Insect Stings and Bites

Essentials

• Insect stings and bites may cause hypersensitivity reactions and skin infections and spread diseases.
• In Finland, insect vectors spread Lyme disease Lyme Borreliosis (LB), tick-borne encephalitis (TBE) Encephalitis, tularaemia Tularaemia, Pogosta disease Pogosta Disease and Inkoo encephalitis.
• Wasp or bee allergy may lead to an anaphylactic reaction.
• In mosquito allergy, prophylactic antihistamines may help.

Symptoms

• An insect sting or bite first causes an erythematous spot on the skin that may be raised like a wheal.
• A pinhead-sized petechia may be seen at the centre of the sting or bite site.
• The skin reaction is a combined effect of the insect venom and an immunological reaction, which explains the wide individual variation in reactions.
• Delayed allergic reactions may appear after some insect bites, and itchy lumps persisting for weeks may develop in the area.

Anaphylaxis

• See also Anaphylaxis.
• Signs of a generalized allergic reaction include:
  • Extensive urticaria, erythema, angioedema
  • Dizziness, dyspnoea, nausea
  • Anaphylactic shock (lowered blood pressure combined with other allergic symptoms) and unconsciousness
• Also general malaise and generalized itching or local itching in palms and soles are possible symptoms. Anaphylaxis may also occur without any notable skin symptoms.
• Possible panic-related symptoms following a sting (vasovagal reaction, for example) should also be taken into account in differential diagnosis as an alternative explanation.
• Anaphylactic reactions caused by insects usually occur in people with Hymenoptera venom allergy. For examinations and treatment, see the section on Hymenoptera below.

Complications caused by bites and stings

• A strong local reaction (large, over 10 cm wheal, may last more than 24 hours)
• A local reaction close to mucous membranes may cause airway obstruction (e.g. in the pharynx or on a lip).
• A skin infection with formation of pus (pyoderma) may develop at the site of the sting or bite. This is particularly common in tourists. See the article on skin disorders in tourists Skin Problems in Returning Travellers.
• An abscess, erysipelas or cellulitis may rarely develop at the site of the sting or bite.
• In people sensitized to Hymenoptera venom, a sting may lead to anaphylaxis.
• Clinically significant infections spread by Arthropoda in Finland include
  • Lyme disease Lyme Borreliosis (LB) and tick-borne encephalitis (TBE), both spread by ticks Encephalitis
  • tularaemia spread by mosquitoes and sometimes by ticks, black-flies or horseflies Tularaemia
  • Pogosta disease Pogosta Disease and Inkoo encephalitis, both spread by certain mosquito species.

Investigations

• Patient history and clinical examination form the cornerstones of diagnosis.
• It is important to assess the site of the sting or bite (erythema migrans, necrotic papule or ulcer, picture 1) and any other skin symptoms (e.g. urticaria, itching) as well as mucosal and general symptoms (fever, joint symptoms, neurological symptoms).
• The diagnosis of early stage borreliosis (erythema migrans) and tularaemia (eschar, febrile lymphadenopathy) is clinical, and the treatment is started based on clinical judgment.
• In patients with a Hymenoptera-induced systemic reaction (anaphylaxis), specific serum IgE antibodies to wasp and bee venoms should be tested for 1-2 months after the reaction (see below).
• In purulent sting/bite site infections, bacterial culture may be needed.
• In unclear cases, histological examination of a skin biopsy may show a typical insect bite/sting reaction but this is not completely diagnostic.

Diptera (mosquitoes and flies)

Hymenoptera

• Because of the venom, a wasp, (honey)bee or bumblebee sting immediately causes severe pain and swelling at the site of the sting.
• In Finland, sensitization to Hymenoptera venom causes many potentially fatal anaphylactic reactions annually.
• People allergic to wasps are not usually allergic to bee venom and vice versa.
• Treatment
  • The sting site should be immobilized to avoid the venom from spreading further.
  • Any bee sting remaining in the skin should be removed quickly to prevent the whole contents of the venom pouch from being emptied into the skin. The best way to do this is by scraping over the surface of the skin with a thin, flat object such as a credit card or coin. The sting of a wasp or a bumblebee leaves the skin when the insect detaches itself.
  • Cold compresses, cold packs or a cooling gel are good first-aid measures.
  • People who have been stung several times should be observed for any systemic symptoms.
• Anaphylactic reactions
  • Acute anaphylactic reactions should be treated like anaphylaxis, and any further exposure to Hymenoptera should be avoided.
  • After a systemic reaction (anaphylaxis) to Hymenoptera, allergy testing and an assessment of the possibility of desensitization therapy is recommended.
  • For first aid, the patient should carry an adrenaline injector, and additionally oral glucocorticoid, for example a single 40 mg dose of prednisolone, as well as an antihistamine at double dose, e.g. cetirizine 10 mg 2 tablets in one go or a preparation that dissolves on the tongue.
• Examinations in case of anaphylaxis
  • In a patient who has had an generalized reaction, the IgE antibodies to wasp and bee venoms, as well as the specific allergen components of the venoms, may be examined already in primary health care, depending on the availability of laboratory examinations.
  • Test packages are also available.
    • Distinguishing true wasp and bee allergy from cross-sensitivity is easier than before with the help of the allergen components.
    • IgE response is usually detectable only 1-2 months after the sting.
  • If the patient has had a typical systemic reaction to Hymenoptera but the results of specific IgE tests are negative, the examinations should be repeated or consulting specialized care should be considered.
  • Measure also serum tryptase concentration to exclude mastocytosis in patients who have had an anaphylactic reaction to Hymenoptera venom. The test should be taken in a non-acute phase (e.g. 1-2 months after the reaction).
• Desensitization (allergy to Hymenoptera)
  • Can be carried out in patiens who have had a severe systemic reaction (anaphylaxis) and who have a verified allergy to Hymenoptera.
  • In the case of a mild systemic reaction (local reaction and urticaria or mucosal swelling) desensitization may be considered if repeated stings are highly probable or the fear of stings affects the quality of the patient's life.
  • An extensive local reaction does not warrant desensitization.
  • In patients allergic to Hymenoptera, desensitization should be continued for 5 years, after which it will remain effective for at least 7 years in most patients (at least 80%).
  • The knowledge that desensitization will probably prevent anaphylactic reactions will improve the quality of life.
  • See also Allergen Immunotherapy.

Lice (Anoplura)

• The blood-sucking species parasitic in man are head lice, body lice and pubic lice Head Lice and Pubic Lice.
• Head lice epidemics may occur at schools and in day care centres.
• Body lice are nowadays rare; they appear primarly in people living in conditions with poor hygiene (e.g. individuals with alcohol or drug abuse problems, homeless persons).
• Ordinary washing of the clothes in a washing machine is sufficient for cleaning them.
• Pubic lice are usually sexually transmitted; symptoms include itching and pustules in the genital area and nits in the pubic hair.
• For treatment, see Head Lice and Pubic Lice

Bedbugs and fleas

• Bedbugs occur in old, dirty buildings. They may be transferred with old furniture. Bedbugs suck blood at night.
• Fleas of birds, dogs, cats (e.g. the Cheyletiella tick) and rodents also bite humans.
  • Symptoms occur most frequently in the spring (people cleaning bird nests or visiting summer houses for the first time after the winter).
  • Bedbug and flea bites cause solid, intensely itching papules with a bite mark (petechia) at the centre. These are most often seen in areas covered by clothes.
  • Skin changes may persist for several days, and only sensitized patients will have symptoms.
  • A wheal or more extensive swelling may develop immediately at the bite site, and it can easily be mistaken for a first symptom of erysipelas or other bacterial infection.
  • Papules are usually seen in groups of a few bites (pictures 3 4) but rows of bites are also common.
  • Apparently, fleas often cause a papular urticaria that children sometimes have in the summer (strophulus or lichen urticatus). The patient usually has no idea of the cause of the rash, and diagnosis is difficult.
  • Sensitization is required, which explains why not all family members have symptoms.
• Treatment
  • Treatment with a class II to III glucocorticoid ointment or solution once or twice daily for 1 to 2 weeks is often sufficient for bedbug and flea bites.
  • Extermination of bedbugs from human homes requires expertise to find where they are hiding and to poison them.

Larvae

• The hair of the larvae of some moths (e.g. fox moth) may cause both toxic and allergic reactions.

Ticks

• The Ixodes ricinus tick spreads borreliosis Lyme Borreliosis (LB), tick-borne encephalitis (TBE) and rarely tularaemia.
• Tick bites are painless and may go unnoticed.
• Ticks are active in Finland from April to October or November, and their favourite sites include lawns, grass, low bushes and the herbaceous undergrowth in woodland areas.
• The best protection is to wear long boots and trousers.
• When moving in areas with ticks, one should remember to examine the skin and clothes for ticks every night and remove any that are found.
• For Borrelia bacteria to be transferred from tick to human, the tick needs to be attached to the skin for at least several hours or even several days; therefore, ticks should be sought and removed as soon as possible.
  • See Lyme disease Lyme Borreliosis (LB).
  • The most typical symptom of the early stage is an expanding erythema, erythema migrans (EM), appearing within about a week around the tick bite.
  • Within a few days or weeks, the erythema often spreads to 5 to 10 cm or sometimes many times this diameter and may subsequently disappear spontaneously.
  • As a rule, it is believed that a skin lesion of 5 cm in diameter observed 5 days after a tick bite is consistent with a diagnosis of EM. Smaller and quicker reactions may represent inflammation due to ingredients in tick saliva.
  • The diagnosis of Lyme disease is always based on clinical signs. Early cases represent the overwhelming majority of all diagnosed infections and it is therefore no use performing antibody tests.
• Laboratory tests (serum Borrelia antibodies) are needed for diagnosis at later stages.
• Removing a tick from the skin
  • Grasp the tick as close as possible to the skin with tweezers (small tweezers with pointed tips).
  • Try to remove the whole tick by slow vertical traction only. Removal with oil or ointments is not worth trying. If the tick's head remains in the skin, it can be dug out with a needle, for example, after disinfecting the skin.

Spiders

• No life-threatening spider species live naturally in Finland.
• Spiders usually bite people only when inadvertently pressed against the skin.
• The water spider (Argyroneta aquatica) is the most poisonous naturally occurring spider in Finland, and the effect of its bite is equivalent to that of a wasp sting.
• The bites of the raft spider and large garden spiders may be painful.
• They are treated like the Hymenoptera stings.

Protection from insect stings and bites

• Clothing should cover as much as possible of the body, and this is particularly important for small children.
• Light-coloured clothes with long sleeves and long legs should be preferred. Mosquitoes, for example, may sting through thin clothing.
• Protect the head with headgear and ankles and feet, particularly, with socks and shoes. Socks can be pulled up over trouser legs. Closed shoes are preferable to sandals.