By Dr Carl Eden BVM&S MRCVS, Virbac
Hypersensitivity to the bites of insects is by far the most common cause of skin disease in horses in Australia. The bites of Cullicoides (Queensland itch, sweet itch, biting midges), Tabanus and Chrysops (horse flies), Stomoxys (stable flies), Simulium (black flies) and Musca (house flies) and the stings of bees and wasps all cause lesions and hypersensitivity reactions in horses skin.
Hypersensitivity is an altered state of immunoreactivity that results in self-harm. There are four different types of hypersensitivity defined by the immunologic process underlying the pathology. The most common type of hypersensitivity is type I and it is predominantly this type of hypersensitivity that results in the clinical syndrome, Queensland itch.
Type I hypersensitivity, or allergy, is mediated by immunoglobulin E (IgE) in response to specific allergens. In the case of Queensland itch, the allergens are primarily salivary antigens of the Culicoides midge but cases have also been attributed to Simulium and Stomoxys species. These antigens result in an intensely pruritic skin disease. IgE is found in tissues bound to mast cells. When two or more IgE molecules cross-link through the interaction with antigen, the mast cells are triggered to release chemical mediators (degranulate). In the case of Queensland itch, these mediators result in localised inflammation of the skin. The range of inflammatory mediators released by the degranulation of mast cells is vast and includes toxic mediators such as histamine and heparin, enzymes capable of remodelling connective tissue, cytokines including IL-4, 5, 13 and tumour necrosing factor alpha. Histamine causes an increase in blood flow to the area and an increased permeability of blood vessels. Enzymes activate metalloproteinases that cause tissue destruction. Cytokines attract inflammatory leucocytes. All of this is appropriate when the mast cell is reacting to an invading pathogen, but not in the case of a reaction to allergens in the bites of Culicoides midges.
The degree of stimulation of IgE responses is dependent upon the characteristics of the antigen and genetic factors of the individual affected, such as major histocompatability complex antigens (MHC). The small soluble proteins (antigens) found in the saliva of Culicoides and simulium species are particularly effective at eliciting an IgE response through presentation to MHC II.
The mast cell degranulation causes immediate allergic reaction, but also a sustained late-phase response that develops over a period of 8 to 12 hours because of the recruitment of type 2 T helper cells (TH2), eosinophils and basophils. During this stage intense pruritus develops.
In the cooler parts of the country,the incidence of Queensland itch is seasonal but the warmer climates may see the disease all year round. There appears to be no breed, sex or age predilection although the condition is rarely seen in horses less than a year of age. Whilst some suggest a familial or inherited component, this has not been verified. Lesion distribution can vary according to the species of Culicoides that is prevalent in any particular area.
Normally Culicoides feed on the dorsal surface of the horse, predominantly along the mane and tail area and around the face and ears. The bite of the fly is particularly painful due to the chewing mouthparts and this can cause significant fly worry for the horse if the midges are present in large numbers. The subsequent hypersensitivity to salivary antigens results in the clinical syndrome termed ‘Queensland itch’.
Within a group of horses, there are normally only one or two horses that show clinical signs of the disease. The distribution of the signs is dependant on the species of Culicoides with the lesions occurring primarily on the ventrum, the mane and tail, dorsally or in all of the above areas. There are varying degrees of pruritus causing thinning of the hair coat, and in the early stages of the disease, papules and wheals may be seen. With progression of the disease, alopecia, crusting, excoriations, hypopigmentation and lichenification may become widespread in affected areas.
Where the skin becomes secondarily infected, the pruritus is often worsened. Staphylococcal isolates are common from secondarily infected areas.
Diagnosis is based on history, physical findings, seasonality (in most parts) and elimination of other causes of pruritus in the horse. Evidence of insect breeding grounds such as stagnant or still water or ponds within a mile of the property supports the diagnosis.
Dermatopathology is supportive of a diagnosis but not conclusive. Biopsies reveal mild to severe perivascular eosinophilic dermatitis and folliculitis. Areas of epidermal damage due to bites may be seen and in chronic cases, eosinophilic vasculitis and dermal fibrosis may be seen.
At present most cases of Queensland itch are diagnosed on the basis of clinical signs and history but overseas, intradermal skin testing is being used as a diagnostic tool. (Greer Labs, Lenoir, NC, USA). Fortunately, therapy and managemental control measures are similar for most biting insects and therefore confirmation of the diagnosis by intradermal skin testing is not critical.
The differential diagnosis list should include all those diseases causing pruritus, alopecia, papules, hypopigmentation and lichenification. Whilst this list is not exhaustive they should include hypersensitivity due to other ectoparasites such as lice, fleas, ticks, fly or mosquito bites and chorioptic mange. Internal parasites such as Onchocerca cervicalis and Oxyuris equi may also cause pruritus and in the case of O. cervicalis, its involvement in cases of Queensland itch must not be underestimated. (The filarial stages of the parasite are transmitted by Culicoides spp and the disease is prevalent in horses not routinely treated with ivermectin or abamectin containing products). Dermatophytosis and dermatophilosis should also be considered, as should the possibility of primary hypersensitivity reactions becoming secondarily infected. On one occasion, the author has seen a case of a cutaneous drug reaction manifesting with similar signs to Queensland itch.
The use of non-pharmacologic and pharmacologic methods is paramount in the successful treatment of any insect related hypersensitivity and Queensland itch is no exception. The use of non- pharmacologic methods such as fly repellents and insecticides significantly reduces the need for pharmacologic preparations in controlling the clinical outcome of hypersensitivity reactions.
Through insect control, knowledge of the crepuscular habit of midges and with the judicious use of glucocorticoids, the majority of horses with Queensland itch can be managed successfully.
The use of an effective permethrin based fly control product is mandatory (Flyaway®). Careful note should be paid to the final concentration of permethrin in the product as many products do not contain sufficiently high concentrations of permethrin, and others contain pyrethrin, which is four time less effective than permethrin. Stabling of horses at dawn and dusk and the use of fans at these times will severely reduce the number of Cullicoides midges gaining access to the patient. This effectively means stabling horses between 1600hrs and 0800hrs during the summer months. In some cases, where clients have been reluctant or unable to install fans, this author has found mosquito netting over windows and around doors (creating an enclosed stable area) has proved very successful. By the same token, the use of fly screen rugs without stabling has not proven totally effective in the authors experience.
Majority of cases will require a short course of glucocorticoid therapy whilst managemental changes take effect. Oral prednisolone (Preddy®-granules) at a dose rate of 1mg/kg should be administered until the pruritus has disappeared. Usually one to two weeks therapy is adequate. Tapering to the lowest effective every other day dose is then indicated.
Whilst at first glance the use of antihistamine preparations may seem advantageous over glucocorticoids, due to the light sedation and occasional behavioural and personality changes observed in horses treated this way, careful consideration is needed prior to instigating this type of therapy. In the authors hands and with careful case selection, there has been no incidence of inducing a bout of laminitis in any case treated with prednisolone at the recommended dose rates. Provided pregnant mares, patients with a history of laminitis and those with internal organ complications are not treated with glucocorticoids the incidence of complications should be minimal.
Many adjuncts to conventional therapy can be employed to aid in the reduction of pruritus suffered by the horse and these may decrease the dose of prednisolone required to manage cases.
The pruritus in many cases can be alleviated through the use of bathing the horse using cool water and colloidal oatmeal shampoos (Epi-soothe®).
The supplementation of horses diets with Essential Fatty Acid (EFA) has increased in recent years. Whilst the jury is still out on their effectiveness, many cases of insect hypersensitivity have shown anecdotal benefit from daily supplementation with EFA (Megaderm®).
Many practitioners have used hyposensitisation and up to 50% of cases have shown a good to excellent response. Treatment based on the results of intradermal skin testing has shown optimal responses. The purchase of insect allergens to the non-specialist veterinarian may be cost prohibitive, but remains an area of interest for future management of chronic cases of Queensland itch.
Culicoides midges are most active at dusk and dawn and require freestanding water in order to propagate. Knowledge of these aspects of their feeding and life cycle can be employed as an adjunct to other management and preventative measures.
Prevention of future episodes
It is important to revisit horses diagnosed with insect hypersensitivity to ensure that advice given is being heeded, and to alter and adjust any pharmacological or environmental management according to season and need. It should be stressed that preventative measures such as fly control on both the horse and its environment (Flyaway®) should be started early and applied consistently throughout the fly season in order to minimise recurrence of hypersensitivity reactions. It should also be noted that the only way of preventing all future episodes is to move the horse to a more desirable location, and as this is not feasible in majority of cases, long-term therapy may be required.
1. Pascoe RRR, Knottenbelt DC. (1999) Manual of Equine Dermatology. WB Saunders, London, England.
2. Perris EE. (1995) Parasitic dermatoses that cause pruritus in horses, Vet clin N. Am Eq Pract: 11: 11-28
3. Reed SM, Bayly WM, Sellon DC. (2004) Equine Internal Medicine. Saunders, Missouri, USA.
Dr Carl Eden BVM&S MRCVS