Dermatology

The Many Faces of Pastern Dermatitis

By Paul Basilio

Pastern dermatitis is one of the most common dermatologic presentations in equine medicine. It is so common that it comprises 38 of the most common dermatologic presentations in equine medicine.

Also known as mud fever, greasy heel, dew poisoning or simply “scratches,” many of the potential causes of pastern dermatitis share a similar clinical presentation. To diagnose the right cause of pastern dermatitis, said Richard Coppack, BVSc, CertAVP (Vet. Dermatology), MRCVS, of the Equine Veterinary Centre in Doncaster, England, you need time and a structured approach.

“You might need to spend 15 to 25 minutes taking a detailed clinical history,” he said during a presentation at the 2024 BEVA Congress in Liverpool, England. “Then you do a clinical examination of the horse that involves the skin and the rest of the body, and then you take some preliminary samples.”

After all of that, these diagnoses can usually be narrowed down to 4 or 5 differentials, which can then be used to guide additional testing, if necessary, to arrive at the final diagnosis.

Time and Samples
“When I’m with the horse, I’ll do the clinical examination,” he explained. “I’ll take some photos of the horse and make note of the type and distribution of lesions. Then I’ll take preliminary samples and a brief history. Later, often at the end of the day, I will call the client and get a detailed history from them. That can take another 15 to 20 minutes. Then I’ll do research and get down to my short list of differentials.”

Dr. Coppack’s preliminary samples typically include a swab or a brushing of the lesion. If it’s a dry lesion, he will moisten a swab with sterile water, roll it across the lesion, and then transfer the specimen to a glass slide for cytology. The swab is then sent for culture and susceptibility testing. For brushings, he will use a toothbrush and a plastic dish to collect the specimen, then transfer it to a container for microscopic examination later. He’ll also draw some blood samples for testing.

Biopsies are particularly useful in horses with nodules or swelling, and Dr. Coppack explained that he typically sedates horses prior to the biopsy procedure. “After sedation, I will administer a nerve block in the abaxial sesamoid, or even a 4-point nerve block. I’ll even put some local anesthetic just under the biopsy site to make sure that the horse doesn’t feel it. It’s a bad place to stick a biopsy punch unless you’ve got total analgesia,” he said.

Many of the horses will be in a significant amount of pain on presentation, and some will not allow the clinician to touch the affected limb. Anti-inflammatories—either topical or parenteral—can provide relief.

Chorioptic Mange
Healthy equine skin is quite resistant to infection, so primary bacterial infections are rare. If a horse is presenting with an infected pastern, the infection could be occurring secondary to some common issues.

Horses with pruritus, scabbing and folds of thickened skin caused by chronic inflammation, for example, may have chorioptic mange caused by Chorioptes bovis.

“These horses commonly have long hair—or ‘feathers’—on their legs, and they’re itchy,” he said. “They’ll rub against anything they can get a hold of, and the lesions are usually due to self-trauma.”

In chronic cases, the skin will be markedly enlarged, and skin folds will become warm and moist—a perfect place for bacteria to thrive. Intertrigo, or infection of the folds, is common.

Treatment is aimed at controlling the mites both on and off the horse and managing the secondary infection. Regular exercise and a clean environment will help with treatment and prevention of re-infestation. Clipping the hair short will encourage air circulation and reduce the likelihood of intertrigo, but some horses may not tolerate clipping due to pain in the area.

Summer Pastern Dermatitis
Photo-activated leukocytoclastic vasculitis—or summer pastern dermatitis—is increasing in prevalence. Characteristic lesions are found mostly on the lateral and medial aspects of limbs with non-pigmented skin. Hind limbs are more often affected vs. front limbs. It’s only when the secondary infection develops that the plantar aspects are affected, he explained.

“One of the theories at the moment is that it’s associated with exposure to buttercups, which are known to cause problems in some humans,” Dr. Coppack said. “The thought is a molecule in the plant called ranunculin is absorbed into the skin, it binds with an antibody, and the antigen-antibody complex adheres to the lining of the artery and causes a vasculitis.”

Treatment involves turning the horse out at night rather than during the day to avoid ultraviolet light, administration of parental or topical corticosteroids, and antibacterial agents for secondary infections.

Mud Fever
“Mud fever,” also known as winter pastern dermatitis, is a complex of bacterial infections that arises due to wet and muddy environments. This condition also primarily occurs on the hind legs, and its more common on non-pigmented skin. Treatment includes strong and effective control of the secondary infection.

“You need to swab, and you treat according to the results,” Dr. Coppack said. MeV

A Note About Shampoo
The use of warm or hot water with antibacterial shampoo is best, as it can open the hair follicles and allow for better penetration of the antibacterial agent, Dr. Richard Coppack said. Also, leave the shampoo on for a bit longer than you would in your own shower.

“These shampoos need about 10 minutes,” he said. “Put the shampoo on, go have a cup of tea, and then come back and wash it off. Allow the shampoo to work.”

While medicated shampoos are useful, Dr. Coppack also said, they can be overused. When used properly, twice weekly applications should be sufficient. If the horse is responding to treatment, you can apply it less often.

Primary Lesions
To narrow the potential diagnoses, the first step is to identify the primary lesion type.
• Alopecia
• Crusting/scaling
• Erosions
• Nodules/swelling
• Pigmentary abnormalities
• Pruritus
• Ulcers

Sometimes, Nothing Is Best
Over-treatment may be occurring more than one might think. To illustrate, Dr. Richard Coppack described a situation in which an owner was treating the horse’s dermatitis for 5 months. After taking the horse’s history, Dr. Coppack suggested watchful waiting instead of additional treatment. After 17 days, the horse had healed. The initial cause of the dermatitis had passed, and the owner was perpetuating it by over-treatment.

“If you are not getting an improvement in a case within 3 to 4 weeks, re-evaluate the diagnosis,” Dr. Coppack said. “Go back to the start and see if you can identify the underlying cause.”