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Systemic allergic reactions can occur from horse fly bites.

Editor’s note: Pest management professionals should never offer medical advice. PMPs always should encourage their clients to see a doctor to address pest-related health concerns. The following is for pest management professionals’ knowledge only since they may encounter such pests/conditions in the field.

Bites of insects and other arthropods are generally only a minor nuisance, but they sometimes can lead to serious medical problems, including transmission of insect-borne illnesses and severe allergic reactions. The bites themselves are only punctures made by mouthparts of the offending organisms and the actual mechanical injury to humans is minimal. Spots of redness, bumps and blisters on the skin may result from the host’s immune reaction to the arthropod’s salivary secretions or venom. If a person is prone to allergies, hypersensitivity may develop to proteins found in arthropod saliva. Insects and other arthropods inject saliva while feeding for a variety of reasons, including: 1) to lubricate mouthparts for insertion, 2) to increase blood flow at the bite site, 3) to interfere with blood clotting, 4) to anesthetize the bite site, 5) to suppress host immune responses and 6) to aid digestion of the blood. The word “bite” probably should be restricted in meaning to purposeful biting by a species for catching prey or blood feeding and not to accidental biting by plant-feeding insects. Plant-feeding and predaceous insects sometimes “bite” in self-defense, piercing the skin with their proboscis, but the injury is actually just a stab wound.

There are two types of blood feeding by arthropods. Some groups, such as mosquitoes, bed bugs, kissing bugs and sucking lice, obtain blood directly from capillaries or small veins — a method called solenophagy. Others, such as ticks, horse flies and deer flies, black flies and biting midges, obtain blood by lacerating blood vessels and then feeding from the pool of blood that forms — a method termed telmophagy.

There’s a difference between stings and bites. Stings involve injection of venom (via a stinger) into the victim, which may cause reactions ranging from local irritation all the way to life-threatening anaphylaxis. Bites also may inject venom (e.g., a spider) but it’s from the mouthparts, not a stinger. In addition, people can develop skin rashes and even allergic reactions from contact with caterpillars, in the absence of an actual bite or sting.

MOUTHPART 411. There are several types of insect mouthparts that can be generally grouped into three broad categories: 1) chewing, 2) sponging and 3) piercing/sucking. Within these categories there are all sorts of adaptations and specializations among the various insect orders. Chewing mouthparts, common in insects like beetles and cockroaches, consist of toothed mandibles which move horizontally to bite/chew food. Chewing mouthpart types are of little health significance, but piercing/sucking mouthparts, and especially the bloodsucking types, are considerably important.

Flea bites on a person’s knee. Note the two on the left side of her knee are recent bites, while the others are several days old.
© 2001 by Jerome Goddard, Ph.D.

Piercing/sucking mouthparts differ primarily in the number and arrangement of the stylets (needlelike blades), and the shape and position of the lower lip of insect mouthparts, termed the labium. Often, what is called the proboscis of an insect with piercing/sucking mouthparts is an ensheathment of several components such as the labrum, stylets and labium. For example, mosquitoes have a proboscis composed of six stylets (two mandibles, two maxillae, the hypopharynx and labrum/epipharynx) ensheathed in an elongated, cylindrical labium. Horse flies, deer flies, black flies and biting midges basically have the same type of bladelike mouthparts. In that case, the stylets are flattened compared to those found in mosquitoes. The mandibles move transversely in scissor-like fashion and the maxillae are thrust in and out of the wound causing pooled blood in the host’s tissues. In tsetse flies, teeth on the labellum aid the labium for penetrating the skin. Movements of the fly’s head further enables the labium to gain access to capillaries in the skin.

Other arthropods such as spiders, mites and ticks also have piercing/sucking mouthparts, but these structures are derived from different morphological features than those of insect mouthparts. Mites and ticks don’t have a true head, but a head-like gnathosoma for feeding. The gnathosoma consists of mouthparts and palps and forms a tubular structure for obtaining food and passing it into the digestive tract. The cutting/piercing mouthparts of mites and ticks are called chelicerae. Chelicerae may cause tearing of skin, as in the case of scabies mites, or piercing, as in the case of chiggers. In ticks, there is an additional anchoring “snout” or hypostome, which is a very prominent structure bearing teeth on the ventral surface. True bugs, such as bed bugs, kissing bugs and assassin bugs, have the labium formed into a prominent three- or four-segmented cylindrical proboscis.

BITE REACTIONS. There are different types of bite reactions. Insect bites may result in local reactions, papular urticaria or “all over” allergic reactions (which can be deadly). Rarely, other forms of systemic reactions can occur, such as serum sickness.

A normal reaction to an insect bite is an inflammatory reaction at the site of the punctured skin, which appears within minutes and consists of itchy local redness and swelling. This is called a local reaction. Symptoms usually subside within a few hours. Local reactions are caused by irritant substances contained in the saliva (anticoagulants, enzymes, agglutinins and mucopolysaccharides). In some cases, a local reaction can be followed by a delayed skin reaction consisting of local swelling, itching and redness. Evolution to a pustule, blister or “hardened nodule” is uncommon, but does sometimes occur.

Horse fly bite on face.
Photo courtesy Lindsey Carpenter Goddard

TREATMENT. Insect and other arthropod bites should be washed with soap and water. Reduction of local swelling may be induced with cooling (ice or cold pack). Topical creams, gels and lotions, such as those containing calamine, may decrease itching. Routine use of topical anesthetics and antihistamines are generally not recommended by doctors because they can sensitize the skin following sun exposure and induce allergic contact sensitivity. Non-sedating oral antihistamines, such as cetirizine or loratadine, may be helpful for patients with troublesome itching. Any serious or infected arthropod bite should be seen by a health-care provider.

Papular urticaria. Papular urticaria is an allergic disorder in which insect bites, most often from fleas, mosquitoes or bed bugs, lead to recurring and sometimes chronic itchy, raised bumps on exposed areas of skin (e.g., arms, lower legs, upper back, scalp). Papular urticaria is reported predominantly in young children (typically 2 to 10 years of age). The diaper/underwear areas, genital, perianal and axillary areas are usually spared. The 0.5 to 1 cm spots at the beginning can become persistent and papular and/or nodular with time. Diagnosis of papular urticaria is made by a doctor, although there may be a delay between the bite(s) and the onset of the lesions. Usually only one child in a family is affected, a clue that infestation at home is unlikely. New lesions may appear sporadically, and renewed itching may reactivate older lesions, leading to a chronic and cycling disorder that may last from months to years.

Treatment. Management of papular urticaria is often overseen by a physician and includes selective and limited use of non-sedating antihistamines for itching, topical steroids applied to individual lesions and reassurance, as the disorder eventually resolves spontaneously.

Systemic allergic reactions. “Systemic” means all over, so systemic allergic reactions to arthropod bites are uncommon, but do occur. Systemic allergic reactions have occurred from Triatoma (kissing bug) bites, mosquitoes, ticks, black flies, deer flies, horse flies and centipedes.

Treatment. Systematic reactions, especially anaphylaxis, need to be treated promptly with epinephrine. Patients who have experienced systemic reactions should be supplied with an epinephrine autoinjector and instructed about how and when to use it. Referral to an allergy specialist should be done. Allergy specialists are able to assess the patient’s clinical history to assure that the correct triggers for allergic reactions are identified. For patients who have suffered anaphylaxis previously, allergists are able to provide effective training in the self-injection of epinephrine.

Dr. Jerome Goddard is the author of “The Goddard Guide to Arthropods of Medical Importance.” He is an extension professor of medical and veterinary entomology at Mississippi State University.