MYIASIS

Anna M. West

Hum Bio 103. Parasites & Pestilence: Infectious Public Health Challenges

Program in Human Biology, Stanford University

Instructor: D. Scott Smith, M.D. 

 

I.  Introduction

Myiasis is the infestation of tissue with fly larvae, commonly referred to as maggots.  It is widespread in the tropics and subtropics of Africa and the Americas, and occurs with significantly less frequency in most other areas of the world.  The infestation is most often subcutaneous and produces a furunculoid or boil-like lesion, but it is also known to occur in wounds and certain body cavities.  As travel to endemic regions becomes more common, physicians, particularly dermatologists, in non-endemic areas are increasingly confronted with cases of myiasis.  Physicians in the northern developed countries may be unfamiliar with this parasitic infection; thus, misdiagnosis and inappropriate treatment regimens are not infrequent.  Greater awareness on the part of physicians about clinical symptoms and relevant exposure histories would improve the expediency and efficacy of treatment for patients with myiasis. 

 

II.  Agent (and Synonyms)

Myiasis may be caused in human hosts by several species of arthropods of the order Diptera, the two-winged true fly.  Two approaches to classification are possible, one entomological and the other etiological, derived from differences in the behavior of the various species of fly and the nature of the parasitic relationship (Noutsis and Millikan).  Only those agents with public health significance will be explored in greater detail, but the taxonomy provides a notion of the breadth of sources and manifestations of myiasis. 

 

A.  Taxonomic Classification of Myiasis-Causing Flies

 

Family Muscidae                 Larvae of the species Fannia canicularis (lesser house fly) and Musca domestica (house fly) can cause wound myiasis when eggs come into contact with ulcers. 

 

Family Calliphorida             Genus Cochliomyia:  The New World screw worm, Cochliomyia hominivorax, is found throughout the southern United States, Central America, and tropical regions of South America.  Adult females deposit eggs in open wounds or discharging orifices, such as the nose.  Larvae invade adjacent living tissue, including cartilage and bone.  Because infestation of ears and nose provides the larvae with access to brain tissue, myiasis caused by C. hominivorax is more dangerous than infestation with other agents.  Photo credit: M.J.R. Hall.

 

Genus Cordylobia:  Cordylobia anthropophaga (tumbu fly) is found throughout sub-Saharan tropical Africa.  Cordylobia (Stassisia) rhodaini (Lund’s fly) is found primarily in rain forests.  Adult females deposit eggs on sand, soil, bedding, or clothing, particularly if soiled with feces or urine.  Larvae hatch and use oral hooks to infect the unbroken skin of a human host.

 

Genus Chrysomia:  Flies of this genus are distributed across Africa, Australia, and Asia.  Infestation with larvae of the Old World screw worm, Chrysomyia bezziana, can be disfiguring.  Like the New World screw worm, C. bezziana can invade bone.  Infestation of the eye is particularly dangerous.

 

Genus Auchmeromyia:  The Congo floor maggot, Auchmeromyia luteola, does not infest human tissue but must feed on human blood.  The larvae inhabit soils in tropical Africa, and feed on persons sleeping on dirt floors.  

 

Family Sarcophagidae         Genus Sarcophaga:  Some species of Sarcophaga are known to cause wound myiasis.

 

Genus Wohlfahrtia:  Wohlfahrtia magnifica larvae infest the ear, eye, and nose, damaging living tissues.  W. magnifica is found in south-eastern Europe, southern and Asiatic Russia, the Middle East and North Africa.  The larvae of the North American species, Wohlfahrtia vigil and Wohlfahrtia opaca, are incapable of penetrating adult skin; infestation occurs only in infants.

 

Family Cuterebridae            Genus Cuterebridae:  Rodent or rabbit flies of this genera cause infrequent cases of myiasis in North America.

 

Genus Dermatobia:  Dermatobia hominis, the human botfly, has a distribution ranging from northern Argentina to southern Mexico, and is usually found in warm, humid lowland forests.  D. hominis causes cutaneous myiasis in humans and other mammals.  Infestation of cattle has important economic consequences.  Adult females deposit eggs on blood-sucking arthropods, which transmit the infectious larvae to the host.

 

Family Gasterophilidae       Genus Gasterophilus:  Gasterophilus intestinalis, the horse botfly, causes “creeping eruption,” a migratory form of cutaneous myiasis with some resemblance to migratory helminthiasis. 

 

Family Oestridae                  Oestus ovis, which commonly parasitizes sheep and goats, and Rhinoestrus purpureus, which usually infests horses, infrequently cause ocular myiasis in human hosts. 

 

Genus Hypoderma:   The warble flies, Hypoderma spp., are cattle parasites.  The uncommon  infestations in human hosts produce migratory subcutaneous swellings or harmful invasion of the eye. 

 

B.  Etiological Classification of Myiasis-Causing Flies

 

Obligatory (Specific) Agents

Both D. hominis and C. anthropophaga are obligatory parasites, whose larval stages can occur only in the living tissue of animal or human hosts.   Myiases caused by larvae of the human botfly and the tumbu fly predominate, but several other species that generally parasitize animals occasionally cause infections in humans as well.  Other obligatory flies include the genera Oestrus, Rhinoestrus, Gasterophilus, Hypoderma, Chrysomyia, and Wohlfahrtia.  Transmission and the type of tissue affected varies with the species of the parasite.  

 

Facultative (Semi-specific) Agents

Flies of the family Sarcophagidae normally develop in decomposing tissue and are considered facultative parasites.  Larvae of these flies parasitize wounds and other damaged tissues, and some species further invade living tissues adjacent to the wound.  Important genera in this category include Musca, Calliphora, and Lucilia. 

 

Accidental (Non-specific) Agents

Accidental myiasis occurs when egg-stage flies are ingested on contaminated food or come in contact with the genitourinary tract.  Flies of the families Muscidae, Calliphoridae, and Arcophagidae may be involved (Powers and Yorgensen).

 

III.  History of Discovery

Maggots have an important role in the history of biology, as they were central to experiments that rejected the theory of spontaneous generation.  Well into the 17th century, European scientists believed that rotten meat itself gave rise to maggots and flies.  In 1668, the Italian poet and physician Francisco Redi (1626-1697) conducted the following experiment, one of the first ever to utilize appropriate controls: he placed samples of meat in two series of jars, half of them lidded and the other half open, and watched the meat for signs of rotting and myiasis.  The meat in the lidded jars began to rot but, having had no contact with adult flies, did not produce maggots, while the meat in the open jars was visited by adult flies, became infested with fly larvae, and eventually produced more adult flies.  Redi correctly judged that the maggots developed from eggs, too small to be seen, that were deposited on the meat samples by the adult flies (Britannica). 

 

IV.  Clinical Presentation in Humans

Furuncular Cutaneous Myiasis

Furuncular myiasis is caused by both the human botfly and the tumbu fly.  The location of lesions varies because of the different means of transmission.  Myiasis caused by the tumbu fly, C. anthropophaga, usually appears on the trunk, buttocks, and thighs, while myiasis caused by the human botfly, D. hominis, commonly occurs on exposed sites such as the scalp, face, forearms, and legs (Lucchina et al).  While abrasions and wounds are commonly inhabited by several larvae, perhaps even by several species of larvae, furuncular and migratory myiatic lesions usually contain one or very few larvae (Bapat).

 

Larval infestation of cutaneous tissue results in the development of a pruritic papule, of aproximately 2 to 3 mm in diameter, within 24 hours of initial contact with the larvae (Purych-Alberta, Swetter et al).  This papule gradually extends, forming a lesion with diameter ranging from 1.0 to 3.5 cm, and a height between 0.5 and 1.0 cm (Purych).  The respiratory sinuses and occasionally the posterior end of the larva itself may be seen through a central punctum of 2 to 3 mm in diameter (Purych, Noutsis and Millikan, Swetter et al, Tsuda et al).  The patient may describe a sensation of pain “caused by the tearing of host tissues as the larva feeds or from the outer spines irritating the surrounding skin as the larvae moves” (Purych).  No systemic symptoms are observed (Purych). 

 

In addition to the symptoms described above, which apply to both the human botfly and the tumbu fly, furuncular infestations caused by C. anthropophaga may become “crusted, odoriferous, purulent, or [may exude] serosanguinous discharge” (Lucchina et al).  Patients infested with tumbu fly larvae often exhibit a greater number of lesions; Jelinek notes that this frequently results in faster referral to a dermatologist than in cases of infestation with the human botfly. 

 

Analysis of tissues exhibiting an inflammatory response to maggot infestation reveals a high concentration of lymphocytes, giant cells, neutrophils, eosinophils, and plasma cells.  There is no inflammatory response to infestation with larvae of Hypoderma spp (Noutsis and Millikan). 

 

Secondary infection by bacteria is uncommon, because “bacteriostatic activity in the gut of the larva seems to prevent undesirable overgrowth of pyogenic bacteria” (MacNamara and Durham).  

 

Furuncular myiasis caused by rodent or rabbit botflies, Cuterebra spp., may present as “subcutaneous abscesses on the face, scalp, neck, shoulders or chest” (Shorter et al).  These cases are acquired in North American, generally during the summer months, and involve a single lesion with only one larva contained in the tissue cavity, whereas other forms of myiasis often present with several lesions, each containing several larvae. 

 

Related Links and Case Reports

                                                        

The New England Medical Journal’s “Images in Clinical Medicine: Furuncular Myiasis” describes a case of myiasis caused by D. hominis and shows a mature human botfly larva spontaneously emerging from a furuncular nodule on a patient’s upper arm. 

A preview…

 

Exotic Myiasis with Lund’s Fly (Cordylobia rhodaini), from the Medical Journal of Australia, describes furuncular myiasis caused by C. rhodaini, and provides SEM photos of larvae, a life cycle diagram, and a distribution map for Lund’s fly. 

 

Dermatobia - Cutaneous Myiasis, a case presentation from the University of Alberta, includes an excellent overview of clinical and epidemiologial aspects of the infestation. 

 

            The Pus is Moving: A Case of Cutaneous Myiasis

 

Non-inflammatory cutaneous myiasis caused by the larva of Cordylobia Anthropophaga.  An illustrated article from the European Journal of Dermatology.

 

 

Wound Myiasis

Also referred to as traumatic or opportunistic myiasis, wound myiasis occurs when flies deposit larvae in decomposing flesh, or in a supparating wound.  Some species of larvae remain within the decaying tissue, while other species continue feeding on living tissue nearby, and may produce subcutaneous nodules (Noutsis and Millikan). 

 

 

 

Figure 1.  Larvae developing in an area of distal gangrene of the foot.  

Photo credit: Larry E. Millikan.

 

Myiasis of Body Cavities

The human botfly, D. hominis, and both the Old World and New World screwworms, C. bezziana and C. hominivorax, may cause invasive myiasis of the eye, orbit, nose, or ear canal, in addition to wounds and open sores.  Destruction of tissue is only the beginning of the problem, as larvae may gain access to the brain, where they can cause meningitis, leading to death (Noutsis and Millikan). 

 

            Related Links and Case Reports 

 

Bilateral Ophthalmomyiasis Interna Posterior: Report of a Case with Severe Visual Loss  Diverse management and treatment options are presented. 

 

Ophthalmomyiasis: Diagnosis and Discussion.  Grand Rounds. 

 

Anterior Orbital Myiasis Caused by Human Botfly (Dermatobia Hominis).  A pediatric case is described and the surgical treatment pictured sequentially. 

 

Creeping/Migratory Cutaneous Myiasis

Creeping or migratory myiasis may be caused by Hypoderm bovis and Hypoderma lineatum.  It generally occurs in persons coming in close contact with cattle (Bapat, Noutsis and Millikan).

 

            Related Links

            “Neonatal Myiasis” from Pediatrics Online.

 

Accidental Myiasis

Accidental myiasis may be caused in two ways.  First, eating food contaminated with fly larvae may produce vomiting and diarrhea, abdominal pain, and anal pruritus (Aguilera et al).  Severity of symptoms depends on the number of larvae that are established in the intestinal tract.  Alternately, because flies are attracted to feces and urine, accidental myiasis of the genitourinary tract is known to occur (Noutsis and Millikan). 

 

            Related Links and Case Reports

 

            Images in Clinical Practice: Urogenital Myiasis.   An editorial in Indian Pediatrics.

 

            Intestinal Myiasis.  A case report by the CDC.  

 

Intestinal Myiasis Caused by Eristalis tenax.   Click on the rat-tailed larvae of E. tenax for a link to the article from the Journal of Clinical Microbiology.  

                            

 

V.  Transmission

Transmission of fly larvae to human hosts differs among the many species of fly.

Dermatobia hominis (human botfly)

The life cycle of D. hominis begins with an unusual process called phoresis, in which the female botfly captures a day-biting mosquito or other blood-sucking arthropod and, in mid air, lays approximately 10 to 50 eggs on its abdomen, cementing them with a glue-like secretion (Gordon et al, Noutsis and Millikan, Millikan, Swetter et al, Tsuda et al).  The temperature-sensitive eggs develop, and hatch when the arthropod takes a blood-meal from a warm-blooded human or other mammal.  The newly-hatched larvae migrate through the bite wound, or along a hair follicle, into the cutaneous tissue of the human host.  Three stages of larval development take place inside the tissue cavity created in the host organism, over the course of 5-10 weeks (Tsuda et al).  The mature third instar larva uses its spines as a burr to widen the central punctum, emerges from the lesion and falls to the ground.  The larva burrows into the soil, where it pupates, then molts after 4-11 weeks, producing an adult fly. 

 

View an illustrated life cycle of D. hominis.

 

Cordylobia anthropophaga (tumbu fly)

The tumbu fly, C. anthropophaga, deposits its eggs on soiled blankets and clothing spread out on the ground to dry, or in soil.  When a potential human host comes in contact with the contaminated article, “the eggs incubate and hatch, allowing the larvae to burrow into the wearer’s skin” (Kpea and Zywocinski).   The timeline for the life cycle of the tumbu fly is similar to that of C. rhodaini, illustrated below. 

 

Cordylobia rhodaini (Lund’s fly)

 

Figure 2Lund’s fly deposits its eggs in soil, where the larvae hatch and may penetrate the skin of barefoot persons.  

 

 

Figure 2.  Life Cycle of C. rhodaini. 

Photo credit:  Merilyn Geary.

 

           

 

 

 

           

Cochliomyia hominivorax (New World screw worm)

Adult females deposit eggs (up to 200 at one time) onto existing lesions on the host’s skin.  The larvae feed on the necrotic or damaged tissue, and then “bore into adjoining normal tissue and cause ulceration.”  Some larvae migrate internally.  The larval stage lasts for 4 to 8 days, after which the larva drops to the soil, pupates, and produces an adult fly.  The entire life cycle is quite short, being about 24 days in length (Powers and Yorgensen).  

 

VI.  Reservoirs and Vectors

In discussing the vectors and reservoirs of myiasis-causing flies, it is important to consider whether a particular species of fly is an obligate, facultative, or accidental parasite of humans and/or mammals.  

 

The mechanical vectors for the larvae of the human botfly, D. hominis, are blood-sucking arthropods such as day-biting mosquitos.  The human botfly is an obligate parasite, in that it requires a mammalian host for its larval stage, but despite the species’ name, it is not restricted to infesting humans.  D. hominis also frequently infests cattle.  The existence of other mammalian hosts implies that even if all human infections were terminated before the larvae matured, larval stages and the very life cycle of the fly would persist in these mammalian reservoirs. 

 

Cochliomyia hominivorax infests both humans and all warm-blooded animals (Noutsis and Millikan, Wyss). Since the adult screw worm deposits eggs directly onto lesions or discharing orifices on human or animal hosts, there is no vector for the screw worm but itself.  Domesticated animals, such as cattle, are important reservoirs for the New World screw worm.  Eradication programs have often been sponsored by the U.S. Dept. of Agriculture because of the economic impact of the screw worm on the livestock industry (Wyss). 

 

In the wild, rats are the principle reservoir for Cordylobia anthropophaga, but in populated areas, humans and dogs are more frequent hosts (Noutsis and Millikan).  There is no organismal vector for C. anthropophaga, but by stretching the boundaries of the concept of vectors, one might consider soiled blankets and clothing to be a form of mechanical vector for transmission of the larvae to human hosts.  The same applies to C. rhodaini. 

 

The most important reservoir for Chrysomia bezziana in Africa, Australia, and Asia is sheep (Noutsis and Millikan). 

 

Rabbits and rodents are Cuterebra spp.  However, they are also a vector, by virtue of transmitting larvae directly to humans who come in contact with rabbits or rodents just as the eggs are hatching (Shorter et al).  

 

VII.  Incubation Period

Larvae of D. hominis develop for a period of 5 to 12 weeks in the tissues of their human or animal host.   This information can be useful for diagnostic purposes in patients who have a history of travel in endemic areas approximately 5 to 12 weeks prior to presenting at a clinic (Tsuda et al).  Larvae of C. anthropophaga have a much shorter incubation period, varying from 7 to 20 days (Jelinek et al).  Most patients seek medical attention before the larvae have completed their developmental stages inside the host; thus, the larvae are usually removed before they reach maturity (Lucchina et al).

 

VIII.  Morphology

 

Three distinct larval stages, or instars, occur inside the host’s tissue. 

 

 

Stage I

The first instar larva is subcylindrical, with small spines circling the larval body (Figure 3.). 

 

 

Figure 3.  Detail of larval cuticle, showing spines. 

Photo credit: Pietro Caramello

http://www.cdfound.to.it/html/myiasis.htm

 

 

Stage II

The second instar larva has a pyriform, or tapered shape (MacNamara and Durham).  Six posterior spiracles (Figure 4.), visible as two black dots in the central punctum of the lesion, are the respiratory sinuses through which the aerobic larva breathes.  Concentric rows of backward-facing spines allow the larva to cling tightly to host tissue and cause pain when the larva moves.  The larva “uses the spines and hooks like a circular burr to enlarge the opening and facilitate its final emergence” (Gorden et al). 

 

Figure 4.  The posterior spiracles of the tumbu fly, C. athropophaga. 

Photo credit: Juan Cabezos

http://www.cdfound.to.it/html/myiasis.htm

 

 

Stage III

The third instar or stage III larva that emerges from the lesion has a fusiform shape (Gordon et al).  Larvae of D. hominis may achieve a length up to 2 cm (Kitching).  Species identification generally cannot be made based on the appearance of the mature larvae, although some differences in the distribution of spines become visible (Figures 5. and 6.)  A comparison of the mature larvae of D. hominis and C. rhodaini may be useful for diagnostic purposes:

 

 

Figure 5.  Larval Cordylobia rhodaini,                                                 

note the scattered spines.  Bar =1mm.                                                            Figure 6.  Larval Dermatobia hominis,

Photo Credit: Dept. of Medical Entomology, USYD                                  note the rows of spines.  Bar =1mm.

                                                                                                                                                Photo Credit: see Figure 5. 

 

 

IX.  Diagnostic Tests

Because myiasis is rare in North America, diagnosis of myiasis is often delayed or prolonged.  Prompt diagnosis is important in avoiding unnecessary and ineffective courses of antibiotics. 

 

Purych outlines the key diagnostic features of myiasis as follows (quoted):

·         recent travel to an endemic area

·         one or more non-healing lesions on exposed areas of skin

·         serous, serosanguineous, or seropurulent drainage from a central punctum

·         a small, white, thread-like structure protruding from the lesion

·         local symptoms of pruritis, pain, movement, or tenderness

Diagnosis of myiasis is generally made by observing the larva as it surfaces periodically in the central puntum of the lesion.  In wounds, the larvae are often more readily visible.  Definitive diagnosis of the exact species of fly responsible for the infestation cannot be made on the basis of the fly’s larval stage.  Larvae obtained from a patient must be reared on meat or a synthetic medium until they pupate and eventually emerge as an adult fly, at which time it is possible to determine the species based on morphological characteristics. 

 

The following chart is provided by Deptartment of Medical Entomology at the University of Sydney, Australia as a diagnostic aid:

 

Species of diptera

Country of origin

Incubation period

Appearance of mature maggot

Cochliomyia hominivorax

Central & South America

4-8 days

Typical maggot shape, 15-17mm long, bands of spines encircling anterior margin of each body segment.

Cordylobia anthropophaga

Central & Tropical Africa

10-12 days

Oval, 11-15mm, 3 curved slits in spiracles, numerous small black spines.

Cordylobia rodhaini
(Fig. 1)

Central & Tropical Africa

10-12 days

Up to 23mm long, scattered spines, 3 sinuous slits in each posterior spiracle.

Dermatobia hominis
(Fig. 2)

Central & South America

5-12 weeks

18-25mm long, pair of flower like anterior spiracles, spines in rows.

 

 

Ultrasound: a new diagnostic tool

A case study from England suggests the use of ultrasound to aid in diagnosing and deciding upon a course of treatment for cutaneous myiasis involving mature larvae.  The authors were able to localize the larva, and to determine its size.  The lesion in question was located on the patient’s face, close to the facial nerve.  The information provided by the ultrasound was of assistance to the surgeons in removing the larva without damaging the facial nerve (Bowry and Cottingham).

 

Differential Diagnoses

Lesions are often mistaken for cellulitis or furunculosis.  While the lesion maintains the appearance of a “nonspecific subcutaneous nodule,” a differential diagnosis of early stage leishmaniasis, onchocerciasis, or tungiasis should be considered (Kpea and Zywocinski).  A second source suggests pyogenic infection and tropical ulcer as additional possible differential diagnoses (Lucchina et al).  One addiitonal source maintains that “myiasis can be easily misdiagnosed because it mimics several common pathological conditions such as adenopathy, cellulitis, skin abscess, insect bites, and subcutaneous cysts” (Powers and Yorgensen).

 

X.  Management and Therapy

In the treatment of furuncular cutaneous myiasis, forcible removal of the larva from the host tissue is not possible because of the larva’s tapered shape and the many rows of spines and hooks that it uses to grip the tissue cavity (Swetter et al).  While myiasis is self-limiting and, in many cases, not dangerous to the host, several authors suggest that the psychological distress associated with maggot infestation alone is sufficient reason to treat even the most harmless cutaneous myiasis (Shorter et al, Bowry and Cottingham, Powers and Yorgensen). 

 

Several methods for extraction exist, and can be grouped generally as surgical/biomedical or as being derived from folk remedies designed to suffocate the larva and force it to the surface of the lesion.   There is a lack of consensus among clinicians as to which method is preferable. 

 

Surgical debridgement

Surgical incision and extraction of the larva is usually done under local anesthesia.  Care must be taken to prevent laceration of the larva; any portion of larva remaining ing the tissue cavity will produce an undesirable inflammatory response, a bacterial infection, or the formation of a granuloma (Purych, Tsuda et al).  Surgery may be unnecessary except in cases in which the larva has died inside the lesion (Swetter et al). 

 

Tsuda et al propose that surgical treatment be accompanied by “systemic administration of antimicrobials to control secondary infections.”  Likewise, Kitching suggests antibiotic and tetanus prophylaxis following surgical removal of larvae.  However, Purych suggests that antibiotics are only necessary when a secondary infection is known to be present. 

                                    

Related Links

View an excellent photo essay depicting surgical treatment of anterior orbital myiasis caused by D. hominis.  

 

Occlusion/Suffocation approaches

The newest JAMA-approved treatment in this category is “bacon therapy” (Brewer et al), but this is only one of several substances which may be used to block the larva’s respiratory sinuses in the central punctum, forcing this aerobic organism to the surface in search of air and allowing removal with the aid of forceps or tweezers.  Other substances that have been used successfully include petroleum jelly, heavy oil, liquid paraffin, beeswax, raw meat, nail polish, adhesive tape, butter, chewing gum, and mineral oil (Shorter et al, Purych).

 

Innovative alternative treatment strategies

An alternative to both surgical and suffocation techniques is the injection of lidocaine at the base of the tissue cavity which the larva inhabits.  The local swelling forces the larva to the surface, where it is easily grasped and removed (Li Loong et al, Shorter et al).  This technique may be of limited use in cases involving multiple larvae, as the necessary dose of lidocaine or other anesthetic could prove toxic (Purych).   Another non-surgical option is the use of two wooden spatulas: by exerting manual pressure on the burrow, the larva may be forcibly expressed (Olumide, Shorter et al).  This technique is particularly useful in endemic areas where adequate medical care may be unavailable (Tsuda et al). 

 

Treatment of non-furuncular forms of myiasis

Creeping or migratory myiasis is self-limiting, but may be prematurely disrupted by blocking the path of the larvae with petroleum jelly, forcing it to the surface.  Alternatively, treatment with machine oil to increase the transparency of the skin allows for removal of the larva using a needle (Bapat).  Treatment of wound myiasis involves washing larvae from the wound, or surgical removal as necessary (Noutsis and Millikan). 

 

XI.  Epidemiology 

Distribution

Myiasis is endemic throughout the African and American tropics and subtropics.  It occurs more readily in warm and humid environments.  In the tropics, cases present year round, but in more temperate zones, myiasis is generally restricted to the summer months (Noutsis and Millikan).  The distribution of C. hominivorax (New World screw worm) has been reduced through aggressive eradication programs in the southern United States and parts of Central America (Wyss).

 

Figure 7.  Geographic distribution of Old World (green) and New World (red) screwworm. 

Photo Credit:  M.J.R. Hall. “Screwworm flies as agents of myiasis,” FAO.

 

 

 

 

 

 

Map of AfricaAs illustrated in Figure 8., C. anthropophaga is found throughout tropical sub-Saharan Africa, while C. rhodaini is confined to a more limited, forested region. 

 

The human botfly, D. hominis, is found throughout the tropical and neo-tropical regions of the Americas. 

 

The rabbit or rodent botfly, Cutebra spp., inhabits North America and is responsible for most cases acquired there.  

 

 

Figure 8.  Distribution of C. anthropophaga (between the dotted lines), and C. rhodaini (the shaded areas). 

Photo credit: Merilyn Geary.

Population

Myiasis is an ephemoral, self-limiting infection.  The vast majority of cases are not fatal, and many are treated at home using occlusion methods not altogether different from those available in a medical setting.  Given these characteristics, it is difficult to determine either the prevalence or incidence of myiasis, or even the  number of people who will experience an infestation of fly larvae at some point during their lives.  While global statistics are lacking, data were available for Panama.  One source notes that six out of seven Panamanian men will contract myiasis caused by D. hominis, the human botfly.  Additionally,  the incidence of primary screwworm infestation is 160 cases per 1000 in Panama and perhaps even higher in other Central American nations (Powers and Yorgensen).  Extrapolating from these numbers, it appears that the lifetime incidence of mysiasis is fairly high in the American tropics. 

 

XII.  Public Health and Prevention Strategies

 

Methods of prevention are determined by the different behavioral patterns of the various species of flies. 

 

Dermatobia hominis (human botfly)

Where D. hominis is endemic, the role of the arthropod vector suggests that avoiding mosquito bites is an important means of preventing infestation with fly larvae.  This can take the form of bedding nets, insect repellents, and protective clothing.  Facultative myiasis can be prevented by properly covering any open wounds. 

 

            Cordylobia anthropophaga (tumbu fly)

Although there are many other reasons (such as malaria!) to avoid mosquito bites in Africa, they are not a vector for the larvae of C. anthropophaga.  Preventive measures in this region of the world consist mainly of sun-drying and/or ironing clothing and bedding (Kpea and Zywocinski). 

 

            Cochliomyia hominivorax (New World screw worm)

Screw worm eradication programs involve the development and release of millions of sterile male flies, and the gradual elimination of the species from certain geographic species.  Accidental introduction of C. hominivorax into Libya in 1988 was dealt with by the introduction of sterile male flies, and by 1991, Libya was declared free of New World screw worm (Markell and Voges). 

 

Chrysomyia bezziana (Old World screw worm)

Efforts are underway to try to duplicate the eradication program that has been so successful with the New World screw worm. 

           

Related Links

            Mass Rearing the Old World Screw-worm Fly, Chrysomyia bezziana. 

           

Accidental myiasis by any species of fly may be prevented by covering food and by practicing proper personal hygiene (Noutsis and Millikan). 

 

XIII.  Maggot Debridgment Therapy

Unlike most forms of commensalism, infestation with fly larvae can, in very specific cases, be mutualistic rather than parasitic.  The concept of facultative myiasis, in which maggots infest decomposing wound tissue but do not feed on adjacent living tissue, provides the theoretical foundation for maggot debridgment therapy (MDT) (Shorter et al). 

 

 

foot with
maggots

Figure 9. illustrates the use of maggot therapy in treatment of a gangrenous wound on a patient’s heel.  While some necrotic tissue persists, healthy red tissue has returned in the locations where maggots are feeding.  

 

Figure 9.  Myiasis in a grangenous wound. 

Photo credit: Ronald Sherman. 

Maggot Therapy Project at UC-Irvine.

 

 

 

 

Related Links 

Wound Care Update: Maggot Therapy.  A podiatrist’s perspective.  

World Wide Wounds: a case history describing the use of sterile larvae (maggots) in a malignant wound. 

 

XIV.  Useful Web Links

 

Clinics in Dermatology: Myiasis

 

Bugs on the Web: Dermatobia—Cutaneous Myiasis.  A very clear and thorough overview of myiasis caused by D. hominis, including detailed photographs of various larval stages.

 

            Atlas of Parasitology: Myiases

 

XV.  References

 

Aguilera A; Cid A; Reguerio B; Prieto J; Noya M.  Intestinal myiasis aused by Eristalis tenax.  J Clin Microbiol.  1999 Sep; 37(9):3082.

 

Bapat, Sonali.  Neonatal myiasis.  Pediatrics.  2000 Jul; 106(1):e6.

 

Bowry R; Cottingham R.  Use of ultrasound to aid management of late presentation of Dermatobia hominis larva infestation.  J Accid Emerg Med.  1997 May; 14(3):177-178.

 

Brewer T; Wilson M; Gonzalez E, et al.  Bacon therapy and furuncular myiasis.  JAMA.   1993; 270:2087-2088.

 

Gordon P; Hepburn N; Williams A; Bunney M.  Cutaneous myiasis due to Dematobia hominis: a report of six cases.  Brit J Dermatol.  1995; 132:811-814.

 

Jelinek T; Nothdurft H; Rieder N; Loescher T.  Cutaneous myiasis: review of 13 cases of travelers returning from tropical countries.  Int J Dermatol.  2000 Sep; 39(9):689-694.

 

Kitching J.  Tropical myiasis: an unwanted holiday souvenir.  J Accid Emerg Med.  1997 May; 14(3):178-179. 

 

Kpea N; Zywocinski C.  “Flies in the flesh”: a case report and review of cutaneous myiasis.”  Cutis.  1995 Jan; 55(1): 47-48.

 

Li Loong PT; Lui H; Buck HW.  Cutaneous myiasis: a simple and effective technique for extraction of Dermatoba hominis larvae.  Int J Dermatol 1992; 31:657-659.

 

Lucchina L; Wilson M; Drake L.  Dermatology and the recently returned traveler: infectious diseases with dermatologic manifestations.  Int J Dermatol.  1997 Mar; 36(3):167-181.

 

MacNamara A; Durham S.  Dermatobia hominis in the accident and emergency department: “I’ve got you under my skin.”  J Accid Emerg Med.  1997 May; 14(3):179-180.

 

Markell and Voges.  Medical Parasitology.  8th Ed.  Philadelphia: W.B. Saunders, 1999. 

 

Millikan L.  Myiasis.  Clinics Dermatol.  1999; 17:191-195. 

 

Noutsis C; Millikan L.  Myiasis.  Dematol Clinic 1994 Oct; 12(4): 729-736.

 

Olumide Y.  Cutaneous miasis: a simple and effective technique for extraction of Dermatobia hominis larvae.  Int J Dermatol.  1994; 33:148-149.

 

Powers N; Yorgensen M.  Myiasis in humans: an overview and a report of two cases in the Republic of Panama.  Mil Med.  1996 Aug; 161(8): 495-497.