Insects play an important role in maintaining the world’s ecosystem (Zhu and Stiller, 2002) but many of them feed on other animals. Humans are relatively hairless and provide an easy target, especially when partly clothed (Cohn, 2003). Biting insects common to the UK include midges, gnats, mosquitoes, flies, fleas, lice, mites, ticks, and bedbugs (Fig 1).
VOL: 100, ISSUE: 22, PAGE NO: 63
Marion Richardson, BD, CertEd, RGN, RNT, DipN, is senior lecturer and programme leader, emergency nursing, University of Hertfordshire
Although their bites rarely cause serious problems, the salivary gland excretions they deposit contain various antigenic substances that may provoke a reaction in susceptible people (Prodigy, 2003).
Insect habits and habitats
Bedbugs - These are nocturnal blood-sucking parasites that feed at night. During the day they hide in mattresses and bed covers, and in cracks in walls, floors and furniture, where they can survive for more than a year without a suitable food source (Fletcher et al, 2002; Zhu and Stiller, 2002). Bedbug bites do not usually cause reactions.
Fleas - These live on small rodents, bats, birds and pets, and move from them to feed on humans. Most flea bites are not associated with disease transmission though rat fleas can transmit plague (Zhu and Stiller, 2002).
Body lice - These live on clothing and move to nearby body areas to feed. Pubic or crab lice live in the pubic or perianal areas - both are blood-feeding parasites.
Body lice bites can cause allergic hypersensitivity, erythematous papules (small elevated palpable lesions), pruritus (itching), swelling, excoriation, lymphadenopathy and conjunctivitis. Crab lice rarely cause these conditions (Zhu and Stiller, 2002).
Mosquitoes - There are many species of mosquito. The females need a blood meal to produce eggs but males feed on plant nectar. Skin reactions are common at the site of bites and, worldwide, mosquitoes transmit diseases from one bitten host to the next. These include malaria, yellow fever, dengue fever (acute arbovirus infection), lymphatic filariasis (caused by the lymphatic filarial parasites), and encephalitis (Zhu and Stiller, 2002).
West Nile virus is the most likely mosquito-borne disease in the UK. It is uncommon because the population density of mosquitoes is relatively low (Prodigy, 2003). In most people the infection is asymptomatic or causes a mild influenza-like illness. It may cause encephalitis or aseptic meningitis, especially in people aged over 50 (Prodigy, 2003; Crook et al, 2002).
Midges - Only female midges attack, often in swarms at sunrise or sunset and with a higher frequency in seasons with increased humidity (Cohn, 2003). Midges are rarely vectors of disease.
Scabies - These mites are found either on animals or in stored goods. They mate on the skin and the female burrows into the epidermis, usually on the hands, wrists or elbows, leaving a small opening and a linear burrow. Scabies causes severe itching, especially at night, not only at the burrowing sites but also over much of the body. Secondary bacterial infection is a complication (Zhu and Stiller, 2002).
Ticks - These are blood-sucking parasites that may embed in the human skin (Storer et al, 2003). They are found in woodland areas with plentiful wildlife (Cutler, 1997) and are especially prevalent in spring and early summer. Ticks often attach in obscure areas such as behind the ear (Storer et al, 2003) and once attached, may feed for many weeks in preparation for egg-laying (Howell, 2001).
Tick bites often cause local allergic reactions such as eczematous changes, urticaria, blistering, and temporary alopecia (hair loss). Susceptible people may have a delayed hypersensitivity reaction including symptoms of fever, pruritus, and urticaria (Storer et al, 2003).
Lyme disease is transmitted by one species of tick (Ixodes ricinus) and, though uncommon in the UK, its incidence is rising - currently there are approximately 200 cases a year (Prodigy, 2003; McGarry et al, 2001). Lyme disease that is caused by a spirochaete can result in arthritis, meningitis, neuropathies, carditis, and encephalopathy.
Distribution and appearance of bites
Typical distribution of insect bites and their possible causes are listed in Box 1. Reactions to bites are varied. The first time a person is bitten there is usually no reaction unless the saliva contains a substance that is likely to cause direct injury, for example, parasites and bacteria.
After repeated bites, sensitivity occurs (Prodigy, 2003); an itchy papule develops about 24 hours after each bite and lasts for several days.
After further bites, an extremely itchy rash develops immediately and is followed by a firm, pruritic papule. Following continued and repeated exposure, no reaction occurs (Prodigy, 2003).
Symptoms - Burns (1998) notes that irritation is an almost constant symptom. Rubbing and scratching may increase the inflammatory changes.
Papular urticaria is common in young children and those with a history of atopic dermatitis. It is caused by a sensitivity to the bites and consists of groups or lines of very itchy, indurated papules that persist for up to two weeks (Prodigy, 2003; Stibich et al, 2001).
Bullous reactions (fluid-filled blisters larger than 5mm in diameter) are common in the lower legs and may occur at other sites, especially in children (Burns, 1998). Chronic cases in adults can appear as lichen simplex - rough thickened epidermis (Prodigy, 2003).
Bacterial infection may be introduced at the time of the bite or may occur as a result of scratching.
Systemic urticarial reactions occur in some people several hours after the bite, and fever and malaise can occur if there are numerous bites or the local reaction is severe (Prodigy, 2003). Anaphylactic reactions to insect bites are uncommon. Typical reactions are listed in Box 2.
Bite reactions can persist for months - for example, tick bites may result in persistent nodules or papules caused by retained mouthparts. However, most tick bites heal within three weeks (Wilson and King, 2003).
Management of bites
Prodigy (2003) notes that there is a lack of good quality evidence regarding the management of insect bites.
Midges, gnats, mosquitoes, flies, and ticks - Bites from these are generally ‘one-off’ incidents and treatment of symptoms is usually sufficient. Creams or lotions with soothing qualities are prescribed and may relieve itching.
Antihistamines are of little help in treating pruritus but a short course of sedative oral antihistamine at night may allow sleep and break the itch-scratch cycle (Prodigy, 2003). If local inflammation is present, a topical corticosteroid may help to relieve itching.
For urticarial reactions, a short course of oral antihistamine may be appropriate to help control itching, the appearance of wheals, and sleeplessness (Drug and Therapeutics Bulletin, 2002). People who suffer severe anaphylactic reactions to these insect bites need to carry adrenaline (Stringer et al, 2002).
Ticks must be removed as soon as possible after the bite to minimise complications. The literature has many suggestions as to how tick removal is best performed to achieve the optimum outcome.
The Prodigy guidelines (2003) suggest using fine-point tweezers to grasp the tick as close to the skin as possible and then pulling gently. It is important to avoid squeezing the body of the tick. The site of the bite should be cleaned with disinfectant.
Routine use of antimicrobial prophylaxis or serological tests for Lyme disease following a tick bite is not recommended but people who develop a skin lesion at the site of the bite or pyrexia within one month of removing a tick should be advised to seek medical advice promptly (ATTRACT, 2003).
Fleas, mites, bedbugs, and lice - Bites from these require symptomatic treatment but it is more important to find the source of the infestation and eliminate it.
Cat and dog fleas should be managed with an appropriate insecticide. It should be used to treat the animal, its bedding, carpets, and soft furnishings (Prodigy, 2003). Regular, thorough vacuuming should be undertaken.
In the case of bedbugs, insecticide should be applied to walls and furniture that is likely to be harbouring the bug. Soft furnishings and bedding should be washed. Mite infestation should be eradicated and clothes regularly laundered (Zhu and Stiller, 2002).
There is a consensus in the literature that the most effective insect repellents are those containing DEET (diethyltoluamide) and the stronger the solution of DEET, the longer the effectiveness of the repellent (Cohn, 2003; Prodigy, 2003; Fradin and Day, 2002). DEET is remarkably safe - very few cases of serious toxic effects have been reported.
However, DEET is a plasticiser capable of dissolving watch crystals, spectacle frames, and certain synthetic fabrics (Fradin and Day, 2002). Stringer et al (2002) advise that DEET should be avoided by pregnant women and children.
While repellents not containing DEET do not provide such effective protection, they may be more acceptable for some people in the UK, where mosquito-borne diseases are not a substantial threat.
There is no evidence that ingested compounds such as garlic and thiamine (vitamin B1) can help to repel biting insects. However, oil-of-eucalyptus appears to confer longer-lasting protection than other plant-based repellents available (Cohn, 2003; Fradin and Day, 2002).
It is vital to reapply repellents regularly, particularly when it is hot and humid (Stringer et al, 2002). Driver (1999) says repellent is needed at sunrise and sunset to prevent mosquito bites but at other times for other insects.
Clothing can be impregnated, especially at the wrist and ankle bands - plug-in vaporisers or mosquito coils can be used. Mosquito nets and other precautions are not usually necessary in the UK.
Wearing appropriate clothing (long sleeves and long trousers tucked into socks) will also help prevent insect bites outdoors, though some insects can bite through clothing. Clothes can be sprayed with DEET for added protection.
Insect repellents against ticks are only useful for a few hours (Howell, 2001) and it is best to avoid tick-infested areas. Ticks are more visible on light clothing (Storer et al, 2003), which should be checked regularly so they can be removed as soon as possible (Cutler, 1997).
The old adage ‘prevention is better than cure’ certainly holds true where insect bites are concerned. For most people bitten in the UK, the reaction will be an irritating nuisance. However, an unfortunate few will experience severe reactions and may develop serious disease.