In warm, wet conditions, an opportunistic organism called Dermatophilus congolensis may infect a horse’s skin causing sticky, crusty scabs that entangle and mat the hair coat. It also invades the skin of ill-kempt goats, sheep, and cattle – and not as commonly – pigs, dogs, cats, and people. Common terms for this disease are Rain Scald, Mud Fever, and Rain Rot.
D. congolenis is an actinobacterium that has fungal qualities in that it produces filamentous hyphae, or threadlike projections that extend from its outer capsule like tentacles or feeler roots. These “tentacles” probe and infiltrate the skin tissues causing a significant inflammatory response from the horse’s immune system resulting in oozing, scabby lesions. The bacteria can live benignly on a healthy animal’s skin, multiplying rapidly and causing disease when conditions become ideal.
Suspected to originate in soil, however yet to be isolated and confirmed, D. congolensis thrives especially on horses exposed to wet, muddy environments for prolonged periods of time. Biting insects, which are more prevalent in warm, humid weather may also play a role in defeating the skin’s natural defenses to bacterial infections such as rain scald.
D. congolensis is considered to be an opportunistic pathogen; rather than being directly contagious to a healthy horse, it takes advantage of an animal whose skin health is compromised by environmental conditions. Animals are the only proven reservoirs of the organism however, so it is wise to separate infected horses from others that are susceptible. In addition, equipment and tack used on an infected horse should be thoroughly disinfected to eliminate a possible source of infection to another member of the herd.
Before rain scald produces visually obvious lesions, the horse owner may feel small bumps along the back and over the rump. While grooming or rubbing down the animal, the bumps may break loose to reveal scabs or irritated patches of hair loss on the skin. Although the inflammation caused by D. congolensis leads to significant skin disease, the horse is not typically itchy or bothered by these infections.
In some cases, rain scald lesions resolve spontaneously in 2 to 4 weeks without treatment. In chronic cases however, smoldering or dormant lesions may seed new sites of infection as environmental conditions become ideal for the bacteria to colonize. A culture and sensitivity test performed in the laboratory is the only method to confirm the causative organism in any bacterial or fungal skin infection. Still, a visual and physical examination along with the horse’s environmental history may lead to a presumptive diagnosis and warrant treatment.
Horses diagnosed with rain scald are treated with antibiotics. A typical protocol includes a single intra-muscular injection of penicillin and streptomycin. Topical antibiotic sprays, washes, and rinses are also often employed to reduce the spread of infection to other parts of the horse’s body and other animals. Scabs should be soaked in betadine or chlorhexidine solution, then gently brushed or rinsed away. The horse should then be towel dried and moved to a clean, well ventilated, dry location. Populations of biting insects must be kept in check to prevent coincidental recurrences of rain scald.
Your veterinarian should always be consulted if you suspect rain scald even though it is typically straightforward to diagnose and treat. Secondary infections are common and can quickly complicate the diagnosis and treatment of skin diseases.