Insect Bites and Infestations

The cause of bite can often be readily diagnosed where insect remains attached as in ticks and with bloodsuckers that are highly visible, e.g. mosquitoes, midges and black flies. Others may not be so easy to diagnose because they bite at night or when patient is asleep, e.g. some mosquitoes, sandflies, bedbugs, triatomine bugs or when it is inconspicuous and does not cause an immediate painful bite, e.g. harvest mites, some fleas and biting flies. Bites typically result in single or grouped pruritic erythematous papules. Some may have a central punctum and others may be bullous.

There is often a skin reaction to an insect bite, and this may lead to pruritus and urticarial papules, and sometimes leads to secondary bacterial infection. As well as a local reaction, the bite may cause an anaphylactic reaction and may act as a vector of disease.

General management measures include cooling the skin, calamine and antihistamines to reduce itching, antibiotics for secondary bacterial infection if develops, and any specific treatment for disease transmitted as a result of the bite.

• Worldwide, these are held responsible for the spread of a large number of diseases including malaria, filariasis, yellow fever, dengue, onchocerciasis, trypanosomiasis, leishmaniasis and loa loa
• In UK, these are usually only a nuisance. Discomfort of bite is followed in sensitive individuals by pruritus with scratching and possible secondary infection
• Where possible, problem can be minimised by wearing clothing that covers the skin and use of insect repellents.

• In UK, only medically significant species are bedbugs (Cimex lectularius)
• No evidence they transmit disease, may cause sleeplessness and bites may be painful and swollen. Bedbugs hide during the day and feed at night. They are found by searching the bedding at night or their hiding places during the day
• They superficially resemble lentils and can live for 6 months without feeding becoming paper thin. Control is by removal or steam cleaning of infected mattresses and treatment of room with insecticide
• In South America, triatomine reduviid bugs transmit trypanosomiasis.

• Worldwide, tick bites are responsible for the transmission of both rickettsial and viral infections and Lyme disease
• In America, Rocky Mountain spotted fever, Colorado tick fever and Lyme borreliosis
• In Australia, Q fever, tick paralysis, Queensland tick typhus and Worldwide tick typhus
• Soft ticks are widely distributed and can cause endemic relapsing fever
• Ticks attach to the skin and feed with a barbed hypostome and then detach when engorged
• The bites are usually painless but can cause local sensitisation and secondary infection
• In UK, most common ticks on humans are sheep tick (Ixodes ricinus), a vector of Lyme disease, and hedgehog tick (Ixodes hexagonus)¹
• Where there is tick infestation, bites can be avoided by tucking trousers into boots and body should be searched after leaving area to allow prompt removal of ticks which can reduce risk of disease transmission
• Management
  • There are many way suggested for removing ticks, including, but not limited to, heat, alcohol, and Vaseline. These methods all have disadvantages and may agitate the tick which, for example, in the case of the paralysis tick can cause more toxin to be expressed into the victim
  • A method that works well and minimises further expression of tick fluids is to lay small forceps along the skin with the ends either side of the tick's head, press down into the skin, and firmly grip the head of the tick. Then steady traction can be applied perpendicular to the skin, without twisting, until the tick is finally released. The aim is not to break the tick so that mouth parts are left in the wound. If remnants do get left behind use local anaesthetic and scrape them away carefully with a scalpel blade
  • In a area of significant Lyme disease incidence, doxycycline for 10 days is the antibacterial of choice for early Lyme disease. Amoxicillin, cefuroxime or azithromycin are alternatives if doxycycline is contraindicated
  • If there is significant paralysis then tick antivenom can be administered in addition to supportive care.

• In Britain during late summer, larvae of the harvest mite (Neotrombicula autumnalis). These are tiny and often not noticed
• They attach under tight fitting clothes, feed and then detach causing itchy lesions that are sometimes bullous.

Scabies

• Scabies is caused by chronic infestation with Sarcoptes scabiei var. hominis
• Adult female mite burrows through the epidermis laying eggs. These burrows may be 1cm in length. More common in overcrowded conditions and outbreaks can occur in nursing homes and hospitals. Usually acquired by close contact
• Usually presents with pruritus appearing approx. 1 month after infestation. May be worse at night and after a hot bath or shower
• Burrows usually found in web spaces between fingers and on wrists but can be widespread
• Often signs of excoriation and possible secondary infection
• Diagnosis can be confirmed by microscopic examination of scrapings from affected area especially between fingers
• In immunosuppressed patients, are prone to Norwegian scabies with crusting lesions accumulating over hands, feet and other sites including eyebrows
• Treatment is by topical application of malathion or permethrin including treatment of close contacts.

Pubic lice (crab lice)

• Pubic lice attach themselves to pubic hairs but may rarely be found on other hairs
• Transmission is usually through sexual contact
• Patient usually presents with pruritus and bluish-grey patches may be seen occasionally
• Diagnosis is by observation of the lice (adults 1-2mm long) or eggs or egg cases attached to hair shafts
• Treatment is with application of malathion to whole body repeating a week later.

Head lice

• Head lice infect the scalp and rarely anywhere else. Eggs are laid at the base of the hair shaft and infestation is more common in children, especially females
• Can be high prevalence in certain institutions, e.g. schools, despite high standards of hygiene. Transmission is usually through close contact and presents with pruritus, with or without excoriation, secondary infection and lymphadenopathy
• Lice can be seen usually close to the scalp and the eggs or their cases stuck to the hairs, these can often be obtained by using a fine comb. Adults are 3-4mm long
• Treatment is by use of malathion, permethrin, or phenothrin.² Very careful use of a nit comb can be used to treat infestation alone but less effective than insecticides.

Body lice

• Body lice infest clothing and body hair laying their eggs on the clothing usually along seams
• Associated with poor hygiene and social deprivation
• Transmission is result of close contact with the person or infested clothing
• Bites on the body cause pruritus often with excoriation, eczematisation and secondary infection. Diagnosis is by observation of lice, usually on the clothing
• Treatment is by topical application of malathion to whole body repeated a week later. Hot washing of cloths will remove lice from them.

Fleas

• Fleas are blood sucking ectoparasites, adults are a few mm long, brown and usually very active
• In developed world, human flea (Pulex irritans) is rare. Most flea bites are now due to cat or dog fleas either through direct contact with infested animal or from an environment exposed to infested animal
• Bites cause intense pruritus at bite site with central punctum and possible bulla formation. They often occur in groups
• If troublesome, bites can be treated with topical steroids and systemic antihistamines.

Tungosis

• In tropical Africa and the Americas, infestation by flea Tunga penetrans is endemic with flea penetrating foot or under toenail causing local discomfort.

Myiasis

• Myiasis is infestation of living animals by the larvae of flies
• Dermal myiasis in American tropics is often caused by the human bot fly
• Female lays her eggs on mosquitoes such that they hatch when in contact with the skin and forms a boil with a small aperture as larvae grows (possibly larger than 1cm in length). This may sometimes be seen in Europeans returning from the tropics
• May be painful but infestation is usually harmless except possible secondary infection
• Can remove larvae through simple incision, avoid squeezing
• Tumbu fly is widespread in African tropics. Eggs may be deposited on drying cloths and then cause a boil, which oozes serosanguinous fluid, sometimes with fever and lymphadenopathy. The larvae can be removed through a simple incision or expressed after applying petroleum jelly.