Insect bites and infestations

So, naturalists observe, a flea Hath smaller fleas that on him prey; And these have smaller fleas to bite 'em. And so proceeds "Ad infinitum"

Jonathan Swift

It is, of course, the internal parasites of biting insects that cause trouble for humans, rather than "smaller fleas" on their surface.

An ornithologist went bird watching in Guyana, where he sustained widespread "midge bites" on the arms. He was referred on account of nodules that developed a few weeks later, then enlarged and ulcerated. Other lesions occurred further up the arms with regional lymphadenopathy. A biopsy specimen showed histiocytic inflammatory changes, and Leishmania braziliensis was isolated from smears; the midges (phlebotomus or sand fly) had acquired the protozoon while feeding on local rodents and transferred it into the ornithologist's skin.

Serious disease from insect vectors is rare in residents of most Western countries but, as in the patient described above, must be considered in those returning from tropical and subtropical countries.

Body louse

Sand fly

Leishmaniasis

Sand fly

Leishmaniasis

Some diseases with skin lesions resulting from insect bites

Condition

Appearance

Organism

Vector

Cutaneous

Chronic enlarging

Leishmania

Sand fly

leishmaniasis

nodules with

protozoon

ulceration

(L. braziliensis)

Oriental sore

Ulcerating nodules

Leishmania

Sand fly

(L. tropica)

Kala-azar

Hypopigmented,

Leishmania

Insect vectors

erythematous, and

(L. donovani)

nodular lesions

Onchocerciasis

Pruritic nodules

Filaría

Black fly

(Onchocerca

(Simuliidae)

volvulus)

Typhus, human

Erythematous rash and

Rickettsia

Human louse

systemic illness

(R. prowazekii)

Typhus, murine

(R. mooseri)

Rat flea

Rocky Mountain

Maculopapular rash

Rickettsia

Ticks

spotted fever

and fever

( R. rickettsii)

Rickettsial pox

Vesicular eruption like

Rickettsia

House mouse,

chickenpox

(R. akari)

louse

Tick typhus

Necrotic lesions,

Various rickettsias

Ticks

maculopapular rash,

and fever

Scrub typhus

Fever,

Rickettsia

Mites

lymphadenopathy,

(R. tsutsugamushi)

maculopapular rash

Relapsing fever

Widespread

Borrelia

Lice, ticks

maculopapular

recurrentis

lesions

Lyme disease

May be annular

Borrelia

Ticks, black fly

burgdorferi

Yellow fever

Flushing of face,

Arbovirus

Aedes

and dengue

scarlatiniform rash

mosquito

Most cases of bites from fleas, midges, and mosquitoes are readily recognised and cause few symptoms apart from discomfort. Occasionally an allergic reaction confuses the picture, particularly the large bullae that can occur from bites on the arms and legs. It may be difficult to persuade patients that their recurrent itching spots are simply due to flea bites and the suggestion may be angrily rejected.

Nevertheless, some patients are convinced that they have an infestation when they do not. Often they will bring small packets containing "insects". Examination shows these to be small screws of wool, pickings of keratin, thread, and so on. Sympathy and tact will win patients' confidence; derision and disbelief will merely send them elsewhere for a further medical opinion. Antipsychotic drugs may help to dispel the delusion of parasitic infestation (delusional parasitosis) and should be used in conjunction with advice from a psychiatrist if possible. These drugs should be used with care and with full awareness of their side effects, particularly in patients with cardiovascular disease and a history of epilepsy. Pimozide has been used in the past but because of its side effects risperidone is preferred.

• Flea bites, including those from Cheyletiella mites in dogs and cats, occur in clusters, often in areas of close contact with clothing, for example, around the waist.

• Grain mites (Pyemotes) and harvest mites (Trombiculidae) can cause severe reactions.

• Tick, and possibly mosquito, bites can produce infection with Borrelia burgdorferi, causing arthropathy, fever, and a distinctive rash (erythema chronicum migrans)—Lyme disease. The condition responds rapidly to treatment with penicillin. Increasing numbers of cases are being reported in the United Kingdom.

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