Agricultura tropica et subtropica, Universitas Agriculturae Praga, Vol. 34, 2001: 97-102











The screwworm caused by Cochliomyia hominivorax, attacking warm-blooded animals and man, was discovered for the first time in the history, outside of American continent. After confirming the occurrence of this horrible myiasis  in North Africa, epizootiological analysis was carried out. Tripoli sheep import quarantine was traced as primary outbreak locality. The myiasis spread rapidly invading vast territory during several months. The invaded territory of 25000 km2 with more than 2.7 million domestic animals was identified between Mediterranean Sea and parallel 32o10' (desert) and between meridians 11o45' and 15o15'. The screwworm strain was compatible with Costa Rica-91 strain confirming American origin. Special national and international diagnostic, surveillance and reporting systems facilitated to identify screwworm occurrence (14111 cases from July 1989 up to eradication in April 1991),  dynamics (seasonality culminated in warmer months) and threatened territories in Africa and Mediterranean basin. Epizootiological analysis served as the background for eradication programme.


Key words: Cochliomyia hominivorax - screwworm - epizootiological analysis - myiasis - surveillance - zoonosis


List of abbreviations: FAO - Food and Agriculture Organization of the United Nations; NWS - New World Screwworm (Cochliomyia hominivorax); SECNA - FAO Screwworm Emergency Centre for North Africa.






 New Word Screwworm (NWS), originally called American screwworm,  spread outside of its natural environment on the American continent for the first time in the history. This horrible myiasis, which was discovered in North Africa in 1998, belongs among Diptera order, Calliphoridae family, Cochliomyia genus and hominivorax (Coquerel) species. The NWS is producing larvae that lead a parasitic existence and result in obligatory myiasis. The fly is a primary invader of fresh skin wounds of animals and man. Adult females lay 200-400 eggs on the edge of fresh wounds. After 12-21 hours, larvae hatch, crawl into the wound, and borrow into the flesh. The larvae feed on wounds fluids and live tissue and complete their growth in 3-7 days. Grown larvae then exit from the wound, fall to the ground, and borrow in the soil to pupate. The pupal period varies from 3 days to 2 months, depending on the temperature. Adults breed only once during their lifetime. They usually mate when 3-4 days old, and gravid females are ready to oviposit when about 6 days old. In warm weather, the life cycle may be completed in 21 days.  Necrotic tissues attract more flies. The wound can become greatly enlarged due to multiple infestation and if left untreated, their victims die. The objectives of epizootiological analysis were to answer questions such as "what, where, when, how and why?".




Material and methods


The paper is based upon official documents (5), personal investigation (2), publications of specialists involved in North African campaign such as Abusova (3), El-Azazy (1), Lindquist (3), Reichard (4) and Toure (2).


Epizootiological analysis consisted in field investigations, laboratory testing, surveillance, monitoring and data processing during NWS occurrence in North Africa. Among working methods belonged anamnesis study, clinical, epizootiological, entomological and ecological on-the-spot investigations, supported by documentation about NWS cases registered in veterinary clinics, veterinary faculty and human hospitals. Collected samples were entomologically investigated in Central Veterinary Laboratory in Tripoli.


Initial data were obtained by the first FAO mission sent to Libya in April 1989 to confirm the presence the NWS and identify limits of invaded territory. The mission was headed by the author of this paper as the Chief, FAO Animal Health Service, responsible for United Nations animal health policy.


After introducing obligatory reporting of this myiasis epizootiological analysis became more precise. NWS occurrence information came from systematic preventive inspection of animals, quarantine stations, animal movement control, sampling and laboratory investigation of all collected flies searching for NWS.


After starting eradication programme epizootiological analyses were complemented by more detailed mapping to identify and delimited myiasis territorial distribution and by more data on dynamics, ecological conditions and other factors important for identifying sterile insect technique targets, timing and intensity.


Key importance for epizootiological analysis were documents of government animal health service and of the SECNA as temporarily established organization to deal with NWS at international level and operating in Libya.






First mission to Libya was sent in April 1989 with following terms of reference: to investigate suspicion of the screwworm Cochliomyia hominivorax occurrence; in case of confirmation to assess epizootiological situation to recommend a programme for the control and eradication and to identify the need for international assistance.


The mission together with the Libyan specialists, carried out a fact-finding survey to determine the reality of screwworm infestation. At Veterinary Clinic in Hadba Al Khadra several larvae were collected from the tail and from the breast of two diseased lambs. Immediate entomological investigation in Central Veterinary Diagnostic Laboratory in Tripoli confirmed presence of NWS. Entomological investigation of specimens led to the clear conclusion that NWS was effectively a myiasis causing agent in Libya confirming the diagnosis made by local specialists (1). Maggots collected from animals were identified by the Faculty of Veterinary Medicine and collected from humans (from head, neck, hand and scrotum wounds) were identified by the Faculty of Medicine. It was found the evidence that the myiases were occurring from summer 1988 with the Tripoli area as the epicenter.


Next step was to identify the limits of invaded territories.  The mission visited nine districts in and around Tripoli (Tab.1). Very helpful were information obtained in district veterinary clinics having good registration of clinical cases. It resulted from the analysis that the situation should be considered as an extremely serious one since in less than a year between 2000 and 3000 cases were reported within a territory ranging 200 km-long coastal strip centered on Tripoli and extending approximately 80 km inland. Infested area in the northwest Libya covered an estimated surface area of 25000 km2 within the following boundaries:  North: Mediterranean Sea;  South: Parallel 32o10'- desert; West: Meridian 11o45'; East: Meridian 15o15'. In infested areas were 2.7 million domestic animals (sheep represented 78.04 %, goats 18.81 %, cattle 2.13 % and camels 1.02 %). In the entire country there were about 7 million heads of livestock, of which 80 % were sheep.


Deep wounds including a great number of larvae were observed mainly on sheep (rams, ewes, lambs) in various parts of the body (fatty tail, face, abdomen, anus, vagina, navel, base of horn, mouth). Other animal species such as cows, calves, camels, dogs, stallions, etc. were affected in various parts of the body. NWS cases were treated by insecticides such as phosphorothiolate (Neocidol) and Dermatox. However, extreme cases were followed by death, mainly in lambs.


Myiases cases were reported also from Tripoli Central Hospital and specimens of maggots were collected from 30 human cases. The main sites of infestation were head and neck.


Following countries were identified as being at high risk where FAO, as recommended by its mission, initiated surveillance, monitoring and training: Tunisia, Egypt, Algeria, Chad,  Niger and Sudan. Specific surveillance and communication activities were also undertaken in other countries at risk: Burkina Faso, Cameroon, Djibuti, Ethiopia, Mali, Mauritania, Morocco, Nigeria, Senegal and Somalia. At high risk were about 120 million domestic animals and in other mentioned countries about 250 million animals, not considering wild animals (Tab.2).


The way of introduction was suspected to be the import of sheep from infested American continent. From Uruguay were imported  236628 sheep in 1988 and 16661 sheep in 1989. The sheep destined for slaughter were accompanied by usual international veterinary certificates confirming the good health of the animals and freedom from all diseases on the OIE lists A and B. No clinical cases of screwworm were reported from amongst the imported animals. (Author visiting  Uruguay in 1990 found that in this country NWS was almost ubiquitous, not yet notifiable and controlled).


Reliable data on NWS occurrence started after establishing obligatory reporting and investigations of all collected larvae from wounds, i.e. from July 1989. That year, additionally to previous ad hoc finding, were laboratory-confirmed 2037 cases, in 1990 12068 cases (peak in September) and in 1991 only 6 cases. (Tab.3).


Very intensive surveillance programme was introduced. For example only in 1991, i.e. last year of NWS occurrence, were carried out 30516633 preventive inspections of animals, 918136 animals were inspected in quarantines, 176936 male flies and 213939 female flies were captured for diagnostic purposes. The last case was in April 1991. During next 10 years of intensive follow-up surveillance no one new case of NWS was detected confirming NWS free status in the East Hemisphere.


Particular study by the USDA laboratory at Fargo, North Dacota, USA confirmed compatibility of the North African strain with American strain (Costa Rica-91) used for mass-rearing at the Mexican facility producing sterile flies. This was key discovery for using sterile insect technique to eradicate this myiasis. British Natural History Museum entomological laboratory was selected as FAO reference center for identification of the screwworm.


The losses reached hundreds of million US $. In case of spreading over Sahara the consequences would be irreparable and supposed losses were estimated to be of billions of US $ (5).




                                                      Discussion and conclusion


For the first time in the history, this relentlessly destructive pest became established outside its natural range in the Americas and, if left uncontrolled, it would inevitably spread to neighbouring countries and eventually into sub-Sahara Africa, the Near East and Mediterranean Europe.


Although the infested area in Libya had the ideal ecology for NWS subsistence and there was little need for the fly to migrate in search of suitable conditions, the major risk was that it could spread through domestic and wild animals movement. Only one infested animal needed to move out of the infested area to cause an outbreak in a neighbouring country. Nomadic animals movement is very difficult to control.


The author presented to General Session of the International Office of Epizootic held in Paris in May 1989 the recommendation NWS to be internationally notifiable and included in OIE International Zoosanitary Code, which was accepted. To support global emergency, special surveillance and preventive measures, the author sent to all Chief Veterinary Officers  in the world personal letters (28 July 1989, 4 October 1989, 12 April 1990) alarming about the screwworm situation and recommending anti-NWS measures, together with the key for NWS identification.


Immense size and complexity of scientifically based continuous monitoring system supported by laboratory etiological investigations followed by immediate successful actions at field and managerial levels, represented one of the most effective surveillance in the history of animal health.






1.      El-Azazy O., 1989. Wound myiasis caused by Cochliomyia hominivorax in Libya. The Veterinary Record, 124, 103.

2.      Kouba V., Toure S., 1989. Study on the situation of the American Screwworm Fly (Cochliomyia hominivorax). Report of the mission to Libyan Arab Jamahiriya, FAO, Rome, 17 pp.

3.      Lindquist D., Abusowa M., 1991. The New World screwworm in North Africa. FAO Wld. Anim. Rev., Spec. Issue: October, 2-7.

4.      Reichard R., 1999. Case studies of emergency management of screwworm,,1999,18(1): 145-163.

5.      Food and Agriculture Organization of the United Nations, 1994. The New World screwworm eradication programme: North Africa 1988-1992. FAO, Rome, 192 pp.


Tab. 1

Screwworm territorial distribution in Libya as identified

by the first international fact-findings mission in April 1989


District                         Distance from Tripoli   Screwworm f i n d i n g s


Zawia                          50 km westward                      first in September 1988

                                                                                              last in January 1989

Sorman                                   80 km westward                      one suspected case

Zahra                           40 km south-westw.                cases started in October


Hadba Al Khadra         5 km southward                      positive cases

Ain Zahr                    100 km south-eastw.                 positive cases

Tajore                         30 km eastward                       first case in July 1988

Garabuli                       60 km eastward                       frequent from November


Gasarkihar                   75 km eastward                       cases from October 1988

Khomes                       120 km eastward                     no cases



Tab. 2

Main domestic mammals in African countries at screwworm risk with special surveillance and measures (1000 heads), 1989, FAO


Country                  Cattle                    Sheep              Goats               Horses             Total


At high risk:


Libya                      238                        5100              1100                22                    6460

Tunisia                    626                        5935              1259                55                    7875

Egypt                    2771                        3310              2407                18                    8506

Algeria                  1366                      17301              2454                83                  21204

Chad                     4298                        1926              2838               184                  9246

Niger                     1712                        3100             4974                 81                   9867

Sudan                   21080                    21304              16257               22                 58663


Others at risk:


Burkina Faso        3937                         5048              6563                   22                 15570

Cameroon            4703                        3407              3428                   14                 11552

Djibuti                    188                          433                502                       -                 1123

Ethiopia              29633                     23320             17733               2650                 73336

Mali                      5007                       6072               6072                    72                17223

Mauritania            1350                      5067               3400                    17                  9834

Morocco             3284                      13528               5059                  190                22061

Nigeria               13974                      12477             23428                   208               50087

Senegal                2616                        3464               2528                  419                 9027

Somalia               4100                      12783             17600                       1               34484


Total                100883                   143575            117602                4058               366118



Tab. 3

Screwworm cases by months in North Africa during 1989-1992

(each case = laboratory-confirmed infestation of one animal)


Months        1989        1990        1991        1992


January          ?               102           3           0

February        ?                 94           2           0

March            ?               190           0           0

April              ?               289           1           0

May               ?               371           0           0

June               ?               917           0           0

July               111         1 570           0           0

August            29         2 145           0           0

September      75         2 932            0           0

October         419        1 701            0           0

November     796         1566            0           0

December      607          191             0           0


Total         1, 937       12 068           6           0