Fungi are primitive plants–one of the oldest and most widespread forms of life on this planet. These elementary plant forms can survive and multiply in a much wider range of temperature than any other micro-organisms, being hardier than most bacteria or viruses. Lacking chlorophyll (the substance that enables complex plants to create their own food through action of sunlight), fungi must feed on other organic matter. Some are parasites, living only on live tissues. Fungi reproduce by sending out spores. These microscopic “seeds” can travel on wind and can withstand extremes in temperature, germinating wherever and whenever conditions become favorable. Most fungi prefer a moist environment. They proliferate most rapidly in warm, humid regions or in wet seasons.
There are several types of fungi that cause skin lesions in horses. Probably the most well-known and widespread type of lesion is ringworm. Other problems caused by fungi include girth itch, “scratches” on the lower legs, and “swamp cancer” (cutaneous pithyosis). The latter occurs mainly in hot, humid climates such as the Gulf Coast states.
RINGWORM – Christine Rees, DVM a board certified veterinary dermatologist at Texas A&M, says that normally ringworm is a crusted, circular-shaped lesion. ”The lesions may be either scaly or crusty, and sometimes multiple horses are affected. The spores can be spread if horses share tack, blankets, etc. There is one soil-borne fungus, Microsporum gypseum, which can look similar to hives. This can be confusing, since people might not always think to do a fungal culture when they see hive lesions. This fungus causes some inflammation in the skin,” she explains.
“The best way to diagnose a skin problem is to do a fungal culture–to have the veterinarian pluck some of the scales and crusts as well as the hair around the periphery of the lesions. This is where the fungus is most active; it likes the growing hair.” The lesion gradually expands outward as the fungus attacks the hairs around the outside edges of the lesion.
“The fungus can be cultured and typed, to determine what kind it is and therefore its source. If it is M. gypseum it is from the soil. M. canis comes from cats. Trychophyton mentagprophytes may be from rodents, such as rats in the barn.” Knowing the source of the fungus can help the horse owner know how to try to prevent more cases from occuring.
“An in vivo study was done with ringworm, to determine which types of topical treatment were most affective. Several different medications were used, like diluted bleach, chlorhexadine, Captan (a garden fungicide often used in treating ringworm and girth itch), etc. Captan is not as effective as some of the others for skin lesions, and there has also been some suggestion that it could be a potential carcinogen. I don’t generally use it, but out in the field it is still fairly commonly used,” says Rees. “The one that was found to be most effective in this study (but which unfortunately has the worst smell) is the lime sulfur dip.”
Some people treat the skin lesions with an oral antifungal medication, which can be very expensive. Intravenous injection of sodium iodide can be used, or oral administration of griseofulvin (an antibiotic used as an antifungal treatment for skin infection in humans and animals). ”There have really been no formal studies on proper dosage for the griseofulvin, but there are some anecdotal suggestions for doses. It’s a very difficult drug to work with, so it hasn’t yet been proven scientifically which dose should be used for this purpose,” says Rees.
“I usually use topical treatments. Some of the things that can be used as shampoos are miconazole (a broad-spectrum antifungal agent) or ketoconazole. These are some of the better shampoos for fungal skin lesions. Iodine-based shampoos also work but iodine can be inactivated by organic debris (scales, crust and dirt). If you have a bad problem, one of the azole antifungals would be better. One anti-fungal shampoo (Malaseb–DVM Pharmaceuticals, Miami, Florida) contains chlorhexidine and miconazole. Another one (Ketochlor–Virbac, Fort Worth, Texas) has ketoconazole and chlorhexidine. If you want to thoroughly treat it, use the lime sulfur dip after the shampoo. If you use the dip in conjunction with a shampoo, you would only have to bathe the horse once a week,” she says. “Bathe the horse with one of the anti-fungal shampoos, then towel dry or let the hair air dry, then mix up the dip. I use about 4 ounces of the medication to a gallon of water, then sponge it on. One of the problems with the dip is that it can stain jewelry, or stain white hair–giving the white areas a yellowed look. It also smells terrible–but it is very safe and very effective,” says Rees.
To disinfect the environment (to kill fungal spores that might infect other horses), she feels that diluted bleach works about the best. ”There is a spray on the market that is already made up, called Chlorox Cleanup. It’s really easy to use, since you don’t have to dilute the bleach. You can use a small hand held pump sprayer to disinfect the stalls, etc.” she says.
To disinfect tack, there are several options. ”Diluted bleach can be used but might dry out some materials too much. Some people wash saddle pads, girths and cinches with a miconazole shampoo,” says Rees. It’s always wise to disinfect any tack or grooming tools that have been used on the affected horse to prevent spread of the fungus.
“Girth itch” can be caused by similar fungal infections, and occurs primarily behind the elbow, in the “armpit” of the horse. Unless the infection is halted, it may lead to ever-widing raw spots. The horse may become so sore that a saddle cannot be used until the infection is halted and the skin heals. This type of infection is most likely to occur after the horse has been ridden with a saddle; the tender skin at the girth becomes irritated by the cinch rubbing the skin, then the fungal spores can enter the broken skin and the skin begins to peel. Damage to the skin (which can lead to girth itch) can often be avoided if the cinch or girth fits the horse properly and is always clean and soft so it doesn’t cause abrasions.
Some animals that might pick up ringworm or girth itch may not have as much immunity as others. Some horses tend to have a higher susceptibility to fungal infections and will get repeated cases, while others in the same environment never get it. To play it safe, don’t use the same tack or grooming tools on multiple animals, or make sure tack is disinfected between animals.
SCRATCHES – Scratches is a skin problem on the lower legs, in which the affected area becomes crusted, scabby and thickened. In severe cases the skin may ooze or the whole lower leg may swell, and the horse becomes lame. This condition usually affects unpigmented skin (white markings) more readily than dark skin, since pink skin is more tender and not as tough–more easily nicked and scraped, allowing the fungal organism to enter. The fungus lives in organic matter and is often present in mud or swampy areas in a pasture; if horses must walk through mud, they are more apt to pick it up.
“Sometimes bacteria are involved as well, which can make it worse. The problem may be caused by a combination of fungus, bacteria and dermatophilosis (which is caused by another type of bacteria). Sometimes an irritant sets it off, such as a contact dermatitis. The latter might result from having the feet wrapped or protected with rubber (and the horse reacting to that) or being in a pasture where there is a lot of mud,” says Rees.
A number of different treatments are used for scratches, she says. Any deworming pastes that contain a chemical in the azole family (thiabendazole, fenbendazole, cambendazole, oxybendazole, mebendazole, etc.) can be used topically, mixed with DMSO to help carry the fungicide into the tissues. Nitrofurazone ointment can be added to this mix to combat possible bacterial components of the infection. “Some people use silver sulfadiazine cream or desitin on scratches. There is no one ‘best’ treatment,” she says. Anything that inhibits the fungus will often work.
“To try to figure out whether there is a bacterial infection involved, the veterinarian can take a sample of some of the crusts and examine these on a slide (with a microscope, to look at the cells), to see whether or not the horse will need antibiotics. Sometimes a yeast will also be present, and this can be picked up on a cytology examination of the crusts. The vet would do a fungal culture to know for sure that a fungus is involved. This will give a better idea about what to treat the horse with,” says Rees.
When treating the horse for scratches, the affected area on the lower leg should be thoroughly cleaned before applying the medication. This may mean clipping the area–and washing and drying it before each treatment. The leg should be kept dry; the horse should be kept in a dry environment rather than in a wet or muddy pasture, and the affected area kept open rather than bandaged.
DEEPER FUNGAL LESIONS – “Some of the other fungal infections that affect horses are deeper, and harder to deal with. Alot of these become ulcerated and nodular, with more drainage. They look much more severe–like an oozing sore. One of these fungi lives in the soil and causes ‘rose growers disease’ in humans. If a person punctures his hands on the thorns of the roses, this gives the fungal spores access, creating a sporotrichosis. A horse can get this same type of infection if he lives in a paddock or pasture where this fungus lives in the soil–and a break in the skin allows the fungus to enter,” she says.
“Almost always there is some kind of wound that introduces the fungus. These infections have to be systemically treated, and sometimes that can be difficult. Usually the veterinarian will use iodine therapy, administering a solution (like sodium iodide) intravenously. This is the most common treatment used. If there are only a few lesions, some veterinarians will surgically remove them and then treat the horse systemically for a little while to get rid of it completely,” says Rees.
Most of the deeper fungal infections look similar. “Another type of deep fungal infection is pithiosis. This is an aquatic fungus that appears to be most prevalent in the Gulf Coast areas of the USA. Other names for pythiosis include phycomycosis, bursatti, hyphomycosis, swamp cancer, Florida horse leeches, Gulf Coast fungus, and oomycosis. The fungus that causes this problem is Pythium species.” A horse might come into contact with this if there are muddy or swampy areas in the pasture. A skin wound would introduce the fungus. Since it is an aquatic fungus, most of the skin lesions are on the lower parts of the horse such as the lower legs, abdomen or chest, she says.
These skin lesions can be very itchy. ”They usually look very raw and ulcerated, and sometimes look similar to squamous cell carcinoma. It can be an angry, nasty, itchy area on the skin. The best treatment for this infection, if you can catch it early, is to surgically remove the affected area, while it’s small and localized. If you find a nasty, open, ulcerated sore on a horse, have it checked by your vet,” she says.
“Some veterinarians use an iodide given intravenously, or a more potent treatment called amphotericin B, which is an I.V. anti-fungal agent. The best luck is usually had by catching it early and getting it surgically removed, however,” she explains.
“On some of these deeper fungal infections that must be treated systemically, the veterinarian will probably want some skin biopsies to look at, and do some biopsies to be ground up and cultured–because you can also encounter some bacterial infections that may look similar to fungal lesions.” Often the organisms are down deep in the tissues, so a culture will help diagnosis these.
“There are two different types of bacteria that might be involved–the ones that grow in air and the ones that don’t. So the vet will probably want to cover all bases with a lesion that looks nasty like this. The horse might need antibiotics, depending on what is found. If a wound that you’ve been treating is becoming ulcerated instead of starting to look better, you should contact a vet for further diagnosis. Usually after a week of treatment, a wound will be making good healing progress. If it is getting worse in that length of time, you may be dealing with a fungus.” The best prevention in most cases, to avoid fungal infections, is to keep a horse in a dry environment, she says.