15 Apr 2019
Tim Adams details various cutaneous conditions that can present in horses during these seasons, as well as ways to treat them.
Tim Adams
Job Title
Figure 1. A small sarcoid on the underside of a horse’s abdomen. The exudate from this lesion attracts flies.
Many skin diseases in horses are exacerbated by insect worry or triggered by insect bites. Treatment of equine skin disease at this time of year often focuses on reducing the horse’s exposure to insects, or mediating their effects.
Warmer, sometimes wetter, weather can also increase the risk of bacterial skin disease; treatment of this focuses on antimicrobial treatments and making the skin a less hospitable environment for these types of pathogens.
Inevitably, for treatment of equids, many of the treatments discussed in this article are not licensed for use in this species. Responsible use of medicines – particularly antimicrobials – must be considered and, when a suitable treatment is not available, the cascade should be followed.
Insect bite hypersensitivity – also known as sweet itch – is an immune-mediated hypersensitivity reaction caused by exposure to saliva from midges (Culicoides species). Clinical signs include an intense pruritus, hyperkeratosis, scaling, and self-trauma to the mane and tail base. The immune response is largely an IgE-mediated, immediate type I hypersensitivity reaction.
Sweet itch can occur in any breed of horse; however, it is particularly prevalent in Icelandics, Shires and Welsh ponies (Pilsworth and Knottenbelt, 2004). A myriad of treatment protocols exist, and many horse owners will have tried both over-the-counter tack shop treatments and treatment from veterinary surgeons, which may have been prescribed for other purposes or other horses.
One of the most effective methods of managing this condition is to prevent the Culicoides species from biting the horse. Many effective rugs exist that cover the horse and prevent the midge from biting. Some of the most effective rugs cover the underside of the horse and have a permethrin coating to act as a repellent.
Many types of fly repellent are also available and are best used in combination with rugs. Active ingredients include deltamethrin; N,N-Diethyl-m-toluamide; and citronella oil. Finally, many horse owners use barrier oils, silicate sprays or gels to coat the skin and discourage insect bites.
The importance of environmental control is worth stressing; turning affected horses into windier paddocks, without standing water and away from muck heaps, can be helpful. When advising owners on preventive strategies, it is important to encourage them to get measures in place early in spring before midge numbers have started to increase. Once skin changes have occurred, treatment should focus on reducing pruritus and the immune response.
Oral prednisolone 1mg/kg once a day, then a tapered dose to the required effect (White, 2015), is useful for this treatment; however, care should be taken when using this treatment in overweight or laminitis-prone equids.
Topical corticosteroid in spray or cream form (for example, betamethasone valerate or hydrocortisone aceponate spray) applied twice daily can be effective in reducing areas of inflammation. Where areas of self-trauma carry an infection risk, topical antiseptic creams – for example, silver sulfadiazine cream – can also be useful.
Antihistamines can be used to reduce pruritus and inflammation in horses; however, their effect can be variable, so the usefulness of this treatment is limited.
One of the most effective antihistamines for sweet itch treatment is hydroxyzine hydrochloride (0.5mg/kg to 1.5mg/kg; Knottenbelt and Malalana, 2014). Nicotinamide with an aloe vera gel is a product also available for the treatment of summer itch.
Insects apart from Culicoides can cause significant skin lesions in equids. Large wheals can result from bites from horseflies (Tubanus bromius), other biting flies and stings from wasps. A single injection of dexamethasone (0.01mg/kg to 0.04mg/kg) with a follow-up of topical corticosteroid can be useful for treating these.
It is also possible to have severe bites from swarms of mosquitoes or midges when the conditions favour them; in these outbreaks, all horses will show signs of pruritus – not simply those susceptible to sweet itch.
In warm weather, geldings can suffer from fly larvae in their sheaths. This causes severe discomfort and kicking of the abdomen. It is common for the clinical signs to be mistaken for colic. Treatment is best achieved by washing the sheath with warm water, under sedation, removing any larvae, and then applying a thin layer of yellow summer fly cream to the penis and sheath.
Areas of the UK affected by ked flies (Hippobosca equina) include the New Forest and South Downs – normally areas where a large number of deer are present. They are robust, flat, winged flies with claw-like limbs that are found around the anus of horses and other thin-skinned areas in spring and summer (Tarry, 1994).
They move fast, appear almost indestructible and are difficult to treat, and do not cause any significant skin lesions, but cause a great deal of distress to the horse when unused to them. The frantic tail swishing and kicking out can be mistaken for colic.
Tick bites are often seen in late summer and, as with ked flies, are often associated with areas where a high number of deer are present. Ticks and tick bites are most commonly seen on the muzzle and face, but can also be found in the axilla and groin areas.
Ticks should be removed quickly to reduce the risk of transmitting Lyme disease (Borrelia). A tick hook (generally used in dogs and cats) is a good way to remove the ticks without leaving behind the head. It is advisable to wash the area where ticks have been removed with an antiseptic wash (such as pevidine solution) and apply an antiseptic cream. Ticks not removed quickly can cause a small abscess or nodule at the bite site.
Prevention of tick bites is difficult as limited efficacious products are available and many fly repellents do not seem to work well. Fipronil spray can be helpful, applied to the muzzle once every two weeks. Deltamethrin and permethrin applied once weekly can also be used.
Sarcoids are equine fibroblastic skin tumours that are locally invasive, but have no true metastatic dissemination. They are common in horses, affecting roughly 10% of the equine population (Bergvall, 2013); the distribution on the horse corresponds to the site of biting flies. Although sarcoids can develop at any time of the year, they often become a problem in warmer months as they attract flies if bleeding or leaking serum – particularly the larger fibroblastic-type sarcoids (Figure 1).
Treatment protocols for sarcoids are beyond the scope of this article; however, it is advisable to apply fly repellent around the site of the sarcoid or treatment site to discourage fly worry.
Horses are particularly predisposed to urticaria – also known as hives – compared to other domestic species and a wide range of potential triggers for the occurrence exist.
The main clinical sign of urticaria is the sudden appearance of oedematous wheals over the abdomen and thorax (Figure 2), with a varied size and appearance. Drugs, food, inhaled agents and even heat can trigger an urticarial reaction, although the precise cause often cannot be identified.
Treatment usually consists of dexamethasone (0.01mg/kg to 0.04mg/kg). Severe cases of urticaria can be followed up with oral prednisolone (0.2mg/kg to 0.4mg/kg every 24 hours) or repeated doses of dexamethasone (Fadok, 2007).
Where cases of recurrent urticaria or equine atopy are suspected, intradermal skin testing can be useful to identify the allergens responsible and use of allergen-specific immunotherapy to try to reduce the severity of the clinical signs.
Bacterial folliculitis and dermatitis normally occurs in early spring and is often caused by over-rugging in spring or isolated cases in mid-summer related to leaving wet rugs over horses in an effort to keep them cool.
This condition sometimes presents as “saddle eczema” associated with the buildup of sweat and dirt under a numnah or saddle cloth and creating a moist environment for bacterial infection.
Bacterial dermatitis often presents as a moist dermatitis with purulent exudate; clumping of the hairs often occurs, creating paintbrush-like scabs (Figure 3). The bacteria generally responsible include Staphylococcus species, Streptococcus species and Dermatophilus congolensis. Saddle eczema usually presents as warm, small papules distributed over the dorsum under the saddle area.
Treatment for bacterial dermatitis is based on making the environment less hospitable for microbes by ensuring the skin is kept dry and clean, and then antimicrobial treatment. Twice-weekly shampooing of affected areas with a miconazole and chlorhexidine shampoo is helpful; it is important warm water is used and a 15-minute contact time is observed to improve efficacy.
Chlorhexidine surgical skin scrubs can be more astringent than shampoos, so – in the author’s experience – should be avoided.
Topical antimicrobial treatments useful for treating bacterial dermatitis include silver sulfadiazine, fusidic acid and topical creams containing amoxicillin. Many of these topical treatments contain corticosteroid, too, to reduce inflammation.
Oral antimicrobials can be a useful adjunct to topical therapy – generally, broad-spectrum antimicrobials such as sulfadiazine and trimethoprim (30mg/kg by mouth twice a day) are most appropriate and, generally, longer treatment courses (two to six weeks) are necessary (White, 2015).
Culture and sensitivity from skin samples can help isolate the pathogens present and guide antimicrobial use.
Although often considered a winter skin disease associated with mud and wet weather, pastern dermatitis is seen in the spring and summer, too. The cause of the pastern dermatitis can be bacterial – usually seen in horses kept on long, wet grass.
These cases are best treated in a similar way to bacterial dermatitis elsewhere on the body by keeping the pastern clean and dry, and with the use of topical antimicrobial treatments. Once the condition has resolved, it can be useful to use a barrier cream – for example, petroleum jelly or udder cream – applied thinly to prevent reoccurrence.
Another cause of pastern dermatitis in summer is pastern leukocytoclastic vasculitis. This condition causes erythematous, crusting lesions around the pastern, fetlock and distal cannon areas (Figure 4). The lesions are often circular and more common on the lateral aspect of the limb; secondary infection with bacteria and fungi is common.
The condition normally occurs in white-haired, non-pigmented limbs, and is triggered and exacerbated by strong sunlight. Diagnosis is often made by clinical appearance alone.
Treatment is best achieved with oral prednisolone (1mg/kg once a day) for a period of one to two weeks.
Topical treatment with corticosteroid and antimicrobial creams can be helpful; clipping the affected areas and shampooing to remove crusting and prevent secondary infection also helps improve outcomes. Treatment is often required for several weeks and reoccurrence is common.
Equids with pink muzzles can suffer from sunburn – particularly if pasture kept. This can be prevented by applying a zinc oxide and cetrimide barrier cream to susceptible areas, or repeated use of a high-factor human sunblock.
Fly masks with a muzzle flap can also be useful to prevent this from occurring. It is sensible to get these horses out of the sun for a few days until the redness has gone, and apply moisturisers to the affected areas.
Horses can also develop areas of photosensitisation around the white areas of coat. This is a much more severe presentation to sunburn, generally characterised as very red, crusting areas that eventually slough and are very painful to the touch. This photosensitisation is usually caused by an underlying hepatopathy.
Horses that present with these clinical signs should undergo a blood test to establish whether any underlying liver disease is present, while another important step is to examine the pasture as ingestion of some photosensitising plants, such as St John’s wort, will cause a mild photosensitisation (Knottenbelt, 2012).
Taking these horses out of the sun is essential, and applying a titanium dioxide, zinc oxide and cetrimide-based cream to the areas affected can be helpful.
A wide range of skin conditions are seen in horses during spring and summer. The treatments listed in this article give some guide to the commonly used protocols available, but insect avoidance remains one of the most important strategies.
