Equine summer eczema, aka; “sweet itch”, insect bite hypersensitivity or summer dermatitis is the most common allergic skin disease of the horse. This recurring, seasonal disease affects animals that have become allergic to proteins in the saliva of biting flies, especially the culicoides midge or “no-see-ums”. The primary clinical sign is itching and local irritation and trauma secondary to scratching. The tiny insect culprits typically become active once the ambient temperature rises to an average of 50 degrees F or above, right about now around here. The muddy, moist habitats which we have in abundance act as a nursery for the upcoming culicoides assault. You may not see ‘em but you’ll know they’re around by your irritated, itchy horse. Depending on the species, gnats tend to feed on either the dorsal (mane, tail) or ventral (belly) areas of the horse. They are most active at dawn and dusk. Pretty much all horses here are exposed to gnats and clearly not all of them react with self mutilation. The disease seems to be more common in some breeds than others and may worsen over the years. Normally, insect caused allergic response is local and transient, without long term consequences,(think mosquito bite). In horses with summer eczema, allergic responses are prolonged with each exposure causing both an immediate and a delayed hypersensitivity reaction with a large cascade of cellular response.
Treatment of seasonal insect bite dermatitis is a challenge since it primarily involves decreasing exposure to the insects and suppression of the allergic symptoms, both of which have a rather wide range of effectiveness and may include unwanted side effects. For many horses management practices are enough to control fly bite dermatitis. That means taking steps to minimize exposure. Culicoides are weak flyers so electric fans in stalls are effective deterrents. Fly sheets and masks are somewhat helpful but leave some areas exposed. Fly repellants like “Swat” (Farnam) or “Belly Balm” (Jeffers) applied to affected areas are helpful when used early and frequently. Fly products with relatively high concentrations of pyrethroid-permethrin have the greatest effect on gnats. We have also had success with a product called “Eco Vet”. Because the flies are crepuscular (active early AM & around dusk) do not turn animals out during those times. Environmental management is also important to decrease fly breeding areas in standing water and moist manure. If you know your animal has seasonal fly allergies now is the time to make a management plan for an itch free summer.
Even with the best management, for severely reactive animals, summer can be several months of torture and misery. If your horse falls into that category, we have another approach to treatment that may provide some hope for both of you. This therapy is another in the autologous, or “heal thyself” category we discussed last month. It includes oral administration of an autologous serum preparation made from the affected animals blood (serum). We’ve been using this therapy on selected horses here at RBE for several years now with good results and there has been some science published recently that backs up our anecdotal evidence. Most scientific research on the use of auto serum for summer dermatitis comes from Finland. A randomized, placebo controlled study (1997-8) established positive clinical effect in 343 horses. A subsequent retrospective study 12 years later determined that, 70% of treated horses benefitted from treatment. Horses treated with auto serum had significantly milder clinical signs than before treatment and no harmful side effects were noted.
The auto serum is made from highly diluted host serum that has undergone serial washings to separate water-soluble molecules from water-insoluble lipids. The extracted lipids are then mixed with ethanol and sugar for oral administration. Just like in the song, "a spoonful of sugar makes the medicine go down" and a delightful treatment it is for most horses. So how does this miracle work? Well, the truth is we really don’t know exactly how it works, but that’s the good news about science, it’s always open to discovery and one of these days someone will understand the mechanism (and no doubt make a flow chart of it). In the meantime, the working hypothesis is that phospholipids found in serum are altered due to allergic reactions and when the animals own (reactive) serum is prepared in a way that concentrates those lipids it appears to effect symptomatic relief. What we know is that lipids are a diverse group of molecules that are main structural components of cell membranes. They are also an integral part of many intra and extracellular reactions and important mediators of mast cells (which are active in allergic reactions). Membrane phospholipids are a prime source for precursors to both inflammatory and anti-inflammatory molecules, helping to maintain cellular homeostasis. We know that lipids play an essential role in a myriad of biological reactions, but their role in pathology is still poorly understood. So this is where science meets faith and experimentation and where that little caveat “no adverse side effects” becomes really important. If you have a horse that you think might benefit from this sweet therapy for summer fly bite dermatitis, give us a call to discuss the possibilities.