Allergic reactions to insect bites are the primary causes of pruritis in horses, of which Culicoides spp (midge) hypersensitivity is the most common and widely documented. It has many synonyms including sweet itch (sweat itch), Queensland itch, equine summer eczema, seasonal summer recurrent dermatitis. This article will, therefore, primarily address summer skin disease related to insect bite hypersensitivity (IBH).
Culicoides spp hypersensitivity
Disease prevalence
There are numerous figures for disease prevalence reported and these include;
There is no sex or breed predilection although the heritable nature of the condition has been documented (Marti et al, 2008).
Aetiology
Insect bite hypersensitivity is complex as a result of a type 1 (immediate) and type IV (delayed) hypersensitivity to antigens (presumably salivary) of primarily Culicoides spp, but also Simulium spp; Stomoxys calcitrans and possibly Haematobia irritans. There are over 800 species of Culicoides worldwide. The species implicated in Western Europe and the preferred feeding sites are:
C. pulicaris — mane, withers, tail base
C. nubeculosis — withers, tail base
C. obsoletus — poll, withers, tail base, legs, belly
C. punctatus — mane withers.
The pertinent factors involved in the vector life cycle which are important both in the geographic distribution, seasonality and control include the fact that:
There is evidence for both a familial and genetic pre-disposition, although all horses exposed to Culicoides bites have IgG antibodies to insect salivary proteins, only horses with IBH have Culicoides specific IgE antibodies (Wilson, et al, 2001). Horses in Iceland which have never been exposed to Culicoides bites have no antibodies to Culicoides proteins (Wilson, 2009) and do not develop IBH because there are no Culicoides in Iceland. However Icelandic ponies born in Iceland and moved to affected areas readily develop the disease when they are exposed to Culicoides and there is a significantly higher prevalence (26–30%) and more severe clinical disease than in those ponies born in Culicoides inhabited areas (28.2%). This suggests that repeated exposure is to some degree protective (Larson et al, 1988).
Eosinophils, lymphocytes, histamine, platelet-activating factor (PAF), Langerhans' cells, and LTB4 also play an important role in the complex pathogenesis of IBH (Scott and Miller, 2003).
Clinical signs
There are a number of clinical signs of Culicoides spp hypersensitivity, as outlined below.
Pruritis
Pruritis may be severe and can result in self trauma due to the horse rubbing Figure 1.
Skin disease
Sloet (2009) suggests that there are three patterns of skin disease; dorsal and ventral distribution and combinations of the above. Secondary lesions include hair loss (hypotrichosis), crusting, alopecia, excoriations (erosions and ulcers with or without serum exudates), pigmentary changes (melanoderma and melanotrichia) and lichenification (thickened skin with rugae) Figure 2 and 3). Many of these changes are reversible out of season but in severe cases they can persist. Secondary infection may follow.
Behavioural changes
There may be behavioural changes such as general restlessness, tail swishing, rubbing, kicking, rolling, rubbing the ventral abdomen on the ground, irritable and difficult to ride especially at the peak midge times of day.
Weight loss
There may be weight loss due to chronic irritation (personal experience).
Diagnosis
There are a number of factors to look for when making a diagnosis:
Fly bites from other genera including Stomoxys spp Lice Oxyuris equi infestation (pinworm) leading to tail rubbing. Onchocercal dermatitis Papular dermatitis Stick fast flea, bee stings Chemical irritation Atopic dermatitis Adverse cutaneous food reactions Stable vices
Treatment
Reduce Culicoides exposure
Animals with Culicoides hypersensitivity should be stabled between 4pm and 8am which includes dusk and dawn at the peak of the Culicoides feeding times with the provision of a fine mesh screen in the stable or a ceiling fan. Wet marshy areas should be avoided and manure heaps and organic matter should be removed.
The use of Culicoides-proof rugs such as the Boett blanket (Figure 5), snug rug or Rambo sweet itch rug is very effective provided they are applied well before the start of the season ideally in February/March.
The use of insecticides and fly repellents, for example permethrin, cypermethrin, Deet, light oils and citronella, may help in mild cases but need to be applied frequently.
Herbal products/feed supplements are available on the market which claim to have repellent properties.
Reduce secondary skin trauma/infection
The use of electric fencing reduces the animal's ability to inflict self trauma. However, severely pruritic individuals will often find ways to relieve their discomfort, by rubbing on the ground for example. This inability to scratch can be very distressing to the animals and should be used in conjunction with other methods such as the rugs. Scratch mats/pads are commercially produced rubber mats which allow the animal to rub and yet minimize secondary skin trauma.
Symptomatic treatments
Treatments that address the syptoms of Culicoides hypersensitivity include:
Immunotherapy
The intension of immunotherapy is to inject small doses of the antigen in order to make the animal tolerant. The mechanism of action of allergen specific immunotherapy is complex with a variety of humoral and cellular changes (Scott and Miller, 2003). There were several studies undertaken in the 1990s without any conclusive evidence. If this approach is to be effective purified preparations of allergens are required and this has now been achieved (Russell et al, 2009). The future research aim is to use the recombinant Culicoides allergens as a diagnostic and therapeutic agent in a pilot study of immunotherapy with oral dosing (Wilson, 2009).
Immunomodulation
Trial work using non-pathogenic bacilli Bio Eos BE-T-101 has been ongoing since 2005. This modulates the immune system from the dysregulation (Th2 mode) that leads to IBH to a more normal response to the midge bites (Th1 mode). The trial now using weekly oral dosing has shown encouraging results and the final blinded, randomized, placebo controlled trial conducted in 2010 is currently being analyzed.
Autoserum preparation
Trial work using an autogenous serum preparation made from their highly diluted sera, showed that over a 10 year period 70% of horses derived benefit from the treatment with milder clinical signs and no harmful side effects (Hallamaa, 2010).
Selective breeding
The familial occurrence of Culicoides hypersentsitivity suggests that selective breeding may be beneficial in reducing prevalence, with the possibility of genetic tests in the future.
Prognosis
The prognosis is guarded with all the above treatment options. Lifelong management is necessary or removing the horse to a different location. Severely affected cases respond less well and euthanasia may be the only option in some cases.
Other insect-related dermatoses
There is evidence in the human literature that cross reactivity among arthropods is possible (Kagan, 1990). In fact, insect allergic humans may exhibit signs when exposed to mites or even crustaceans which suggest that there is cross reactivity between distinct genera.
Dermatitis can result from the bites of several insects including Tabanus spp (horse fly); Stomoxys spp (stable fly); Culicoides spp (midge); Haematobia spp (buffalo and horn flies) and various varieties of wasps, spiders, mosquitoes, bees and ticks (Fadok, 1988).
In addition to the local and systemic reaction to their bites, insects are also important vectors of diseases (e.g. African horse sickness), and provide the mechanical transfer of many fungae (e.g. ringworm) and viruses (e.g. sarcoids).
Clinical signs and diagnosis
Most bites/stings have an obvious central bite mark surrounded by an oedematous plaque. It is often not possible to establish the cause unless the episode is observed. However, in bee stings the remains of the sting may still be present and the feeding pattern of some insects may be apparent. Clusters of bites may indicate several feeding sites of one insect or a group of insects. Other signs include irritability, foot stamping, unpredictable behaviour. Pruritis may develop leading to secondary alopecia, excoriations and crusts with the secondary development of multiple granulomas (Fadok, 1998).
Treatment
Systemic antihistamines and glucocorticoids may be necessary if the clinical signs are severe.
Control
Control measures should be directed at improving stable and paddock hygiene. Removal of faeces should be at least twice weekly and ideally daily. The removal of weed piles, rotting vegetation, trimming long grass and avoidance of cattle may help. Topical insecticides (permethrin, cypermethrin) and repellents may be of value and the use of stable fans, fly rugs and masks may offer some protection.
Harvest mite infestation
Clinical signs associated with the larvae of harvest mites (Trombicula spp), usually occur in late summer and autumn and affect primarily the head and legs of grazing horses. Affected animals are irritable with reported leg stamping, nose rubbing, and head shaking. Small papules and wheals may be evident with secondary alopecia and crusting (Figure 6). Eggs are laid on the soil and the hatched larvae normally feed on vegetation. The red/orange mites fall off the host after feeding and therefore may be difficult to identify. Treatment is symptomatic and may not be necessary since the disease is self limiting if further contact with the trombiculids is avoided. A single application with topical pyrethrins will kill any larvae still feeding (Scott and Miller, 2003)
Conclusion
In conclusion equine summer skin disease is primarily dominated by insect hypersensitivities (primarily IBH) where there is current work at the research level. However, the sun is responsible for another series of commonly encountered dermatoses which are discussed in another article in this series.