Sweet itch or sour rash?

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Sweet itch or sour rash?

By Michael Fisher

Colorado Area Livestock Extension Agent

Have you ever thought about how things get their name? Especially those items that have a two word name. Occasionally, you will find one that uses words which mean opposite things when used individually and just don't fit together well. Some questionable examples that I have heard over the past few days are silent alarm, sure bet, young adult, dull roar, and firm estimate. These are known as oxymorons.

Another oxymoron that has spoken around the Golden Plains Area in recent weeks is sweet itch. While the name conjures up a notion of something pleasantly distracting, the condition that it refers to is far from pleasant.

Sweet itch is a skin condition that can affect some horses. It is caused when a susceptible horse is bitten by biting midges called Culicoides, the same parasites that are responsible for transmitting bluetongue among cattle and sheep. These very small gnats (1 to 3 mm) swarm together for feeding. Typically, the midges are nectar feeders; however, the female of the species requires a blood meal in order to mature her eggs. The saliva that is left behind following the insect's bite contains a specific protein molecule that the horse's immune system considers to be a threat. In sweet itch cases, the immune system overreacts and releases an overload of the antibody IgE. This is followed by a cascade production of both cytokins and histamine to kill the invader. However, the overproduction of histamine inflames the skin and causes intense, uncontrolled itching.

Hypersensitive horses may pace endlessly and seek an unusual amount of mutual grooming from other horses penned with them. They are also prone to excessive yawning and may be easily distracted when being ridden. Additionally, they will rub, roll, paw, and bite at the affected area in an effort to relieve the itch. In turn, this activity tears away at their skin causing hair loss, abrasions, weeping soars, open wounds, and leading to secondary infections. Often times the inflamed area will be around the tail, over the withers and through the mane, or about the ears and face. Additionally, some cases may be along the spine or belly. This variability is a result of a difference in feeding preference among the various varieties of midges.

As is the case with many allergens, a horse may develop a greater intolerance to sweet itch the more that it is exposed to the allergen. Therefore, it is possible that the symptomatic horse's condition may worsen with each passing summer.

There currently is no real good treatment for the condition. Steroids and antihistamines have been used to relieve the itching; however, these are only masking the condition and may lead to side effects that can be more harmful, such as laminitis. Prevention is the best chance of avoiding a problem. This may include the use of insect repellants in the horse's stall or topical repellants regularly applied to the pastured horse. Insecticides that appear to work the best in these situations are those that use either a permethrin or benzyl benzoate as an active ingredient. Additionally, some equine enthusiasts will place a blanket or hood over the horse to prevent the midges from being able to bite into the horse. Another successful tactic is to understand the midge in question and manage around it. Most of the Culicoides will do their feeding from 4 a.m. to 10 a.m. and 4 p.m. to 10 p.m. ; therefore, it is possible to keep your horse stalled during those times, in the airflow of a strong fan. Midges are poor fliers and require calm air to be able to fly. It is also advisable to drain still or stagnant water and remove decaying vegetation near the horse, as these are considered prime breeding habitat to the midges.

Personally, I think that sour rash would be a better name for the condition. At least it does not make it sound pleasant. If you have questions about sweet itch, or would like to discuss it at greater length, feel free to contact me at the Yuma County Extension office, 970-332-4151 or by e-mail at mj.fisher@colostate.edu.