Rain scald is a relatively common skin infection caused by the bacteria Dermatophilus congolensis. The infection often affects the skin on the back and rump of horses.

Rain scald can develop when skin is exposed to excessive moisture which weakens and damages the skin. Once the skin barrier is compromised, Dermatophilus congolensis enters and spreads in the top layer of the skin.

In mild cases, the most common symptoms are dry and scaly skin. Advanced cases involve the development and spread of crusty lesions and scabs.

Prompt treatment is required to stop the spread of rain scald. Treatment involves cleansing the skin with an antiseptic wash and applying topical antibacterial agents. In severe cases of the condition, systemic antibiotics may be administered.

Key management strategies to prevent rain scald include reducing exposure to moisture by providing proper shelter and blanketing your horse appropriately. Regular grooming and good nutrition also help to prevent the condition.

What is Rain Scald

A relatively common skin condition first identified in cattle in Africa, rain scald is also referred to as rain rot, mud fever, dermatophilosis, and streptothricosis.

Rain scald is a type of infection (dermatitis) caused by the bacteria Dermatophilus congolensis. [1][2][3] This pathogenic bacteria lives on the skin in a dormant statem but can cause an inflammatory infection if activated.

When skin is exposed to moisture, the bacteria can replicate and spread in the top layer of the skin (epidermis) by causing infectious lesions.

Rain scald can cause lesions to form on the back, rump and sometimes on the face of affected horses. Dermatophilus congolensis is sometimes present in the skin of horses that have mud fever lesions on their pasterns. [6]

Rain scald is contagious through direct contact and through sharing of blankets, tack, brushes or other equipment. Horses living together may also contract the infection at the same time because they are exposed to the same environmental conditions.

Risk of Infection in Horses

Horses affected by rain scald often have cracked, damaged skin that facilitates the entry of additional types of microorganisms such as Staphylococcus. When additional microorganisms enter the skin, they can cause secondary infections.

Because Dermatophilus congolensis thrives in damp conditions, skin infections caused by this type of bacteria are most common during the wettest seasons of the year.

Rain scald affects horses around the globe, especially those living in areas where it is humid and when rain is abundant and frequent. [4][5]

Skin infection with Dermatophilus congolensis affects horses of all breeds and ages. Horses with lowered immunity or poorly developed immune systems also have an increased risk.

Horses affected do not become immune to dermatophilosis infection once they have had rain scald. The infection can reoccur again in the future, especially in wet and muddy conditions.

The spores of Dermatophilus congolensis can remain infective for months if present on skin, hair, and dried scabs from healed lesions. These spores can infect new animals through direct contact or can reinfect the original host. Biting insects can also transmit the infection by spreading the bacteria’s spores to other animals.

Signs of Rain Scald

The bacterial infection caused by Dermatophilus congolensis can vary from mild to severe. If left untreated, the condition can spread along the skin covering an increasingly larger area over time.

In mild cases, the infection may appear as dry and scaly skin. Crusty lesions and scabs develop as the condition progresses to a more advanced state.

The infection results in the discharge of a sticky secretion that causes the hair to matt together and leads to the development of crusty scabs of varying sizes. [7] These scabs have attached hairs that stand up like the bristles on a paintbrush. Hair loss occurs in areas where the scabs have come detached from the skin.

Underneath scabs, the skin may be raw and ooze yellow exudate (pus). In serious cases of rain scald, large areas of skin can become covered in hardened scabs and the skin is left raw in areas where they have come off.

Risk Factors

Environmental

Horses living in humid and tropical climates where rainfall occurs often have a greater risk of rain scald. Excessive exposure to rain weakens the skin’s protective barrier leaving it susceptible to bacterial infection with Dermatophilus congolensis, an opportunistic microorganism with a complex lifecycle.

Skin Damage

Any physical trauma to the skin leaves it vulnerable to infection with Dermatophilus congolensis. Bug bites can cause damage to the skin and increase infection risk by facilitating an opening for bacteria to enter the skin.

Rough plants can scratch, cut, and puncture the skin, also leaving it prone to infection by microorganisms including Dermatophilus congolensis.

Poor Hygiene

Inadequate grooming promotes unhealthy skin that becomes prone to infection. Proper grooming is necessary to allow the pores to breathe by removing dirt from the skin to reduce the accumulation of microorganisms in the hair and on the skin.

The bacteria that cause rain scald are easily transmitted from horse to horse through contact with one another. Using shared tack and grooming tools on horses can facilitate the spread of bacteria between horses.

Lack of Skin Pigment

Some types of dermatitis are more common in horses with unpigmented skin or light coat colours. [16]

Impaired Immunity

Rain scald is more likely to affect horses with poor immunity due to stress, poor nutrition, or health conditions such as pituitary pars intermedia dysfunction. [ren n=”8″] Horses that are at a very young or advanced age are also at a higher risk of infection if their immunity is less than optimal.

Diagnosis

Identifying rain scald and initiating treatment as soon as possible is important as the condition can worsen quickly causing pain and hair loss. Prompt treatment of the condition may also avoid the development of secondary skin infections.

Consult your veterinarian to provide a diagnosis. This condition causes clinical signs that are similar to other skin diseases, including allergic dermatitis and parasitic infections. To diagnose rain scald, your veterinarian will assess your horse’s overall health condition and complete diagnostic testing.

Testing may include microscopic examination of scabs and/or taking impression smears from any moist or greasy skin surface to identify the presence of microorganisms. Skin biopsies may be conducted to identify the cause of the infection and if it is rain scald.

Treatment

Depending on the severity of the infection, many cases of rain scald subside in two to three weeks and the wounds heal without scarring. Avoid covering affected areas of skin with tack or equipment, since pressure can cause pain.

Rain scald may prevent you from riding your horse for up to several weeks in severe cases. Pressure from tack or other equipment may also cause the wounds to reopen or expose them to dirt and debris.

A veterinarian will provide appropriate treatment recommendations for your horse. Common treatment strategies include the following.

Management

Gently groom the area of affected skin to remove loose hair. Clipping the hair away from affected areas may be beneficial for some horses. Removing hair allows the skin to dry more easily and may slow the rate at which the infection spreads.

Horses should be kept in a dry environment while they are being treated for rain scald. Keeping the skin clean and dry will aid in the healing process and reduce recovery time.

If the horse must be turned out and the skin will be exposed to moisture, a waterproofing agent should be applied to cleansed skin in advance. An application of zinc oxide cream, Vaseline, or an antibiotic ointment with a petroleum base can be used to create a waterproof barrier on the skin.

Covering the horse with a waterproof and breathable blanket/rug is also recommended to protect the skin from moisture.

Cleansing the Skin

Areas of skin affected by rain scald should be gently cleaned with an antiseptic wash, spray, or powder. Gently washing helps prevent the spread of the infection and loosens scabs so they come off easily.

It may take several days of treating the affected area(s) of skin to remove the scabs. Avoid removing scabs by using force as this is painful for the horse and may promote further infection.

The skin should be dried completely before applying any topical preparations.

Topical Medications

The most common topical treatments for rain scald are disinfectant washes containing povidone-iodine or chlorhexidine.

Topical antibiotic creams and ointments are also used to treat the condition. Silver Sulfadiazine (Flamazine) is an antibacterial cream that can be applied daily.

Systemic Drugs

Analgesics:

Your horse may require medication for pain management if he/she has a severe case of rain scald. Drugs commonly used to relieve pain include phenylbutazone (Bute) and flunixin meglumine (Banamine).

Antibiotics:

Oral or injectable antibiotics may be prescribed to treat rain scald. Common types of antibiotics used for treating the condition include oxytetracycline [9], sulfadiazine, sulfamethoxazole, trimethoprim, procaine penicillin, gentamicin, enrofloxacin, and ceftiofur.

Follow-Up

Monitor how lesions and abrasions are healing. Contact your veterinarian if healing is particularly slow or the skin condition worsens.

Prevention

Raid scald is a problematic skin condition that can often be prevented. Consider the following management strategies to reduce the risk of rain scald.

Provide Shelter or Keep Your Horse Stabled Temporarily:

Horses require adequate shelter to protect them from the elements. When the skin is constantly wet, the risk of infection with the bacteria that causes rain scald increases.

If your horse is turned out, give them shelter so they can take cover and ensure their coat can dry out during rainy periods. Alternatively, keep your horse in the stable to let their coat dry.

Blanket Properly:

If your horse is turned out in wet or snowy weather, blanket him/her with a waterproof rug to protect the skin from prolonged moisture which can promote infection.

Make sure your horse isn’t sweating under the blanket he/she is wearing. If the weather is warm and dry, remove/change the blanket so the skin can breathe and dry out.

Avoid blanketing your horse immediately following a workout, bath or exposure to wet weather. To help prevent rain scald, ensure your horse is completely dry before covering him/her with sheets or blankets.

Assess Skin Health Regularly:

Do a complete inspection of your horse’s skin and coat condition at least every other day. Check for any abrasions or skin injuries that require treatment.

Practice Good Hygiene:

Your horse’s coat should be brushed regularly. Keeping your horse’s coat and skin clean and free from dirt and debris helps prevent the hair from matting and reduces the risk of infection.

The bacteria that cause rain scald can be transferred from one animal to another and to humans through direct contact. [10][11] All equipment that contacts the skin of an affected horse should be disinfected and not shared with other horses.

Wash your hands thoroughly after touching the skin of affected horses to avoid spreading the bacteria.

Use Insect Repellent:

Use insect repellent to protect your horse’s skin from bug bites that could promote him to scratch his skin and potentially allow bacteria to enter.

Nutrition:

Proper nutrition helps reduce the risk of skin infections by supporting the immune system and promoting the maintenance and repair of healthy skin.

Feed your horse a balanced diet that includes adequate zinc and vitamin E – these nutrients are vital for proper immune function and the formation of proteins that form the skin barrier. [12][13] Feeding probiotics and yeast can also promote immune function and support gut health. [15]

Supplementing your horse’s diet with an oil that is high in omega 3 fatty acids can also promote skin health and provide natural anti-inflammatory properties. [14]

Is Your Horse's Diet Missing Anything?

Identify gaps in your horse's nutrition program to optimize their well-being.

References

  1. Nielsen, Soren S. et al. Assessment of animal diseases caused by bacteria resistant to antimicrobials: Horses. EFSA J. 2021.
  2. Larrasa, J. et al. A simple random amplified polymorphic DNA genotyping method for field isolates of Dermatophilus congolensis. J Vet Med B Infect Dis Vet Public Health. 2002.
  3. Rosser, E.J. Infectious crusting dermatoses. Review Vet Clin North Am Equine Pract. 1995.
  4. Branford, I. et al. Comprehensive Molecular Dissection of Dermatophilus congolensis Genome and First Observation of tet(Z) Tetracycline Resistance. Int J Mol Sci. 2021.
  5. Ambrose, NC. Et al. Preliminary characterisation of extracellular serine proteases of Dermatophilus congolensis isolates from cattle, sheep and horses. Vet Microbiol. 1998.
  6. Aufox, EE. et al. The prevalence of Dermatophilus congolensis in horses with pastern dermatitis using PCR to diagnose infection in a population of horses in southern USA. Vet Dermatol. 2018.
  7. Burd, E.M. et al. Pustular dermatitis caused by Dermatophilus congolensis. J Clin Microbiol. 2007.
  8. Miller, AB. et al. Effects of pituitary pars intermedia dysfunction and Prascend (pergolide tablets) treatment on endocrine and immune function in horses. Domest Anim Endocrinol. 2021.
  9. Awad, W.S. et al. Diagnosis and Treatment of Bovine, Ovine and Equine Dermatophilosis. J Appl Sci Res. 2007.
  10. Moriello, K.A. Dermatophilosis in Animals. Merck Veterinary Manual. 2020.
  11. Kaminski, G.W. Et al. Human infection with Dermatophilus congolensis. Med J Aust. 1976.
  12. Stafford, Sian L. et al. Metal ions in macrophage antimicrobial pathways: emerging roles for zinc and copper. Biosci Rep. 2013.
  13. Petersson, KH. et al. The Influence of Vitamin E on Immune Function and Response to Vaccination in Older Horses. J Anim Sci. 2010.
  14. Goh, YM. et al. Plasma n-3 and n-6 fatty acid profiles and their correlations to hair coat scores in horses kept under Malaysian conditions. J Vet Malaysia. 2004.
  15. Furr, M. Orally Administered Pediococcus acidilactici and Saccharomyces boulardii–Based Probiotics Alter Select Equine Immune Function Parameters. Equine Vet J. 2014.
  16. Federici, M. et al. Assoziation zwischen Hautgesundheit und Fellfarbe sowie weissen Abzeichen
    bei dreijährigen Freibergerpferden
    . Schweiz Arch Tierheilkd. 2015.