i204 Horse fly

Tabanid flies are among the most commonly encountered blood-feeding insects worldwide, however, tabanid allergy may be under-reported as reliable identification of a biting insect is rarely possible. Tabanus spp. introduce allergens to humans and animals via a painful bite, usually on the upper part of the body. Local transient cutaneous reactions to a tabanid bite are common, and systemic reactions of variable severity from generalized urticaria to anaphylaxis have been reported. Three allergenic molecules have been characterized in Tabanus spp. that cross-react to some degree with homologous proteins of wasp (Hymenoptera spp.) and mosquitoes.

Living environment

Tabanid flies are most prevalent in areas with plentiful grazing livestock (2). Eggs are grey in color, approximately 2 mm long, and laid in clusters of 100‒1,000 on moist soil, vegetation, or water (5, 6). Larvae are 6 cm long, white/tan in color, and undergo several molts lasting between 3 months to 3 years before moving to dry soil to pupate (5). The pupal stage lasts 5‒21 days depending on the species, and adults emerge from the soil to mate almost immediately (5, 6).

Worldwide distribution 

Tabanus spp. can be found worldwide, except in extreme northern and southern latitudes and some islands (2, 6). 

Main 

Exposure to venom allergens from horseflies is primarily via a bite. Female Tabanus spp. are hematophagous and require a blood meal to complete egg development, whereas the males are phytophagous (nectar feeders) and incapable of biting (2, 5, 6). Most tabanids feed on mammals, while some feed on birds, reptiles, or amphibians (6). Female tabanids wait in vegetation for a suitable host to pass nearby and are attracted to large dark moving objects and to carbon dioxide (6). Tabanids are strong fliers (4) and have been described as “relentless” biters that bombard and follow people outdoors (5, 6).

Tabanus spp. are telmophages (pool feeders) and use two sharp and slender mandibles within their mouthparts to penetrate the skin to the superficial dermal vessels, macerate the tissue, and lap up the resulting pool of blood with their tongue (2, 5, 6). Larger species can collect up to 200 mg of pooled blood in 1‒3 minutes, and these bites can cause open bleeding wounds (4). The horsefly bite is reported to be very painful and is usually located on the upper part of the body (4-6). Of note, tabanid larvae, voracious predators of invertebrates, have also been reported to inflict a painful bite to humans (6). 

Tabanid bites usually leave little more than a transient immediate wheal and flare reaction, however large local cutaneous reactions are common, and systemic allergic reactions of varying severity have been reported (2, 7).

An observational report of 7 Caucasian adults who were bitten by Tabanus bovinus in Bolivia described identical clinical presentations with multiple, erythematous, roundish, flattened, large plaques, often with a central point corresponding to the horsefly bite, surrounded by satellite smaller and similar lesions (5). All of the lesions described in this report appeared between 1‒3 hours after a painful bite and were located on the upper body (e.g. face, shoulders, neck, and/or upper chest) (5). No systemic signs or symptoms other than pain at the site of the bite were recorded, and rapid recovery in all cases was achieved with topical corticosteroids (5).

Horsefly bites can also cause hemorrhagic blisters, which could lead to diagnostic confusion with other causes of hemorrhagic blisters such as autoimmune bullous diseases, polymorphic erythema, orf, and other bullous diseases which have a different clinical evolution (3). In Brazil, an adult female stung by horseflies on both hands developed local hemorrhagic blisters of various diameters, which were treated by emptying the liquid content and applying topical antibiotics in the exulcerations (3).

Freye & Litwin (8) published the first report of anaphylaxis caused by Tabanus spp, in a male aged 56 years who developed rapid onset generalized urticaria, presyncope, lip angioedema, and collapse following a bite from Tabanus americannus in North America (8). The patient recovered after receiving emergency treatment with epinephrine, diphenhydramine, intravenous steroids, and oxygen (8).

While a number of severe allergic reactions to tabanid bites have been published, the extent to which these reports were categorically attributable to Tabanus spp., versus species belonging to related genera such as Chrysops or Haematopota, is unclear (9, 10). Reliable identification of a biting insect is rarely possible, and diagnosis of insect bite hypersensitivity is often based on medical history (9). As such, allergy to tabanids may be under-reported (10).

Other reactions

The saliva of Tabanus spp. contains a wide range of physiologically active molecules including antihemostatic, fibrinolytic, and immunosuppressant compounds that can interact with host processes (3, 11-13). Tabanids take frequent and rapid meals from many different individual hosts, and in so doing can transmit a variety of viruses, bacteria, parasites, and protozoa, either biologically (as an intermediate host) or more commonly mechanically via contaminated blood on the mouthparts (2). Tabanids are considered a serious pest due to their important role in spreading diseases that can impact livestock production, such as anthrax, tularemia, and trypanosomiasis, among many others (2, 6, 14). Tabanids may also be potential vectors of disease between animals and humans: a sheep breeder in Italy developed skin lesions attributable to cutaneous anthrax after being stung by gadflies, despite being 3 km away from the affected farm (15).

Allergen immunotherapy

The importance and feasibility of allergy diagnostics for insect bites may be limited for a number of reasons, including high rates of sensitization in the general population, low discrimination by tests between ‘normal’ and ‘hypersensitive/anaphylactic’ reactions, significant variation in locally relevant insect species, and the numerous pathomechanisms that underlie skin reactions (9). Importantly, some authors have cautioned that the exact identification of the insect is essential before beginning or changing any treatment, such as immunotherapy (10, 16).

Author: RubyDuke Communications

Reviewer: Dr. Christian  Fischer

Last reviewed:March 2022