New world screwworm in Libya
Eradication programme
Conclusion
D.A. Lindquist and M. Abusowa
The presence of the New World screwworm (NWS) in the Libyan Arab Jamahiriya, once confirmed in 1989, set a highly elaborate defence system into motion. It entailed massive livestock inspection, treatment and control as well as the release of sterile NWS flies from Mexico which commenced at the end of 1990. This highly technological eradication campaign has been essential in preventing the spread of the NWS into the rest of the African continent, the Mediterranean region and the Near East.The authors are Field Programme Director and Field Programme Co-director, respectively, of the Screwworm Emergency Centre for North Africa (SECNA), Gorji Road, PO Box 83252, Tripoli, Libya.
The New World screwworm (NWS), Cochliomyia hominivorax (Coquerel) was discovered in the northwest Libyan Arab Jamahiriya in the spring of 1988 by Libyan veterinarians. The NWS was introduced into Libya from the Americas, the only area of the world where it previously existed. The mode of introduction and the country from which the pest came are unknown. Positive taxonomic identification of the pest was made by British scientists, and FAO confirmed the presence of the screwworm myiasis disease in Libya in March 1989.
The Libyan Government implemented a large-scale control and containment programme in mid-1989 which involved inspection of livestock by veterinary teams, collection and identification of worms associated with wounds, treatment of animals and control of animal movement to prevent the spread of the disease.
The presence of the NWS in Libya was viewed as a potential major disaster for North Africa, the Mediterranean Basin and the whole of Africa. Based on the cost of controlling the disease in the Americas, FAO estimated that the cost of inspection and treatment of livestock would be about US$5 per head per year. Thus the cost of control in North Africa, with about 50 million head of livestock, would be US$250 million per year. Furthermore, the impact on wildlife in Africa south of the Sahara would be severe. Data from the United States had indicated that up to 75 percent of newborn deer died from the disease. Such high mortality rates could result in the extinction of some of the African wildlife.
Movement of the NWS is both by long-distance flight of adult insects in search of food or hosts and, much more commonly, by the movement of infested livestock and wildlife. Since control of livestock movement is difficult and never complete and control of the movement of wildlife impossible, FAO calculated that the disease would spread inevitably. FAO decided therefore that action should be taken rapidly to prevent the disease from spreading throughout North Africa, Africa south of the Sahara and much of the Mediterranean Basin. Early in 1989 FAO concluded that the only solution to the potential disaster was to eradicate the NWS from Libya, and planning of the eradication programme was initiated.
The only proven technology available for eradication was the Sterile Insect Technique (SIT) combined with intensive surveillance, animal treatment and animal movement control (see Figures 1 to 4). With the assistance of FAO, the United Nations Development Programme (UNDP), the International Fund for Agricultural Development (IFAD) and the International Atomic Energy Agency (IAEA), the Libyan surveillance and animal movement control activities were strengthened with the object of preparing for the eradication programme. The neighbouring countries (Egypt, the Sudan, Chad, the Niger, Algeria and Tunisia) were also assisted in strengthening their surveillance and quarantine activities. Under FAO sponsorship veterinarians and biologists from Libya and neighbouring countries were trained in NWS control and identification, and planning for the eradication programme was undertaken by IAEA and FAO.
Initial funding for the programme was obtained from FAO, UNDP, IFAD and IAEA. Donors were contacted and, as additional funds became available, staff and equipment were deployed to Libya. IFAD provided special funds to conduct a Preparatory Phase Project on SIT and to strengthen the quarantine/animal movement and communications activities.
After the discovery of NWS in Libya, the veterinary services established surveillance teams throughout the country, although most are in the NWS-infested area. Each three-person team covers a prescribed area and is equipped with a vehicle, insecticide and larval sampling supplies. The team inspects all animals in its assigned area every 15 to 20 days. The team members collect worms from myiasis cases and treat all wounds with insecticide. They discuss with the animal owners the overall eradication programme and control techniques and encourage the owners to inspect, collect worms from and treat their animals.
The data and worm samples collected by the teams are submitted daily to the central veterinary clinic of their municipality. These data are sent to the programme's headquarters in Tripoli at least once a week, and identification of worm and fly samples is carried out in the laboratory (see Figures 5 and 6). The data are summarized and combined with data from other municipalities on a daily, weekly and monthly basis.
In the primary agricultural area of Libya, which is along the coastline, animal production is not a major income-producer since most of the livestock owners have full-time jobs and keep relatively few animals, predominantly sheep. In addition, veterinary treatment and drugs are provided free of charge in Libya. Thus there is little incentive for owners to inspect and treat their own animals. Consequently, the surveillance teams are extremely important to obtain essential data on the number and distribution of NWS-infested animals.
During 1989 the impact of the NWS on Libyan livestock production was not great. The number of myiasis cases was not enough to disturb the livestock producers. In 1989 many individuals and organizations, including FAO and IAEA, predicted a very serious NWS problem based on the usual population explosion of insects invading new areas. In view of this, the Libyan Government implemented an effective publicity campaign to alert livestock owners and the general public of the serious problem to be faced in 1990. However, when the problem did not occur, the livestock owners became sceptical about the serious nature of the NWS and failed to inspect their animals as thoroughly as was necessary.
In 1990 the reported number of livestock cases of NWS myiasis increased dramatically, primarily because of the mild 1989/90 winter. The increased experience and efficiency of surveillance teams in locating cases may have contributed to the larger number of cases reported. In addition, the strain of NWS in Libya probably adapted better to local conditions and hosts, becoming more virulent. There were sufficient cases therefore in 1990 for the livestock producers to become concerned and to cooperate well with the surveillance teams.
Although the NWS attacks humans, few cases of myiasis in humans have been reported in Libya. The first cases were reported from hospitals in 1988. After the medical profession was alerted to the NWS threat, special care was taken in hospitals, and few cases were reported in 1989 and 1990.
The increase in NWS cases detected in livestock from 1989 to 1990 was not accompanied by a great increase in the total infested area. In late 1989 the infested area was estimated at 20000 km². This increased to an estimated 25000 km² by December 1990. Natural barriers were probably the primary factor in limiting the spread of the disease.
Quarantine and animal movement control activities were started in the autumn of 1990, after delivery of equipment. The quarantine stations are either fixed, located on roads used by livestock producers, or mobile, consisting of the quarantine vehicle and personnel seeking out the animals. Data on animal movement into and out of the NWS-infested area, number of wounds, NWS-positive cases and number of animals treated are recorded.
Communications are important in large, visible programmes of this type. Livestock owners, the rural population and the general public must be kept informed and must support the eradication programme activities. Communication activities, including the use of television, radio and posters, were initiated by the Libyan Government as soon as the NWS surveillance and control activities began. By the autumn of 1990 few people in Libya were unaware of the threat of NWS to the country and region and of the ongoing and planned eradication programme activities (see Figure 7).
The aerial release of sterile NWS was initiated in mid-December 1990 with 3.5 million flies released per week. The number of released sterile flies was increased to 28 million by late February 1991. The area treated was enlarged as well: the initial dispersal of sterile flies was over an area of about 5000 km² in the northwestern part of the NWS-infested area. By April 1991, 28 million sterile flies were being released over the entire NWS-infested area twice a week (see Figure 8).
Various quality control tests on the sterile NWS received from Mexico showed that the transatlantic shipment and handling in Libya did not measurably reduce fly quality. These tests, standard in the Mexico-United States screwworm eradication programme, measured adult emergence, longevity and ability to fly. Previous studies had demonstrated that the NWS strain in Libya was sexually compatible with the Mexican strain.
The number of reported NWS-infested animals fell dramatically during the winter of 1990/91, primarily because the cold weather reduced both NWS reproduction and the effectiveness of the surveillance teams in locating and treating infested animals.
This reduced NWS population now offers an opportunity to eradicate the NWS in a shorter period. Providing that sufficient numbers of good-quality sterile NWS can be continuously dispersed over the infested and barrier areas twice a week, eradication can be achieved by approximately January 1992. This estimate assumes, however, that there will be no great increase in the infested area and that no new infestation will occur at a distant location to require additional sterile flies and resources.
By the end of March 1991, the full eradication programme was scheduled to be operational. This involved the weekly aerial release of 40 million sterile NWS throughout the infested area and the border areas. An average of about 1000 sterile flies per square kilometre per week were released over the entire area, somewhat more than were routinely used in the Mexico eradication programme. However, there was the possibility that long transport from Mexico to Libya could cause a reduction in quality that would not be detected by the standard quality control tests, and increased numbers of sterile flies were required to effect rapid eradication. Ideally, sterile NWS should be released twice a week over the entire area, with three- to four-day intervals between releases.
Fly traps
Traps baited with an attractant (swormlure) for the NWS fly are deployed throughout the country, primarily in the infested area (see Figure 9). The purpose of these traps is twofold. First, the numbers of sterile NWS adults caught indicate the distribution of sterile flies. Sterility is easily determined in female flies, using a dissecting microscope. However, sterility in male flies is very difficult to detect and is often surmised inaccurately. Thus sterility can only be determined for females. Those traps that do not catch sterile flies are investigated immediately to determine whether the trap location is suitable and whether there is sufficient attractant. If these factors are satisfactory then visual observations of the aerial release are made to verify that the release aircraft is flying the correct pattern. When all of these factors are satisfactory, sterile flies are captured in 95 percent of the traps.
The second purpose of traps is to estimate the ratio of sterile to wild NWS adults. A ratio of 10:1 is required initially. The ratios increase as the released sterile NWS influence the reproductive rate of the wild population. When the wild population is very low, the capture of fertile NWS adults also indicates a potential problem area that needs special surveillance and treatment.
Sentinel animals
To estimate the degree of sterility in the wild population, sentinel animals are used. The sterile female does not lay egg masses. The wild female that has mated with a sterile male will lay egg masses, but they will not hatch. If she has mated with a wild male, the egg masses will hatch. Sentinel animals (sheep) are surgically wounded with a sterile scalpel after the wool has been shaved from the area. This wound is an attractive site for deposition of egg masses by the female NWS. Sentinel animals are located throughout the infested area and inspected twice daily. Any egg masses found are carefully removed, taken to the headquarters laboratory and placed in an incubator to determine whether they will hatch.
As the eradication programme proceeds and reduces the wild population, traps and sentinel animals become increasingly important but also increasingly difficult to evaluate. Few wild flies will be captured and few egg masses will be obtained from sentinel animals because of the very low population of adult NWS. Thus surveillance to detect infested animals becomes the major and finally the only indicator of eradication. Surveillance is the foundation of the entire eradication campaign.
The flexibility of quarantine and animal movement control activities becomes more important as the eradication campaign progresses. Changing the location of quarantine stations is necessary as areas are declared free of NWS. The intensity of movement control activities must also increase, particularly with regard to unofficial movement of animals.
Communications must continue to inform producers and others of the importance of inspecting and treating animals, reporting to programme personnel all myiasis-diseased animals and following quarantine regulations.
Any large programme requires research and development support for immediate solution of problems that prevent success. Long-term research is not a part of the NWS eradication programme in North Africa. However, there have been problems that have required attention. The first of these was to determine whether the Libyan
NWS strain was compatible with the strain reared at the Mexico plant. Libyan material was taken to the Laboratory of the Agricultural Research Service of the United States Department of Agriculture in Fargo, North Dakota, where the Mexican NWS strain is maintained in a quarantine facility. Tests showed the two strains to be compatible. Other tests demonstrated that the sterile NWS could be successfully shipped by air from Mexico to Libya.
If an infestation of NWS is discovered in a neighbouring country or in another part of Libya, it could take several days or weeks before sterile flies could be released in the newly infested area. Before new releases are initiated, everything possible should be done to reduce the chances of further spread and to reduce the infestation. Animal inspection and treatment is the first activity.
It took two years from the time that the NWS infestation in Libya was confirmed to initiate the SIT part of the eradication programme. Fortunately the activities initiated by the Libyan Government and subsequently strengthened by FAO and other UN organizations have helped control the pest. The NWS has not spread excessively but remains only in Libya. Had the pest moved into Tunisia, the eradication programme would have become much more difficult. The release of sterile NWS in Tunisia along the border with Libya is now insurance against the pest moving into Tunisia and becoming established before detection. If the pest should become established in Tunisia, the total cost of the eradication programme would increase dramatically.
The apparently low NWS population in early 1991 offers the opportunity for rapid eradication, providing the essential basic requirements of utilizing the sterile insect technique are met. In early March 1991 it appeared likely that the NWS could be eradicated from North Africa within nine months. However, the programme does not end here, as the release of some sterile flies will be required for an additional four to six months and surveillance activities are required for at least an additional 12 months.
