Insect stings and bites

Essentials

• Insect stings and bites may cause allergic reactions and local infections and spread diseases.
• In Finland, insect vectors spread Lyme disease , tick-borne encephalitis (TBE) , tularaemia , 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 can often 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 .
• In anaphylaxis, extensive erythema and swelling often develop at the site of the bite or sting, and general symptoms, such as malaise and lowered blood pressure, usually appear within a few minutes.
• Acute urticaria is also possible, as well as generalized itching or local itching on the palms and soles.
• However, anaphylaxis may also occur without any notable skin symptoms.
• Anaphylactic reactions are particularly common in people allergic to Hymenoptera. For examinations and treatment, see the section on Hymenoptera below.

Complications caused by bites and stings

• A strong local reaction (painful erythema, an extensive weal, angioedema) close to mucous membranes may lead to respiratory obstruction (in the pharynx or on the lips, for example).
• 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 .
• 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 and tick-borne encephalitis (TBE), both spread by ticks
  • tularaemia spread by mosquitoes and sometimes by ticks, black-flies or horseflies
  • 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 ) 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) is clinical, and the treatment is started based on clinical judgment.
• In anaphylaxis due to Hymenoptera, skin prick tests should be performed and specific serum IgE antibodies to wasp and bee venom tested for.
• 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)

Mosquitoes

• A mosquito bite causes a rapidly developing urticaria-like papule that almost always disappears spontaneously but that may also result in a long-lasting, itching papule in sensitized persons.
• Large papules and even vesicles may occur.
• Antihistamines, such as 10 mg cetirizine once daily, have been shown to alleviate the symptoms of mosquito allergy, particularly if taken prophylactically

Black flies

• Black flies cannot bite through clothing but may bite by crawling underneath. Many people develop a reaction of papules that may last to up to a few weeks.
• People allergic to the venom may develop a more extensive local oedematous reaction.
• Black-fly bites usually produce severe itching, and scratching may cause ulceration or infection

Biting midges

• The biting midge is smaller that the black fly and may be found in large swarms.
• They penetrate mosquito nets and also get into city houses where they may bite if they get under the bed covers.

Deer louse flies

• Deer louse flies make their way to the hairy scalp and under clothing so the bites are located mainly around head and back.
• At the site of the bite a papule appears that may suppurate and last for several days, and some people develop nodules that may last up to several months (picture ).
• Cause problems late in the summer.
• Most common in Southeast, Southern and Central Finland
• Insect repellents are not efficient.

Horseflies

• A horsefly bite often causes a large swollen papule with a watery spot in the middle.
• Treatment
  • For topical treatment, class II to III corticosteroid ointments or combinations of a corticosteroid and an antiseptic agent may be used once or twice daily for 1 to 2 weeks.
  • For clearly infected bite sites, an antimicrobial ointment can be used 2 to 3 times a day for 1 to 2 weeks.
  • Moist or cold compresses or a cooling gel will alleviate the symptoms.
  • For strong reactions, a 3-day course of oral corticosteroids, such as 40 mg/day of prednisolone, may be given.

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 is recommended.
  • For first aid, the patient should carry an adrenaline injector, as well as a single 40 mg dose of e.g. prednisolone, and an antihistamine product e.g. in the form of a tablet that dissolves on the tongue.
• Examinations in case of anaphylaxis
  • The patient is considered allergic to Hymenoptera venom if he is found after the reaction to have IgE antibodies to bee or wasp venom. An IgE response can only be detected about 1 to 2 months after the sting.
  • If both are positive, allergen-specific IgE antibodies can generally be tested for to find out the true cause of allergy.
  • If such a service is available, it is useful in these cases to also send a serum sample for an immunochemical test (immunospot).
  • 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 and special examinations considered, consulting an allergologist.
  Measure 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)
  • A severe systemic reaction (local reaction and cardiac, vascular or respiratory symptoms) to stinging by Hymenoptera (wasp or bee) is an absolute indication for desensitization.
  • 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.
  • Extensive local reactions do 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 .

Lice (Anoplura)

• The blood-sucking species parasitic in man are head lice, body 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 alcoholics and socially excluded persons.
• Ordinary washing of the clothes in a washing machine is sufficient for sterilizing 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

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 ) but rows of bites are also common.
  • Apparently, fleas often cause a papular urticaria that children sometimes have in the summer (strophulus). 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 corticosteroid 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 , 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 .
  • 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.

Insect repellents

• The active ingredients in insect repellents sold in Finland are DEET (diethyltoluamide), icaridin or IR3535.
• The effect of insect repellents usually lasts a few hours but intensive sweating shortens the period of activity.
• The strengths and periods of efficacy of repellents correlate to each other, i.e. the strongest repellents always have the longest period of activity.
• A mosquito repellent should preferably have a concentration of at least 20%.
• A DEET product of over 35% can be recommended if there are really many mosquitoes in the area, if the mosquito repellent quickly evaporates from the skin or for people moving in nature in a terrain where there may be ticks.
• 50% solutions are effective against ticks and 20% solutions against mosquitoes, black-flies and biting midges if used according to the instructions given on the package.
• Insect repellents are not effective against deer louse flies, wasps, bees or bumblebees.