Screwworm Eradication Program Research Articles

A new formulation of screwworm fly attractant with reduced hazardous chemicals and transport restrictions.

New World screwworm flies, Cochliomyia hominivorax (Coquerel), are obligate parasites of warm-blooded animals. They were eradicated from North and Central America during the mid-20th to early-21st centuries using the sterile insect technique (SIT), a method presently employed to maintain a permanent barrier between Central and South America. Lures are an important component of the screwworm eradication program, where they are used for surveillance, sample collection, and strain evaluation in the field. The first chemical lure, later named swormlure, was developed based on the attractiveness of C. hominivorax to volatile organic compounds (VOCs) produced from decomposing animal tissues. The formulation has changed little over the years and presently contains 10 chemicals, one of which is dimethyl disulfide (DMDS). Restrictions on the transport of DMDS have recently impeded its use in swormlure-4 (SL-4). However, dimethyl trisulfide (DMTS) is not as highly restricted and can be shipped via air transportation. Both chemicals are produced by microbial decomposition of animal tissues. Here, we conducted field trials using three releases of sterile C. hominivorax, each comprising approximately 93,000 flies, to test the efficacy of SL-4, containing DMDS, to swormlure-5 (SL-5) containing DMTS. Traps baited with SL-4 and SL-5 captured 575 (mean = 191.7, SD 17.9) and 665 (mean = 221.7, SD 33.2) C. hominivorax, respectively (df = 19, F = 1.294, P = 0.269). However, traps baited with SL-5 captured considerably more Cochliomyia macellaria (Fabricius), a closely related but nontarget fly.

The microbiome of wild and mass-reared new world screwworm, Cochliomyia hominivorax

Insect population control through continual releases of large numbers of sterile insects, called sterile insect technique (SIT), is only possible if one can mass-rear large quantities of healthy insects. Adaptation of insect stocks to rearing conditions and artificial feeding systems can have a multitude of negative effects such as inbreeding depression, reduced compatibility with wild strains, unintentional selection for traits that lower fitness after release, and an altered microbiome. Changes to insect microbiomes can have many effects on insects ranging from a reduction in sex pheromones or reduced fitness. Thus understanding these systems is important for mass rearing and the performance of the sterile insect control programs. In this study we explored the microbiome of the New World screwworm, Cochliomyia hominivorax (Coquerel) (Diptera: Calliphoridae) an economically important parasite of warm-blooded animals. Samples from myiases in cows and wild adults were compared to and mass-reared flies used by the SIT program. Significant differences were observed between these treatments, with wild captured flies having a significantly more diverse microbial composition. Bacteria known to stimulate oviposition were found in both wild and mass-reared flies. Two bacteria of veterinary importance were abundant in wild flies, suggesting screwworm is a potential vector of these diseases. Overall, this study provides the screwworm eradication program a platform to continue exploring the effects associated bacteria have on screwworm fitness.

Review of research advances in the screwworm eradication program over the past 25 years

AbstractNew World screwworms, Cochliomyia hominivorax (Coquerel) (Diptera: Calliphoridae: Chrysomyinae), are devastating pests of warm‐blooded animals that cause significant economic damage to livestock. The successful campaign to eradicate screwworms from continental North America, led by the US Department of Agriculture and using the sterile insect technique, continues to receive research support that has resulted in improved technologies for all aspects of the program. The process and ingredients for mass‐rearing screwworms is more efficient and sustainable, and there is now a standardized protocol for developing new strains used in mass rearing. Cryopreservation of screwworm embryos allows strains to be preserved and recovered if necessary and also reduces rearing requirements for backup and research strains. Sterile fly release procedures and equipment have been updated leading to optimized sterile fly release rates. Surveillance for screwworm infestations and outbreaks have incorporated new trap designs, habitat analysis, and molecular genetic techniques that enhance monitoring the progress of the program as well as early detection and response to outbreaks. Genetic analyses of screwworm populations across their current range have increased the understanding of genetic differentiation, which may aide in developing new strains and determining the geographic origin of screwworms causing outbreaks when they occur. The ability to release only sterile males, which has been a goal of the program for over 60 years, has recently been accomplished through the development of transgenic sexing strains. The strains carry a conditional female lethal gene and are comparable to the wild‐type strain for several biological parameters that are important for mass production and performance in the field. The strains should improve efficiency of population suppression of the current and future eradication and prevention programs against screwworms. These research advances as well as future considerations are presented.

Managing Ammonia Emissions From Screwworm Larval Rearing Media.

Mass production, sterilization, and release of screwworms (Cochliomyia hominivorax (Coquerel)) that were competitive in the field significantly contributed to the successful application of the sterile insect technique for eradication of screwworms from continental North America. Metabolic byproducts resulting from protein-rich diets required for larval screwworms lead to ammonia liberation, sometimes at high levels, within the mass rearing facility. Until recently a sodium polyacrylate gel bulking agent was used for the larval media and adsorbed much of the ammonia. A need to replace the gel with an environmentally "friendly" bulking agent, while not increasing ammonia levels in the rearing facility, led to a series of experiments with the objective of developing procedures to reduce ammonia emissions from the larval media bulked with cellulose fiber. Additives of ammonia-converting bacteria, potassium permanganate, and Yucca schidigera Roezl ex Otrgies powder extract, previously reported to reduce ammonia levels in organic environments, were evaluated. Ammonia-converting bacteria did not have a positive effect. Addition of Y. schidigera powder extract (∼1% of total volume), potassium permanganate (∼250 ppm), and a combination of these two additives (at these same concentrations) kept ammonia at equivalent levels as when larval media was bulked with gel. Potassium permanganate also had sufficient antimicrobial properties that the use of formaldehyde in the diet was not necessary. Further testing is needed, at a mass rearing level, before full implementation into the screwworm eradication program.

Sequence-characterized amplified regions that differentiate New World screwworms from other potential wound-inhabiting flies.

New World screwworms, Cochliomyia hominivorax (Coquerel, 1858), were once devastating pests of warm-blooded animals in the United States before they were successfully eradicated using the sterile insect technique. Guarding against the introduction of screwworms to North America or any other screwworm-free area relies on rapid, reliable identification of suspected cases. In the current study, the DNA from excised markers generated by randomly amplified polymorphic DNA polymerase chain reaction was used as the basis to generate 2 species-specific sequence-characterized amplified region molecular markers. Resulting primer pairs, named CR92A1 and J1A2 (each with forward and reverse components), produced amplicons of 852 and 848 base pairs, respectively. The 2 primer pairs successfully discriminated between C. hominivorax, Cochliomyia macellaria (Fabricius, 1775), 8 other species of blowflies, 3 noncalliphorid dipterans, and 1 nondipteran outlier. These primers may become important tools for veterinary laboratories and the screwworm eradication and exclusion program for rapid identification or verification of suspicious larval samples in presumed outbreaks.

Effect of Adult Screwworm Male Size on Mating Competence

Screwworms, Cochliomyia hominivorax (Coquerel) (Diptera: Calliphoridae), were devastating pests in parts of North America and Central America before their eradication by the Sterile Insect Technique. Now a barrier is maintained to prevent re-entry of screwworms from endemic regions to those free of screwworms. Barrier maintenance requires mass rearing millions of competitive insects, sterilization by irradiation, and release into infested areas. Weight of the mature larvae (or pupae) is used for assessing fly quality for mass-reared screwworms. A method for producing flies of different sizes in the laboratory was developed. Small and large male flies were compared for sexual performance. A standardized measure of the wing length was used to characterize adult size. This measurement correlated positively with the larval and pupal weights. Although small males mated successfully, they mated with fewer females than did large males when each was placed alone with a group of females. When placed together with large males, small screwworm males were out-competed (41.2% as competitive) by the large males. Although behavior of male screwworm flies in laboratory conditions has not been demonstrated to be similar under field conditions, knowledge of mating competitiveness of sterile males of different sizes is valuable to the screwworm eradication program.

Successful and currently ongoing parasite eradication programs

The eradication of parasitic diseases is not a new concept. The most successful programs of parasite eradication have occurred with species of veterinary importance. The first such program, the eradication of Texas Cattle Fever from the United States, is one of the great success stories of disease eradication. The American screwworm eradication program is ongoing and is serving as a guiding impetus for many of the ongoing or proposed vector eradication schemes around the world. The success of these programs prompted similar successful operations in human health. Although they once led the way, veterinary parasitologists have taken second place in eradication planning. The only three parasitic diseases of veterinary importance that have been targets of recent eradication programs are Hypoderma species in Great Britain and Europe, Cochliomyia hominivorax after its introduction into Libya from the Americas, and Echinococcus granulosus in Tasmania, Australia. There is also work on the eradication of the tick, Amblyomma variegatum, from the Caribbean Islands. Some animal diseases are targeted under the auspices of the human eradication programs, most notably the eradication of the tsetse fly from parts or all of Africa. This paper reviews some of the past or ongoing successful eradication programs and presents a brief summary of the history of the programs, the methods used or planned, and potential controversies surrounding their success and implementation.

PiggyBac transformation of the New World screwworm, Cochliomyia hominivorax, produces multiple distinct mutant strains

Sterile insect technique (SIT) programs are designed to eradicate pest species by releasing mass-reared, sterile insects into an infested area. The first major implementation of SIT was the New World Screwworm Eradication Program, which successfully eliminated the New World screwworm (NWS), Cochliomyia hominivorax (Coquerel) (Diptera: Calliphoridae), from the Continental US, Mexico and much of Central America. Ionizing radiation is currently used for sterilization, but transgenic insect techniques could replace this method, providing a safer, more cost-effective alternative. Genetic transformation methods have been demonstrated in NWS, and verified by Southern blot hybridization, PCR and sequencing of element insertion junctions. A lethal insertional mutation and enhancer detection-like phenotypic expression variations are presented and discussed. In addition to supporting the eradication efforts, transformation methods offer potential means to identify genes and examine gene function in NWS.

Seasonal and spatial distributions of adult screwworms (Diptera: Calliphoridae) in the Panama Canal area, Republic of Panama.

The distribution of screwworms, Cochliomyia hominivorax, (Coquerel) was studied in a seasonally moist lowland tropical forest in the Republic of Panama using a combination of field collections and satellite imagery. We found that different forest types could be distinguished and mapped using remotely sensed data. To determine the temporal and spatial distribution of flies, we collected flies coming to rotted liver at 82 sites in ten vegetation types (open areas, edge forest, dry scrub forest, forest successional stage 1, forest successional stage 2, forest successional stage 3, forest successional stage 4, forest successional stage 5, mature forests, palm swamp forest, and forest along streams) over three seasons (dry, transitional, wet). Nine of the vegetation types (excluding dry scrub forest) were identified and mapped using SPOT XS and Landsat 5 TM satellite data. Screwworm flies were most abundant during the transition from wet to dry season. Fly numbers were consistently higher in forest habitats, particularly those with trees 20-30 m in height and a fairly open canopy composed of many deciduous species that shed their leaves during the dry season. Screwworm numbers were also high in palm swamp forest, edge forest, and mature growth forest. Traps sampled in open areas had fewer flies and were unrelated to proximity to cattle. Females accounted for 88% of the total fly counts. This study further substantiates the importance of forests in the ecology and behavior of screwworm flies and demonstrates that remotely sensed data can be used to construct the spatial distribution of these flies in a tropical landscape. We discuss implications of this information to the screwworm eradication program.

A PERSONAL ACCOUNT OF DEVELOPING THE STERILE INSECT TECHNIQUE TO ERADICATE THE SCREWWORM FROM CURACAO, FLORIDA AND THE SOUTHEASTERN UNITED STATES

The history is recounted of developing the sterile insect technique to eradicate the screwworm, Cochliomyia hominivorax (Coquerel), from the Caribbean island of Curacao, Florida and the southeastern U.S. Observations of screwworm biology and challenges faced in conducting these eradication projects are described by the author who worked on all aspects of the research and field operations. Eradication was first demonstrated on Curacao, essentially a 170 mi2 outdoor laboratory. The population dynamics of the wild screwworm was determined and overflooding ratios and dispersal patterns essential for population suppression were defined. Eradication was achieved with minimal resources by attacking the pest during the time of year when it is least abundant. In Florida, eradication was greatly facilitated by an unusually cold winter that reduce the range and density of the target population. Eradication could not be attained easily by only suppressing the reproduction of adult screwworms. The larval population also had to be reduced to increase the ratio of sterile to wild males. It was essential to control the immature stages by diligent inspection and treatment of animal wounds. Leadership of the cattle producers was critical not only for securing program resources from the clientele, state legislatures and U.S. Congress, but also for gaining the cooperation of virtually all livestock owners. Additionally, leadership was required to acquire adequate research, extension and public information resources. Lessons learned from this work were corroborated repeatedly as the screwworm eradication program moved into the Southwest, Mexico and the Central America.

Identification of screwworms, Cochliomyia hominivorax (Coquerel) (Diptera: Calliphoridae), with a monoclonal antibody-based enzyme-linked immunosorbent assay (MAb-ELISA)

Myiasis caused by screwworms, Cochliomyia hominivorax (Coquerel), is devastating to warm-blooded animals and economically important to livestock producers. It is difficult to distinguish these pests, immature screwworms, from immatures of other non-pest fly species that often occur in animal wounds; it would be helpful to have tools available that do not rely on morphological characteristics. We developed two monoclonal antibodies (MAbs), highly specific for the screwworm, and used them in an enzyme-linked immunosorbent assay (MAb-ELISA), that differentiated screwworm eggs, larvae, pupae, and adults from those of the closely related secondary screwworm, C. macellaria (Fabricius) as well as Phormia regina (Meigen), Phaenicia sericata (Meigen), Calliphora vicina Robineau-Desvoidy, and Chrysomya rufifacies (Macquart). In a blind study, the microplate MAb-ELISA, which took about 4 h to complete, displayed high specificity (99%), sensitivity (92%) and overall accuracy (97%) in distinguishing all life stages of the screwworm. Electrophoresis results suggested that the two monoclonal antibodies recognized identical conformational epitopes present in all screwworm life stages. The screwworm eradication program, successful in eradicating this pest from the US, Mexico, most of Central America and Libya (after an accidental introduction), could benefit in future eradication, surveillance, and exclusion efforts by developing a reliable field identification kit based on MAb-ELISA that accurately and quickly distinguished cases of screwworm myiasis.

Screwworm eradication in the Americas.

The screwworm fly, Cochliomyia hominivorax (Coquerel), is a parasite that attacks all warm-blooded animals including humans. This parasite has caused significant losses to the livestock industries of the Americas. Since the screwworm eradication program was initiated in the Southeastern United States in 1957, the eradication program has successfully progressed to its current location in Panama. A variety of technologies and tools have been used in the eradication programs. The cooperative agreement has been a significant tool in the success of the program. In the United States, the State-Federal Cooperative programs provided the mechanism for carrying out screwworm eradication. Once screwworms were eradicated from the United States, the need to expand the program internationally, in order to protect the United States, became evident. A cooperative agreement created the Mexico-United States Commission for the Eradication of Screwworms (Commission). Commission-Guatemala and Commission-Belize Cooperative Agreements were used to eradicate screwworms from these countries. Followup programs in El Salvador, Honduras, Nicaragua, Costa Rica, and Panama were implemented by cooperative agreements between the United States Department of Agriculture and the individual countries. The positive and negative aspects, as well as the necessary elements of successful cooperative agreements, are discussed.

A New Starch-Grafted Gelling Agent for Screwworm (Diptera: Calliphoridae) Larval Diet

Larvae of screwworm, Cochliomyia hominivorax (Coquerel), were reared on diets solidified with an inexpensive starch-grafted sodium polyacrylate gel (brown and white formulations) and on a control diet solidified with Water-Lock G-400 gel, the standard gelling agent used for larval diet in the Screwworm Eradication Program (of the United States Department of Agriculture in collaboration with the government of Mexico). Mean pupal weight, number of pupae, percent emergence, sex ratio, adult longevity, fecundity, and fertility recorded from the test diets solidified with the brown starch-grafted gel were compared with the same parameters obtained from diets prepared with Water-Lock G-400 gel. There were no significant differences in any of the parameters examined between the tests and the corresponding control. Diets solidified with the white variety of the starch-grafted gel performed poorly. The brown starch-grafted gel has potential for use in the larval diet for mass production and may effectively replace the more expensive Water-Lock G-400 gel.

Estimating Population Density Per Unit Area from Mark, Release, Recapture Data

In this paper, we describe a method for estimating animal population density per unit area from mark, release, recapture (MRR) data. Standard procedures are available for estimating population size from MRR data, but not density per unit area, which is often of more practical value. If data on dispersal movements are available in addition to capture/recapture records, they can be used to estimate the area over which the study population ranges. These area estimates can then be combined with MRR abundances to yield estimates of population density. As a concrete example of how these methods can be applied, we calculated population density estimates for seven MRR data sets for the screwworm, Cochliomyia hominivorax (Diptera: Calliphoridae), from Costa Rica. As a standard of comparison, we also used a different method to calculate estimates of screwworm density for five sets of USDA APHIS Screwworm Eradication Program data from Mexico. Mean densities for both sets of analyses were similar, ranging from 10 to 120 adult flies/km2. General agreement between the Mexican and Costa Rican estimates lends support to the dispersal approach we propose here. Although we have applied these methods to the screwworm as a specific example, they are entirely general and can be applied to virtually any organism for which both MRR records and dispersal data are available.

Screwworm Eradication Program in Central America

The Screwworm Eradication Program has been extremely successful in its efforts to achieve its goal of eradication of screwworms through Central America and establishment of a permanent biological barrier in the eastern half of Panama. Following eradication of screwworms from Mexico in 1991, eradication was achieved in Belize in 1992, in Guatemala in 1993, and in El Salvador in 1994. Honduras has been free of screwworms since January 1995, and the number of cases in Nicaragua has dropped, as of April 1995, to about 4% of the average number of cases found during the period June-August 1993.

Central America Regional Screwworm Eradication Program (benefit/cost study).

Central America Regional Screwworm Eradication Program (benefit/cost study).

The New World Screw Worm Eradication Program

Australian Veterinary JournalVolume 70, Issue 12 p. 463-463 The New World Screw Worm Eradication Program AN Sinclair, AN SinclairSearch for more papers by this author AN Sinclair, AN SinclairSearch for more papers by this author First published: December 1993 https://doi.org/10.1111/j.1751-0813.1993.tb00865.xAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume70, Issue12December 1993Pages 463-463 RelatedInformation

Estimation of Native Screwworm (Calliphoridae: Diptera) Reproductive Activity by Release and Recapture of Sterile Females

Reproductive capacity and activity patterns of native screwworm (Cochliomyia hominivorax (Coquerel)) males were evaluated by release and recapture of sterile females in central Belize. On six occasions 4,000 virgin, 1-d-old sterile females were released at a central location. A total of 382 sterile females and 66 native females were recaptured on sentinel sheep at four locations. Ratios of virgin to mated recaptured females were used to calculate an estimated number of matings per release. During the 65-d period of the test, numbers of recaptured females and mating percentages increased, then decreased for the last release. Location of sentinel animals, day after release, and individual releases were significant factors explaining capture rates for mated and nonmated released females and captures of native females. Mating capacity for the native population of screwworm males over the 10-d recapture period was estimated to range from 70 to 1,735 matings for the dispersal area of these females. These data and estimates suggest that although screwworm males have relatively low population densities, they have significant excess mating capacity, and that production and release of competitive sterile males is important for the success of the screwworm eradication program.

Integrated pest management of dipteran pests in the New World

Although the screwworm eradication program, so successful at ridding much of North America from this pest, was not called an IPM program as such when first initiated, it nevertheless met the current definition of such a program. Considering the enormous success and the unbelievable numbers of insects reared for sterile release, this eradication program is one of the most outstanding entomological feats in history. Similar programs for other livestock pests are not yet assured of the same success. Hypoderma eradication may become a definite reality provided in vitro rearing methods can be developed to provide large numbers of sterile males. This development would not only indicate another possible international control effort but could also provide impetus for a Dermatobia hominis control program in the subtropics of Mexico, Central America and South America. The cost/benefit analysis of a number of the major pests needs to be completed to determine if their control is beneficial to the health of the host or provides a profitable return on the producer's investment. Greater knowledge of the pest biology-ecology, including dispersion, migration, rate of population increases, and behaviour is needed. Pest population models used to simulate the effect of weather, host density and pest density on outbreak situations need to be developed. Insecticide resistance in insect populations now needs to be considered in these models. Even sampling procedures are inadequate for most pests. Also, alternative methods to insecticide control are needed for many of the dipterous pests. Some pests cannot be controlled with current delivery systems. For instance, flies of the family Tabanidae cannot be controlled because of lack of information on their biology-ecology; also, methods of chemical application to effect their control are not available. Much effort has been made to control the dipterous and myiasis-causing pests in the new world. For the most part, chemicals have been effective, and until recently, resistance has not been a problem. Although control has been satisfactory for most pests, the implementation of IPM programs against some pests could make control more permanent and cost effective. Many parts of an IPM program are available for use but need to be put together in a control system and validated. Only after a system has been shown to be cost effective can the technology be transferred to the producer. A few pests of the New World can be controlled within the concept of IPM, but by and large, operational programs remain something to be achieved.

Control of insects of veterinary importance by genetic techniques

The practicality of genetic methods for control of arthropods that affect livestock production in the tropics is best documented in the results of the screwworm ( Cochliomyia hominivorax ) eradication program, conducted first in the southeastern United States, subsequently in the southwestern United States, and currently in Mexico. The screwworm eradication program integrates the use of insect sterility with other appropriate measures to obtain population elimination. The basic sterilization mechanism utilized for control of screwworm reproduction is the induction of dominant lethality in the chromosomes of the released insects by iradiattion. As a result of the practical success of that program, a substantial amount of laboratory and field study has been conducted on the use of artificially induced dominant lethality for control of tsetse flies ( Glossina species), house flies ( Musca species), the stable fly ( Stomoxys calcitrans ), the horn fly ( Haematobia irritans ), cattle grubs ( Hypoderma species) and mosquitoes. As an alternative to dominant lethality, the use of chromosome rearrangements (translocations, compound chromosomes, etc.) or hybrid sterility has been studied in mosquitoes, the sheep blowfly ( Lucilia cuprina ) and Boophilus ticks. The potential for substantial economic impact by the increased use of these techniques for control of arthropod pests of livestock in the tropics is very promising.