Horse fly

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Code: i204
Latin name: Tabanus spp.
Source material: Whole insect bodies
Family: Tabanidae


An insect, which may result in allergy symptoms in sensitised individuals. Biting insects are a world-wide problem and can elicit severe allergic reactions.

Allergen Exposure

Allergen exposure is through bites.

The insect family Tabanidae (Order: Diptera) comprises approximately 4 500 species of flies worldwide, with several species that feed on human beings (1) including the commonly known horseflies and deerflies. Other taxonomically poorly defined subgroups such as yellowflies and or clegs also belong to this family. Over 1 000 species are in the genus Tabanus. Three subfamilies are widely recognised: Chrysopsinae, Pangoniinae and Tabaninae.

About 350 species of tabanids and 165 species of simulids inhabit North America, and some are vicious biters. (2, 3)

Often considered pests for the bites that many inflict, they are among the world's largest true flies. They are known to be extremely noisy during flight. They are also important pollinators of flowers, especially in South Africa. Tabanids are often encountered as bee-like insects that often follow people outdoors and bombard them, acting as a nuisance factor. They tend to favour the head and upper body, where they usually alight completely unnoticed by the victim until the painful bite gives them away. Females of most species require a blood meal to complete egg development, and many species are important in human and animal medicine. (4)

Tabanids occur worldwide, being absent only at extreme northern and southern latitudes. (5) Flies of this type are among those known sometimes as gadflies, breeze flies, zimbs or clegs. In Australia, they are known as ‘March Flies’ (not to be confused with the Marchflies of North America, which belong to the non-biting fly family Bibionidae). (4) In some areas of Canada, they are also known as bulldog flies. (6)

Tabanids are mostly stout-bodied, fast-flying flies ranging in size from approximately 6mm to 30mm. (5) Horseflies are known for their very painful and often relentless biting. The bites may become itchy, sometimes causing a large swelling afterwards if not treated quickly. In those species that bite, it is only the females that search out prey, for blood meals to nourish their developing eggs. Male tabanids are nectar feeders and are incapable of biting, as they lack the modifications of the mouthparts found in females. Most tabanids feed on mammals, but some species feed on birds, reptiles or amphibians. (7)

As with all other true flies, horse flies follow the following life stages: egg, larva (a.k.a. maggot, when referring to most fly larvae), pupa and adult. Eggs are usually laid in large, layered clusters of 100 to 1 000, on vegetation or other objects overlying water or moist soil. Depending on the species, the larvae may be aquatic, semi-aquatic, or terrestrial; they hatch from the eggs and drop to the water or soil below, where they become voracious predators of other invertebrates or small vertebrates. The head contains two sharp, slender mandibles with a hollow canal for transmitting venom into their prey. The larva undergoes several moults as it grows, and depending on the species, the larval stage may last several months or as long as two to three years. Once the larva is fully developed, it moves into drier soil to pupate. Depending on the species, the pupal stage lasts approximately 5 to 21 days, and then the adult flies emerge from the soil. Mating occurs shortly after the adults emerge. (4, 5, 7)

Tabanids are very good vectors of the equine infectious anaemia virus, as well as some Trypanosome species. Species in the genus Chrysops are biological vectors of Loa loa, transmitting this parasitic filarial worm between humans. They have also been known to transmit anthrax in cattle and sheep, and tularemia between rabbits and humans. (6)

Many species of tabanids are known to play important roles in spreading diseases of livestock and other animals. (8) In addition, several species in three or four genera are medically important to humans. (4, 7) The mouthparts of female tabanids are modified into the equivalent of miniature scalpels or steak knives, ideal for macerating the skin to the depth of the superficial dermal vessels. A pool of blood collects in the tissues and is lapped up by the tongue-like component of the mouthparts. (4) Horseflies rely heavily on the pharmacological propriety of their saliva to get blood meals and suppress the immune reactions of hosts. (9) Horsefly saliva contains a wide range of physiologically active molecules that are crucial for attachment to the host or for the transmission of pathogens, and that interact with host processes. (10, 11) For example, Tabanus anticoagulant protein has anticoagulant activity on the blood coagulation system. (12, 13)

Allergen Description

Attempts have been made to purify the allergenic components from Tabanus spp. (14) A 69 kDa IgE-binding protein has been isolated from the salivary gland protein of species of Chrysops. (1)

The following allergens have been characterised to date:

Tab y 1, a 70 kDa protein, an apyrase. (15, 16, 17)

Tab y 2, a 35 kDa protein, a hyaluronidase. (15, 16, 18)

Tab y 5, a 26 kDa protein, also known as Antigen 5. (15, 16, 18, 19)

Tab y 1 derives from the salivary glands of the horsefly. Seventy per cent (7/10) of patients with horsefly allergy tested positive to Tab y 1 in skin-prick tests, and sera from 81% (30/37) of patients were shown to react to Tab y 1 on western blots. Tab y 1 has also been shown to display enzymatic activity to hydrolyze ATP and ADP, as well as potent antiplatelet aggregation and antithrombotic activities. (17)

Tab y 2 has been demonstrated by immunoblotting to bind IgE in 91.8% of subjects' sera. (16)

Tab y 5 has been demonstrated by immunoblotting to bind IgE in 86.5% of subjects' sera. (16)

Tab y 2 and Tab y 5 were shown to have some IgE-binding capacity in sera of subjects with wasp-sting allergy, suggesting that these allergens are thus not only found in stinging insects but also in hematophagous insects, providing support for the presence of a so-called ‘wasp-horsefly syndrome’. (16)

Potential Cross-Reactivity

No studies have specifically evaluated the cross-reactivity of horse fly, and in particular between various species of horsefly. The suggestion of a degree of cross-reactivity has been reported between horsefly (Tabanus spp.) and the closely related deer fly (Chrysops spp.) (1) However, the few other studies that considered potential cross-reactivity between these species found none.

Nevertheless, coexistent anaphylaxis to Diptera and Hymenoptera has been reported, with concomitant sensitisation attributed to hyaluronidase present in both horse fly and Hymenoptera venom. (1, 20)

Clinical Experience

a. IgE-mediated reactions

A horsefly bite is painful, but often leaves little more than a transient wheal and flare reaction with minimal bleeding from the wound. Occasionally, secondary bacterial infection is a problem. Some individuals have significant urticarial reactions to horsefly bites, and cases of anaphylaxis have been reported to various horse fly species in the literature. (21, 22, 23, 24, 25, 26)

More than 30 cases of horsefly allergy (Tabanus spp. and Chrysops spp.) have been recorded so far, involving severe allergic reactions including generalized urticaria, angioedema, bronchial constriction, and shock. (1), 21, 22, 23, 27, 28, 29, 30, 31, 32) It has been suggested that allergy to Tabanids may be underreported, as a result of poor diagnostic facilities and a lack of medical vigilance. (1) Furthermore, some horsefly species mimic vespid pigmentation patterns and may have been mistaken for Hymenoptera. (1)

In early surveys (conducted prior to 1969), 6 patients were identified who experienced systemic symptoms from horsefly ‘bites’. The symptoms consisted of urticaria, itching, shortness of breath, nausea, vomiting, and faintness. One young woman experienced shock. (27, 31)

Case reports are illustrative.

A female farmer allergic to house fly (Musca domestica), confirmed by serum-specific IgE analysis, experienced anaphylaxis following a horsefly (Tabanidae spp.) bite, on two occasions. The IgE response was highly specific for Musca domestica, with no cross-reactivity observed with flies and midges from other families. (33)

A 51-year-old man allergic to Hymenoptera venom – and being treated with Vespula spp. subcutaneous immunotherapy (SIT) was bitten on the neck by a yellow, black and green insect. Five minutes after the bite he developed generalised itching, urticaria, and paraesthesia of the mouth and lower limbs, followed by loss of consciousness. The captured insect was recognised as a horsefly (Tabanus bovinus), which resembles Hymenoptera. Skin-prick test and RAST was positive for Tabanus. (34)

Two male patients aged 57 and 62 and known to be allergic to stinging Hymenoptera venom described the development of systemic symptoms after a horsefly bite. The 57-year-old male, who had previously experienced urticaria, dyspnoea, dysphonia and hypotension after a Vespula spp. sting, was bitten on the neck by an unidentified insect. Within minutes he developed angioedema of the face and general urticaria, paraesthesia of the feet and tongue, and loss of consciousness. The insect was identified as a horsefly (Haematopota pluvialis). The second patient, a 62-year-old allergic to Hymenoptera venom and undergoing specific immunotherapy to Vespula spp. venom, was bitten on the arm by an insect. He experienced pain at the site of the bite, followed (after 5 minutes) by general urticaria, dyspnoea and loss of consciousness requiring adrenaline. Following this episode he was bitten again, about 6 months later, and experienced the same symptoms. The insect was identified as a horsefly (Chrisops spp.). Skin-prick tests with Tabanidae spp. whole-body extract were positive, and RAST was positive (5.4 U/ml) for Tabanidae sp. The authors proposed that hyaluronidase present in the Hymenoptera venom may have contributed to the sensitisation to horsefly, and proposed the term ‘wasp-horsefly syndrome’. (35)

A 57-year-old man was bitten on the left foot by an unidentified horsefly species. Within 10 minutes he had generalised itch, urticaria, and angioedema, followed by pharyngeal constriction, shortness of breath, and a final 30-minute episode of unconsciousness. A second horsefly bite, received 1 week later on the chin, again provoked a systemic reaction. The patient developed nausea, vomiting, shivers, profuse perspiration, and impairment of the sense of taste. No horsefly species could be used for skin testing. RAST horsefly (Tabanus spp.) was negative. Immunoblotting with Chrysops spp. showed IgE binding to a protein of around 69 kDa. (1)

An anaphylactic reaction to horsefly bite (T. americannus) was documented in a 56-year-old white male. Within five minutes of the bite he felt dizzy, developed generalised urticaria, swollen lips and tongue, and collapsed. He had previously experienced anaphylaxis to yellow jacket and white-faced hornet. Intradermal testing was positive for yellow hornet, yellow jacket and wasp, but negative for bee. RAST tests were positive for honey bee, wasp, white-faced hornet, yellow hornet, and yellow jacket. Scratch testing to the Tabanidae family was positive, and the RAST markedly positive. (20)

In a study in Queensland, Australia 11 (1.5%) of 742 patients evaluated for adverse reactions to insect stings or bites had allergic reactions to horsefly (March fly) (Tabanus spp.). (29) In context, of the 742 patients, 452 (61%) had allergic reactions to honey bees, 244 (33%) to wasps, 30 (4%) to various ants, 11 (1.5%) to March flies and 5 to tick infestation. (29)

A case of coexistent Tabanid and Hymenoptera anaphylaxis has also been reported. (20)

Importantly, authors have cautioned that patients may misidentify a horse fly bite as a Hymenoptera sting, and that the exact identification of the insect is essential before beginning or changing any treatment, e.g. immunotherapy. (25) At present, no commercial immunotherapy products are available.

b. Other reactions

Various diseases in humans are vectored by tabanids; e.g. loaiasis caused by the helminth Loa loa, and vectored by species in the genus Chrysops. Tabanids have been suspected as vectors of tularemia (Francisella tularensis) and anthrax (Bacillus anthracis) in humans, though there is little hard evidence. (8)

Compiled by Dr Harris Steinman,


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As in all diagnostic testing, the diagnosis is made by the physican based on both test results and the patient history.