Screwworm

Cochliomyia hominivorax, the New World screw-worm fly, or screw-worm for short, is a species of parasitic fly that is well known for the way in which its larvae .
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When the sterilized males were in competition with normal males, the ratio of sterile-to-fertile egg masses deposited by normal females was essentially equal to the ratio of sterile-to-fertile males caged with the females. A field trial was conducted on Sanibel Island, Florida, which is separated by 3 km of water from the mainland peninsula. As much as 80 percent of the egg masses collected from goats in the release area were sterile failed to hatch , but eradication could not be demonstrated because of the immigration of normal, mated female screwworm flies from the mainland.


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Screwworms were mass-reared in a facility near Orlando, Florida. Each week sterile flies were released per square kilometre during the eradication phase. The zone was treated weekly with sterile flies per square kilometre. Because immigration of mated female screwworm flies into the test zone from the untreated area was likely, eradication was not expected. Rather, the scientists anticipated a high weekly egg mass sterility level of 50 percent. After three months of sterile insect releases, the level of egg mass sterility reached 70 percent near the centre of the test zone, and the natural population in the test zone appeared to have declined by about 90 percent Baumhover, The winter of was one of the coldest in history in Florida.

To take advantage of the resulting reduction of the screwworm population, the research facility at Bithlo, Florida expanded its production from 2 to 14 million sterile flies per week Graham and Dudley, The distribution of the sterile flies in the southern Florida peninsula further reduced the screwworm population that survived the winter and was responsible for slowing the migration north during the spring and summer.

A new rearing facility was constructed near Sebring, Florida and initiated operations in July Within three months the weekly production of sterile flies from this new facility was in excess of 50 million per week, with an average of 50 to 75 million per week for the duration of the eradication programme Smith, Sterile flies were packaged in boxes, with flies per box, which were dispersed at the rate of 76 to sterile flies per square kilometre from single-engine planes flying about m above the terrain.


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Planes flew over treatment areas in Florida, southern Georgia and southern Alabama each week on 3. Sterile fly distribution was supplemented by a field organization that inspected livestock, educated livestock owners and practicing veterinarians, investigated reported screwworm infestations and monitored sterile and native fly activity through use of liver-baited traps.

Florida agriculture officials operated a quarantine line across the northern part of the state where all livestock moving north were inspected and permitted movement only if they had been sprayed with insecticide at the point of origin. USDA officials operated a quarantine line along the Mississippi River from Memphis, Tennessee south to the Gulf of Mexico where all livestock moving from west to east were inspected and treated to prevent transport of screwworms to the eradication area. The last reported screwworm infestation in the southeastern United States was reported in Florida in Meadows, Southwestern United States As a result of producer interest, the eradication programme was expanded to encompass the southwestern United States Scruggs, , see Figure 3.

Objectives were to prevent the reintroduction of screwworms into the southeastern United States; to eradicate screwworms from Texas, Arizona and California; and to prevent reinvasion into the United States from Mexico. Prior to , livestock owners estimated that one million cases were treated in Texas in a normal year.

Thousands of cases were reportedly treated each year in Louisiana, Arkansas, Oklahoma, Kansas and Missouri as a result of the migration of screwworms from Texas. In a massive educational campaign was conducted. Hundreds of thousands of sample vials and forms to be completed were distributed to livestock owners.

They were instructed to collect a sample of larvae from each animal treated for a suspected screwworm infestation and to return the sample and form with pertinent data to the designated laboratory for identification. The winter of was very severe, and the screwworm population in the overwintering area was reduced to an unusually low level.

USDA Agencies Work Together to Eradicate an Old Foe: the Screwworm | USDA

Weekly production was initially 5 million but increased to over 22 million flies by May Sterile flies were dispersed at the rate of 80 per square kilometre over central Texas. The effects were monitored by collection of egg masses from wounded sheep, and as many as 75 percent were found to be sterile Bushland, In June , production of sterile screwworm flies began in a new plant in Texas. Production of flies gradually increased from 28 million per week in July to 75 million per week by October Subsequently during many livestock owners found no screwworm-infested animals on their ranches for the first time in modern history, and in Texas and New Mexico were declared free of endemic screwworm infestation.

The screwworm population remained low in the southwest because of hot, dry weather much of the year, and for the first time in recent history no cases were reported in Oklahoma, Arkansas and Louisiana. The programme was extended into Arizona in Pupae were packaged, and the emerged adult sterile flies were distributed over Arizona and northwestern Mexico.

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As a result, only screwworm cases were reported from Arizona, compared to the several thousand cases reported annually in previous years. California was added to the programme in , thus forming a complete barrier with strategic and selective distribution of sterile flies each week along the Mexico-United States border from the Pacific Ocean to the Gulf of Mexico, an expanse of about km see Figure 4.

Unfavourable weather for screwworm survival for at least three months of each year eliminated the need to release sterile flies over the entire expanse all through the year.

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The highest screwworm populations were on the east and west coasts of Mexico, and sterile fly dispersals were concentrated in these areas during the winter months. This reduced the screwworm population in areas adjacent to the border with the United States and delayed the spread of screwworms northward each spring. Approximately 80 percent of the production of sterile flies was distributed along the eastern half of the United States border, where more animals were at risk and where the climate was more suitable for screwworm than in New Mexico, Arizona and southern California.

Boxes containing instead of sterile flies were used, and the weekly fly dispersal swaths were widened to Sterile fly production was increased in ; million flies were released weekly. This increase, combined with hot, dry weather in New Mexico, Arizona and California, helped keep the screwworm population low. The winter of in Texas was cold, and no screwworm cases were reported for the first three months of , but favourable weather conditions contributed to a steady increase in infestations during the year until cooler weather again reduced the area and numbers by November.

From to , livestock owners and officials worked hard to improve screwworm case reporting from northern Mexico and the United States. An intensive educational campaign by the Agricultural Extension Service supported livestock owners in their continuing need to inspect animals, to treat screwworm infestations, to treat wounds as a prevention and to avoid making wounds.

Another cold winter in , following a year when the screwworm population was already low, created an almost ideal situation in Texas remained nearly screwworm-free that year, except for a summer outbreak in one area. Dry weather continued in southern Texas during the first half of Livestock movements out of this drought-stricken area in April or May were implicated in the introduction of screwworms into the Red River Valley in extreme northeast Texas Novy, , more than km northeast of the previous reported screwworm infestations that year.

The first case, reported toward the end of June, probably resulted from the second or third generation of screwworms after introduction into the area. From June to October 54 cases were reported, confined to an area 16 km north-south and 40 km east-west, with the Red River passing centrally through the infested area.

Most livestock in the area were sprayed with coumaphos insecticide at least twice in July and August as a preventive measure. Sterile fly releases were made from June to October at the rate of to per square kilometre on 1. Three tropical storms moved inland along the Texas and northeastern Mexico Gulf Coast in the late summer and autumn of There was an abundance of rain.

The screwworm population began to build up despite releases of to sterile flies per square kilometre, averaging Livestock owners had reduced the amount of labour and the use of trained horses over the years because screwworms were under control. The number of absentee livestock owners had increased and cattle populations in southern Texas had increased Deer populations had also increased because of the lack of screwworms, and brush clearing with improved range conditions allowed the land to support the increasing animal population.

Warmer than normal weather during the winter of contributed to an exceptionally high screwworm population in northeastern Mexico. Screwworm survival in southern Texas was also significant. The number and range of screwworm-infested animals in Texas increased rapidly during the first four months of Maximum production in Texas of By mid-May, the screwworm situation was completely out of hand.

Perhaps even if maximum production had been achieved earlier, it would have had little effect because of the existing favourable conditions. Mother screwworm flies are attracted by the smell of open wounds. Upon finding one, the fly lays its eggs, which hatch and go to work. Little spines on the outside of the screwworm anchor into the tissue, making the creepy crawlers hard to remove.

Aside from open wounds, screwworms can invade mucus-covered tissue — like that in eyes, ears, nostrils and the anus. The buggers can also squirm into the belly buttons of newborn mammals. The screwworms then expand their feeding area from skin into deeper tissue, such as muscles. The larvae eventually crawl out of an animal and fall to the ground, where they mature into adult flies An infestation of screwworm maggots on a human body is called myiasis. In fact, human infestations, in general, are rare. No medications are available , at least none approved by the FDA. Therapy options are limited to surgical removal of the pests and antibiotics to prevent secondary infections by other germs.

Ranchers can tend to tackle livestock infestations by rubbing approved insecticides into the wound , which is followed by pulling out individual larva with tweezers.

7 reasons flesh-eating screwworms are as gross as you think

Slide of treating ear of screwworm infested pig. Any open wound is attractive to the New World screwworm, either natural or human-inflicted. Female screwworm eggs reach maturity within five days after hatching. This can result in up to 3, larvae being present in a single wound.

For now, she says the most effective prevention method is to watch your animals. A 4-year-old girl in India was once treated for a screwworm infestation inside her rectum. Her rectal ulcer had grown to 1. Another case of a year-old girl was reported in , after she returned to the U.

Early eradication programs in the s involved sterilizing male screwworm flies with cobalt gamma ray radiation. Knipling pictured and Dr. Bushland pioneered the technique. In , the U. To achieve maximum eradication, the Mexico-United States Commission developed flight paths to release millions of sterile flies in Mexico, Panama, El Salvador, Costa Rica and other areas at risk including Texas, Arizona and California. The mutant males competed with normal ones for females, and ultimately wiped out the New World screwworm population from the U.

But eradication labs tend to move south, once an affected area has been eradicated. Today, USDA and its partners maintain a permanent sterile fly barrier between Panama and Colombia to prevent the establishment of any deadly flies from South America. This could result in a post mortem interval being off by a few days at the most if the C. Secondary screwworms have the stereotypical metallic green body of the genus, and the larvae are extremely similar to those of C.

The most effective way to differentiate the two is to note the absence on C. Interaction between humans and C. However, given the medical stigma surrounding the screwworm fly, and the slight potential for even C.

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Primary screwworms are primary, obligate parasites in the larval stage, and as a result are capable, unlike secondary screwworms, of initialising the penetration of the skin barrier to create an entry wound. Despite this, they are most commonly seen as colonisers of previously existing wounds, and frequently are hatched from eggs laid at the perimeter of a wound.

Once the infestation commences, a dark brown or reddish-brown discharge begins leaking from the wound, sometimes accompanied by an unpleasant smell as the flesh begins to decay. This is often the first sign in both livestock and human victims that something is amiss, and often initiates consultation with a professional. As the infestation increases, the victim begins to experience escalating tissue irritation, and in the case of domesticated animals, may be observed to become withdrawn, listless, and anorexic. Once the process of clinical diagnosis begins and myiasis is recognised, the larvae are fairly easy to identify.

Their overall body structure resembles the spiraled screw shape on which their common name is based, a shape fairly unusual within parasitic larvae. The cranial end of the larvae has two sharply curved hooks, generally dark in color, and distinctive spiracle patterns are observed at the caudal end. The most identifying features, and the easiest way to differentiate between C. Treatment of the victim can be time-consuming and, due to the high incidence of secondary infection, frustrating, but with decisive treatment, a surprisingly positive result is often achieved in all but the worst cases.

The obvious first step is the manual removal of the maggots, generally using tweezers or forceps to seize the larva at the posterior end as the spiracles emerge to allow respiration. Once all larvae have been removed, a topical antibiotic smear is applied, often with an oral antibiotic accompaniment. Necrotic tissue may need to be debrided, which can be a painful process. A loose dressing is essential to allow continued fluid drainage from the wound.

As with many things, prevention is the best cure. Any open wound, even one so small as a blister, is a potential infestation site, and should be treated accordingly with approved pesticides. At times of the year when livestock are most vulnerable castration, birthing, etc. Infestations detected early are quite treatable, but fatalities can and do result from advanced infestation, particularly in sheep and newborn calves. Secondary screwworms appear only around an existing wound wound in this case being defined as including the opened cord area of newborn livestock , so practice facultative myiasis.

Flies can also be induced to lay their eggs in hair or wool that is matted thickly and stained with any bodily fluid. The larvae can be diagnosed and identified by a trained professional based on larval size, shape again following the spiraled screw-shape , and ultimately on spiracle pattern. These larvae lack the distinctive pigmented tracheal tubes that mark C.

As the secondary screwworm does not have the biological characteristic of single lifetime breeding as does C. However, due to its being associated with facultative myiasis, its impact on livestock in the United States is not nearly as severe. It can also be associated with human infestation in poorly run medical facilities and areas stricken by poverty, so vigilance is essential.

Both species thrive in tropical areas which are warm and humid. However, the bordering Central American countries serve a challenge to keep the species eliminated since these countries still have populations of this fly. Many of these countries continue to implement elimination programs. From Wikipedia, the free encyclopedia. Cochliomyia Townsend , [1]. Journal of the Washington Academy of Sciences. Washington Academy of Sciences. Calliphoridae of America North of Mexico". Proceedings of the Entomological Society of Washington.

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