by Michael Fisher, Pueblo County Extension Director
While it is nice to see the rain following a lengthy drought, there are also issues that excess moisture can create or enhance. One of those issues is an increase in bothersome insects. Standing and stagnant water can be a breeding habitat for many of the biting insects. With that in mind, here is an excerpt from an article that I wrote several years ago. It addresses the affliction of sweet itch in horses and how this is caused by biting midges.
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 over reacts 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 relief 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 4am to 10am and 4pm to 10pm. 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.