Blood-sucking flies (Diptera)

Many flies are haematophagous (Table 7.12.1). Most blood-sucking flies are in the suborder Nematocera (mosquitoes, sand flies, blackflies, biting midges) and family Tabanidae of the suborder Brachycera (horse flies, deer flies, clegs). The tsetse flies, Glossina spp., are in the suborder Cyclorrhapha. Parasitic louse-like flies known as keds, such as the deer ked Lipoptena cervi, have been placed in a separate group, Pupipara, although they are considered by many authors to be specialized members of the Cyclorrhapha. All blood-sucking flies are at least a nuisance: the bites are often painful and associated with sensitization. More important, biting flies may transmit infection. Mosquitoes (Culicidae) are vectors of filariasis and numerous viral diseases, including yellow fever and dengue fever. Mosquitoes of the genus Anopheles transmit malaria. Depending on species and location, mosquitoes bite at various times of the day. Mosquitoes may be controlled by reducing their access to stagnant water needed for development of their larval stages and by application of insecticides to dwellings. Use of permethrin-impregnated bed nets has been shown to reduce malaria transmission. Sand flies (Phlebotominae) are mainly tropical and subtropical in distribution and transmit leishmaniasis. In South America, sand flies of the genus Lutzomyia transmit Bartonella bacilliformis. Blackflies (Simuliidae) occur worldwide and are vectors of Onchocerca volvulus and Mansonella ozzardi. Blackfly larvae require well-oxygenated water. Female blackflies pierce the skin and suck blood from the edge of the puncture. Substances in blackfly saliva inhibit platelet aggregation, impair the final common pathway of the coagulation cascade, and encourage vasodilatation. The bites, oozing blood, have a characteristic appearance and may be associated with severe reaction, sometimes referred to as simuliosis or simuliotoxicosis. Puncture sites often become surrounded by a wide zone of haemorrhagic erythema and oedema (Fig. 7.12.1). Rarely, haemorrhagic shock may occur. In Brazil, the haemorrhagic syndrome of Altimira has been epidemiologically associated with exposure to blackflies. Blackfly saliva appears to contain immunomodulating substances. In Brazil, the autoimmune condition ‘fogo selvagem’ (a form of pemphigus foliaceus) occurs in simuliid-infested areas (Fig. 7.12.2a). In Britain, blackflies are rarely troublesome to humans except in certain localities. In southern England, the Blandford fly, Simulium posticatum occurs on the river Stour, Dorset, on tributaries of the river Thames in Oxfordshire, and on other rivers. In 1993, 16% (22% female, 9% male) of Blandford’s inhabitants reported bites. Use of the biological larvicide Bacillus thuringiensis as a control measure was associated with a marked drop in the number of people complaining of bites. In Scotland, S. reptans, S. argyreatum, and other species may bite humans.

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Fig. 7.12.1

Reaction to blackfly (Simulium sp.) bites, 48 h after exposure. Algonquin, Ontario, Canada.

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Fig. 7.12.2

(a) Endemic Pemphigus foliaceus (‘fogo selvagem’ meaning ‘wild fire’) in a man from a rural area of São Paulo State, Brazil infested with Simulium flies. (b) Deer ked or deer fly without wings (Lipoptena cervi Diptera, Hippoboscidae).

(a) (Copyright DA Warrell.) (b) (Copyright J Paul.)

Biting midges (Ceratopgonidae) (Fig. 7.12.3) are vectors of the filarial worms Mansonella (Dipetalonema) perstans and M. ozzardi. In Africa, tabanid flies transmit Loa loa. Tsetse flies are vectors of African trypanosomiasis (see Chapter 7.8.10). Deer keds Lipoptena cervi are highly evolved louse-like flies with biting mouth parts that feed on deer. Occasionally, deer keds bite people, such as forest workers, hunters, or entomologists (Fig. 7.12.2b). Deer ked dermatitis is a condition where itchy papules exist for weeks to months at the site of bites. Eventually, papules resolve without specific treatment. Recently, it has been suggested that Bartonella schoenbuchensis may have a role in the aetiology of the conditions as this agent has been detected in roe deer and in deer keds but not as yet from humans.

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Fig. 7.12.3

Reaction to midge (Ceratopogonidae) bites, 24 h after exposure. Sligachan, Isle of Skye, United Kingdom.

True bugs (Hemiptera)

Bedbugs

The common bedbug Cimex lectularius (Fig. 7.12.4) is cosmopolitan. In recent years, reports of infestations in developed countries such as the United Kingdom and United States of America have increased. The resurgence has been linked to the emergence of resistance to pyrethroid insecticides used for bedbug control. Deep sequencing has revealed a number of candidate genes likely to account for metabolic resistance. Infestation may be unrelated to lack of general hygiene but associated with translocation of personal effects or furniture. The tropical bedbug C. hemipterus occurs in tropical and subtropical countries. Epidemiological studies have failed to produce clear evidence of bedbugs as vectors of infections, such as hepatitis B. They are nocturnal, hiding during the day and feeding at night. Although in some cases bites may go unnoticed and there may be no allergic reaction, bedbugs may cause sleeplessness and the bites may cause pain and swelling and, exceptionally, disseminated bullous eruptions (Fig. 7.12.5). Rooms that are heavily infested may acquire an unpleasant odour. Bugs may be found by making special searches at night or by seeking their hiding places during the day. They resemble lentils superficially, being round and flat. Adults reach a length of about 5 mm. Nymphs pass through five instars to reach adulthood after about 4 months. Bedbugs can live for 6 months without feeding, becoming paper-thin. Related bugs which occasionally bite humans derive from pigeons, bats, and martins (C. columbarius, C. pipistrelli, and Oeciacus hirundinis respectively). Infestation may be managed by restricting access of host species to dwellings, but in the United Kingdom, for example, bats are protected under the Wildlife and Countryside Act.

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Fig. 7.12.4

Bedbugs Cimex lectularus.

(Copyright J Paul.)

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Fig. 7.12.5

Erythematous macules of bedbugs.

(Courtesy of D Hill, Adelaide, South Australia.)

Prevention

Bites are discouraged by keeping the light on all night, sleeping under a permethrin-impregnated mosquito net, and putting newspaper under the undersheet. Eradication is by thorough cleaning of the environment and application of residual insecticides. Sheets should be steam cleaned or exposed to the sun and treated with insecticide. However, insecticide resistance has developed and bedbugs are becoming more abundant.

Ticks (Ixodoidea)

Hard ticks (Ixodidae) and soft ticks (Argasidae) occur worldwide. Stages of the life-cycle are egg, larva (six-legged), and nymph and adult (both eight-legged). Ticks attach and feed with a barbed hypostome and detach when engorged. Smaller stages and ticks in inconspicuous sites, such as the perineum may feed unobserved. Bites are usually painless but may result in local sensitization, secondary infection, and transmission of infectious agents, including numerous viruses, rickettsias, and Lyme disease (Table 7.12.2). Local reaction to bites may be confused with erythema migrans of Lyme disease, (which expands as a ring with a central punctum—see Chapter 7.6.32). Ticks may be removed by gripping with forceps (or, in the field, with finger and thumbnail), between the skin and the tick’s head and pulling gently. Special tools for removing ticks have been made widely available by the pet industry and such devices should work just as well with humans. Toothed devices that work in the manner of combs or forceps that are curved in profile (tick tweezers) have the advantage of allowing removal while avoiding squeezing the tick. Careless removal may detach the head or hypostome, leaving a potential source of inflammation and secondary infection. In the United Kingdom, the ticks most often found on humans are the sheep tick Ixodes ricinus (a vector of Lyme disease) and the hedgehog tick I. hexagonus (Fig. 7.12.6).

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Fig. 7.12.6

Upper side of hedgehog tick Ixodes hexagonus, to show sucking mouthparts (hypostome).

(Copyright J Paul.)

Harvest mites (Trombiculidae)

In the United Kingdom, larvae of the harvest mite Neotrombicula autumnalis are a common cause of bites in late summer, especially in chalk downland. They are tiny and seldom noticed, crawling rapidly on to the body, attaching (often under tight-fitting clothes), injecting proteolytic enzymes, feeding on tissue fluid and then detaching, leaving pruritic, sometimes bullous lesions hours later. For many victims, the cause of irritation remains a mystery. Red bugs or chiggers (confusingly, a term also applied to the flea Tunga penetrans) are names given to trombiculids in the Americas. Bites to the penis, associated with swelling and dysuria, have been described in the paediatric literature as ‘summer penile syndrome’. In Asia, trombilucids are vectors of scrub typhus.

Accidental bites

Arthropods which do not normally bite humans but can inflict painful but usually trivial bites when provoked by handling, e.g. by children and entomologists, include predatory true bugs such as the water boatman Notonecta glauca and the assassin bug Reduvius personatus in the United Kingdom and wheel bugs Arilus spp. in the Americas; larger beetles (Coleoptera); dragonflies (Odonata); and bush-crickets (Orthoptera) such as the wartbiter Decticus verrucivorus.

Spines used in defence by the great silver diving beetle Hydrous piceus and larger tropical grasshoppers of the subfamily Cyrtacanthridinae can cause penetrating injury when handled. Pincers of larger crabs and lobsters (Crustacea) can cause crushing injuries of digits and their spines may cause penetrating injury.

Treatment

Treatment of scabies is by topical application of acaricides. Aqueous lotions of 0.5% malathion or 5% permethrin are currently recommended in the United Kingdom, given as two treatments a week apart. γ-Benzene hexachloride is also effective. The lotion is applied to the whole body surface of all affected people and left on for 24 h before being washing off. Itching may persist for several weeks and requires a topical counter irritant and corticosteroid (e.g. crotamiton and hydrocortisone) and a sedating antihistamine (chlorphenamine at night). Ivermectin (200 µg/kg single dose) is used for Norwegian scabies and in patients whose severe excoriations make topical treatment intolerably irritating and painful. During outbreaks, it may be necessary to treat whole cohorts of patients or health care teams.

Pubic lice (crab lice)

The lice (Pthirus pubis) attach themselves to pubic hairs. Rarely, lice may be found on eyebrows, eyelashes (phthirosis palpebrarum), axillary, head, or chest hair. Eggs are deposited on hair shafts. Most infestations are probably acquired through sexual contact with an infested case. Children may acquire phthirosis at atypical sites through close contact with adults. Lice seldom stray from the body. Transmission is possible but unlikely without close contact with an infested case. The main symptom is pruritus, sometimes with excoriation and secondary infection. Grey patches (maculae caeruleae) may occur on the skin. Diagnosis is by observation of the lice, which may be difficult to find, or of eggs or egg cases attached to hair shafts. Adults are 1 to 2 mm long. The anterior legs are smaller than the other two pairs. The body is squat and crablike (body length, excluding head, c.1.2 times body width) (Fig. 7.12.11). The original description contained a printing error (pthirus) for phthirus (Greek: louse).

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Fig. 7.12.11

Adult specimen of Pthirus publis..

(Courtesy of D Hill, Adelaide, South Australia.)

Head lice

Head lice infest the scalp and rarely other body sites. They lay their eggs at the base of hair shafts. Infestation is more common in children than in adults and more common in females than in males. Prevalence rates vary but may be very high in certain communities or institutions, such as schools. Prevalence rates may be high despite good standards of hygiene. Most cases probably occur as a result of close contact. The main symptom is pruritus which may be associated with excoriation, secondary infection and lymphadenopathy. Diagnosis is by observation of lice, which generally remain close to the scalp, or of eggs or egg cases, attached to hairs (Fig. 7.9.12). A fine comb (nit comb) may be used to collect material to make the diagnosis. Adults are 3 to 4 mm long.

Body lice

Body lice infest clothing and body hair. They lay their eggs on clothing, often along seams. Body lice are morphologically like head lice but slightly larger. Body louse infestation is associated with poor hygiene and social deprivation, as may occur in wartime. Transmission occurs as a result of close contact or through contact with infested clothing. Bites occur on the body, resulting in pruritus which may be associated with excoriation, eczematization, and secondary infection. Diagnosis is confirmed by finding lice, usually on clothing.

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Fig. 7.12.12

Nits attached to hair. Photograph from a patient with pediculosis showing several hair fibres with numerous egg cases attached.

(Courtesy of D Hill, Adelaide, South Australia.)

Tungosis

Tungosis is infestation by a flea Tunga penetrans, known as the jigger, chigger, chigoe (popular names shared with trombiculid mites) or sand flea. Tungosis is a zoonosis that affects a range of domestic animals as well as humans. The gravid female, about 1 mm long, burrows into exposed skin (usually the foot), or under a toenail, and swells to about 1 cm in diameter, causing local inflammation and discomfort. The wearing of footwear prevents infestation. In the developed world tungosis is regarded as an easily treatable condition that is occasionally seen in returning travellers. Lesions may be enucleated surgically and the diagnosis confirmed by histology. Local remedies in endemic areas (tropical Africa and the Americas) of shelling out fleas may leave cavities prone to secondary infection and lethal tetanus. In endemic areas of the tropics, such as in parts of Brazil and Uganda, high levels of infestation may cause significant disability and morbidity.

Dermal myiasis

The human bot fly Dermatobia hominis is a common cause of dermal myiasis in the American tropics. The female fly lays her eggs on biting arthropods, such as mosquitoes. The eggs hatch when in contact with skin into which the larva burrows. The larval stage lasts about 10 weeks, a boil with a small aperture forming as the larva grows. Such boils are not infrequently seen in Europeans returning from the neotropics. The larva may grow to more than 1 cm in length (Fig. 7.12.15). An early symptom is sporadic pain caused by the spiny larva. Unless in an unusual anatomical site, such as close to the eye, infestation is generally harmless. Secondary infection of the wound is the most common complication. Larvae may be removed through a simple incision. Alternatively, a commercially available snake venom extractor (not recommended for snake bite) has been shown to serve as a useful tool for removal of the larvae. Remedies which include application of raw meat or glue to the lesion may not be successful. Squeezing may rupture the larva to evoke a local granulomatous reaction.

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Fig. 7.12.15

Two third larval instars of the human bot fly Dermatobia hominis (c.13 mm long) extracted from a facial ‘boil’ in a European who had been visiting Guyana.

The tumbu fly Cordylobia anthropophaga is widespread in the Afrotropical region. There have also been rare reports of apparent acquisition in Spain and Portugal. The female oviposits on sand and also on drying clothes. Ironing destroys eggs. Contact with viable ova on clothing leads to infestation. The larvae pierce the skin and grow rapidly. An uncomfortable boil forms which oozes serosanguinous fluid (Fig. 7.12.16). Fever and lymphadenopathy may occur. Larvae reach maturity in about 10 days. Larvae may be removed through a simple incision, but with care it may be possible to express larvae following application of petroleum jelly (Fig. 7.12.17).

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Fig. 7.12.16

Skin lesion caused by larva of the Tumbu fly Cordylobia anthropophaga in a Peruvian man who had been working in Zambia.

(Copyright DA Warrell.)

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Fig. 7.12.17

Larvae of African tumbu fly Cordylobia anthropophaga, a common agent of dermal myiasis.

The larvae of warble flies Hypoderma spp. occasionally cause dermal myiasis in humans. Larvae of horse bot flies Gasterophilus spp. cannot complete their life cycle in humans but they can pierce human skin, where they wander for a week or so, causing intense itching (creeping eruption).

Wound myiasis

Many dipterous species are known to cause wound myiasis, but most of them are facultative feeders on necrotic tissue and are rarely destructive to the host although the presence of maggots in a wound may cause distress. Debridement of nectrotic tissue will control such infestation. In contrast, under controlled conditions, clinicians may introduce maggots to promote healing.

Causes of malign myiasis include the New World screw-worm Cochliomyia hominivorax in the Americas and the Old World screw-worm, Chrysomya bezziana and Wohlfahrt’s wound myiasis fly Wohlfahrtia magnifica in the Old World. Their larvae are obligate parasites of living tissue. Eggs are laid on wounds, in ears and on mucous membranes. The larvae (Fig. 7.12.18) burrow in groups into healthy tissue, causing widespread destruction which may be mutilating or fatal (Fig. 7.12.19). Secondary bacterial infection or secondary wound myiasis may ensue. All species may cause nasopharyngeal, aural, orbital, genital and malign wound myiasis. Infestation is best avoided by cleaning and dressing wounds as they occur. Treatment involves surgical removal of the larvae, debridement of affected tissue, and treatment of secondary infection. Reconstructive surgery may be required.

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Fig. 7.12.18

Larvae of the New World screw-worm Cochliomyia hominivorax (c.8 mm long) extracted from the wound illustrated in Fig. 7.12.19. These were sent to the Natural History Museum in London where they were identified. Larvae of the second myiasis species (C. macellaria) were also found in the sample and were probably collected from the edges of the wound.

(Courtesy of Dr Martin J R Hall, Medical and Veterinary Division, Natural History Museum, London.)

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Fig. 7.12.19

Fatal myiasis (New World screw-worm). Historical illustration of a 50-year-old Honduran woman who complained of a small chronic ulcer on the right cheek; on admission to hospital she was found to have a huge ulcer exposing the bones of the face and forehead and destroying the tissues of the cheek and face, right eye, and orbit. More than 300 larvae were removed (see Fig. 7.12.18).

(From Harrison JHH (1908). A case of myiasis. J Trop Med Hyg, XI, 20.)

Ophthalmomyiasis (ocular myiasis)

Fewer than 5% of cases of human myiasis affect the eye. Usually only external structures such as the lids and conjunctivae are infested but some fly larvae can penetrate the conjunctiva or sclera, causing corneal ulceration and damage to anterior and posterior internal structures that may result in blindness. The usual cause of ophthalmomyiasis externa is Oestrus ovis, the cosmopolitan sheep and goat nasal botfly, whose natural host is herbivorous mammals. Although most common in tropical developing countries (especially North Africa, the Middle East, and the Caribbean) it still occurs rarely in Western cities. Female flies eject their larvae into the nostrils of the host, where they mature. Human victims may give a history of having been buzzed in the face or struck on the eye by an insect and later of developing irritation and redness of the eye, foreign body sensation, pain, lacrimation, palpebral oedema, and signs of purulent conjunctivitis or a stye (hordeolum). O. ovis larvae rarely develop beyond the first instar in humans and so symptoms are self-limiting, but they may be more rapidly relieved by slit lamp examination and removal of larvae which cling to the conjunctivae and may cause follicular conjunctival reaction and pseudomembrane formation. Other causes of human ophthalmomyiasis include Rhinoestrus purpureum, Dermatobia hominis, Hypoderma spp., (Oestridae); and Cochlyomyia hominis, Lucilia spp., Phormia spp. (Calliphoridae). Larvae of Hypoderma, Cochlyomyia, Dermatobia, and Oedemagena tarandi are more dangerous as they may burrow into the eye, resulting in pain, nausea, and destruction. They must be surgically removed.

Nosocomial myiasis

Hospitalized patients with exposed wounds, ulcers, or medical devices that breach the skin are susceptible to wound myiasis. Patients with impaired mobility or decreased levels of consciousness are also susceptible to infestation of the airways and urogenital tract. Cases of nosocomial myiasis have been reported from developed countries as well as from hospitals in resource-poor settings. Many reported cases involve infestation with opportunistic species including members of the genera Lucilia, Sarcophaga, and Musca. Cases of infestation with the New World screw-worm Cochliomyia hominivorax in hospitalized patients have been reported in the Americas. Flies are attracted to necrotic tissues and readily gain access through open windows but may even reach patients through open doorways in air-conditioned rooms.