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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


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


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


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


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.


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


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


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


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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