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The new world screwworm in Mexico and Central America

Distribution in central and south American countries
Control and eradication programmes in Mexico
Activities in Central America
Economic aspects
Disease prevention

M. Vargas-Terán

The New World screwworm (NWS) has existed on the American continent from ancient times to the present. The diffusion of NWS in Mexico and beyond the border into the southern United States led to a signed eradication agreement between the two countries in 1972. The success of this international cooperation, which culminated in Mexico being declared NWS-free in 1991, is one of the greatest achievements in animal health in this century. The sterile insect technique is now in use to eliminate the fly in other areas of Central America. Significant difficulties will be encountered, however, including inaccessibility of the areas that the fly inhabits, language barriers, and the need for constant control of the movement of live animals to the pest-free zones. Eradication of NWS is important for increasing livestock production for a growing population without encroaching on the forests, jungles and environment of Central and South America.

The author is Animal Health Officer (Insect Control), Animal Production and Health Division, FAO, Rome.

The New World screwworm (NWS), Cochliomyia hominivorax (Coquerel), has existed on the American continent (see Figure 1) from ancient times, and its name is found in all the principal indigenous dialects. References to the species have also been found in some documents from the time of the Spanish American colonies. Over North and South America the NWS is known by various local names, the most common being screwworm, queresa, gusanera, bichera and bicheira. In North America NWS was one of the most harmful ectoparasites of wild and domestic animal populations and even sometimes of human beings (see Figure 2). In Central and South America NWS is still an obstacle to increased animal production. In Mexico two other species of parasitic Diptera follow NWS in importance, in the northern plains Hypoderma bovis (locally called barro del ganado) and in the tropical southern zones Dermatobia hominis (colmoyote). The latter undoubtedly competes in animal and public health and economic importance with the NWS in the countries of Central and South America.

1. Geographical distribution of NWS in the Americas before implementation of eradication programme - Répartition géographique de la lucilie bouchère avant l'exécution du programme d'éradication - Distribución geográfica del gusano barrenador del ganado antes de iniciarse el programa de erradicación

2. Nasal myiasis from NWS in a man - Myiases nasales causées par la lucilie bouchère chez un homme - Miasis nasal causada por el gusano barrenador del ganado a un hombre

Photo/Foto: Drs Torres & Guerrero-Sotres, Mexico

As an obligatory parasite in warm-blooded animals (see Figure 3), the NWS inflicts damage that causes deaths of newborn livestock by frequent infestation of the navel, diminished growth, susceptibility to secondary infections and diminished weight gain and milk production. NWS can also damage skins and pelts and can generally reduce the production of protein of animal origin for human consumption. Such effects necessitate expenditures for increasing the work force needed to care for animals and prevent infestations, the purchase of antibiotics and insecticides, confinement of sick animals, veterinary honorariums and additional time for selling of animals. In wildlife the effects of NWS can accelerate to such an extent that a species can become extinct, with catastrophic consequences. Populations of common wildlife are mainly attacked through infestation of the navel. A loss of 80 percent of white-tailed deer fawns was reported in Texas from this type of infestation. The eradication of this parasite in the United States has made it possible to diversify rural production by raising wildlife for the market; hunting is also becoming an important economic activity in the southern United States.

3. Wound of adult bovine infested by NWS - Plaie causée par la lucilie bouchère chez un bovin adulte - Bovino adulto con una herida infestada por el gusano barrenador

Photo/Foto: Mexico-US Commission

The serious veterinary and economic consequences of this parasitosis depend on the number of animals, the size of the area concerned, the animal species involved, husbandry techniques, the density of the cattle-raising areas, the presence of other ectoparasites or diseases that make wounds and the time of year. The wild NWS population fluctuates during the year, being most abundant in the most hot and humid season.

Infested wounds are treated using diverse products. Home remedies such as vegetable oil or animal fat mixed with tobacco or other plants are smeared on the wound to achieve larval asphyxia. Petroleum derivatives are used to provoke irritation of the larvae on the surface of the infested wound and effect their exit. More sophisticated is the use of organophosphate larvicides to cause intoxication and death of the larvae. This last method has proved to date to be the most effective. Regarding the efficiency of systematic insecticides against NWS there is not yet sufficient information.

4. Biological barriers with NWS sterile flies - Obstacles biologiques grâce aux mouches stériles - Barreras biológicas con moscas estériles del gusano barrenador del ganado

Prevention and control measures aim to preclude the movement and introduction of infested animals into the NWS-free zones, to reduce the numbers of wounds and to treat those detected with a cicatrizant product that contains a non-repellent insecticide to avoid reinfestation but also to eliminate adult flies that land on the wound. It is recommended that those animal husbandry practices requiring the wounding of animals be carried out during the dry and cold months, and that if possible grazing be allowed during the night.

Distribution in central and south American countries

Belize, Guatemala, Mexico and the United States are the only countries of the American continent that have established programmes aimed at the control and eradication of the NWS. Consequently, there is no accurate information available about its seasonal distribution, geographic distribution and affected species. It would be useful to have this information to be able to establish control and to effect eradication strategies, possibly at a national level and with bordering countries.

Control and eradication programmes in Mexico

In 1966, a cooperative programme was initiated between the United States Department of Agriculture (USDA) and the Secretary of Agriculture and Livestock (SAG) of Mexico. This programme affected the states on both sides of the United States-Mexico border. Its objective was to control and reduce economic losses caused by the NWS and to avoid the pest's natural migration toward the control and eradication zones in the United States. These zones were protected by a biological barrier of sterile NWS flies (see Figure 4) along almost the entire 2000 km frontier.

In spite of these efforts, the success expected was not achieved because of the presence of microclimates where the parasite could overwinter, the impossibility of controlling wildlife migration and the absence of natural barriers that could impede the parasites' movement. Therefore during 1966 and 1967 the veterinary authorities of both countries had to study the seasonal abundance and geographical distribution of the parasite in Mexico in order to evaluate the feasibility of establishing an eradication programme. When the study was completed, the parasite was found to be distributed over 95 percent of the country, with a greater abundance in coastal areas and the south. The exceptions were the mountainous snowy areas and the Altar Desert in the state of Sonora, where no screwworms were found. Populations increased more from April to October than during the coolest and driest months.

Control activities included attempts to encourage livestock owners to inspect their animals, to send larvae samples from the animals' wounds to the programme officers and to treat the wounds with an adequate insecticide to reduce the wild population of NWS. The results, however, were not satisfactory. It was decided, therefore, to programme a new strategy.

By the end of the 1960s, when the union of Mexican livestock owners became aware of the preliminary results of the distribution study, they were alarmed at the wide distribution of the parasite in the country. At the same time, they learned from their American counterparts of the effectiveness of the sterile insect technique. They also learned of the American livestock owners' concern over the constant reinfestations that were occurring in United States territory through natural migrations of the parasite. This clearly demonstrated the failure of the sterile fly barrier.

The respective associations of Mexican and United States livestock owners, therefore, decided to request their governments to take combined action in both countries to eradicate the screwworm from the territories. Finally, after innumerable negotiations, inauguration of the Mexico-United States Commission for the Eradication of the Screwworms (henceforth referred to as "the Commission") took place on 28 August 1972.

The Commission

The Commission is governed by a Board of Directors formed of officers from both countries. The United States contributes 80 percent of its budget and Mexico 20 percent. This proportion was determined by the number of animals that would finally benefit in both countries. The Commission has functioned under a counterpart system in the departments of insect mass production, field inspection animal movement control and quarantine, air operations and administrative services.

The initial objective of the Commission was to eradicate NWS in the northern and central regions of Mexico as far south as the 93° meridian. Here at the narrowest point of Mexico (the Tehuantepec Isthmus) a smaller and less expensive biological barrier than one along the northern border of the country could be established with sterile flies (see Figure 4).

Once the organization had been established and professional personnel had been recruited to carry out its various services, the intensive and systematic monitoring of the disease began with the free distribution of sample collection tubes and organophosphate insecticides for the treatment and prevention of infestation in wounds (see Figure 5). It was decided that the sterile fly factory would be located in the central part of the sterile insect barrier, and construction began in Tuxtla Gutierrez, Chiapas in February 1974. The original plan to build the factory in three sea vessels was discarded because of the high operation costs involved.

5. Sample larvae kit distributed free for the monitoring of the disease and for local treatment of wounds - Matériel distribué gratuitement pour suivre la maladie et traiter localement les plaies - Tubo colector para larvas de distribución gratuita usado en el monitoreo de la enfermedad y el tratamiento local de las heridas

Photo/Foto: Mexico-US Commission

The strategy for initiating the eradication was based on epizootiological characteristics, geographical and seasonal distribution of the disease, field inspection resources and production of sterile insects available from the plants in Mission, Texas and Tuxtla Gutierrez. The latter plant initiated production in August 1976.

The decision was made to fight the parasite from the wide area in the north toward the narrower area in the south, commencing from the Baja California peninsula, where the last positive case was reported in June 1979. During 1981 and part of 1982 the northern states of Mexico were liberated from NWS, and the last six positive cases were registered in the United States in 1982. The Mission, Texas plant was closed to avoid the escape of fertile insects in the screwworm-free zone. The installation and equipment were conserved as a strategic resource in case of eventual reinfestation. At the same time the Tuxtla Gutierrez plant increased its monthly production to the record level of 600 million sterile flies per week, which were dispersed in the central part of Mexico in 1982 and 1983. In 1984, a year before schedule, the biological barrier of sterile insects was established in the Tehuantepec Isthmus, thus achieving the primary objective of the Commission. In February 1991 eradication was complete and the country was declared free of NWS.

From 1972, when the Commission was set up, to February 1991, 286752 positive cases of NWS were reported in Mexico. The species most affected, in order of importance, were young bovine (less than six months), adult bovine, equine, porcine, ovine, caprine canine, feline, avian and other. Occasionally cases were found in human beings. The sites most frequented by the NWS were navels of newborn livestock; wounds from castration and dehorning; wire cuts; wounds from tail-cutting, shearing and branding; dog, vampire bat and insect bites; lesions of ear and eye; plus a group of wounds classified as "other type" unspecified by the livestock owners.

Critical lines

During the course of the programme, once NWS was eradicated from a defined geographical zone a critical line (see Figure 6) would be established dividing the freed zone from control zones. Along this critical line a sterile insect barrier would be set up as a protection from reinfestation. Quarantine belts were also created to avoid the introduction of infested animals.

6. "Critical lines" dividing Mexico's screwworm-free northern zones from southern control zones - Mexique. «Lignes critiques» séparant les zones du nord exemptes d'insectes des zones du sud sous contrôle - Las "líneas criticas" dividen, en México, las áreas libres del norte de las áreas bajo control del sur

Programme difficulties

Obstacles encountered throughout the eradication programme were:

· the enzootic character of the disease and its fluctuating presence throughout the year;

· the diversity of climates and microclimates in the mountains and canyons where the parasite can survive, and the inaccessibility of these locations;

· language barriers with some ethnic groups;

· difficulties in controlling the movement and transport of live animals;

· the operational challenge of reaching a maximum production of sterile flies;

· transport of sterile flies over long distances and systematic and continuous aerial dispersion.

The problems of producing sufficient good quality sterile flies economically were solved by the construction of an extra insect colony and the improvement of methods for rearing insects. Language difficulties were overcome by the employment of a bilingual inspection team with knowledge of the work areas and the livestock-raising activities of the zone. Regional and national information programmes relating to diagnosis, control, eradication and prevention were adapted for more than 20 dialects.

It was necessary to determine where the commercial livestock routes should be set up and to establish quarantine stations functioning 24 hours per day. At these stations an average of two animals per minute were inspected; in all more than 5000 animals per week passed through these stations from the infested areas. The bulk transport of pupae was changed from DC-6 aircraft to refrigerated trucks of large capacity. This was to avoid the constant interruptions in insect delivery to the packaging and dispersal centres caused by mechanical maintenance problems and adverse meteorological conditions for flying. In addition, the C-45 and DC-3 aircraft originally used to disperse the flies were later replaced by a fleet of smaller aircraft, such as the Cessna-206 and PA-31 (see Figure 7). This reduced the constant mechanical failures and diminished the duration of the missions so that the quality of the dispersed sterile flies was improved.

7. Loading of small aircraft (Cessna-206) for release of cardboard boxes containing sterile NWS flies - Chargement d'un petit avion (Cessna-206) transportant des boites en carton contenant des mouches stériles - Carga de una avioneta (Cessna-206) con cajas de cartón que contienen moscas estériles del gusano barrenador del ganado aura su dispersión

Administratively, it became essential to decentralize the control of activities to the five regions of the country. In addition, the hydroponic media production system was replaced by the use of gel media. This achieved savings in sterile insect production without diminishing biological quality.

New eradication goal

Two factors, first the success of the results so far obtained and second the constant risk of reinfestation of the freed areas in the United States from the enzootic zones of southeastern Mexico and Central America, caused the Commission to modify its original agreement. This modification was insisted upon by the organized cattle ranchers of the United States, Mexico and Central America. Consequently, a medium-term plan was agreed to whereby eradication activities were continued throughout the rest of Mexico, Belize and Guatemala. Once all the countries of Central America were freed of screwworm, a sterile insect production plant and biological barrier (see Figure 4) were established in the Isthmus of Panama, thereby taking advantage of geographical and ecological factors and the convenient position of the isthmus for ground communications with the countries of South America.

Activities in Central America

Puerto Rico. In 1974 an eradication programme was initiated on the island of Puerto Rico and sterile insects were transported from the plant in Mission, Texas. The disease was widely distributed, and one of the biggest problems in controlling and eventually eradicating NWS was the population of stray dogs, which were difficult to inspect and treat. However in 1975 eradication of NWS was fully achieved.

Curaçao. In 1954 the island of Curaçao, 65 km from the coast of Venezuela, was free from the parasite for the first time. However, because of a lack of control measures, reinfestation subsequently occurred. In 1975 NWS was again eradicated from the island.

Guatemala. In December 1986 the Government of Guatemala signed a technical and scientific cooperation agreement with the Commission, marking the beginning of the eradication programme. The country was divided into three large regions to facilitate inspection and the dispersal of sterile insects. These activities began in 1987 in the northern region (Peter) and continued until the whole country was covered. The results were excellent, as shown by the reduction in positive cases recorded. Difficulties in the operation have been numerous, the most outstanding being the aerial dispersal of insects because of the high altitude of the mountains, which are permanently covered by clouds. Other difficulties occur in locating ecological niches in the jungle and mountainous zones because of the difficulty in communications. These problems have been solved by using strategic dispersal of insects and by implementing inspection brigades in problem zones.

Belize. In August 1988 the Commission and the Government of Belize signed an agreement to eradicate NWS from Belize. Good results have so far been obtained. Very soon after the dispersal of sterile insects and control work began in 1989, there was a marked reduction in cases presented. One of the biggest challenges to eradication has been obtaining information about the wild NWS population because of the inaccessibility of the jungle areas. In the southern part of the country a system for dispersing flies without boxes and on a large scale was evaluated. The flies are maintained in a lethargic state at a temperature from 3.5 to 5.5°C and are dispersed from the aircraft in bulk with a special chute. The preliminary results were encouraging. Since this type of operation is more economical and because the quality of the insects is similar to that of flies dispersed by box, this procedure could in future be applied to other eradication programmes.

Economic aspects

In the enzootic areas costs for treating and preventing parasitosis fluctuate between US$4 and $10 per animal per year. A 1975 study found that the cost-benefit ratio for the eradication programme in Mexico was satisfactory, 1:4.5.

Disease prevention

According to available information any evolutionary stage of the NWS can produce infestation in areas free of the parasite. Nevertheless, the introduction of animals with an infested wound seems to be the most common form of introduction, as shown by the detection of the following positive cases originating from enzootic areas. In the United States two cases were detected in 1987 in hunting dogs transported from Honduras and Venezuela; one case was detected in 1988 in a horse from Argentina; and one case was diagnosed in 1989 in a soldier coming from Panama. In Mexico, 130 cases detected in 1985 in the northeastern zone of the country, mostly in bovines, were caused by the introduction of a single female sheep from the State of Yucatan (enzootic zone); and six cases were detected in 1989 in the port of Acapulco Guerrero in bovines transported from Panama. In Puerto Rico, one case was detected in 1989 in a horse transported from Venezuela.

From these cases it is evident that health controls must be established in the international importation and exportation of live animals as well as in the movement of animals between enzootic and free zones within the same country. Control measures should be adequate to avoid introduction of the disease without preventing free trade of live animals at national and international levels (see Figure 8).

8. Veterinary inspector carrying out routine animal inspection at the quarantine station in Mexico before the animals pass into the NWS-free areas - Inspecteurs vétérinaires effectuant une inspection de routine à la station de quarantaine mexicaine avant que les animaux ne passent dans les zones exemptes de lucilie bouchère - Inspector veterinario en una estación de cuarentena en México, efectuando la revisión de rutina de un animal antes de ser transportado a las zonas libres del gusano barrenador del ganado

Photo/Foto: Mexico-US Commission


Despite the topographical, communicational, social and economic difficulties involved, the eradication work in the isthmus of Central America should continue. The culmination of the programme will be the establishment of a biological barrier in the Darien zone of Panama, supported by new techniques of sterile insect production with gel media and cheaper dispersal of insects, as already described. The regional eradication programmes in the Caribbean region and in South America should be continued to bring economic and public health benefits to a large number of countries. This will also reduce the risk of outbreaks in other continents, such as the present one in the Libyan Arab Jamahiriya, which if it extends will jeopardize wildlife and animal production in countries south of the Sahara (see Figure 9).

9. Geographical distribution of the New World screwworm fly, June 1991 - Répartition géographique de la lucilie bouchère (juin 1991) - Distribución geográfica de la mosca del gusano barrenador del ganado (junio de 1991)


The control and eradication of the New World screwworm in the United States, Mexico and Central America has been one of the greatest achievements in animal health of this century. The pacific use of atomic energy in sterile fly production and the work and cooperation between scientists, technicians, cattle ranchers and governments to end a historical problem in animal production and wildlife conservation on the American continent represent an achievement of unusual proportions.

Throughout Latin America population growth makes it necessary to optimize food production, yet without extending the agricultural frontiers to avoid the destruction of forests and jungles. Eradication of NWS through the sterile insect technique provides a great opportunity to achieve this without using environment-polluting chemicals.


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