Two common issues to be on the lookout for in our horses during the wetter months are rain scald and ringworm – two conditions that can be confused with each other but require very different treatments.
Coming into the wet weather, horses are very prone to contracting skin ailments with wet skin more susceptible to penetration by bacteria. Often, long-hair coats stay wet for longer and hide small lesions that fester and become large, painful infections requiring treatment.
Coming into the wet weather, horses are very prone to contracting skin ailments with wet skin more susceptible to penetration by bacteria.
Adding to this is the practice of rugging horses when they are still wet, allowing bacteria to thrive in moist, warm environments. In some of these cases where the horses are rugged wet, lesions are not identified until the rugs are removed days to weeks later, revealing large areas of crusty, scabby skin and hair loss.
Two of the skin lesions we often need to differentiate are rain scald and ringworm, as treatment varies between the two.
Rain scald, also referred to as mycotic dermatitis, is a skin disease caused by a bacteria called Dermatophilus congolensis. These bacteria can survive and thrive in environments with or without oxygen, and so can develop in both exposed sites and areas covered by rugs that limit the available oxygen. When the skin is wet, it is susceptible to microdamage, allowing D. congolensis to readily enter through these minuscule skin lesions, infecting the skin cells and producing an exudative dermatitis. An exudative dermatitis is one in which fluid seeps out from damaged cells onto the skin and this exudate then causes further irritation and inflammation to the surrounding cells.
Rain scald is often found along the back and over the rump of paddock horses, as these are the sites most likely to become wet when the horse is not rugged and outside in the rain. It can also occur on the head and neck, and the bacteria are common contributors to pastern dermatitis, commonly known as “greasy heel” or “mud fever”. When infected back and rump areas are viewed from a distance, the lesions taper as they go distally, following a pattern that mimics the flow of water as would occur if poured onto the topline and allowed to run down the side. Pastern dermatitis is a frequently annoying problem for horses (and their owners!) during winter and early spring, as it is difficult to keep the pasterns clean and dry when horses are housed in wet and muddy paddocks.
Rain scald is usually painful for the horse, as the exudate that seeps from the skin causes more irritation to the cells that then causes further exudate to be produced. This results in a continuous cycle of damage to the skin. The exudate then dries in and around the hair follicles, and when the scabs are removed the hair is also removed, leaving behind areas of inflamed, reddened and hairless skin. If the scab is plucked off, a pus-like substance can be seen on the underside of the scab.
FUNGAL INFECTION
Ringworm is caused by a fungal infection and is not a bacterial infection like rain scald. Image by Maxine Brain.
“There are several different
fungi that cause ringworm.”
Ringworm is caused by a fungal infection and is not a bacterial infection like rain scald. There are several different fungi that cause ringworm, typically from Trichophyton spp and Microsporum spp, with Trichophyton equinum being the most common in horses. It can occur year-round, but as most fungi require wet and warm conditions to thrive it is more commonly seen in Autumn and Winter. Although it can occur in horses of any age, it is more common in young horses (< 2 years old) and older horses that are immunocompromised and/or in poor condition. This is because most horses will develop an immunity to the fungus and show only mild, self-limiting lesions if exposed as a healthy adult. It is very contagious in young horses and in those horses undergoing “stressful” situations, so is seen very commonly when young horses are brought together during weaning and breaking-in periods.
The fungi can survive for months and even years in the hair shafts that are expelled from the skin, and in the scaley skin cells that are shed. It can also survive just as long on objects such as brushes, combs, horse gear and wooden fence posts, making it very difficult to eradicate it from properties. Although the initial presentation may appear like rain scald in that there are areas of hairlessness, there are some key differences.
SELDOM PAINFUL
Unlike rain scald, ringworm is seldom painful unless the horse has had puritis (itchiness) that has resulted in self-inflicted trauma to the skin. The skin lesions are rarely inflamed and often appear as grey, scaley circular skin areas, as the fungi invade the hair follicles and do not cause any exudate to weep from the epidermal cells. It would be rare to see any pus-like substance unless there has been a secondary infection (usually by an opportunistic bacteria).
“Unlike rain scald, ringworm
is seldom painful…”
As it is often transmitted by contaminated gear, it can be common for large numbers of horses to have the ringworm in a similar anatomical position, and this should alert the owner to the offending piece of equipment that is contaminated. For instance, infected reins will result in outbreaks occurring on the neck, saddle pads on the flanks and head collars around the ears or face. I once had a client ring and tell me that they had a strange infection that was only affecting the left fore (LF) leg of their horses. This was in fact a case of ringworm that was being spread by a contaminated LF tendon boot.
This horse has a mix of skin ailments, one that looks like ringworm and a second skin lesion that could be a secondary infection. Further laboratory tests could be required.
Ringworm is often transmitted by contaminated gear.
Differentiating between rain scald and ringworm is easy in many cases but can be difficult when secondary factors become involved and complicate the initial presentation. In cases where the clinical signs are not well demarcated, or don’t show the classic presentation, laboratory tests can be used to acquire a correct diagnosis.
Submitting hair that has been plucked from the edges of a ringworm can allow the laboratory to identify fungal hyphae or spores attached to the hair shaft. A PCR (polymerase chain reaction) test can also be used to detect ringworm in horses and relies on submission of hair follicles and hair shafts that can be assessed for a provisional diagnosis within 1-3 days. The most reliable way to confirm ringworm and identify the causative fungus is to culture the hair from and around a lesion. The downside to this is the time it takes for a culture to grow and can be as long as three weeks before a diagnosis is confirmed.
With rain scald, the causative bacteria can often be seen when the crusts are examined under a microscope. Cultures of the crusts can be performed, although sometimes requiring more complex methods to grow and isolate the bacteria, and gives an answer in 48-72 hours, which is more time-effective than the fungal cultures. A real-time PCR test is also available to confirm Dermatophilus congolensis.
Ringworm around the ear. Image by Maxine Brain.
TOPICAL WASHES & MEDICATION
Treatment for these two skin lesions relies predominantly on topical washes and medication, although both can be treated with systemic medications if they are chronic (ongoing) and non-responsive to topical medications. Rain scald is usually responsive to keeping the horse dry and well groomed, and removal of the crusts and matted hair is often all that is required.
Medicated skin washes containing chlorhexidine, iodophors or lime sulphur can be used daily, especially those that aid in the removal of scabs, to hasten the resolution of the infection. It is important that these horses are not left with wet coats as keeping the coat dry is paramount in resolving these infections. In severe cases, especially those where the horse is too painful to tolerate topical treatment (applied directly to the skin), systemic antibiotics can be used to kill the bacteria and reduce the amount of exudate being produced.
Ringworm around the girth. Image by Maxine Brain.
Cases of pastern dermatitis can be more difficult to cure as these infections commonly become complicated by additional bacteria and fungal elements invading the damaged areas. Such infections often require the use of additional topical medications to resolve the secondary invaders. In cases where the horse is in poor health, malnourished or housed in unhygienic environments, management and general husbandry requirements need to be addressed to help the horse to recover. This involves ensuring good nutrition, a dry and clean environment, and treatment for any underlying illness, including the removal of any parasites (lice, biting flies and mosquitoes) that damage the skin and allow further infiltration of the D. congolensis bacteria.
Horses affected with rain scald do not develop significant immunity to stop them becoming re-infected with D. congolensis if subjected to the same wet conditions that caused previous infections. Contrary to this, ringworm can resolve without any treatment in about 3-4 months if the horse is healthy and not in a situation of high stress. These horses may develop a resistance to the ringworm fungus that lasts for years, and this resistance can then reduce or prevent that horse becoming re-infected with ringworm if it remains in the same environment.
CONTAMINATION RISKS
However, leaving a horse to recover without treatment only increases the risk of further contamination to the environment and infection to other horses, which is not desirable. Therefore, the mainstay of treatment for ringworm is usually topical treatment, using an antifungal agent. For small areas, this can often be achieved with creams or ointments, but in cases where the horse has large areas of skin involved, washes that can treat large areas of the horse are far more effective.
Austrazole would be the most used topical fungicide available here in Australia and has replaced the previously available imaverol. The main ingredient in the product is enilconazole and comes as a concentrate that requires dilution (1:50) before being applied to the horse. The horse should be treated three times at three-day intervals and the rinse should be allowed to dry on the skin. All in-contact horses should also be washed to minimise the spread of the disease and the environment and any tack, brushes and equipment also disinfected.
Whilst enilconazole can be used to wash gear and decontaminate the environment, products such as bleach and Virkon (a multi-purpose disinfectant) can also be used at a lower cost. Other treatments that can also be used topically include chlorhexidine, lime sulphur, miconazole, clotrimazole, and nystatin. In situations where ringworm is persistent in the environment, removal and destruction of items such as bedding, towels, rugs, combs and brushes may be necessary to minimise the potential for contamination should new horses arrive on the property.
In cases where the horse has large areas of skin involved, washes that can treat large areas of the horse are far more effective. Image by Maxine Brain.
There are systemic medications such as griseofulvin, ketoconazole and itraconazole that have been suggested to treat horses with ringworm, but in my experience are of limited value, or too expensive for routine use. I have, however, commonly used systemic (intravenous) sodium iodide (trade name Sodide) to treat ringworm infections and had a lot of success in resolving these cases. It should be noted, however, that this product is extremely irritant if administered outside the vein and should only be administered by a veterinarian using a catheter.
Thus, my advice for winter is to stay on the lookout for both rain scald and ringworm, and frequently check the skin of horses that are exposed to the elements. For horses that are rugged, take the time to check beneath the rug at least once a week, as this can help prevent any small skin problems inadvertently becoming bigger ones.
Hopefully, this brief description of these two skin conditions means if your horse ends up with either one you can differentiate one from another and know the most effective and appropriate way of dealing with it. EQ
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