THE INSECT

 

The screwworm is the larva of a fly whose scientific name is Cochliomyia hominivorax (Coquerel, 1958). This parasite is harmful, in terms of production and mortality for all warm- blooded animals, i.e. cattle, pigs, sheep, horses, wild animals and pets; including humans which are not exempted from being attacked by this insect.

 

MORE ABOUT THE SCREWWORM:

  1. Taxonomy of the insect
  2. Pathology
  3. Life cycle
  4. Clinical signs of Infestation

TAXONOMY OF THE INSECT

KINGDOM: Animal
PHYLUM: Arthropoda In this group there are invertebrates with jointed limbs.
CLASS: Insecta Includes all arthropods with three pairs of legs.
SUBCLASS: Pterygota Insects with wings.
ORDER: Diptera Insects with a pair of wings and one pair serving as modified flight stabilizer (halteres). To this order belong flies and mosquitoes.
DIVISIÓN: Endopteygota Insect with internal wings. The formation of wings begins in the larval phase internally and fully manifested in the adult stage.
FAMILY: Calliphoridae Flies that produce miasis. Most feed on dead tissue, except hominivorax screw-worm, which feeds on living tissue.
GENDER: Cochliomyia Cochlio (Latin): In a spiral or screw form
Myia (Latin): Fly
SPECIES: hominivorax Man eater

PATHOLOGY

The pathological effects of screwworm fly infestations in the parasitized host can be divided into four main components:

  1. A traumatic effect, caused by the larvae when ripping off the tissues of the host with their hooked mouthparts.
  2. An irritant effect, caused by the constant borer movement of the larvae inside the wound.
  3. Secondary infections of exuding wounds, caused by other contaminating organisms such as bacteria, viruses, protozoa and fungi.
  4. The toxic effect caused by larval excretions of waste products.

An infected animal can survive only a few days if the infestation is severe and not treated early. Even with treatment, particularly if delayed, secondary infections can spread through the bloodstream and cause arthritis, enteritis and septicemia.

The excretions of screwworm produce necrosis to the infested tissue, which by its smell attracts other species of Diptera that infest the outside screwworm area while still widening and deepening the wound.

Factors contributing to death are:

  1. Secondary infections caused by bacteria and other microorganisms.
  2. Toxemia and fluid loss.

LIFECYCLE

Like other insects, screwworm fly has four phases or stages in its life cycle which are: adult, egg, larva and pupa.

Adult: They are blue – bright green (metallic) colored, their eyes are reddish orange. In the case of the male they are united (holoptical) and in the female there is a separation between both eyes (dichoptic), and they are about twice the size of a housefly.

Males are sexually active and can copulate several times in the course of their adult lives. While females mate only once in their lifetime and an oviposit can put between 10 and 1,600 eggs (3,000 in some cases) into three or four groups of 10 to 400 each. This oviposition performed in the dry edges of the wounds of warm-blooded animals (looking the hardest part without fluid, usually to one side thereof). It can be anything from a tick bite up to a larger wound, newborn navel cut, dehorning, castration, tears due to wiring, fresh cow vulvas, etc.

Adults have an exceptional ability to travel up to 290 kms, in less than two weeks. This translation is done by taking advantage of shades offered by the vegetation.

Egg: They are of short duration so they are not usually collected at this stage for identification.

They are deposited in masses (maggots) at the edges of the wounds of warm-blooded animals, usually presenting a placement or array (as tiled roofs), being all oriented in the same direction and joined together by a solid concrete, making their separation difficult. This order can be altered by the lack of rest or stillness of the host at the time of posture (blows from the tail onto the wound, biting, kicking, scratching with the limbs, wallowing in the grass or dirt, among others.)

Each little egg has a tough outer shell called corium (with reticulated or lace appearance) containing an embryo which is completely transformed into first instar of larvae before hatching. The tape hatching or dorsal suture is a weak line along the ovule from the operculum to the opposite end, which opens at the time that the first instar larvae is ready to go, and between 11-21 hours after posture.

Larvae: this stage is usually found more frequently in cases of screwworm. The fitting of worm is caused by the larvae of the fly (obligate parasite) called Cochliomyia hominivorax.

Their injuries are typical, large and deep, with highly unpleasant odorous exudates which become more attractive to gravid females.

In this evolutionary cycle there are three larval instar which last from 4 to 8 days, as detailed as follows:

  1. First instar larvae: When the larva leaves the egg, it is introduced into the wound and its differentiation from other species at this level of development is not reliable. Employing the external morphological observation technique it can be observed that it has two spiracles and each one has one or two oval blowholes extremely close together towards the bottom, looking like a “V”. The peritreme is not visible; tubers surrounding the anal cavity are not yet clearly defined. During this stage perhaps the most important element for taxonomic identification is the cephalopharyngeal skeleton.
    The two elements used for this level are:
    – The size of the skeleton is shorter than in other species
    – The presence of oral escleritos. Properties that are not present in the next larvae instar of the same species.
  2. Second instar larvae: each segment of the body has large amount of spinets from 1, 2 or 3 tips (bifida or trifid). The anterior spiracles are fully visible and they have from 7 to 9 extensions or lobed in shape of a finger. The anterior spiracles are also visible and can be appreciated with an incomplete periderm, which surrounds the two respiratory holes that have larger separation between each other than in the anterior instar. Pigmented tracheal trunks appear at half of the last segment. The process can be observed with more details from the anal protuberance.
  3. Third instar larvae: This instar larva is the best to identify the screwworm. At the end of the “molt” it has a white creamy color and the larva matures to a reddish color before passing to the next pupa phase. It is a typical calliphoridae larva, with a cylindrical body, truncated later and the segments surrounded by spine rings. These spines are large and robust and mostly they have one or two tips, been able to have three tips (trifurcated). The anterior spiracles have an incomplete periderm of dark color, existing inside of each one of them three respiratory holes or clefts. The tracheal trunks have a dark pigmentation from the spiracles until the tenth or ninth segment and can be seen in the posterior dorsal surface. In the anal protuberance the spiracles are disposed horizontally.
  4. Pupae: when the larvae of third instar has reach full maturity it falls naturally from its host, falling into the ground and moves a few centimeters and buries itself from 10 to 12 cm. Is in this period when the pupa phase begins in which the larva suffers a serious series of changes to give origin to the adult phase. In this phase the puparium is formed, product of the larva skin hardening and in its surface can be seen the bands of large spines surrounding the segments of the larva; it is reddish brown color and has cylindrical shape, rounded in both ends and measures approximately 10.2 mm long by 4.3 mm wide. The term of this phase is highly influenced by the variations of temperature, humidity and the ground in which is exposed.

CLINICAL SIGNS OF AN INFESTATION

The infestation begins when the screwworm eggs incubated in the wound of an animal or host and the larva proceeds to feed of the live flesh. It is difficult to see on first sigh in a wound, the presence of screwworm larvae in its first stages of life; only can be observed by very slight movements. As the larva feeds, the wound gradually enlarges, becoming larger and deeper.

In this stage is frequent that the female flies have laid its eggs inside the wound, developing in this way a multiple infestation. The infected wound often bleeds and releases a very peculiar smell. Can also be that the wounds on the skin look little; however right under these there are large cavities which house larvae that are actively feeding.

The animals infested with screwworm may die in a few days if the wounds are not healed. This animal suffers general discomfort; loss of appetite and the females produces less milk.

Typically these animals are separated from the rest and seek for isolated areas or with shadows to get laid.

The screwworm has seriously affected the livestock in the American Continent, and specially the cattle. The wounds caused by dehorning, castration, iron marking, ticks bites, and in particular birth are very attractive to the oviposition of the female flies.

The economic losses produced are large, because even when the affected animals not always dies, it susceptibility to others diseases, the meat and milk production decreases and the livestock remains harmed.

In addition, the cost of labor necessary for the inspection and treatment of the cattle and the cost of the proper treatment can be very expensive. The wild animals probably suffer and even worst devastating effects because they are unmanageable and cannot be benefited by the measures of treatment and protection that domestic animals get.