What are arthropods?
Arthropods include insects, spiders (arachnids), mites and ticks.
- Arthropods can infest human skin, especially scabies and head lice
- They can inflict bites and stings
- They can carry diseases such as malaria, yellow fever and filariasis
- They can give rise to allergic conditions such as hay fever, asthma and atopic eczema
What causes arthropod bites and stings?
Insect bites and stings can be divided into two groups: venomous and non-venomous. A small number of spiders are also venomous.
Venomous insects (stingers)
A sting is usually an attack by a venomous insect that injects toxic and painful venom through its stinger as a defence mechanism. Venomous insects include:
- Yellow jacket
- Fire ant.
Venomous spider bites are rare but have potentially serious systemic neurotoxicity (eg, Lactrodectus species), or usually local, cytotoxic effects (eg, Loxosceles species). The majority of household spiders are harmless. Identification of the offending spider is essential to determine management including antivenin.
Non-venomous insect bites
Non-venomous insects pierce the skin to feed on blood. This usually results in intense itching. Non-venomous insects that bite include:
Caterpillars and moths
Some caterpillars and moths have irritating hairs and sharp spines, causing stinging, short-lasting papular urticaria, dermatitis and allergic reactions.
- Calyptra moth.
Who gets arthropod bites and stings?
Anyone exposed to arthropods can be bitten or stung. Arthropod habitats are variable and individual risks depend largely on geographic and climatic factors. Time of day may be important. For example, some mosquitoes bite at night and others during daytime. Other factors to consider include:
- The fact that less clothing is worn in tropical areas or in summer elsewhere
- The fact that there is increased exposure to insects in gardens or forests
- Overcrowding, travel and poor hygiene
- Current or previous pets.
Mosquitoes are attracted to body heat, carbon dioxide in exhaled air, human sweat, and human microflora.
What are the clinical features of arthropod bites and stings?
The reaction to encounter with an arthropod depends on the species involved, whether it carries disease, and individual factors such as host immunity.
A venomous sting from a bee or wasp usually causes a stinging sensation or pain with redness and swelling of the area. Sensitisation to the venom affects response.
- A large localised reaction causes swelling to spread more widely over several hours.
- Anaphylaxis results in immediate angioedema, urticaria and bronchospasm and can be life-threatening.
An insect bite presents as one or more intensely itchy papules on a body site exposed to the insect.
- Insect bites often arise in crops.
- The papule usually subsides within a few hours.
- It may have a central clear or haemorrhagic blister and persist for several days.
- Scratching results in an open sore.
More images of insect bites ...
Complications of arthropod bites and stings
Complications of arthropod bites include:
- Secondary infection with staphylococci and/or streptococci (impetigo, cellulitis)
- Papular urticaria
- Persistent insect bite reaction
- Arthropod-borne infection.
Papular urticaria is a hypersensitivity reaction, most often in a young child due to fleabites and/or mosquito bites. New bites are accompanied by reactivation of old ones and present as symmetrical crops of itchy urticated papules. Papular urticaria resolves with the development of immunological tolerance.
Persistent insect bite reaction
Solitary persistent insect bite reactions can be urticarial, bullous, vasculitic or granulomatous.
Diseases in which specific arthropods are the vector occur worldwide but are particularly prevalent in tropical and developing regions.
- Parasites: malaria, leishmaniasis, trypanosomiasis
- Bacteria: Lyme disease, plague, bacillary angiomatosis, relapsing fever, tularaemia, babesiosis, Carrion disease
- Viral disease: dengue fever, chikungunya fever, zika fever
- Rickettsial disease: typhus.
How are arthropod bites and stings diagnosed?
Generally people are aware of bites, especially if they have observed the arthropod, but occasionally they are not. The clinical appearance is usually typical.
Skin biopsy can be suggestive if it shows central punctum, eosinophilic spongiosis, and a wedge-shaped mixed dermal infiltrate distributed around the sweat ducts/glands.
What is the treatment of arthropod bites and stings?
If the reaction is mild, insect stings should be treated by first removing the stinger. This is necessary as the stinger continues to pump venom from its sack until it is empty or removed.
- Place a firm edge such a knife or credit card against the skin next to the embedded stinger.
- Apply constant firm pressure and scrape across the skin surface to remove the stinger. This is preferred to using tweezers or fingers, which can accidentally squeeze more venom into the patient.
- Clean the site with disinfectant.
- Apply ice or cold pack to reduce pain and swelling. Topical steroid cream or calamine lotion may be applied several times a day until symptoms subside. If necessary, oral antihistamines can also be taken.
If an insect sting causes a severe reaction or anaphylaxis, urgent medical attention should be sought. If a patient is known to have an allergy to insect stings they may carry with them an allergy kit containing adrenaline (epinephrine).
The main treatment aim of insect bites is to prevent itching.
- Cool the affected area.
- Apply topical calamine lotion or local anaesthetic agent.
- Oral antihistamine reduces itch and weals.
- Use moderate potency topical steroids for papular urticaria or persistent reactions.
Bites from insects carrying disease require specific antimicrobial therapy to treat the disease.
How can arthropod bites and stings be prevented?
The following simple measures can prevent arthropod stings and bites:
- Wear fully covering clothing.
- Keep windows and doors closed at night.
- Avoid perfume and bright-coloured clothing to reduce the risk of bee stings.
- Control odours at picnics and garbage areas that can attract insects.
- Destroy or relocate hives or nests close to the home.
- Drain pools of stagnating water that attract mosquitoes.
- Use electrical insect repelling devices and lit coils.
- De-flea cats, dogs and other household pets regularly.
- Apply insect repellents containing DEET (diethyltoluamide) to exposed skin.
- Apply permethrin to clothing for 2-week protection, through two wash-cycles. It can also be applied directly to exposed skin keeping the insects away for a few days.
- Thiamine (vitamin B1) can be used as a systemic insect repellent (the skin has a characteristic smell).