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Page 1 Field Technician Handbook
Field Technician Handbook
The screwworm is a blow fly with the scientific
name, Cochliomyia hominivorax and is an obligate
parasite that requires a warm-blooded living animal
in order to reproduce. The female screwworm fly
mates once in her lifetime and deposits eggs on the
edge of open wounds. The eggs soon hatch and
burrow into the wound where the maggot tears the flesh with
hooks and feeds on the resulting fluids. If untreated, the infested
wound attracts more females and the wound may have thousands
of maggots and the animal may die from toxins and infection.
The maggots characteristically form a pocket in the flesh and
burrow head first into the wound and resemble a screw. The
infested wound produces a unique and easily identified odor and
may the first indication that an
animal is infested with
screwworms. All warm-blooded
animals are susceptible to
screwworm infestation including
wildlife, domestic livestock,
companion animals, and even
chickens. Humans are also
susceptible to screwworm infestation and the screwworm fly often attacks elderly indigent
persons or children with scalp wounds resulting from lice.
The life-cycle of the screwworm is approximately
21 days. The gravid female deposits a mass of 300-
500 eggs on the edge of a moist wound. The female
may deposit 3-4 clutches of eggs every few days,
often in the same wound. The eggs hatch within
24 hours and the larva burrows into the wound to
feed. The maggot spends approximately 5-7 days
in the wound and grows through three stages
known as instars. The mature maggot then crawls
out of the wound and drops to the ground and
quickly burrows into the soil. Buried under 3-5 centimeters of soil, it forms a hard puparium and
undergoes metamorphosis. Soil that is excessively dry or wet is not well suited and the puparium
may not survive and while on the surface the worm may be preyed upon by ants, spiders, or birds.
In approximately 5-7 days the adult fly breaks the puparium and crawls to the surface where it
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dries its wings and takes on a shiny metallic blue-green color and then flies off. The fly spends
several days feeding on flowers and after 3-5 days the female is ready to mate with a male. The
male waits in shrubs and grasps a female that passes by; contact pheromones play a part in the
attraction. The female mates only once but males are promiscuous and will mate with several
females. The gravid female is then ready to complete the cycle.
In the natural state, the screwworm
fly maintains a low density
population with only a few
individuals that inhabit areas of
secondary forest growth and shrubs
near sources of running water. The fly is reclusive and not
attracted to decaying material associated with human habitation.
Man intrudes on this environment clearing land for pasture and
introducing domestic livestock and an abundance of wounds allows
the screwworm fly population to expand, which results in many
more infested wounds and economic loss for the livestock producer.
A wound as small as a tick bite may attract a female screwworm fly;
the umbilical cord of a new-born animal is especially attractive and
the new-born frequently dies or is stunted. Free-ranging livestock
that are infested with screwworms often seek out sheltered areas
where they die unobserved. Typical animal husbandry such as
dehorning, castrations, branding, and unattended births produce
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ideal wounds for screwworm flies; foot lesions caused by infection are often infested and in areas of
high population of both flies and sheep nearly all animals will be affected.
It was this economic loss for livestock producers in the United States that prompted investigation
into how to control this pest. In the 1950’s scientists with the United States Department of
Agriculture discovered that when a female mates with a male whose reproductive chromosomes
have been damaged by irradiation, the eggs do not hatch. The Sterile Insect Technique (SIT) is the
release of large number of artificially produced sterile males that overwhelm the native fly
population resulting in gradual elimination of the native population. However, the SIT is most
effective when the native screwworm fly population is low and eliminating wounds susceptible to
screwworm infestation is critical for successful eradication of screwworms.
A large industrial factory in Tuxtla Gutierrez, Mexico produces millions of sterile insects daily. A
brood stock of fertile flies produces larvae that feed on an artificial diet of blood and other sources of
protein such as milk and egg. Temperatures and humidity in the production facility are strictly
regulated. The pupae are then irradiated with gamma rays produced by a radioactive isotope such
as Cesium or Cobalt. The irradiation dose is calculated to alter the reproductive chromosomes in
the gonads but does not affect the body of the fly. The insect does not retain any of the irradiation
and is not dangerous. The sterile pupae are then sent to a processing center located in an
eradication area where the life-cycle continues and the sterile adult fly emerges from its puparium.
Then it is released into the environment where the sterile male then mates with a native female.
Even though it is the sterile male that achieves the eradication when it mates with a fertile female,
sterile female flies are also produced and released. Investigations are being carried out to develop
the capacity to produce only male flies, however, current technology results in both sexes.
The eradication program releases the sterile flies from low-flying airplanes and the release is
carried out in a regular and routine manner following a grid pattern. Approximately 3,000 flies are
released per square nautical mile (2,266/square mile; 875/square kilometer; 9/hectare) every week.
In areas of persistent or high incidence of screwworms; special treatment grids or spot treatment
may be carried out and Field Technicians help to identify these areas and verify the release of
sterile flies. In some instances Field Technicians may help with the ground release of sterile flies.
There are two techniques for the aerial release of sterile flies, flies that are contained in small
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cartons or the free release of chilled
flies. With the box system, 1600-
2000 pupae are placed in small
carton boxes and then under
controlled environmental conditions
allowed to emerge as flies inside the
box. The box contains a food source
of sugar. The boxes are loaded into
aircraft and then ejected from the
aircraft and fall to the ground where
the fly disperses into the
environment. Field Technicians are
often asked to verify that boxes fall
to the ground on target. The other technique involves sterile flies
that are emerged and held in large environmental chambers. The
flies are immobilized with cold temperatures and the chilled flies are
loaded into refrigerated boxes and ejected from the aircraft in a
continuous stream. The fly warms as it falls to the ground and
disperses into the environment. Both techniques have proven to be
successful in eradication programs in Mexico and Central America.
Additionally, ground release chambers have occasionally been utilized to target release of sterile
flies in localized areas where aerial dispersal may not be effective and Field Technicians may be
called upon to assist with this activity.
The Eradication Program may carry out activities in the field to test the quality of the sterile insect
and the Field Technician may be asked to help. Quality Control Technicians capture flies in
specially designed traps that are serviced every day and the quantity of flies captured gives an
indication of the successful dispersal of sterile flies. They also use sentinel animals to determine if
the sterile male is successfully competing with the native male. Animals, usually sheep, are held in
pens and then an open cut is used to attract the female to deposit her eggs. The Quality Control
Technician collects the egg mass, which is incubated for 24 hours and then examined under
microscope to determine if the mating was sterile. The traps and pens are located in areas of
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natural habitat for the screwworm fly and Field Technicians are called upon to help identify these
areas.
The Field Technician is the primary
component of the Eradication
Program surveillance system. The
Field Technician is the face of the
eradication program and it is critical
that the interaction with the public is
courteous, respectful, and
professional. The primary objective
of the Field Technician is to educate
the animal owner about the
screwworm program and solicit the
individual’s collaboration by
inspecting animals for wounds,
collecting diagnostic samples,
treating all wounds infested or not with a topical insecticide, and only move or market animals that
are healthy and free of screwworm infestation. Animal owners are encouraged to treat all open
wounds with a topical insecticide that prevents infestations and livestock owners are instructed to
carry out husbandry practices that minimize the number of susceptible wounds. Surveys conducted
in other screwworm eradication programs demonstrate that the Field Technician’s regular and
routine contact with the animal owner is the most effective manner to suppress the screwworm fly
population. The Field Technician is equipped with propaganda material that helps to reinforce the
message to reduce susceptible wounds.
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The primary duty of the Field Technician is to educate the animal
owner and obtain support and collaboration with the eradication
effort. The Field Technician collaborates with Public Affairs
Officers to educate the animal owning public and helps to organize
public meetings with stakeholder groups and other public events. A
successful strategy employed to potentiate the effectiveness of the
Field Technician is the Honorary Inspector. The Field Technician
identifies individuals who serve as a liaison with the Eradication Program and the public and thus
exponentially increases the Program interaction with the public. The Field Technician must
exercise care when selecting Honorary Inspectors and must emphasize that the position is without
remuneration and that the Honorary Inspector is willing and able to act as an agent of the Program
in the community. The Field Technician provides instruction and Program material for the
Honorary Inspector and group orientation and frequent follow-up meetings help to keep the
Honorary Inspector motivated. The Public Affairs Officer assists to publicize these individuals.
A method used by the Eradication Program to measure success is
the incidence of cases of screwworms identified by diagnostic
samples. The Field Technician must encourage the animal owner to
collect and submit diagnostic samples. The Eradication Program
provides, free-of-charge, a kit to collect and submit samples that
includes a specimen tube with a 70% alcohol preservative and a
laboratory data form. Forceps and topical insecticide can also be
included in the kit. The Field Technician instructs the animal
owner to collect up to ten maggots extracted from the deepest part of
the wound; maggots on the edge of wounds may not be C.
hominivorax but instead may be maggots of secondary blow flies.
However, diagnostic samples should not be selective and all persons
should be encouraged to collect any and all suspicious worms. The
fact that the animal owner has collected a sample indicates that the
person is cognizant of the program and an excessive number of
negatives only suggest that more emphasis on education is needed.
Likewise, an excessive number of samples with only 3rd instar
worms suggest that the animal caretaker is not routinely inspecting
his animals and does not detect the
smaller 1st and 2nd instar worms.
Also, some of the 3rd instar worms
may have crawled off the wound and
go on to perpetuate the screwworm
fly population. The Field Technician
also instructs the animal owner to
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extract all maggots from the wound and kill them. The Coumaphos topical insecticide effectively
kills the maggot but may not work immediately; however, the insecticide effectively promotes rapid
healing of the wound. Unlike other insecticides, the Coumaphos powder does not repel flies and
will kill a fly that comes to a treated wounded, thus preventing infestation. The powder can be
mixed with a small amount of vegetable oil and worked into the wound. The animal caretaker
frequently uses home remedies to treat screwworm infestations that may or may not be effective.
The caretaker should be educated about effectiveness of the topical insecticide and its use strongly
encouraged. The Field Technician informs and educates the animal owner that it is important that
the laboratory data form is complete and accurate and the Field Technician should review these
forms and correct information when necessary. The Field Technician is responsible for the prompt
submission of the sample to the laboratory and the Program should support expenses incurred for
shipping the sample to the laboratory. A strategy employed to facilitate animal owners submitting
samples is to set up a sample collection center. Secondary sensors that are easily accessible by the
public are identified and provided with a supply of sample collection kits and Program propaganda.
Public Affairs Officer assists to publicize these collection centers.
The Field Technician is assigned to a defined geographical area
and is responsible for the routine contact with animal owners in
the area. The Field Technician sets up a network of
epidemiological sensors that report on the incidence of
screwworms in the area. Primary sensors are individual animal
owners or their agents who are in direct contact with animals
while secondary sensors are individuals or entities that have a
routine and regular contact with the animal owning public and
may be in a position to obtain information regarding the
occurrence of screwworms in the area. Primary sensors usually
are livestock producers or their agents or other animal owners
while secondary sensors may be veterinarians, agricultural supply
outlets, government offices and officials, livestock point-of-sales,
slaughter houses, public health units, etc. Regular and routine
contact with the sensor network is critical to success; primary
sensors should be visited every 30 days and epidemiological
information regarding the premises is collected in a data bank.
Secondary sensors can be prioritized and visited every 30-45 days.
Honorary Inspectors and Sample Collection Centers are part of this network and should be visited
as frequently as possible. A method to ensure this contact is to design sectors and work routes
whereby sensors are identified and mapped and are visited according to schedule. A sector is
covered in its entirety in one month and a work route is completed in 1-3 days. This method also
serves to facilitate the Field Technician’s activity reports and schedule, supervision, and location of
the Field Technician during an emergency. Program authorities may consider a communication
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system of radios or other two-way devices. The Program Epidemiologist can use the sector and
work route information and the data bank to determine the most efficient deployment of resources
and design response to outbreaks or other special operations. A measure of the effectiveness and
efficiency of the surveillance system is the days lost from when a diagnostic sample is collected and
when it is identified in the laboratory. Average less than 21 days, the life-cycle of the screwworm
fly, is the goal while averages greater than 21 days suggest that improvements in the surveillance
system are needed.
Areas of persistent or high incidence of screwworms or outbreaks of screwworms in areas free of the
pest may require an active monitoring of susceptible animals. The Field Technician may be
required to inspect animals, treat wounds, and collect diagnostic samples. The Eradication
Program Epidemiologist and Chief of Field Operations with the help of Field Technicians plan
these special operations. The Field Technician should be provided with appropriate equipment to
restrain animals and be familiar with restraint procedures and the Field Technician should be
aware of the danger involved when working with animals. There may be occasion to treat groups of
animals with insecticide with spray equipment or immersion vats and the Field Technician should
be provided with appropriate instruction and protective equipment when using insecticides.
The screwworm fly is a strong flier and may travel great distances in search of appropriate habitat
and in particular, wounds to deposit eggs. However, screwworms are more commonly introduced to
areas through the movement of infested animals and the control of animal movement is critical to
Program success. The Field
Technician may be called upon
to assist the Animal
Movement Control unit to set
up inspection posts or other
activities. Importantly, the
Field Technician must educate
the animal owner to only move
or market animals that are
healthy and free of wounds.
The Field Technician is responsible for reporting on activities carried out and the information
provided is useful for Program authorities for planning and reporting on Program advances. The
Field Technician maintains a daily record of activity and prepares a weekly summary. The
premises data bank and work routes need to be current and an update provided to the Supervisor
at least monthly. The Field Technician provides the Supervisor with a weekly work plan indicating
activities and work routes that will be covered during the week. The Daily Activity Report, Weekly
Summary and Work Plan reports are useful for the Supervisor and Program authorities to evaluate
how well the Program is delivered to the field. The Field Technician is under the supervision of an
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Area Supervisor who helps the Field Technician perform the job with the most impact and
efficiency.
The public’s acceptance of the Eradication Program and collaboration with the Program is critical
for success. The primary goal of the Field Technician is to achieve this acceptance and
collaboration. The Field Technician is the primary contact with the animal owning public and
performance of the Field Technician is the key to the successful eradication of this costly pest.
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