Horsefly (genus Tabanus) bite: allergy or infection?
- Katherine Kelly and
- Pauline Grose
- General Medicine, Glasgow Royal Infirmary, Glasgow, UK
- Correspondence to Dr Katherine Kelly; katherine.kelly3@nhs.scot
Abstract
Summer heatwaves are often associated with increased reports of insect bites as people enjoy spending time outdoors. The horsefly (genus Tabanus), or ‘cleg’ as it is colloquially known, often receives negative publicity due to peak activity in summer. Following an increase in local hospital presentations due to complications of bites, discussion among colleagues suggested a lack of knowledge regarding best management.
We present a case of a woman in her 30s with a large erythematous swelling on her thigh following a suspected horsefly bite. Review revealed no clinical evidence of sepsis but a localised inflammatory response. Inflammatory markers were normal and she was treated with intravenous antibiotics and oral antihistamines to good effect.
This case highlights secondary care presentation of a common condition and discusses potential complications, pathogenesis and uncertainties around best practice guidelines for treatment. We highlight strategies to aid antibiotic stewardship, reduce complications and secondary care presentation.
Background
Horseflies are found worldwide and are particularly prevalent in warm and wet locations.1 They can range in size from 0.5 to 3 cm.2 Their bites are not only painful and itchy but can be distressing for patients due to the severity of reaction. Response between individuals varies significantly, ranging from localised to severe systemic reactions.3
Bites are difficult to prevent and carry the possibility for infection—primarily secondary bacterial infection but also the possibility of transmission of infection from the bite itself. Literature suggests allergy plays a large role raising the possibility that antibiotics are being used inappropriately to treat ‘cellulitis’.4 Increasing awareness and further research could promote antimicrobial stewardship, as these patients are commonly referred to secondary care for intravenous antibiotics.
The main learning objectives were to explore the relationship between inflammation and infection with regards to horsefly bites, the potential variation in management options, complications and strategies for future prevention.
Case presentation
This woman in her 30s with no significant medical history and on no regular medications, presented with progressive erythema and swelling of her left thigh over a 48-hour period. She reported walking her dogs in a grassy area and recalled suffering a horsefly bite.
Within 24 hours, she noted an initial area of swelling with a small punctum which rapidly progressed to a painful, itchy, erythematous area covering the outer aspect of her left thigh and extended towards her groin. She was started on oral flucloxacillin but despite 24 hours of therapy, her symptoms progressed and she was referred to acute medical receiving for further management and exclusion of sepsis.
Of note, she had a previous similar reaction to horsefly bites and had presented to the emergency department a year earlier. At this time, she was noted to have an inflamed and possibly infected horsefly bite on her right arm with associated pain and swelling. She was systemically well and was discharged with oral flucloxicillin.
On this admission, her initial observations showed she was apyrexial and haemodynamically stable with a blood pressure of 130/84 and heart rate of 90. On examination, on the outer aspect of left thigh, there was a 10 cm × 10 cm indurated erythematous swelling with central punctum. It was mildly tender and erythema appeared to be tracking towards her groin (figures 1 and 2). She had a normal range of movement of that leg and no pain or crepitus on movement of the hip joint.
Horsefly bite with associated erythema and swelling on day 1 of admission (lateral view).
Horsefly bite with associated erythema and swelling on day 1 of admission (anterior view).
Investigations
Her blood results showed C reactive protein (CRP) 2 mg/L, White CellCount (WCC) 10.1×109/L, creatinine 67 μmol/L, estimated Glomerular Filtration Rate (eGFR) >60 mL/min, normal liver function tests (LFTs) and a normal coagulation screen. Blood cultures taken on admission showed no growth at 2 days.
Differential diagnosis
The main differential was thought to be inflammation secondary to a horsefly bite, however, other differentials included localised cellulitis. It is difficult in presentations such as these to confidently exclude infection, however, with normal observations, normal inflammatory markers and no exudate or break in the skin, a localised inflammatory response is possibly more likely.
Treatment
She was managed with intravenous flucloxicillin 2 g four times daily and antihistamines with a plan to consider steroids if worsening but no signs of sepsis. The area of erythema was demarcated and she had also been taking pictures on her mobile phone for comparison. Within the first 24 hours, the erythema seemed to improve and regress slightly within the demarcation. She had ongoing discomfort and itch, felt nauseated but otherwise systemically well. The pictures were particularly useful for objective comparison given lack of continuity of medical and nursing staff.
Over the next few days, it remained indurated but the erythema started to improve. She continued on intravenous flucloxicillin and regular chlorphenamine for 4 days. Inflammatory markers remained normal—CRP peaked at 7 mg/L, WCC 10.1×109/L and she was apyrexial. Clinically there was a marked improvement with significant regression in erythema and size of indurated area (figure 3).
Horsefly bite day on 4 of admission.
Outcome and follow-up
She was switched to oral flucloxicillin from intravenous after 4 days with a plan to keep her in hospital for a further 24 hours. However, she self-discharged with worsening advice and was given flucloxicillin to complete the 7-day course.
After discharge, she continued to feel non-specifically unwell for a week but then felt back to her normal self. She was advised if it happened again with similar worsening despite oral antibiotics to represent to the emergency department early. She was not given prophylactic medication for possible future bites and there was no follow-up arranged.
Discussion
There has been an increase in the frequency of presentation of patients with reactions to horsefly bites (genus Tabanus) colloquially known as ‘cleg’ bites, particularly in warm and damp weather.2 These patients have had significant erythema and swelling with variable systemic and inflammatory response and have required admission for further management such as antibiotics and supportive care. Discussion among colleagues suggested a lack of knowledge regarding best management of such bites and potential complications. There is limited literature available regarding similar cases and the treatment and management, particularly in the UK. However, horseflies are noted to be one of the most common biting insects in Scotland.3 Literature suggests varying responses to bites ranging from localised inflammation to systemic involvement and even anaphylaxis.4
Haematophagous insects such as horseflies, ticks and mosquitoes have proteins in their saliva to optimise the feeding process. These interfere with many physiological pathways including platelet aggregation, vasodilatation and pain and can potentially trigger an immune response in the host.4–6
Female horseflies are anautogenous so require a blood meal for reproduction. Horsefly bites can transmit disease between animal vertebrate species such as horses and cattle.7 8 There is very limited evidence in case reports to suggest that they possibly could transmit infections such as Borrelia burgdorferi and Bartonella to humans,9 10 however, this requires further research.
The most frequently reported insect transmitted organism in humans to Public Health Scotland was B. burgdorferi which causes Lyme disease.11 There were 287 reported cases of Lyme Disease in Scotland in 2020 and more recent data is yet to be published.12 Public Health England reported Louping Ill Virus to be the only other zoonotic disease transmitted via insect bite in the UK. It predominantly affects sheep and birds but very rarely causes disease in humans. There are further zoonotic diseases exotic to the UK, which have been reported on occasion.13
B. burgdorferi is most notably transmitted via tick bites and has increased prevalence in the Scottish Highlands and South of England, particularly in wooded or grassy areas. Lyme disease typically presents with a characteristic rash called erythema migrans or a ‘bullseye’ rash but can also present with more generalised symptoms such as headache, fever and fatigue, usually several weeks after the initial bite.14 This patient did not present with any of these features and was able to identify the horsefly at the time of bite so we were confidently able to exclude this as a differential.
A study by Li et al found that there were two particular IgE binding proteins in Horsefly saliva which seemed to be significant in promoting an allergic response.5 These were also found in stinging insect venom which has supported a phenomenon described as ‘wasp-horsefly syndrome’.15 This suggests in certain individuals there is an overlap of sensitisation to the bites and stings of both types of insects at the time of insult. This relationship was initially described between wasps and mosquitoes.16 A study by Sabbah et al suggested that there could be an increase in severity of reaction associated with the frequency of bites.17
Limited evidence suggests that early treatment with antihistamines and possible addition of steroids would be appropriate for many patients in early stages of this presentation. This would reduce unnecessary antibiotic usage and reserve these solely for patients with lack of response or suggestion of sepsis, although at times it can be difficult to confidently differentiate infection from inflammation. With reference to our own patient, given her previous allergic history, normal inflammatory markers and no systemic features, initial steroids and antihistamines may have reduced the risk of hospitilisation. It is unclear whether the treatment in hospital was appropriate—perhaps a prolonged stay was unnecessary and steroids should have been trialled? Overall, this area requires more research to further determine this.
Patient’s perspective
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‘I felt it was very worrying when the infected area from the bite grew rapidly in size at such a rate.The bite area was so tight with pressure, aching and a lot of heat from the site. It took such along time for antibiotics and antihistamines to make any difference. I have previously had sepsis and suffer from post sepsis disorder which added to my stress and worry. Being in hospital for days brought back memories as my cannulas were failing daily.’
Learning points
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Difficulty of differentiation between infection vs inflammation—if no systemic features and low inflammatory markers then early initial treatment with steroids and antihistamines may be indicated.
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Important to ask in the history their reaction to other insect stings and bites.
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Possibility of a course of steroids and antihistamines to take home in patients with multiple previous reactions—‘just in case’.
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Benefit of patient taking pictures for comparison and continuity given doctors and nurses change frequently.
Ethics statements
Patient consent for publication
Footnotes
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Contributors KK: Internal medical trainee—submitting author, involved in patient care and writing of the case report. PG: Consultant Acute Physician—supervising consultant for patient care plus reviewing and editing case report Patient—Reviewed case report and provided her perspective.
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Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
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Case reports provide a valuable learning resource for the scientific community and can indicate areas of interest for future research. They should not be used in isolation to guide treatment choices or public health policy.
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Competing interests None declared.
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Provenance and peer review Not commissioned; externally peer reviewed.
- © BMJ Publishing Group Limited 2023. No commercial re-use. See rights and permissions. Published by BMJ.
References
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