Overview of equine skin diseases
Once the integrity of a horse's skin is compromised, it is not only the site of the irritation that's a problem, but the general health of the horse since the immune barrier has been breached.
Skin diseases in horses are prevalent throughout the year, although some may be seasonal. They may be due to infectious agents (e.g., bacteria, viruses, fungi, parasites) or environmental irritants. Here are some important skin diseases and conditions encountered in horses.
DermatophytosisDermatophytosis, or ringworm infection, usually manifests as a crusting dermatosis with hair loss and circular lesions on the body.
"Sometimes you see lesions that look almost hivelike," says Christine Rees, DVM, DACVD, a veterinary dermatologist who practices at Hillside Veterinary Clinic, Dallas. "Since there are various ringworm fungal genus species, the best method to diagnose the disease is to do a fungal culture, which also helps to determine the potential source," she says.
For example, it's helpful to know if the horse has contracted ringworm found in the ground (Microsporum gypsum). "If so, you can get more aggressive in bleaching or treating the ground or stall floor," says Rees.
Ringworm infection is common in most animals and is zoonotic, so it's easily passed between individuals. If it's the type usually seen in cats, evaluate any barn cats to determine if one has skin lesions, thereby transmitting it to the horses. Ringworm infection doesn't seem to be species-specific—unlike infections caused by lice or other organisms—so it can be indiscriminant.
"Normally, you'll see it in a stressed animal or one whose immune system is slightly compromised; therefore, it's more common in older or younger horses," says Rees.
Moreover, it can be difficult to treat since the organism can linger. So if horses are sharing tack or grooming aids, ringworm can be spread easily among horses. "One has to treat the environment as a source of infection, which is as important as treating the animal," Rees says.
Rees usually treats with topical therapy. Miconazole works slightly better than ketoconazole, although Rees calls the latter a reasonable alternative. (Note: It's been reported that, in small animals, some fungal infections are resistant to ketoconazole.) If the owner doesn't want to shampoo the horse two to three times a week, a lime-sulfur dip can be used, Rees notes. Although it has a pungent, rotten-egg-like odor, it's effective against fungus, bacteria and parasites at a higher concentration.
Oral griseofulvin also may be used to treat ringworm, although Rees says she hasn't seen a pharmacokinetic study for it's use. Terbinafine is another option, but side effects can include increased liver enzyme activities. Occasionally, fluconazole is used.
Staphylococcus aureus infection
These bacteria may cause warm, painful skin; focal crusts are most common in the pastern region, but similar lesions may occur elsewhere on the skin. Methicillin-resistant S. aureus (MRSA) is a concern in horses.
Rees notes that some horses that get recurring Staphylococcus infections seem to be most at risk for contracting MRSA, especially those repeatedly treated with various antibiotics.
And as the bacteria are zoonotic, horses can contract MRSA infections from their human handlers, particularly those who work in the health field and may carry the bacteria to the stall in their nasal passages. Since the bacteria are zoonotic, people working with an affected horse should wear heavy latex gloves.
Horses commonly get opportunistic secondary infections that result in itchy skin disease, especially those animals with allergic skin conditions. "In an allergic individual, we don't think the barrier function of the skin is exactly the same as with a normal individual," says Rees. "When they're having an allergic flare-up, they may be more predisposed to picking up things that are out in the environment than a normal animal would be."
If the infection is recurrent, do a culture to determine what it is. Many laboratories will test a set of panels for several medication options. "Unfortunately, when a horse contracts a resistant infection, it can get a bit pricey as far as antibiotics and repeated treatments, which can make it more difficult for owner compliance," Rees says.
Owners for whom cost is a problem can bathe the horse with a 2% to 3% chlorhexidine or a benzoyl peroxide shampoo, using it more aggressively (e.g., every other day) before trying a more expensive injectable or oral antibiotic, Rees notes.