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.
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.
Papular urticaria
Persistent pruritic (itching) papules in groups on the trunk and legs may be due to bites from fleas, bed bugs, or mites.
A seasonal incidence suggests bites from outdoor insects, while recurrence of the papules in a particular house or room suggests infestations with fleas.
The term is sometimes used for other causes of itchy skin.
Spider bites
In Europe spider bites rarely cause problems, but sometimes noxious species arrive in consignments of tropical fruit.
The patient shown had been bitten by a spider the day before leaving Nigeria and developed a painful necrotic lesion. Bites from the European tarantula are painful but otherwise harmless.
In tropical and subtropical countries venomous spiders inject neurotoxins that can be fatal.
The “black widow” (Latrodectus mactans), “fiddleback” (Loxosceles veclusa), and Atrax species of Australia are better known examples.
Scorpions cause severe local and systemic symptoms as a result of stings (not bites).
Infestations
Scabies
The commonest infestation encountered is scabies, and it is easily missed or misdiagnosed.
Scabies is due to a small mite, Sarcoptes scabiei.
The female mite burrows into the stratum corneum to lay her eggs; the male dies after completing his role of fertilisation, and the developing eggs hatch into larvae within a few days.
Intense itching occurs some two weeks later, during which time extensive colonisation may have occurred.
The infestation is acquired only by close contact with infected people.
Diagnosis
Finding a burrow the small (5–10 mm long) ridge, often S shaped can be difficult as it is often obscured by excoriation from scratching. Without finding a burrow, however, the diagnosis remains uncertain.
Isolation of an acarus with a needle or scalpel blade and its demonstration under the microscope convinces the most sceptical patient.
Always ask whether there are others in the patient’s household and if any of them are itching. Treatment
10% sulphur in yellow soft paraffin is traditional, effective, and safe.
There are several more modern treatments, including 25% benzyl benzoate emulsion, 0·5% malathion cream, 1% gamma benzene hexachloride (lindane) lotion, and 1% permethrin.
In children benzyl benzoate should be diluted to 10% and used with care as toxicity results from absorption.
In infants over two months old permethrin or 2·5% sulphur ointment can be used.
Gamma benzene hexachloride should not be given to children under 10 years or pregnant women in the first trimester.
Important points are:
(1) The patient should wash well: a hot bath was formerly advocated but it is now known that this may increase absorption through the skin.
(2) The lotion should be applied from the neck down, concentrating on affected areas and making sure that the axillae, wrists, ankles, and pubic areas are included.
If there is any doubt about the thoroughness of application the process should be repeated in a few days.
(3) All contacts and members of the patient’s household should be treated at the same time.
(4) Residual papules may persist for many weeks.
Topical steroids can be used to relieve the itching.
(5) Secondary infection as a result of scratching may need to be treated.
Demodex
Demodex folliculorum is a small mite that inhabits the human hair follicle, the eggs being deposited in the sebaceous gland.
It is found on the central area of the face, chest, and neck of adults.
It may have a role in the pathogenesis of rosacea, in which it may be found in large numbers.
It may be associated with a pustular eruption round the mouth and blepharitis.
Larva migrans
The patient in the illustration had been on holiday at a tropical coastal town and regularly visited a beach frequented by dogs.
Two weeks after returning to Britain he started itching on the buttocks and subsequently noticed a linear, raised area a condition known as larva migrans, due to the larvae of the hookworm of dogs and cats, Ancylostoma caninum.
The ova are shed in the faeces and in a warm moist environment hatch into larvae that invade “dead end” hosts.
They do not develop any further, so systemic disease does not occur.
Treatment
This is either by freezing the advancing end of the lesion with liquid nitrogen or by applying thiabendazole (10%) suspension.
Similar lesions in patients returning from tropical countries raise the possibility of larva migrans from strongyloides infestation, myiasis from the larvae of flies, or gnathostomiasis. Visceral larva migrans caused by Toxocara canis and Ascaris lumbricoides may produce a transient rash.
Pediculosts (lice)
Infestation with lice became less common in the postwar years, but the incidence has recently increased.
There are three areas of the body usually affected by two species of wingless insects Pediculus humanus, infecting the head and body, and Phthirus pubis, the pubic louse.
The wingless insects feed on blood aspirated at the site of the bite, and each female lays 60–80 encapsulated eggs attached to hairs ”nits” in common parlance.
Head lice are transmitted via combs, brushes, and hats, being more common in girls than boys. The infestation is heaviest behind the ears and over the occiput.
If the eyelashes of children are affected this is with “crab lice” (Phthlrus pubis); it is not pediculosis.
Body lice are less common in western Europe.
Transmission is by clothing and bedding, on which both lice and their eggs may be found in the seams.
Poor hygiene favours infestation.
Pubic lice infestation occurs worldwide and is generally transmitted by sexual contact.
Infestation of eyelashes may occur with poor hygiene.
As a result of scratching there may be marked secondary infection that obscures the underlying infestation.
Treatment
Gamma benzene hexachloride 1% is usually effective as a single application.
