Screwworm Program

Brief history of the Screwworm Program in Mexico and Central America

The Screwworm Eradication Program is a long-term project, developed by the Animal and Plant Health Inspection Service of the United States Department of Agriculture (USDA-APHIS). The Program had its origin in the southern United States where the livestock industry was suffering great losses due to the damage caused in screwworm-infested cattle. The United States successfully eradicated screwworm in 1966. Since then, the livestock industry has grown, producing a better quality product with higher market prices resulting in profits of hundreds of millions of dollars for both producers and consumers.

In 1972, Mexico and USDA-APHIS initiated a Screwworm Eradication Program in order to establish a biological barrier farther south, covering the area from the US-Mexican border to the Isthmus of Tehuantepec with sterile screwworm flies.

Later, the Program was expanded with the goal of covering the entire Central American Isthmus and Panama and eventually reaching the Darien Gap. The Screwworm Eradication Program was initiated in Guatemala in 1987, Belize in 1989, El Salvador and Honduras in 1991, Nicaragua in 1992, Costa Rica in 1995 and Panama in 1997. The success of the Program has resulted in the eradication of screwworm from the United States in 1966, Mexico in 1991, Belize and Guatemala in 1994, El Salvador in 1995, Honduras in 1996 and Nicaragua in 1999. On October 4, 2000, the President of Costa Rica, Dr. Miguel A. Rodríguez accompanied by Ambassador Thomas J. Dodd, U.S. Ambassador to Costa Rica, declared Costa Rica free of screwworm.  Assisting in the ceremony were Minister of Agriculture, Ing. Alberto Dent and Under Secretary of Agriculture from the United States, Mr. Michael V. Dunn. Panama was declared free of the screwworm in 2006.

What exactly is Screwworm?

A screwworm infestation is caused by larvae of the fly Cochliomyia hominivorax. These larvae can infest wounds of any warm-blooded animal, including human beings. The screwworm fly is about twice the size of a regular house fly and can be distinguished by its greenish-blue color and its large reddish-orange eyes.

Infestations can occur in any open wound, including cuts, castration wounds, navels of newborn animals, and tick bites. The wounds often contain a dark, foul-smelling discharge. Screwworm larvae distinguish themselves from other species by feeding only on the living flesh, never dead tissue. Once a wound is infested, the screwworm can eventually kill the animal or human, literally eating it alive.

How can screwworms be eradicated?

The Screwworm Eradication Program takes advantage of two simple biological factors: 1) the male screwworm fly is very sexually aggressive, and 2) the female only mates once in her lifetime. Using a highly sophisticated technique, the pupae are exposed to a low dose of atomic radiation which inhibits the development of the ovaries in females and the testes in males without affecting any other body part. This results in normally developed but sterile adult flies. The Program disperses sterile flies where screwworm flies are indigenous. The sterile males mate with fertile wild females which results in non-viable egg masses and interrupts the insect’s life cycle.

In order to achieve eradication, the Program supported a fly production facility in Mexico where hundreds of millions of pupae are produced each week and transported to the countries where eradication programs were being carried out. The pupae were placed in large units with a controlled environment adequate for the development of the flies. The flies are later dispersed using specially equipped planes. In 2006 a new sterile fly production facility was built in Panama and sterile flies began to be dispersed from that facility along the Panama / Colombia border in 2009, in order to maintain a sterile fly barrier protecting Central America, Mexico and the U.S. from screwworm re-infestations.

In the case of Costa Rica, an average of 60 million sterile flies were dispersed weekly over every area of the country. The sterile flies were dispersed at an altitude of 6000 feet at a rate of approximately 3000 per square nautical mile.

Screwworm Program Benefits

The overall cost of the Screwworm Eradication Program in Central America has been approximately 200 million dollars; 41 million dollars in Costa Rica. The benefit-cost ratio of the Program in Costa Rica ranged from 4.8 to 12.8. In other words, if the Program in Costa Rica cost 41 million dollars, the benefits received ranged from 168 to 448 million dollars. Once screwworms were eradicated in Costa Rica, the direct benefit to the producers was estimated at over 13 million dollars per year (in 1997 dollars). Taking into consideration the direct economic benefits to the producers as well as the impact on the consumer, a study done by Texas A&M University indicates that the overall benefit of the Screwworm Eradication Program to Costa Rica is more than 50 million dollars per year (in 1997 dollars).

The governments of the United States of America and Costa Rica shared program costs the same as in the rest of Central America, contributing 85% and 15% respectively. The contribution of the United States in Costa Rica was equivalent to approximately 28 million dollars, non-reimbursable.

Although the Screwworm Eradication Program mainly focused on the agricultural sector and the positive results are most obvious in the livestock industry and the national economy, there have been great benefits to the public health sector. In Nicaragua, for example, Program personnel reported 138 cases of screwworm infestations in humans, 70 of which were children. Three of the affected persons died and two other lost body parts. In El Salvador there were 530 cases in humans between 1990 and 1992.

It is also important to point out the Program’s impact on the environment. In Costa Rica, a country known for its natural beauty and growing eco-tourism, the screwworm fly also affected wildlife populations. The program therefore helped improve the environment and the health of wild animals by reducing their suffering and aiding in the preservation of endangered species.