Horses with unusually sensitive immune systems may develop hives in reaction to allergens, medications, or other stimuli.
Horses with unusually sensitive immune systems may develop hives in reaction to allergens, medications, or other stimuli.
The horse’s skin is his first line of defense. It is subject to problems caused by bacteria, fungi, biting flies, allergies, and other things that range from being mere annoyances to serious conditions. Chapter 10 discusses skin problems related to external parasites such as flies, lice, and mites. This chapter focuses on dermatitis related to allergies and hives, photosensitization, fungal infections, rain rot, scratches, warts, and sarcoid tumors.
An allergy is a reaction to a substance that would normally cause little or no response by the body. Some horses are allergic (hypersensitive) to insect bites, certain plants, drugs, vaccinations, or medications. If the horse’s defense system works overtime, an inflammatory response may become worse with each encounter. Sometimes, an allergic reaction creates raised areas in the skin. This condition is called “urticaria”; the bumps are called “hives” or “wheals.” They are caused when small blood vessels near the skin surface dilate and blood seeps from the damaged capillaries. The hives develop suddenly after the horse comes in connect with the allergen. If you are with the horse when it happens, you can see the bumps growing as you watch. They usually pop up on both sides of the body and sometimes on the head and neck.
Insect bites or stings, contact with certain plants and/or pollen, and ingestion of certain weeds or plants can cause hives. Moreover, some drugs or vaccines produce allergic reactions. For example, penicillin causes hives (or some other manifestation of allergy) in some horses.
In mild cases, the lumps subside without treatment within a few hours or by the next day. In severe cases, they persist for several days and may be accompanied by diarrhea, fever, or breathing difficulties. The affected horse may need an injection of antihistamine or corticosteroids.
Hives may recur if the allergen is not removed, the diet is not changed, exposure to the causative insect bites or plants is not prevented, or the same offending drug or vaccines is used again. Many times, the cause of hives is readily obvious, as when a horse reacts to a certain injection, drug, or feed change; walks through a patch of stinging nettle; or suffers insect bites. At other times, it may be difficult to discover the cause. If hives recur or do not disappear within a few days, have your veterinarian do sensitivity skin tests to determine the cause of your horse’s allergy.
Horse with hives
This is a skin condition in which unpigmented skin is damaged by a reaction to something the horse has eaten, and occurs when the skin is exposed to sunlight. Photosensitization is most common during summer (long days and stronger sunlight) when horses eat pasture plants that may contain certain photodynamic agents — chemicals that cause a toxic reaction when exposed to sunlight. A horse with long winter hair is not as susceptible because the hair shields the skin from the sun.
If a horse eats a plant containing photodynamic agents, the chemicals are absorbed into the bloodstream and travel to all parts of the body. If they accumulate in the skin, the sun’s rays cause a reaction, with inflammation and cell death. This problem is triggered by a number of plants — especially fescue grasses and some species of clover — and usually occurs when they are lush, green, and growing rapidly. The disease can also occur when a horse’s liver is impaired and unable to filter out the by-products of chlorophyll breakdown in the digestive tract. When these by-products accumulate in the skin, the reaction kills skin cells.
The ultraviolet rays penetrating the sensitized skin tissues trigger release of histamine, which causes dilation of capillaries and swelling of tissue. Irritation and itching may be intense because of painful swelling deep within the layers of skin. The condition may be mistaken for sunburn at first because the area turns red and starts to peel. Then it becomes more swollen, breaking out in blisters and cracks, and the skin develops thick crusts and painful scabs. In severe cases, the horse becomes quite ill, with patches of skin sloughing off. The horse may go into shock.
Remove the affected horse from the pasture and put him in a barn or covered area out of direct sunlight. In serious cases, your veterinarian may give laxatives such as mineral oil (1 quart [1 L] or more by stomach tube) to help get rid of toxic materials still in the digestive tract. Antibiotics and antihistamines may be necessary as well. Corticosteroids reduce soreness and swelling but should be used only on the advice of your veterinarian. You may, however, apply soothing ointments or creams to raw areas to help alleviate the pain and itching.
Areas most affected by photosensitization are pink or unpigmented skin, such as white face markings and white legs. Pigmented skin is more resistant to the sun’s rays.
Many types of fungal infections lead to skin problems in horses, including the fungi that cause ringworm and girth itch. Some fungal infections are most common in winter when daylight hours are less (sunlight tends to inhibit them), and several can be spread from horse to horse through tack or grooming tools.
Though it sounds like a parasite, ringworm is caused by several types of fungi, including Microsporum and Trichophyton, that send out spores to start new infections when rubbed into skin. After the fungus spores enter the skin, patches in which hair falls out or breaks off become noticeable about a week to a month later. These areas are often, but not always, circular.
Ringworm
Ringworm is highly contagious, and all the horses in a group may become affected. It is spread from horse to horse by infected brushes, curry combs, cinches, saddle pads, blankets, and the like. Young horses; older horses; those deficient in vitamin A, zinc, or selenium; and any horse with an immature or compromised immune system may be quite susceptible. Some families of horses have less resistance than others. Occasionally, the horses who get ringworm most often on a farm are related.
Some of the fungi that can cause ringworm in horses can infect other animals, including humans.
A horse with ringworm should be treated to prevent spread. It is also important to isolate the horse and disinfect all grooming tools and other equipment used on him. Several topical medications kill ringworm fungi. You can obtain them from your veterinarian.
Ringworm will crop up sporadically if the fungal spores are present in the environment, but incidence of cases can be minimized by keeping horses healthy (adequate vitamin A and selenium in the diet) and not sharing grooming tools and tack among horses.
Girth itch is a fungal skin problem that occurs in the girth area, generally after the horse has been ridden with a saddle. When tender skin at the girth (especially behind the elbows and in the armpits) becomes irritated by the cinch or girth rubbing the skin, the fungus gets a start in the broken skin and then the skin begins to peel. Unless it is halted, the infection leads to ever-widening raw spots, and the horse may become so sore that he can’t wear a saddle until the infection is halted and the skin heals.
Most good fungicides halt girth itch, but take care to use one that will not be irritating to tender girth skin. Tincture of iodine is too harsh, but “tamed” iodine (Betadine) is fine. Another product that works well for halting serious cases of girth itch is the garden fungicide captan (Orthocide), used on fruits, vegetables, and flowers. It is a wettable powder. Follow label directions, read label precautions, avoid getting it in eyes or inhaling it, and wear rubber gloves when applying it to horses. The solution can be sprayed or wiped on.
Don’t use the same equipment on more than one horse (each should have his own brushes and tack). If you must use the same saddle, be sure to change cinches. If you use the same cinch or girth on more than one horse, wash it with a fungicide or bleach before using it on another horse. A neoprene girth is handy; you can wash it with warm water and chlorhexidine (Nolvasan) and have it dry and ready to reuse in just a few minute because the rubbery material does not absorb moisture.
Rain rot (also called rain scald, mud fever, dermatophilosis, and cutaneous actinomycosis) often affects horses in wet climates, especially during fall and winter when they have long hair and it takes longer for the coat to dry. Skin lesions commonly occur along the horse’s back, neck, and withers but can also spread down the sides, chest, and legs. Affected horses exhibit crusting hair loss, predominantly over the back, rump, and shoulders. The pattern of lesions are the areas of the body that have been constantly wet. If horses are standing in mud or wet conditions, you might see the problem on the lower legs. If the horse has been standing out in the rain for days and the skin becomes soft and moist, any parts of the body that have been wet will be vulnerable, since moisture is one requirement for these bacteria to flourish.
In earlier years rain rot was thought to be caused by a fungus. Now we know it is caused by a bacterium called Dermatophilus congolensis. It is often present in a dormant state on healthy animals and only causes problems when conditions are favorable for infection. The bacteria can live on the skin or on tack or grooming tools. They are activated with moisture, and infect an animal when they find a break in the skin.
This infection is contagious. Horses can pass it when rubbing on each other or when grooming tools or tack are shared. If an infected horse rubs on a post or fence, another horse may pick up the bacteria when rubbing on the same place. There are probably carrier horses, and if conditions are right (lots of moisture, and breaks in the skin) these horses can spread the infection to susceptible horses through direct contact.
Once they find a way into the skin, the bacteria cause inflammatory reactions and form pustules. Tiny tufts of hair rise up, with the bacteria living and growing beneath them. In early stages it is easier to feel the emerging bumps than see them, and the horse may be sensitive when you brush him. The bumps and tufts are painful. In an advanced case, clumps of hair may fall out and the horse has raw patches of skin. This condition may also be complicated by staph infection.
Dermatophilosis is a zoonotic disease, which means humans can get it from animals. If your horse has rain rot and little scabs on its back, and you ride bareback in shorts on a hot day, you might get this skin disease.
Daily bathing with medicated shampoo for at least a week will usually kill the bacteria and help dry up skin lesions. A common misconception is that iodine is a good treatment for rain rot, but there are better products now, such as benzoyl peroxide, the same ingredient in human products for acne. It’s a good antimicrobial agent and also opens the hair follicles, helping the scabs come off. Another effective product is chlorhexidine, a versatile antimicrobial; it doesn’t work as well for dislodging the crusts and scabs.
While bathing the horse, leave the lathered shampoo on for 10 minutes before rinsing. This softens and loosens the scabs and allows the medication to reach the affected areas. Use a soft brush or your fingers to gently remove loosened tufts and scabs as you bathe the horse. You should not pull the scabs off or pick at the lesions. This is painful, and if the skin is not ready for the scab to come loose, it leaves open skin and can lead to scarring. When the scabs are ready, they will come off on their own. After each bath, rinse and dry the horse, and keep him dry.
In severe cases your veterinarian may recommend topical or systemic antibiotics. If there are just a few infected areas, you can rub or pour antibiotic medications onto those places if you don’t want to do a complete bath — especially in cold weather.
Some horsemen use a product called M-T-G, which contains sulfur, paraffinic distillates, petroleum distillates, zinc stearate, cade oil, and glycerin. It often works well, but test it on a small area first, as some horses may react adversely with swellings and skin irritation.
Rain rot is rarely a problem in dry weather because the organism needs moisture to multiply. The best prevention is to keep the horse clean and dry. In a rainy climate, a roof or run-in shed and dry bedding will help. You can also blanket a horse, as long as the horse is clean and dry when you put the blanket on. In climates where rain rot is a recurring problem, regular grooming can keep the horse from being constantly muddy. Mud keeps hair matted and slower to dry.
A dirty horse is more at risk for rain rot when wet.
Grooming several times a week enables you to feel any bumps or scabs that are starting to form, and treat a small area with topical medication—and halt the infection—before it affects the whole horse. Brushing or using a horse vacuum to get dirt out of the coat is healthier than constant washing, since you want to keep the horse dry. Don’t use a metal currycomb, or you may scratch the skin and introduce infection.
Disinfect tack or grooming tools that have been used on an infected horse. Otherwise you may spread infection to another horse, or keep infecting the same horse.
Some skin problems on the lower legs are the result of infection that enters through nicks or sores. One example is a condition that many horse owners call “scratches.” This is just a general word for dermatitis and crusty lesions on the pasterns and lower legs. A horse can have pastern dermatitis due to photo-active dermatitis or other allergies, or bacterial and fungal infections. Any photo-aggravated problem on the lower legs is often called scratches.
Inflammation and infection in the pastern area or fetlock area is common in wet weather, especially if horses are standing in or wading through mud. There is usually some trauma to the skin that enables the problem to get started, which can include damage from photosensitization.
Some types of Culicoides (tiny gnats and midges) feed on the horse’s legs, and this can cause scratches. They start the problem with itching and inflammation, which leads to a secondary bacterial infection. The horse is allergic to insect bites, then gets a little staph infection, making the situation worse.
The infected leg area becomes crusty, scabby, and thickened. In severe cases, the skin may ooze or the lower leg may swell, causing lameness. This condition affects horses with unpigmented (white) skin more readily than dark-skinned ones.
Traditional treatments for scratches comprise astringents such as methylene blue, iodine and glycerine mix, ointments made with zinc oxide, nitrofurazone, and steroids. A better treatment involves thoroughly scrubbing affected areas and applying a mix of one part nitrofurazone ointment (antibiotic), one part dimethyl sulfoxide (DMSO), and one part dewormer paste containing a benzimidazole (thiabendazole, fenbendazole, cambendazole, oxibendazole, oxfendazole, or mebendazole — good fungicides as well as dewormers).
DMSO reduces swelling and inflammation and also helps the fungicide penetrate the area deeply, taking the medication into the underlying tissues. The nitrofurazone combats bacterial infection and buffers the DMSO so it won’t burn or irritate the skin. The wormer paste kills the fungus. Applied daily, this mixture clears up most cases of scratches quickly.
These infections can also be treated with chlorhexidine or benzoyl peroxide to kill bacteria that cause the crusting. Bandaging is not necessary. In fact, it’s actually detrimental; you don’t want to hold in the moisture.
Prevent scratches by keeping the horse in a drier environment so he doesn’t have to walk through mud. Prevent serious cases by treating small lesions when they first appear, never allowing them to become extensive and deep.
Warts are a common problem in horses, especially young ones (yearlings and 2-year-olds), and are caused by a virus that lives at the surface of the growth. They spread from animal to animal by direct contact, but this virus does not cause warts in humans. On horses, they are often spread by grooming equipment or tack used on more than one horse, or by a person who touches an infected horse and then a susceptible one. Small cuts and scrapes in the skin can lead to wart infection if the virus is present. Once established on the animal, warts can last for several months up to a year. After exposure, a susceptible animal may break out with warts a few weeks later. Infected horses often pass the virus to other horses before their own warts are evident because of the long incubation period.
Warts are unsightly but not a serious problem for the animal unless they grow large enough to interfere with breathing. In horses, they are usually found in clusters around the muzzle, but occasionally they appear on other parts of the body or inside the ear. They are distinguished from other skin growths by their dry, rough appearance. They may be pale or gray.
Horse’s muzzle covered with small warts
Warts usually run their course and then disappear after the animal builds immunity. When left alone, they often go away after a while. If they must be removed for health reasons, have your veterinarian remove them surgically. You can hasten their disappearance by crushing them with forceps or twisting off the larger ones; in fact, the veterinarian may choose this method. It encourages the immune system to fight them because some of the wart tissue makes contact with the bloodstream, stimulating the animal to make antibodies.
Like warts, sarcoids are also caused by a virus. Some horses develop them following an injury or break in the skin; others for no apparent reason. There are several types of sarcoid. Some are slow-growing and some are more aggressive. Some are flat, hairless, and often small and circular; they often appear around the mouth or eyes, or on the neck or inside of the legs, or hairless regions of the body. Sometimes these types are hardly noticed; you may just see a little bit of pigment change in the skin.
Others are wart-like, gray, and scaly but extend deeper into the tissues. They may have a circular hairless patch around them that might look a little like ringworm, and others might be mistaken for warts. Some are nodules underneath the skin (usually with the hair still covering them), while others are hairless. Sometimes a tumor first looks like proud flesh (excessive granulation tissue that protrudes from a healing wound; see here) but continues to grow. At first, a sarcoid may be hard to differentiate from proud flesh. The virus attacks young fibrous cells in the proud flesh and causes them to multiply rapidly. This excessive growth cannot be controlled by medications used to control proud flesh.
Wounds in areas where there is motion (joints or moving parts on the legs) seem most susceptible to sarcoid tumors. The growth is sometimes mushroom shaped; the base of the attachment is generally smaller than the outer surface. Wounds that are not properly treated to prevent contamination, irritation, and delayed healing are susceptible to sarcoid.
Sarcoid tumors do not travel through the bloodstream or lymph system to other parts of the body, but many of them do continue to spread and grow locally. They are moderately malignant in that they usually grow back if removed — often in a larger, more aggressive form. If not removed early, a tumor may become so large that it disables the horse. Because removal often stimulates regrowth, some veterinarians leave small, nongrowing sarcoids alone.
Sarcoids can sometimes be successfully removed with surgery, followed first by cauterization of the area to control bleeding and kill stray tumor cells, and then by radiation treatment. The area is kept clean and bandaged while it heals. Surgery is probably one of the more common treatments for sarcoids that have to be removed for some reason, but the goal is to remove all of the tissue so it won’t grow back.
Other removal techniques include freezing and burning. About 50 percent of growths will return, however, so other treatments are often tried. Radiation therapy is used on some sarcoid lesions, but only a few equine hospitals offer this form of treatment.
For sarcoid lesions around the eyes, where surgical removal is not an option, a common treatment is an injection of BCG (Bacillus Calmette-Guerin), which is an immune stimulant. This produces a lot of inflammation, and hopefully stimulates an immune response so the body will fight the tumor. If the treatment works, the sarcoid becomes ulcerated and then falls off. The horse may be put on drugs to help control the swelling and inflammation. One risk when using BCG is the possibility of side effects, which could include anaphylactic reactions. Another option is cisplatin, which is a type of chemotherapy.
Another treatment involves use of Aldara cream, which contains a drug (imiquimod) that fights the virus by stimulating the body’s immune system. This is a human prescription drug that your veterinarian can prescribe for your horse, to be applied three times a week for several months until the tumor shrinks and disappears.
Sarcoids are very likely to recur, regardless of the treatment selected. Probably the most effective treatment is surgical removal followed by some sort of chemotherapy (such as cisplatin or carboplatin injections) or radiation.