4 Jun 2018
Jamie Prutton discusses methods of controlling this condition, including when to stable, the use of rugs and advice to give owners.
Jamie Prutton
Job Title
Figure 1. Evidence of alopecia, thickening of the epidermis and self-excoriation secondary to insect bite hypersensitivity.
Insect bite hypersensitivity (IBH) is a chronic, recurrent seasonal dermatitis known to affect between 5% and 50% of horses, dependent on their location, breed and genetics.
Caused mostly by allergens present in the saliva of Culicoides midges, it is the most common allergic skin disease affecting horses worldwide. Research has looked at the potential genetic involvement in horses suffering from IBH, as well as various treatment modalities.
This article will address the pathogenesis, diagnosis and treatment modalities available.
Insect bite hypersensitivity (IBH) is a chronic, recurrent seasonal (spring, summer and autumn) dermatitis known to affect between 5% and 50% of horses, dependent on their location, breed and genetics (Schaffartzik et al, 2012).
The pathological state is an allergic, pruritic dermatitis predominantly caused by IgE-mediated type-one hypersensitivity reaction to the antigens present in the saliva of Culicoides midges. Worldwide, it is considered the most common allergic skin disease affecting horses (Gortel, 1998).
Allergic reactions are seen to many types of biting insects, but the most severe response is generally to the genus Culicoides.
Culicoides midges have a worldwide distribution – apart from Iceland, polar regions, New Zealand, parts of South America and the Hawaiian Islands. Their habitats must include damp ground or static water to facilitate breeding. Only females are required to feed on blood, for the development of eggs.
Although not in the UK, it is important to consider the pathogens the midge can carry include African horse sickness (AHS) and bluetongue. Having seen incursions of bluetongue into the UK, it is feasible AHS could affect the UK horse population in future.
Culicoides are found in greatest numbers when the horse’s grazing area is surrounded by hedges or wooded areas – particularly if static water exists for them to breed in. They are most active during dawn and dusk – especially when hot and humid. They are poor fliers and, therefore, vulnerable to exposed and windy areas, or by the use of fans in stables – both of which can play a pivotal role in control.
Many biting insects can cause immune responses leading to similar clinical signs, but it appears Culicoides are the source of the allergen associated with IBH. This has been shown in a study undertaken in Iceland, where IBH does not occur as Culicoides are not present, but other biting insects exist (Broström et al, 1987).
Multiple allergens are found in the saliva of Culicoides – many of which are involved in the immune response. However, not all allergens produced in saliva cause the immune response seen in IBH. The differentiation of the allergens may play a significant role in the production of immunotherapy in future.
The normal immune response starts as a type-one hypersensitivity, which progresses to a type-four delayed hypersensitivity reaction – leading to many of the chronic signs (Wagner, 2016). The inflammation is associated, specifically, with production of interleukin 4 and interleukin 13. All horses possess this response, as it is involved in the host resistance to any ectoparasite, but those that stay clinically healthy have a reduced response due to the presence of anti-inflammatory cluster of differentiation 4+ T cells – leading to interleukin 10 and transforming growth factor beta production, and suppressing interleukin 4 production. This immunosuppression leads to less severe, or absent, response to the bite of the midges.
Histamine, among other endogenous ligands, is involved in the pruritic reaction seen. Although over-activated in horses suffering from IBH, it is likely involved in reducing parasite burden and, therefore, could be a beneficial adaptation. This link between IgE and a reduction in parasite burden has been suggested, but not confirmed, in horses.
A genetic component to IBH has been investigated by numerous groups, with varying results. Although the possibility of genetic involvement exists, a paucity of evidence confirms whether it is a simple dominant or recessive mode of inheritance. Without this information, no sensible guidance can be made regarding the possibility of selective breeding to reduce the incidence of IBH.
In a questionnaire-based study, Peeters et al (2014) showed a degree of heritability existed, which varied from 0.65 to 0.78, in the Belgian warmblood population – indicating IBH is a heritable trait. However, the heritability is more variable depending on the breeds studied, with some breeds showing very little heritability.
Peeters et al did not try to find genetic markers, therefore not opening a window for genetic testing. A thesis has looked into genetic markers associated with IBH and some were found, but with borderline significance (Shrestha, 2017). This area may become an area of research that could facilitate prevention of the disease.
Clinically, horses are most affected in the summer, although disease can also be seen in spring and autumn.
Horses will show severe pruritions of the mane (Figure 1) and tail (Figure 2) areas, while, to a lesser degree, some will be affected along the ventral midline and face. Clinical signs will often start with papules, urticaria and tufted hair, or changes in hair structure (Figure 3). These changes can progress to self-excoriation, due to the severity of the pruritus.
Winter normally leads to relief, as Culicoides midges are not present in the environment.
When clinically affected, the welfare of the horse can be seriously affected. This must be assessed and discussed when formulating a treatment plan. Often, horses will show a worsening of the clinical presentation in succeeding years of exposure to Culicoides midges; therefore, prevention is essential, rather than expecting a cure to be achieved.
Peeters et al (2014) found most horses showed their first clinical signs by the age of 2 years (32%) or 3 years (20%), but the average age of onset was 3.6 years. In total, 88% of cases in the study had shown symptoms by the age of 7 years.
The severity of clinical signs are variable, based on geographical location, habitat control, managemental issues, nutrition, genetics and previous exposure of the horse to the allergens. Various risk factors have been postulated, but several contradictory studies exist. One showed black paint Shetland ponies were at a higher risk compared with bay mares, but two other studies have shown no correlation between colour and incidence of disease (Steinman et al, 2003; van Grevenhof et al, 2007; Schurink et al, 2013).
One study has found IBH is frequently associated with equine asthma – and horses with IBH, but no signs of equine asthma, had as sensitive airways as those with asthma when tested with a histamine provocation test (Lanz et al, 2017). In humans, atopic dermatitis is frequently associated with asthma and many patients that initially present with dermatitis progress to uncontrolled asthma. Therefore, care should also be taken with the horse’s environment to reduce the risk of progressing to equine asthma.
Diagnosis of IBH is frequently made on clinical signs alone, as they are pathognomonic for the disease if the environment is also correct.
Intradermal (Figure 4) and serological tests have been trialled, but the results have been variable, with cases of both false positive and false negative results occurring (Quinn et al, 1983; Sloet van Oldruitenborgh-Oosterbaan et al, 2009). This is likely due to the commercially available tests relying on whole-body Culicoides mixes, rather than specific antigens, leading to low sensitivity and specificity (Frey et al, 2008).
Therefore, if an intradermal skin test is performed, it is important to interpret the results in correlation with clinical signs seen in each patient.
Biopsies can be of some use in confirming the immune response, or ruling out other disease processes. However, in most cases, they are not very informative.
Treatment and prevention will be lifelong and require intense management on the part of the owner.
A multimodal approach is the most likely to secure comparative success and should include:
Immunotherapy should also be considered.
Peeters et al (2014) found stabling alone resolved clinical signs in 59% of horses, while blankets did so in 49%. This confirmed, to achieve the best success rate, multiple treatment protocols should be used and treatment should not be based on a single method.
When formulating a treatment protocol with owners, it is helpful to consider a three-tier system to ensure all aspects of management are covered. The tiers should include:
Control of the pruritic response can be done with either systemic or topical medication. Systemic steroids can be used, with oral prednisolone (1mg/kg by mouth every 24 hours) the initial treatment of choice. If a limited response is seen, dexamethasone (0.1mg/kg IM every 24 hours) can be trialled.
Antihistamines are of limited use in most cases, but can be used as an adjunctive therapy. Topical treatments – including hydrocortisone spray and hydrocortisone/triamcinolone cream – have their place, but will only be efficacious when limited hair is present.
It should be highlighted steroids can be used palliatively to break the itch-scratch cycle, but will not be curative while exposure to the midges is ongoing.
Topical treatment can include antimicrobial shampoo, if a secondary infection has occurred following self-excoriation. Systemic antibiotics may be warranted in the most severe cases, but a sensible antibiotic stewardship approach should be taken to their use.
The final tier – prevention of further exposure – is frequently the most challenging and should include multiple techniques, including:
Turnout must be addressed as part of the prophylactic treatment. Proximity to hedgerows or static water increases the frequency of clinical signs due to the increased burden of midges, so changing the habitat is essential. This can be achieved by either moving fields or location to a more windy, open place, or ensuring standing water is removed when possible.
Chemical repellents or insecticides could be effective in reducing the biting rate, or even reducing the population size in an environment. Baker et al (2015) found pyrethroid-based insecticides available in the UK caused 100% mortality in exposed Culicoides, so the application of these products to rugs may be very beneficial in controlling the population. Additionally, 2% permethrin-based products reduce the number of Culicoides on horses (Marsella, 2013).
The number of products on the market – and their variable contents and responses to them – makes it difficult to recommend one product; a logical approach should be applied, with different treatments trialled during the Culicoides period.
Various rugs are on the market that reduce the exposure of horses to midges and dramatically reduce the incidence of disease. They are a pivotal, if frustrating, part of the treatment protocol.
Fatty acid supplementation has been trialled on multiple occasions, often with limited success. O’Neill et al (2002) found the addition of flaxseed (1lb/1,000lb bodyweight per day) to diets mitigated the skin test response to Culicoides, so should be considered an adjunctive therapy in these cases.
Theoretically, immunotherapy should be the most appropriate and functional treatment for IBH. Frustratingly, multiple attempts have been made to create an immunotherapy, but none to date have been efficacious (Ginel et al, 2014). So far, these studies have looked at extracts of whole midges, rather than allergens specifically from their saliva.
Therefore, although it can be trialled in appropriate cases that have shown a positive response to the intradermal skin test, it should not be relied on as a sole treatment.
It is clear management and treatment of IBH is, and will continue to be, challenging. This is mostly due to the limited effect of each treatment, individualised care and advice required for each case, and the intense nature of care owners must provide.
Often, client compliance can be poor due to the intensive nature of the care; ongoing veterinary support will be required for those cases.
