SCREWWORM (COCHLIOMYIA
HOMINIVORAX) AND ITS ERADICATION IN
KOUBA, V.
Abstract
The discovery of the screwworm Cochliomyia hominivorax
in
Key words:
Cochliomyia hominivorax - screwworm - disease control - disease
eradication - sterile insect technique.
List of
abbreviations: NWS - New World Screwworm (Cochliomyia
hominivorax); SECNA - FAO Screwworm Emergency Centre for
Introduction
The discovery
of the screwworm Cochliomyia hominivorax
Coquerel) in
The first
mission sent by FAO in April 1989 recommended using sterile insect
technique of Bushland and Knipling (1938). The screwworm flies are reared
artificially and exposed to irradiation by caesium 137 shortly before they
emerge from the pupae. Male flies are sterile but able to mate. The female
mates only once, and when mated with a sterile male,
lays eggs that do not hatch. Therefore, release of sufficient numbers of
sterile males in an area over a period of time leads to eradication. This
programme along with the use of insecticides that killed adults, led to
eradication of screwworm in
Material and Methods
The paper is
based upon personal experience (1), publications of Abusowa (3), Reichard (2), Lindquist(3) and Vargas-Teran (4). Other sources represented
publications of the FAO (5,6) and documents of FAO Screwworm Campaign Action
Group, FAO/IFAD Screwworm Campaign Action Group and FAO Animal Health Service
Screwworm Task Force, all chaired by the author of this article as the Chief,
Animal Health Service, FAO.
To apply the STI for NWS eradication in
- identifying programme phases and key moments for STI application
in time and space based upon epizootiological analyses;
- comparing
North African strain how far was compatible with the strain used for mass
production of sterile insect in only factory in the world located in Tuxtla
Gutierrez in Mexico;
- sterile screwworm surviving in the best possible conditions
from the factory up to final dispersion and mating in target zones in a very
distant territory;
-
transcontinental transport of sterile flies and their storing under North
African conditions up to reach the optimal stage for release and mating;
- effective system of aerial dispersion of sterile flies in
terms of frequency, intervals, size and location paths;
- continuous control from production factory up to
effectiveness of STI practical application;
- continuous surveillance in invaded and threatened zones and
countries.
Results
Preventive
(containment) phase
This phase
consisted in: establishment of specific diagnostic system to detect and monitor
NWS in the North Africa region; controlling live animal movement (individually
examined for wounds and screwworm larvae) to prevent introduction of NWS to
territories free of the pest; reporting on the worldwide situation of infested
and NWS-free countries and on risks involved in international trade;
improvement of methods for rapid and economical deployment of the STI.
One of the
first action was to arrange for production of coumaphos (5%) powder for
treatment of wounds and prophylactic spraying (0.25%) as well as to reorganize
inspection activities within 21 days (the average life cycle of the NWS). Millions of sampling/treatment kits which
consisted of a small amount of insecticide powder and larval collection tubes were
routinely provided to livestock owners.
In December 1989, tests confirmed that Mexican and
Libyan NWS strain were compatible. Then, it was necessary to determine how the
pupae would be transported to
After not easy
legislation process, on
Sentinel pens
with surgically wounded sheep were established and wind-oriented traps baited
with the NWS fly attractant (swormlure-4) were set up to gather live NWS to
test suitability for STI under
Sterile flies production capacity of the Mexican facility was
significantly increased to be able to meet additional requirements for
eradication programme in
Pilot phase
This phase was
necessary to develop the infrastructure and test the feasibility of
transferring the proven technology, i.e. to test the logistic support necessary
to apply SIT successfully in
The sterile
NWS is a living insect and requires particular conditions with regard to
temperature, packaging, shipping, storage, handling
and must be transported within a limited period. In order to ensure good
quality flies, a decision was taken to transport the insect in the pupal stage
by packing 1600 pupae into small biodegradable cardboard dispersal boxes. Each
box contained a small cup of special gelled diet for the emerged flies. The
temperature during ground and aerial transportation of the pupae must remain at
10oC. The pupae were transported in refrigerated trailers from the
factory to
Then the boxes
were loaded on twin-engine aircraft, each fitted with a specially designed
chute through which the boxes were released at a predeterminated rate. The
boxes were designed to open during their fall from the aircraft, or on impact
with the ground, thus releasing the flies.
In December 1990 started dispersion with more than 5 million of sterile
flies. Pilot dispersion continued in January 1991 into eradication phase
without interruption.
The dispersal
aircraft flew along predetermined paths, 4 km apart, and boxes were dispersed
at a rate of 3 to 10 per minute as the plan flew at 240 km/hour at a altitude of 500 m. On each dispersal day an area of
approximately 6400 km2 was covered, with an average distribution of
800 flies per km2.
Eradication phase
The main
activity consisted in intensive dispersion of sterile flies over infested area
of about 25000 km2 and protective barrier of 15000 km2 including 2500 km2 in
northeastern Tunisia. From January to
Activity of
sterile flies and the ration of sterile to fertile flies in each area were
monitored. Laboratory tests were targeted to pupal weight, emergence, malformed
pupae/adults, sex ratio, longevity without food and water, longevity with food
and water, sterility,
Effective
international technical and financial assistance together with strong national
public veterinary service were considered the crucial pre-requisites for
success. The government provided a range of facilities, support staff and other
assistance to the project. Department of
Veterinary Service employed approximately 300 veterinarians and supporting
staff of about 1200 persons. In total more than 50 million inspections were
recorded (e.g., during eradication phase - every months the surveillance units
were carrying out about 3 million inspections and treating nearly 20000
wounds). National authorities ensured also: landing, take-off, parking,
servicing, refuelling and repair facilities for project aircraft; facilities
for unloading, storage and loading of boxed flies; flight control through
aeronautic maps and ground-to-air communications; access to national and
international communication facilities for project staff; ground support staff
to assist with air dispersal and biological and veterinary monitoring; sentinel
animals for biological assessment; office, laboratory and storage facilities
for diagnosis and identification. Particular role had specialists with
experience from screwworm eradication in
Because no
further evidence of NWS in the six months following detection of the last case
in April 1991 (Tab. 1), SIT activities were terminated.
Posteradication phase
The continuous
compilation of NWS negative results of surveillance throughout the country
confirmed the evidence that NWS no longer exists in
Discussion and conclusion
The fly's
mobility makes containment extremely difficult. Adult flies can travel up to
200 km and, where conditions are favourable, the size and range of the
population can expand rapidly. The screwworm presence in
In addition to the economic impact, a
continent-wide infestation would pose a major human health risk. In remote
areas where medical facilities are limited, human would fall easy victims -
particularly children, the sick and the aged. For
Eradication was achieved in very short period
before the parasite spread and become enzootic in the
For the pre-STI programme were allocated 10.6
million
The responsibility for the programme
preparation, management and coordination of antiscrewworm action was with above
mentioned FAO Screwworm Campaign Action Group (from
Some fortunate features should be mentioned:
The outbreak occurred in a fertile oasis area surrounded by desert and the
Success of NWS programme provided useful
precedence: rapid recognition of the problem, determination of its extent,
early development of a sound technical programme of action, establishment of a
single executive unit with necessary funds and authority to implement the
programme, clear delineation of the responsibility of different agencies
involved and implementation of an extensive communication campaign to ensure
that people in the affected regions are fully informed on the programme (using
radio, TV, leaflets, posters, etc.). The population was informed why low-flying
aeroplanes would be dropping cardboxes full of foreign flies over large
sections of the country (before 1989 NWS was known as American screwworm).
If the NWS infestation had spread into other
areas of
References
1.
Kouba, V., 1990. Information note on FAO activities to
combat screwworm in the Near East Region. 20th FAO Regional Conference for the
2.
Reichard R., 1999. Case studies of emergency
management of screwworm. Rev.sci.tech.Off.int.Epiz.,1999,18(1):
145-163.
3.
Lindquist D., Abusowa M., 1991. The
4.
Vargas-Teran M., 1991. The
5.
Food and Agriculture Organization of the United
Nations, 1989. Manual for the Control of Screwworm Fly Cochliomyia hominivorax,
Copquerel, 93 pp.
6.
Food and Agriculture Organization of the United
Nations, 1994. The
Author´s address:
Prof.MVDr. KOUBA Václav,
DrSc.
170 00 Praha 7
Tab.1
Larvae collected from wounds in
==============================================================
Year Total Screwworm larvae Percentage positive
--------------------------------------------------------------
1989 5805 1938 33 %
1990 12557 12068 96 %
1991 1090 6 0.5 %
-----------------------------------------------------------
Total 19452 14012 72 %
==============================================================
Tab. 2
Allocation of funds for the preventive
(containment) phase (in US $)
============================================================
Personnel 405 000
Equipment and
supplies 1 105 918
Purchase of
flies 157 843
Transport of
flies 664 674
Dispersal of
flies 138 965
General
operating expenses and
direct
operating expenses 137 965
IAEA
contribution 507 000
-----------------------------------------------------------
Total 10 617 365
===========================================================
Tab. 3
Eradication
programme breakdown of overall funding sources (in US$)
==============================================================
Initial phase
(FAO,
Pilot phase
(FAO,
Eradication
programme
60 600 000
multidonor
funds 32 600 000
Libyan contribution (cash) 11 400 000
Libyan contribution (in kind) 14 600 000
Neighbouring countries (in kind) 2
000 000
Bilateral
donations to
neighbouring
countries 5 100 000
---------------------------------------------------------------------------------------------------------
Total 80 100 000
==============================================================