm flies and foot-and-mouth disease is shared with the US Department of Agriculture.

Dr. John H. Wyss, regional director of the USDA's Screwworm Program, explained that although the fly larvae that feed upon living flesh will eat into any warm-blooded creature, including a human being, the main threat is agricultural. Screwworm infestations cause weight losses, secondary infections and deaths among cattle, and require ranchers to spend money on insecticides, veterinary care, medicines and the labor of extra inspections. The losses can be considerable: now that screwworms have been eliminated in the US, the Department of Agriculture estimates that American livestock producers are saving over $896.1 million per year.

The USDA's battle against screwworms dates back to the days of the Dust Bowl, when unusual weather, economic disruptions and changing migration patterns moved the insects from its southern Texas reservoir, where the flies could survive a winter, to George from where they spread through the southeast. The previous pattern had been for the insects to move north and west from their strongholds near the Mexican border, infesting the American southwest and sometimes hitching a ride on cattle cars as far north as the Great Lakes. Once established in Florida wintering grounds, however, screwworms quickly conquered the old Confederacy during the warm months.

It wasn't until 1933 that scientists even knew what they were dealing with. That's when Doctors Cushing and Patton published a study identifying Cochliomyia hominivorax, the New World screwworm fly, as a distinct species from a look-alike insect whose larvae feast on dead flesh. By contrast, the agricultural pest, attracted by the smell of blood, lays its eggs near open wounds and when the eggs hatch the larvae dig into the flesh, scraping it with tiny hooks and feeding on the ooze coming from their little wounds. Screwworms aren't interested in dead meat.

A series of scientific discoveries ensued. In 1936, Melvin and Bush land figured out how to breed screwworm flies in captivity. After a lull in most non-military scientific research during World War II, in 1950 the earlier work of Doctor Muller came to light showing that exposure to gamma rays disables the flies' reproductive systems but otherwise does not seem to affect them. It was a short step from that discovery to the knowledge that mating with a sterile fly leaves one that's capable of breeding without offspring, and the development of techniques to raise, sterilize and release flies over infested areas in experiments on Florida's Sanibel Island and on Curaçao in the Netherlands Antilles.

The USDA went on the offensive in Florida and the southeastern states from 1957 to 1959, completely eradicating screwworms in that region. Texas ranchers saw a good thing and clamored for a program to get rid of their old nemesis. In 1962, then Vice-president Lyndon B. Johnson inaugurated a sterile screwworm production facility in Mission, Texas and the battle was carried to the southwest. By 1966 the southwestern states were declared screwworm-free.

However, southern Texas continued to get occasional outbreaks, spawned by flies coming across the Mexican border. Thus in 1972, by popular demand from both Texas ranchers and their Mexican counterparts, a binational agreement was signed and the USDA took its offensive south of the border. By 1984 the screwworms were gone, all the way to the Isthmus of Tehuantepec. Then cattle breeders in the Yucatan wanted equal treatment with their northern counterparts, who in turn voiced the same concerns about re-infestations that Texas ranchers once raised. The program was extended to all of Mexico, which was declared screwworm free in 1991.

Agricultural air force: MIDA and the USDA do battle against screwworms from the old Tocumen Airport, where a new facility to breed and sterilize the insect pests will soon be added near the complex.

However, flies kept migrating up from Guatemala, so Belize and Guatemala joined the US-Mexican program, which now operated out of a fly breeding and sterilization center in Tuxtla Gutierrez, Mexico. A series of binational agreements with Panama and the remaining Central American nations ensued over the next few years. Belize and Guatemala were declared screwworm free in 1994, El Salvador in 1995, Honduras in 1996, Nicaragua in 1999, and Costa Rica in 2000.

In 1997 US and Panamanian agricultural agencies joined forces to create COPEG, the Panama-US Commission for the Eradication and Prevention of Screwworms, and opened a fly emergence and dispersal center at the older part of Tocumen Airport. Screwworm pupae that were exposed to radiation were flown in, from the facility in Mexico, emerged flies are kept under control in special rooms where the temperature and humidity are carefully controlled, then dispersed over Panama by a fleet of small aircraft. The program proceeded step-by-step from Chiriqui east, and within 30 months most of Panama's screwworm flies were extinct. To prevent comebacks, the releases have continued for a time after no reproducing flies can be found. Currently releases are taking place from western Chiriqui and Bocas del Toro provinces to near the Colombian border (not too near, so as to avoid exposing agricultural workers to a war zone), and Panama's last wild screwworm flies hold out in the jungle and mangroves around the Gulf of San Miguel.

South America still has a screwworm problem, so it won't be possible to just declare victory and dismantle the program. In fact, there are now plans to build a breeding and sterilization facility near Tocumen, which may very well make agricultural pest control a new export industry for this country. That will depend on whether South American governments are willing and able to join the effort.

For Dr. Wyss, all this has been a sound investment for the United States. "It costs a lot less to maintain a barrier here," he noted. In order to maintain that barrier, he urges people to continue checking their animals — dogs and cats as well as barnyard livestock — for infested wounds, collect sample from worm-infested lesions, treat with commercially available larvacides, avoid transporting animals that have screwworms, and report any incidents to COPEG or other agricultural authorities.

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