Insect causes of skin disease

Insect causes of skin disease

 

Insect bites

 

Insects produce their effects on the skin by a variety of mechanisms including mechanical trauma and injection of irritant, toxic or allergenic substances. Insect bites may readily become secondarily infected. Certain occupations, eg gardeners, farmers, dog breeders, and removalists handling hessian, carry a high risk of insect bite reaction. Patients vary in the degree to which they respond.

 

Insect bites most often present as an itchy wheal that develops into a firm, itchy papule which may persist for days to weeks. Occasionally, insect bite reactions can be much more persistent, lasting for months. The centre of the papule may show a punctum or a vesicle. Papules are often grouped or in lines and appear at irregular intervals.

The number and distribution of the lesions depend on the type of insect and degree of exposure. Usually exposed areas are involved, but fleas and mites may cause reactions under clothes. New bites by the same species often reactivate old lesions. On the lower legs, blistering reactions are common, particularly in children, see Papular urticaria. In elderly persons, insect bites on the lower legs may be haemorrhagic or may ulcerate.

Occasionally, a severe reaction to a bite may result in an area of induration and swelling that mimics cellulitis. Numerous bites may result in fever and malaise. Anaphylactic reactions may be encountered after stings by bees and wasps (see below). Very occasionally, a bite may result in a necrotic reaction.

Attempt to identify and eradicate the source of the insect bites. This may involve help from a veterinarian, or a pest control company. If exposure is occupational, a change of job may be required. Insect protection measures, using insect repellents, mosquito coils and screens, and protective clothing, should be used if the source cannot be eradicated. Topical antipruritics such as calamine lotion may provide rapid relief. If this is inadequate, use

a potent topical corticosteroid applied twice daily until itch has settled (maximum of 1 week on the face).

 

Persistent nodules may be treated with

triamcinolone acetonide 10mg/mL OR betamethasone acetate/sodium phosphate 5.7mg/mL, 0.1mL injected into each nodule.

 

If ulceration or necrosis has occurred on the lower legs, the wound should be debrided and treated expectantly with a topical agent. Use

silver sulfadiazine 1% cream OR mupirocin 2% ointment or cream.

 

Healing may take 6 to 12 weeks.

A very severe acute reaction to insect bites may be treated with

prednisolone 0.5mg/kg orally, daily until settled.

 

Bee and wasp stings may be treated with an oral antihistamine, see Acute urticaria, but if anaphylaxis occurs immediate treatment should be given with

adrenaline 1 in 1000 solution 0.3 to 1mL IM or SC, repeated every 10 to 15 minutes as required.

OR

1 in 10 000 solution [Note] 1 to 2.5mL IV over 5 to 10 minutes, repeated every 10 to 15 minutes as required.

 

Patients who have experienced this severe reaction should be taught to self-administer subcutaneous adrenaline.

 

Related topics:

Papular urticaria Corticosteroids Oral antihistamines - Treatment of acute urticaria

 

[Note]

If using adrenaline 1 in 1000 solution, dilute to 1 in 10 000 before IV injection.

Scabies (Sarcoptes scabiei)

 

 

Scabies is a dermatosis caused by infestation with the mite Sarcoptes scabiei var hominis. This insect is a human pathogen and is spread by close physical contact between infected persons. Human scabies is not acquired from animals. Scabies is common in school-age children and in closed communities such as nursing homes. If untreated, it will usually spread to all members of a patient’s family.

The diagnosis of scabies is often difficult, especially if it is not suspected. This is because the clinical picture is variable and signs may be subtle. An allergic reaction to the presence of the mite is responsible for signs and symptoms. The degree of this reaction is variable and may sometimes be absent. Sometimes the only complaint is itch without an obvious rash. The itch exacerbates at night and after hot showers. Typically there is an itchy, excoriated but nonspecific rash on the trunk, associated with scaly burrows on the fingers and wrists. Papular lesions are often seen around the major flexures in children, on the penis in men, or on the nipples in women. In babies and young children, there are often vesicles and pustules on the palms and soles and sometimes on the scalp. Nodules may occur in axillae, groin and sometimes other parts of the skin. Secondary bacterial infection may occur.

Confirmation of the diagnosis is made by microscopy of scrapings from a burrow. The main pitfall in using this technique is the selection of a suitable burrow, as these are few in number and difficult to identify. This procedure therefore requires some skill. Response to antiscabetic medication may therefore be used as the most practical diagnostic test. In doubtful cases, or where the patient has difficulty accepting the diagnosis, a scraping is very useful. The burrow is a superficial lesion, which involves only the epidermis, and its contents may easily be scraped from the skin surface using a scalpel blade. The material is smeared onto a drop of oil or potassium hydroxide 10%. Microscopy of this material reveals the mite, eggs or faecal material.

 

If secondary bacterial infection is present, treat as for impetigo at the same time as scabicide treatment.

1	permethrin 5% cream (child >6 months) topically, to the whole body including face and hair (avoid eyes and mucous membranes), leave overnight
	OR
2	benzyl benzoate 25% emulsion (child <2 years: dilute with 3 parts of water; child 2 to 12 years and sensitive adult: dilute with equal parts of water) topically to the whole body, including face and hair (avoid eyes and mucous membranes), leave for 24 hours.

 

Treat family and close contacts even if free of symptoms, since these can take several weeks to develop and contacts may by then have become a source of re-infection. After the recommended treatment, wash clothing and bedclothes and hang in the sun. Clothing or bedding that cannot be washed should be placed in plastic bags for 7 days.

Children under 2 months of age can be treated with sulfur 5% cream daily for 2 to 3 days or crotamiton 10% cream daily for 3 to 5 days, according to the manufacturer's instructions. These agents are less efficacious and cure may not occur. If relapse occurs, consider treatment with permethrin.

For moderate and severe infections, repeat scabicide treatment in 14 days.

 

Contact tracing, notification and treatment are essential to prevent treatment failure. All members of the patient’s family and close contacts should be treated simultaneously. In closed communities such as nursing homes, all patients and staff require treatment. If a school-age child has had scabies, the school should be notified, but treatment of clinically uninvolved children is not required.

 

The itching of scabies does not resolve immediately after treatment and may take 3 weeks to subside. Part of this itching may be irritation from the antiscabetic agent itself. Patients must be warned and instructed not to apply further antiscabetic agents. During this time, relieve the itch and dermatitis that occurs secondarily.

Use

	a moderately potent topical corticosteroid applied 2 to 3 times daily
	AND
	an emollient.

 

Postscabetic nodules may last for months despite treatment with a topical corticosteroid.

 

In this variant of scabies, the mite population on the patient is very high due to a poor host response. This is seen in physically incapacitated and immunocompromised patients, including those with HIV infection. It presents with gross, fissured thickening of the skin, particularly on the back of the neck and around the hands and feet and under the nails. Often there is secondary dermatitis but, paradoxically, itch may be absent. Diagnosis is easily made by a scraping because of the vast number of lesions.

For crusted (Norwegian) scabies, consider ivermectin (child >5 years) 200 micrograms/kg orally initially as a single dose, in association with scabicides. Topical keratolytics containing, for example, lactic acid 5% and urea 10% can be applied daily after washing, on days when scabicides are not applied. It may be necessary to repeat scabicides twice-weekly for 2 to 6 weeks, together with repeated ivermectin doses. One regimen for severe crusted scabies is ivermectin (child >5 years) 200 micrograms/kg orally on days 1, 2, 15, 16 and 29.

 

The source of a scabies outbreak in this situation is frequently a new, infested patient. If possible, this patient should be identified, as they may have Norwegian scabies. The safety of ivermectin in elderly and debilitated patients has not yet been established.

The affected ward should be quarantined and all patients, medical and nursing staff and their families should be treated. Bedding, clothes and towels should be laundered and communal sitting areas sprayed with insecticide. If staff from the affected ward have worked elsewhere, that area should also be treated.

 

The recommended treatment is permethrin 5% cream, see Background and treatment. Permethrin is classified as category B2; however, this must be balanced against the significant morbidity of untreated scabies. Patients may elect to postpone treatment if scabies is acquired in the first trimester of pregnancy.

Alternative treatment with sulfur 5 to 10% in sorbolene cream may be used if the clinician judges the risks of treatment with permethrin to outweigh the benefit.

 

Scabies in HIV-infected individuals may be resistant to repeated attempts at topical therapy. In this situation, use

ivermectin 200 micrograms/kg orally, weekly.

 

Note:

Ivermectin is available in Australia but is not specifically approved for use in scabies. It is not recommended in geriatric patients.

 

If treatment fails after the use of permethrin, consider the possibility of a wrong diagnosis, an unidentified source of reinfestation, inadequate contact tracing or noncompliance with instructions. Recommended measures are to consider other diagnoses and treat accordingly, to supervise treatment (inpatient, community nurse) or specialist referral.

If topical treatment fails, and the diagnosis of scabies is not in doubt, use

ivermectin 200 microgram/kg orally, as a single dose.

Infestation with animal or bird mites

 

Infestation with animal mites is an uncommon condition, but one the clinician should be aware of in rural areas or where humans make close contact with pets and other animals. Bird mites may cause a similar problem. The clinical presentation may seem identical to scabies; however, typical burrows are absent. The rash results from bites from the mite, which produce an allergic reaction in the human host but not an infestation. Antiscabetic medications are ineffective.

The following measures are recommended:

           cease contact with the animals or birds

           use a potent topical corticosteroid, to relieve the itch and treat the rash

           treat the animal(s) if possible with an appropriate insecticide effective against animal mites.

Lice

 

 

Head lice are common and not indicative of poor hygiene. The only complication is secondary infection from scratching.

Approximately one-third of cases can be cured by wet combing (applying hair conditioner or olive oil to wet hair and using a fine nit comb) every 3 to 4 days for several weeks after detection.

Alternatively, topical insecticides can be used

1	maldison 0.5 to 1% topically, according to the manufacturer’s instructions. Not to be used in children <6 months
	OR
1	permethrin 1% topically, according to the manufacturer’s instructions
	OR
1	pyrethrins 0.165% + piperonyl butoxide 1.65% to 4% topically, according to the manufacturer’s instructions.

 

Apply to hair (avoiding contact with eyes and mucous membranes), leave for the time advised, then rinse thoroughly with warm water. Wash hands thoroughly after use. Two to 3 days after the treatment, comb hair with a fine comb. If pyrethrins or permethrin are used, treatment should be repeated at 7 to 10 days.

Family and other intimate contacts should be examined and treated.

After 1 or 2 treatments, persisting adult lice suggest resistance, so re-treatment with an alternative regimen may be necessary. Pyrethrins and permethrin are chemically similar, but maldison has a different mode of action. This may be important, as head lice may become resistant to each group of agents.

 

For head lice that are resistant to topical insecticides, the recommended treatment is

olive oil applied thickly to scalp and hair for 8 hours on treatment days 1, 2, 5, 9, 13, 17 and 21, to coincide with the lifecycle of the louse. Then comb out with a fine comb.

 

The basis for this treatment is that the oil suffocates the lice. If this fails, use

trimethoprim+sulfamethoxazole 80+400mg orally, twice daily for 3 days. Repeat after 10 days.

 

It is thought that the effectiveness of trimethoprim+sulfamethoxazole is due to the destruction of symbiotic bacteria in the gut of the lice.

 

Phthirus pubis colonises pubic, axillary, beard and body hair. It may also involve eyebrows and eyelashes. It is transmitted by close physical contact, often sexual. It is most often seen in adults. In children sexual abuse should be considered but is not invariable. The main symptom is itching with the louse and eggs visible on hairs. Treatment is the same as for head lice (above). Contact tracing is essential. The whole body surface should be examined, including eyelashes and eyebrows. Shaving pubic hair is also helpful. Underwear and bedclothes should be washed. Treatment failure may be due to re-infection, and family and sexual partner(s) should therefore be checked and treated as appropriate.

 

Treat as for head lice (above) applying the preparation to the whole body, but avoiding contact with eyes and mucous membranes. The parasites and eggs are found in clothing and bedclothes, which should be discarded, hot washed or sealed in plastic bags for 30 days.

 

White soft paraffin is applied thickly to the eyelashes twice a day for 8 days to suffocate the insects. The nits may then be physically removed with fine forceps. This may be difficult, requiring slit lamp control. In this situation, referral to an ophthalmologist is recommended.

Cercarial dermatitis (swimmer's itch)

 

This is an acute allergic dermatitis that develops after contact with members of the Schistosoma family. Schistosomes are parasites of birds, ducks or cattle, which use snails as their intermediate host. The snails are found in certain freshwater lakes.

Initially, a pricking sensation is felt as the cercariae penetrate the skin, followed by a an itchy macular rash and then 10 to 15 hours later by a very itchy papular and vesicular eruption on areas not covered by the swim suit. The rash settles spontaneously in a week but may leave pigmentation that takes months to settle. For treatment, see allergic contact dermatitis. Patients should avoid further immersion in infested waters.