We realize that this is likely an inappropriate time of
the year to publish an article on Screwworm. But because
it was recently in the news that positive and false positive
cases were reported in the U.S.,
we will print the following article which was plagiarized
from a USDA/APHIS article found on the world wide web as a
reminder of its potential for entrance. The U.S.
has spent millions of dollars in successfully eradicating
Cochliomyia hominovirax; if they are reintroduced,
it could well be a veterinarian who would make the first identification.
The diagnosis of this screwworm occurrence is classified as
an exotic myiasis and, therefore, a reportable disease. Being
aware that it would be rare to find this parasite in Indiana,
we are also cognizant of the importance of identifying it
early if it should arrive here so that immediate eradication
actions can be taken.
A screwworm infestation is caused by larvae of the fly
Cochliomyia homino-virax. These larvae can infest
wounds of any warm-blooded animal, including human beings.
The screwworm fly is about twice the size
of a regular house fly and can be distinguished by
its greenish-blue color and its large reddish-orange eyes.
Infestations can occur in any open wound, including cuts,
castration wounds, navels of newborn animals, and tick bites.
The wounds often contain a dark, foul-smelling discharge.
Screwworm larvae distinguish themselves from other species
by feeding only on the living flesh, never dead tissue.
Once a wound is infested, the screwworm can eventually kill
the animal or human, literally eating it alive.
After mating, the female screwworm fly fly lays her eggs
in open wounds. One screwworm fly can lay up to 400 eggs
at a time and they can hatch into larvae in as little as 12
hours. A single adult female can lay as many as 2800 eggs
during its 31-day lifespan. The screwworm larva grows
inside the wound to greater than one-half inch within 5 to
7 days of entering the wound. The full-grown larva
then drops from the wound, tunnels into the soil, and forms
an immobile protective case that houses the pupa. An adult
screwworm fly emerges from the pupa.
Screwworms are eradicated through a form of biological control.
Millions of sterile screwworm flies are raised in a production
plant located in Tuxtla Gutierrez
in the southern Mexican
State of Chiapas
near the Isthmus of Tehuantepec. During
the pupal stage of the flys life cycle, the pupae are
subjected to gamma radiation. The level of radiation is designed
to leave the fly perfectly normal in all respects but one:
it will be sexually sterile. Thus, when the artificially
raised flies are released into the wild to mate with native
fly populations, no offspring will result from the matings.
These unsuccessful matings lead to the gradual reduction of
native fly populations. With fewer fertile mates available
in each succeeding generation, the fly, in essence, breeds
itself out of existence. The screwworm fly can travel up
to 180 miles in several days and under warm, favorable conditions
can complete a life cycle in as few as 3 weeks. Left
untreated, screwworm-infested wounds lead to death. Multiple
infestations can kill a grown steer in 5-7 days.
As early as 1825, western States reported serious screwworm
problems. Infestations spread to the Southeast by the 1930s.
Losses to livestock producers exceeded $400 million annually.
A plan for eradicating the pest began in the early 1950s,
when USDAs Agricultural Research Service developed a
new control method. Under this method, laboratory-raised
flies sterilized by gamma rays are spread by aircraft over
infested areas. As millions of sterile flies flood an area,
the sterile males mate with fertile female flies. The resulting
eggs do not hatch. This sterile insect technique was tested
in a field trial on the Dutch island
of Curacao in 1954
and then used operationally in Florida
by 1957. By 1959, screwworms had been eradicated from the
The sterile insect technique was next applied to the more
extensively infested Southwest starting in 1962. Self-sustaining
screwworm populations were eliminated from the United
States by 1966. A barrier
zone of sterile flies was set up along the 2,000-mile-long
U.S.-Mexican border to prevent reinfestation from Mexico.
However, constant reinfestation from migrating flies or larvae
carried by animals, which are then transported by people,
remained a problem. The United States-Mexico Joint Commission
was formed in 1972 between Mexico
and the United States
with the goal of eliminating the pest from Mexico
and pushing the barrier to the Isthmus of Tehuantepec,
just north of Guatemala.
A new sterile screwworm plant at Tuxtla
Gueierrez, Chiapas, Mexico,
was dedicated in 1976. With a production capacity of more
than 500 million sterile flies per week, it replaced the former
production plant in Mission,
Texas, which was closed
in January 1981. APHIS also is cooperating with Central American
countries and Panama
in efforts to eradicate screwworms from those countries and
establish and maintain a barrier of sterile flies at the Darien
Gap between Panama
As a result of these cooperative efforts, Mexico
was officially declared free of screwworms in 1991, Belize
in 1994, and El Salvador
in 1995. In addition, Honduras
is considered technically free, with no pest detections since
January 1995. Currently, screwworm program officials are
focusing their efforts on eradicating the pest from Nicaragua
and Costa Rica.
APHIS hopes to begin eradication activities in Panama,
the final frontier of the program, in 1997. Eradication activities
include regulation of cattle movement, wound treatment, and
the release of sterile flies. To date, the program has been
The screwworm programs resources were strained by
introduced outbreaks that occurred in Mexico
in 1992 and 1993. Of the 66 cases that were identified, the
closest one was only 121 miles from the U.S.
border. While these outbreaks were eliminated, the resources
that APHIS spent in eradicating them pushed back the entire
Central American program by 12 to 18 months.
As part of the screwworm programs overall strategy,
a new sterile fly rearing facility in Panama
will be established to replace the existing one in Mexico.
The Panamanian government has agreed to provide land for the
new facility. Establishing the new facility well outside
the area where screwworms have been eradicated will reduce
the risk of reinfestation through an accidental release of