Sweet Itch: Itching for a Cure
There’s nothing appealing about sweet itch; here’s the latest on managing this chronic skin condition
Sweet itch, seasonal equine dermatitis, Culicoides hypersensitivity, summer dermatitis—they’re all one and the same. So what is this seasonal skin condition, exactly? Biting midges or gnats, namely Culicoides (colloquially known as ‘no-see-ums’), can trigger an allergic reaction when they bite a horse.
It’s not the simple puncture of the skin that causes the reaction but, rather, the insects’ saliva. In the same way some people develop a rash after exposure to certain allergens, but others don’t, some horses become abnormally sensitive—or sensitized—to midge saliva.
“Insect-bite hypersensitivity is the most common allergy in horses,” says Rosanna Marsella, DVM, Dipl. ACVD, professor at the University of Florida’s College of Veterinary Medicine, in Gainesville. Here, she and Julia Miller, DVM, assistant clinical professor in the section of dermatology at Cornell University College of Veterinary Medicine, in Ithaca, New York, will share current information about sweet itch prevention and treatment.
Little Insect, Big Troublemaker
Culicoides are small, weak-flying insects that can’t cover long distances or fly against a breeze, so they tend to feed at night and live near water.
In many areas sweet itch is seasonal, says Marsella, although where she lives Culicoides are active year-round. In addition, there are many Culicoides species that feed on different parts of the horse. “The species of Culicoides that are present on the farm determine the distribution of lesions on affected horses,” she says. “In a place like Florida we have over 20 different species of Culicoides; the more species present, the more generalized the distribution of lesions on the horse.”
Affected areas of the body might include:
- Mane and tail;
- Ventral midline (the center of the belly);
- Face and ears; or
- A combination of these.
Two Conditions and Lots of Allergies
Horses sensitized to Culicoides saliva can develop two types of hypersensitivity. “One (Type 1) is an immediate reaction, which is why some horses may present with hives,” says Marsella. “The other (Type 4) is a delayed reaction, with signs of itch in particular body locations 24 to 48 hours after the bite. These two types of allergies are mediated by different chemicals in the body. Many horses have both.”
Marsella says effective treatment options for Type 1 hypersensitivity include antihistamines, as well as a customized vaccine to desensitize the horse to the allergen. For Type 4 sensitivity a vaccine does not work. “When it comes down to horses with insect allergy, the majority of management is avoidance,” she says.
Further, Culicoides hypersensitivity is rarely a stand-alone condition. “To complicate things, the majority of horses that are sensitized to gnats are also atopic, a genetically inherited condition that occurs in various species, including people, where there is a propensity toward building allergies to whatever comes your way,” Marsella says. “In people, an example is eczema (an itchy skin inflammation): If you have eczema as a child, you may become sensitized to foods, ragweed, trees, pollen, dust. In the management of these horses, it’s important to identify the different components that add to that itch.”
Thankfully, owners of Type 1 horses with multiple allergies have options. “We can design a custom-made vaccine that desensitizes with allergen-specific immunotherapy,” says Marsella.
She explains that the body can make the allergic type of antibodies called immunoglobulin E (IgE) or the more protective immunoglobulin G (IgG) antibodies. The goal of vaccination is to encourage the immune system to switch to the more protective immune response.
“It takes months to reach full efficacy, but it does re-educate the immune system to become tolerant rather than to overreact,” she says.
To deter the insects that instigate hypersensitivity in your horse, look for a quality repellent. “There are many fly sprays that are not true repellents—they are just insecticides,” says Marsella, which are designed solely to kill. “An insecticide is good for the average horse. It reduces the insect burden, but a horse that is very allergic needs something strong enough that insects won’t land.”
She explains that while product names might suggest effective solutions, you should read the label to understand the repellent’s composition, bearing in mind pyrethrin, a naturally produced insecticide, is not a true repellent. Active ingredients to look for include:
- 1% permethrin;
- 0.15% cypermethrin, a synthetic pyrethroid;
- 10% concentrated permethrin marketed for equids and livestock, diluted one part concentrate to 10 parts water to make a 1% solution; and
- 45% permethrin spot treatment (applied to the poll to repel insects from face and ears).
The product should also contain a substance that binds to the hair, particularly when humidity and sweat come into play.
While many product manufacturers advertise long-term coverage, you typically must apply the 1% spray products up to twice daily, says Marsella.
For horses that have developed a sensitivity to chemicals, Marsella recommends neem oil repellents.
Miller says she’s had limited success with topical sprays and recommends focusing on avoidance measures such as full-body fly sheets. While these sheets can help prevent bites, Marsella discourages using them in hot and humid climates. The horse might overheat, creating a warm, moist environment that’s an ideal breeding ground for bacteria. In cooler and drier climates, look for a breathable stretch fabric with neck and belly coverage. Many fly sheets come impregnated with insect repellent, and you can also spray them. But use the fly sheet before sweet itch takes hold; once itchy, the horse will scratch despite the sheet, even rubbing its fibers into his skin, says Marsella.
For face and ear masks, she cautions owners to keep them clean and dry, as wet, dirty fabric might promote secondary bacterial and fungal infections.
Stable management strategies can help mitigate midges, as well. Because we know Culicoides are poor flyers and most active between dusk and dawn, stabling your horse indoors at night with stall fans and insect screens over doors and windows will reduce the likelihood of encounters. Barn sprayers emitting a permethrin spray at a 0.2%-0.5% concentration might also help, says Marsella. And because water attracts midges and provides them a breeding ground, turn your horse out as far as possible from water sources such as creeks or ponds. Midges do not breeds in manure, however, so, unlike with flies, stall or paddock cleanliness is not an avoidance measure.
She also suggests installing a Mosquito Magnet, a propane-powered device that uses carbon dioxide to lure mosquitoes—and midges—into a trap. “You should locate this in a spot to attract insects that would otherwise go to the horse,” she says. “I live on a river in Florida, and the use of this machine has significantly cut down on the insect burden.”
Both veterinarians agree that preventing Culicoides contact with the horse is at this point the most effective strategy for avoiding sweet itch.
The Trouble With Testing
Currently, testing options to definitively diagnose sweet itch are lacking. “Unfortunately for Culicoides hypersensitivity, the best way to diagnose is the clinical picture,” says Miller, listing signs such as:
- Lesion seasonality, which is dependent on where you live;
- Lesion distribution; and
- Response to removing Culicoides from the horse’s environment.
While some veterinarians have attempted to skin-test for Culicoides hypersensitivity, Marsella says it can be problematic. During this process, the practitioner injects the horse with a Culicoides allergen. Positive horses with the IgE response will react within about 20 minutes. Horses without the IgE response but with Type 4 hypersensitivity won’t respond until 24 hours later. Therefore, an immediate negative skin test does not rule out the disease. Also, Marsella says some horses might show a false positive if they have IgE antibodies against Culicoides, but aren’t showing clinical signs, raising the question of whether it has not yet reached the threshold for a reaction or that not all IgE antibodies cause disease.
“At this point in time, it is not recommended to place too much importance on Culicoides allergy skin testing,” she says.
Blood serum testing (serology), Marsella says, is not a good option for Culicoides hypersensitivity testing, either. “A blood test measures circulating IgE, not what is found in the skin,” she says. “Nobody has demonstrated at this point in time that there is a correlation between what is circulating in the blood and what is in the skin. Studies have shown a skin test and serology many times don’t match.”
Research has also shown serum allergy testing to be inaccurate. “Intradermal allergy testing (in which a dermatologist injects a small amount of allergen into the skin), has variable results in the literature regarding accuracy,” says Miller, explaining that tested horses often react positive to Culicoides because they are fly-bite hypersensitive, not Culicoides hypersensitive.
Sweet itch treatment options include topical or systemic medications or both, says Marsella. Topical therapies might include the corticosteroids dexamethasone or hydrocortisone, while systemic treatment might entail antihistamines for Type 1 hypersensitivity and glucocorticoids for Type 4 hypersensitivity. Oral therapies are not without risk, however.
“Systemic glucocorticoids may cause laminitis,” she says. “We try to use them as little as possible—only when needed, for a short period of time.”
Regular systemic glucocorticoid use can also lead to decreased efficacy over time and increased infection risk due to these drugs’ immunosuppressive nature.
With Type 4 cases that don’t respond to antihistamines and do require steroids, careful management is key to avoid steroid overuse. “Insect allergies can’t be ‘fixed,’ ” says Marsella. “You can manage them, but you can’t cure them at this point in time. Many of these horses have a chronic history, so you need to find a long-term sustainable plan. You may use steroids a few times here and there when acute, but you cannot and should not rely on giving dexamethasone every summer.”
Nevertheless, “you have to nip pruritus (itchiness) in the bud,” says Miller. “A lot of people are afraid of steroids, but you have to treat itchy horses or they will scratch into a secondary infection (e.g., Staphylococcus), which can also be difficult to treat. You have to break the cycle.”
She recommends getting a head start on insect control each year, which in her area of New York is as early as April.
And if you compete with your itchy horse, learn about association-legal dosage and withdrawal times for medications.
Supplement for Health Skin
While supplements alone don’t offer Culicoides hypersensitivity solutions, omega fatty acids can help manage skin conditions and atopic disease, says Marsella. “In atopic disease in other species—we know very little about horses—there is a defect of the skin-barrier function, so the skin is drier. In this regard supplementation of fatty acids is beneficial. Fatty acids are not strong enough by themselves to take care of the problem but, if people want to supplement, that’s fine.”
Miller adds that research in dogs has shown omega-3 and -6 fatty acids to be anti-inflammatory, but in horses this effect is only anecdotal.
On the Horizon
While an effective off-the-shelf vaccine against Culicoides hypersensitivity is not yet available, Marsella says research is ongoing. Some scientists are looking at a preventive vaccine, she says, which would be particularly helpful for horses shipping from cooler to warmer climates to prevent hypersensitivity from developing. She says work is also underway to develop a vaccine to address Type 4 hypersensitivity.
Miller describes current Culicoides hypersensitivity work worldwide, including:
- Scientists working to replicate a purified Culicoides salivary antigen (a substance that induces an immune response in the body, especially the production of antibodies), which could potentially become available as a vaccine for commercial use;
- Researchers studying which antigen creates an allergic response in horses, as different Culicoides species potentially introduce different antigens;
- Investigators looking into the effects of injecting a small amount of purified allergen-specific vaccine along the lymphatic nodes; and
- Scientists examining sublingual (under the tongue) antigen administration via a special bit that allows the substance to linger in the mouth, ultimately being absorbed under the tongue.
Don’t play the waiting game with Culicoides hypersensitivity. “I encourage people if they have a horse with allergies not to wait until things are out of control,” says Marsella. “At that point it is so difficult to turn the horse around. Many of these strategies work better in a younger individual, also early in the course of the disease, when the animal has not developed 60 different allergies, because it is impossible to desensitize for so many.”
Being proactive and getting help for your horse early allows him to have a better quality of life. “To live itchy is a miserable life,” says Marsella. “Animals can’t talk, but I see some whose quality of life is so bad their whole personality changes. I encourage people to be proactive.”
Stay on top of the most recent Horse Health news with