Insect bites and stings

When insects bite, they inject their saliva into the skin and ingest blood, which usually causes localised areas of discomfort and itching (Figure 17-1), which may be clustered or in a linear distribution (Figure 17-2). Stinging insects inject venom through the sting that causes a more severe local reaction. More serious effects are due to (i) anaphylactic reactions from the bite/sting or (ii) the introduction of infectious diseases.

Most cases of bites from fleas, midges and mosquitoes are readily recognised (Box 17-1) and cause few symptoms apart from discomfort/itching. Occasionally, an allergic reaction confuses the picture, such as large bullae (Figure 17-3). Persuading patients that their recurrent itchy spots are due to flea bites can be difficult and they may reject the suggestion (Box 17-2). Some patients develop a persistent insect bite reaction which lasts for many months (Figure 17-4).

Delusions of parasitosis (DoP)

Patients are convinced they have an infestation with parasites when they do not; the term pseudoparasitic dysesthesia may helpfully be used with patients. Those affected (usually women over the age of 50 years) often bring samples in jars of 'insects' (Figure 17-5). Morgellons is virtually identical to DoP except that patients believe that threads of cotton are parasites/spirochetes in their skin. Examination in most cases shows these to be pickings of keratin, cotton, or thread. Sympathy and tact should help reassure the patient and help build confidence; derision and disbelief will tend to trigger seeking a second opinion elsewhere. Two-thirds of patients have the unshakable belief that they are infested with parasite. Application of topical creams and bandaging may help heal the affected excoriated skin; however, antipsychotic drugs are usually required to treat the psychosis and should be prescribed in conjunction with advice from a psychiatrist. Drugs such as risperidone (start with 0.5 mg daily and build up slowly each week, usually 2-4 mg OD is sufficient) can be of benefit, but care must be taken because of potential side effects, particularly if the patient has epilepsy or cardiovascular disease. More recently, aripiprazole (2 mg for two weeks, then increasing by 2 mg every two weeks up to 12 mg daily) has been shown to be effective. Antidepressants including citalopram have also shown benefit especially in those with concomitant depression.

Allergic reaction to bites

Commonly, insect bite reactions cause local irritation, and rarely (often to stings rather than bites) a generalised anaphylactic reaction may occur. For localised allergic reactions oral antihistamines and topical steroids are effective and for more generalised reactions oral antihistamine, intramuscular (IM) adrenaline (injected into the lateral thigh), and systemic steroids may be required. In those identified as having severe reactions to stings they should be given a self-injectable epinephrine pen to use in subsequent severe reactions.

Management of bite reactions

• Over-the-counter preparations can be used in the majority of cases, such as bite-soothing spray, lidocaine, and hydrocortisone ointment.
• Antihistamine creams (crotamiton and doxepin) or tablets (cetirizine and desloratadine) can help reduce itching.
• If blisters are present, they can be deflated with a sterile needle.
• Topical steroids such as Betnovate ointment twice daily can be applied to reduce inflammation, swelling, and itching.
• If bite reactions are secondarily infected, topical or oral antibiotics may be needed. Topical fucidin ointment or Fucibet cream (a combination steroid and antibiotic) can be used twice daily. Flucloxacillin usually covers secondary staphylococcal infections (erythromycin in penicillin-sensitive patients).
• In very severe cases, a short course of oral prednisolone may be needed (30 mg daily for five days).

Prevention of bites

Keep the skin covered with clothing (especially dark colours), wear insect repellent (WHO recommends Icaridin (Autan^®, chemical KBR 3023) and DEET (N,N-diethyl-meta-toluamide)), and sleep under bed nets off the ground.

Insect bites transmitting parasites

The identity of the biting insect can be valuable information if a parasitic infection is suspected. Insects that bite humans include mosquitoes, midges, bed bugs, fleas, sand flies, mites, ticks, and lice. Each insect has its own specific distribution, preferred location, seasonal activity, and preferred skin sites. All these factors can help pinpoint the offending insect (see Table 17-1). It may therefore be significant to know where the patient has been and his or her activities. Travel to tropical areas raises the possibility of parasite infection, while Lyme disease may occur from walking in endemic areas. Handling grain at harvest time may lead to harvest mite (Pyemotes spp.) bites.

Lyme disease

Tick bites can lead to infection with Borrelia burgdorferi, causing arthropathy, fever, and a distinctive rash (erythema chronicum migrans) (Figures 17-6 and 17-7). The bite may not be recalled by the patient. The tick needs to stay on the skin for many hours to take the blood meal and transmit the disease. If the patient is unwell and has visited an area where Lyme disease is endemic (parts of the United States and Europe) or gives a history of a tick bite, then check serology for B. burgdorferi. Do not wait for the results, however, but treat with doxycycline 100 mg twice daily for 10-30 days. Children under eight years and pregnant or breastfeeding women can be given amoxicillin or azithromycin.

Spider bites

Bites from spiders found in the tropics and subtropics can be quite severe (Figure 17-8). The bite of the brown recluse spider (found in parts of the United States) can become necrotic, resembling pyoderma gangrenosum (i.e. a necrotic ulcerated lesion). Some spiders inject venomous neurotoxins that may be fatal - for example, bites from the 'black widow' (Latrodectus mactans), 'fiddleback' (Loxosceles veclusa), and Atrax species found in Australia. European spiders may cause a painful bite reaction, but they are not venomous. However, with increasing transportation of fresh produce from topical/subtropical countries, there are incidents of venomous spiders arriving by ship in consignments of fruit to non-endemic areas.

Wasp and bee stings

The Hymenoptera are a large order of insects, which inject venom through the sting apparatus. Both bee and wasp venom contain histamine, mast cell-degranulating peptide, phospholipase A2, and hyaluronidase. Local reactions are usually insignificant but generalised systemic reactions with massive respiratory tract oedema can be fatal.

Treatment

Mild local reactions can be treated with oral antihistamines. If anaphylaxis occurs, then an epinephrine autoinjection can be given (currently available in two fixed doses: 0.15 and 0.30 mg) immediately if available. In the emergency room, epinephrine is given IM (1 : 1000 adrenaline, 500 μg in 0.5 ml IM given in the lateral thigh to children >12 years and adults, children 6-12 years 300 μg IM, children 6 years 150 μg IM), repeated every five minutes if necessary. Consider giving additional IV fluids, chlorpheniramine (chlorphenamine), and hydrocortisone. IV adrenaline should only be given by experienced specialists. Desensitisation with venom extract carried out in a specialised unit is effective in those sensitive to bee venom.

Infestations

Scabies (Sarcoptes scabiei)

The most common infestation worldwide is scabies (recognised as a Neglected Tropical Disease by the World Health Organization), which causes intense itching that characteristically keeps those affected awake at night. The female mite burrows into the epidermis and lays eggs which hatch into larvae within a few days. See Box 17-3.

Transmission occurs because of close personal contact (at least 15 minutes of skin-to-skin contact) with an infected individual. The first symptoms of itching occur two weeks later when the immune system reacts to the proteins in the mites, eggs, and faeces in the skin. Most infestations in immunocompetent individuals carry 10 adult mites, but in crusted scabies mite numbers will be in the hundreds because of failure of the host's immune system.

Diagnosis

Scabies infestations may be difficult to diagnose because of the wide variation in clinical presentations. However, key points in the history include several individuals in the same household/institution/classroom/ward being affected simultaneously by a rash that is intensely itchy at night. Clinically, burrows can be seen, especially in the finger-web spaces and on the genitals. Burrows are linear palpable ridges on the skin with a black speck indicating the position of the mite (Figure 17-9), which can be teased out of its burrow using a sterile needle and mounted onto a microscope slide. Patients often have a widespread papular rash, which is due to a reaction to the infestation, with multiple excoriation marks which can become secondarily infected with staphylococcus.

Scabies in children

Babies and young children infested with scabies characteristically present with erythematous cutaneous papules and nodules in the axillae and on the soles of the feet (Figures 17-10 and 17-11). It is not unusual for the lesions to blister. Classic burrows are rarely seen in this age group.

Crusted scabies

Crusted scabies can look similar to dry scaly skin rashes such as psoriasis and eczema, and consequently can be misdiagnosed. Patients are usually immunosuppressed or elderly and do not complain of itching, as their immune cells are not reacting against the mite proteins. Consequently, mite numbers are usually in the hundreds. Clinically patients have a crusted fine scaling on the skin, which is superficial with very little erythema (unlike psoriasis and eczema) (Figure 17-12). If the diagnosis is missed, then numerous close contacts such as nurses and carers develop classic scabies and small outbreaks can occur.

Management

A full explanation of how to use topical therapy is essential if infestations are to be successfully managed. The most common cause of treatment failure is incorrect use of insecticides. Patients should be told that they and all their close personal contacts need to be treated at the same time, the lotions should be applied from the neck downwards (although the head and neck of babies should also be treated), the treatment left on overnight, and then repeated after seven days. They should pay attention to the web spaces and genital areas. They should reapply the lotion after washing their hands (Box 17-4).

Towels, bedding and underwear should be washed. Patients should be advised that the skin itching will take six to eight weeks to subside. Persistent itching often leads patients to conclude that the mites are still active, and they subsequently treat themselves repeatedly, leading to an irritant dermatitis. The itching resolves when the mites, eggs, and faeces have been removed from the skin by the host's immune cells.

Lice

Head lice

Head lice have infested humans for thousands of years. They have a worldwide distribution and can affect anyone. Children are the most common hosts. The exact prevalence is difficult to estimate with 0.1-66% of schoolchildren shown to be infected in different areas of rural Africa and 11% in Australia.

Lice are transmitted by head-to-head contact, and on combs, brushes and hats. Girls are more commonly affected than boys; this is thought to be due to their close contact with others during play. Mild itching may be the only symptom of head lice. Careful inspection of the hair close to the scalp may reveal adult lice and nits (white empty egg cases) in infested individuals (Figure 17-13). Patients may develop an itchy irritant-looking dermatitis on their upper back and neck area (Figure 17-14). Fine-toothed combs can aid detection.

Management

• Only treat individuals with live lice visible on the scalp. Fine-toothed nit-combs can be used to comb out lice and eggs over a basin ('bug-busting'). An application of hair conditioner usually allows the comb to pass more easily through the hair. Wet combing alone (30 minutes every 3 days for 2 weeks) has been shown to be inferior to insecticides, but in highly motivated families it can be very successful.
• First-line treatment is dimethicone 4% gel (an inert silicone that physically kills the lice through obstruction of its homeostatic workings) applied to dry hair and left on overnight (eight hours) and repeated after seven days (70-92% effective in studies).
• Second line is permethrin 1-5% crème rinse applied to dry hair and left on overnight. This should be repeated after seven days.
• Alternative agents include phenothrin 0.5% or malathion 0.5% (applied as above).
• Combinations of insecticides and bug-busting (wet combing) can be used.

Body lice

Body lice are the vectors of several human pathogens including Bartonella quintana (agent of trench fever, bacillary angiomatosis, and endocarditis) and Rickettsia prowazekii (agent of typhus). Body lice tend to affect individuals from poorer economic backgrounds and those sleeping rough. The lice live in the host's clothes and bite the skin. Close inspection, especially of the clothing seams, reveals the adults and eggs. Infested individuals have a widespread papular eruption with excoriations. If the patient has a fever or constitutional symptoms, then the possibility of louse-borne systemic infection should be raised.

Management

• Wash clothing in hot water, 'tumble-dry', or place infested clothes in a sealed plastic bag for two weeks to kill adults and eggs.
• Treat the skin reactions with a moderately potent topical steroid, plus topical antibiotic if secondarily infected with bacteria.
• Take blood for culture and serology if louse-borne systemic disease is suspected. Refer to cardiologists if endocarditis is suspected.

Pubic lice

These lice prefer the sparser hair-bearing sites on the skin such as the pubic, axillary, and eyelash areas (Figure 17-15). The so-called crab lice are slow-moving and are spread by close personal contact. Check the patient for other sexually transmitted diseases.

Management

• Use topical permethrin 5% cream or 0.5% malathion to the skin from the neck downwards, left on overnight, repeated after seven days.
• If the eyelashes are involved use petrolatum only, as insecticides can damage the eyes.

Cutaneous larva migrans

Larvae from nematodes such as the dog/cat hookworm (Ancylostoma caninum) and Strongyloides parasites accidentally penetrate human skin and then wander aimlessly, unable to invade the deeper tissue, causing a superficial creeping eruption (Figure 17-16). In their animal host, the larvae eventually make their way to the gut to complete their lifecycle. Treat the patient with albendazole 400 mg daily for three days or ivermectin as a single dose of 0.2 mg/kg body weight.