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Streptothricosis, also known as rain rot or rain scald, is a skin infection that is common in horses. The organism that is responsible for the condition is dermatophilus congolensis, which is present and multiplies in a warm, damp environment where temperature and humidity are high.
Fortunately, the scabs produced by the condition tend to not be itchy. The condition is not life-threatening, though can be challenging to eliminate.
Caused by the organism dermatophilus congolensis, streptothricosis is a common skin condition in horses that leads to the development of large, crusty scabs.
In the early stages of streptothricosis you may feel small lumps on the skin or hair of your horse when you stroke his coat. The condition can present as big, crusty scabs or little matted tufts of hair around ¼ inch in size. Often there are dozens of very small scabs that are entrenched in the hair of your horse. The lesions can grow larger and join together. These scabs can typically be scraped off easily, and once they are removed you will often notice pink skin with pus, which will become gray and dry when healed.
Typically, the condition is seen on the back, rump and down the legs of the horse. It can also be seen on the tips of your horse’s ears, along his mouth and near his eyes.
Streptothricosis can often lead to a secondary infection like staphylococcus, streptococcus and rhodococcus, which will require treatment by antibiotics. These infections can be challenging to eliminate. Treatment of streptothricosis should occur as soon as possible to avoid the development of a secondary infection.
Streptothricosis is caused by the organism dermatophilus congolensis. The organism is an actinomycetes, which behaves like bacteria and fungi. It is thought, though not proven, that the organism is found in soil. When conditions are not clean, they support the growth of this organism. Horses can be carriers of the organism in their skin and can pass it along to other horses even if they are not affected by it (perhaps having a natural immunity). The organism can also be passed through blankets, brushes, saddles, halters, leg wraps and other items that have the organism on them. There needs to be open skin present for the organism to enter in order for the condition to develop.
Certain horses seem more susceptible to the organism; chestnuts, grays and those with a thick coat (the coat keeps moisture close to the skin of the horse).
Diagnosing streptothricosis is not difficult. Your veterinarian will conduct a physical examination of your horse and can usually diagnose the condition based on viewing the skin lesions. Diagnosis of the condition can be confirmed by scraping skin from the lesions and viewing it under a microscope for Gram-positive bacteria.
Streptothricosis can heal on its own, though it is recommended that you treat your horse so that the lesions don’t spread and grow and so a secondary condition does not develop.
In minor cases, the scabs can be removed through a bath with antimicrobial shampoos and rinses to resolve the problem. Products that include chlorhexidine are effective for eliminating the organism. When scraping the scabs, do so gently in order to cause minimal pain for your horse. Horses are typically not enthusiastic about the scrubbing and will resist.
When the infection has progressed to deeper layers of your horse’s skin, it may require that your horse receive an injection of an antibiotic like procaine penicillin or streptomycin. It is a good idea to avoid ointments as they will hold moisture in, when in order to resolve the issue you will want to keep the lesions dry.
You will also want to disinfect all equipment, including blankets, that your horse has been in contact with in order to prevent the spread of the organism. The organism responsible for streptothricosis does not like oxygen; therefore, the more oxygen it gets the more quickly it is eliminated.
When treating streptothricosis in your horse, it is a good idea to communicate with your veterinarian about what will work best for your horse as well as for disinfecting any equipment that your horse has had contact with. It is important to work closely with your veterinarian and attend follow up appointments as necessary to ensure that the condition is resolved and that any secondary infection that has developed is treated appropriately.
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