Posted on 17th October 2022 at 12:01
What Is Ringworm?
Ringworm, or dermatophytosis as it is sometimes called, is a fungal infection of the skin surface i.e. the hair and the top layer of skin. Unfortunately, ringworm is endemic across equine populations and therefore occurs very frequently in equine establishments. It can affect horses, ponies, and donkeys alike, and can occur at any age. It may, however, be more likely to occur in immunocompromised individuals such as older horses, ponies, or donkeys, those suffering from Cushing's disease, or in densely stocked youngstock. It can also occur in individuals who have contracted ringworm previously, so owners should not be complacent if their horse, pony, or donkey has had ringworm before – it does not mean they cannot get it again!
Why Should We Worry About Ringworm
Ringworm is of particular concern to horse owners and yard managers due to its highly contagious nature (meaning it will spread rapidly within a group of horses unless appropriate precautions are in place), and also its zoonotic potential, i.e. its ability to infect humans as well as horses, ponies, donkeys, etc. Racehorses cannot enter racecourse premises if they are suffering from ringworm, nor can horses be exported to other countries if they have active ringworm lesions. It may also prevent competition horses from competing in other disciplines if they become infected.
Ringworm can be defined as “self-limiting” which means if left untreated, it will run its course without significant detrimental effect to the individual or individuals concerned. However, during that time (which can be prolonged!) it has the potential to spread considerably, which is inconvenient for all involved; that uncontrolled spread can cause much greater problems than treating individual cases themselves.
What Causes Ringworm?
Ringworm in equines can be caused by several different fungal species:
There needs to be damage to the skin for the fungal infection to take hold, however, this damage can be very minor. Rubbing from the girth, reins rubbing the sides of the neck, or rug rubs on the chest or shoulder region can be all it takes to traumatise the skin and allow the infection to start.
What Does It Look Like?
Initially, the ringworm lesion often has small circular patches of hair that stick up, tuft, or change direction. This then progresses to circular bald areas where the damaged hair is lost from the centre, leaving a reddened, scaling, crusting, or oozing wet surface. Swelling can occur around the edge of the lesions. If the infection is extensive, the lesions may merge, and no longer appear circular. Secondary bacterial infection can also occur, which would result in discharge from the skin. It has to be said that the appearance of ringworm can be very variable, so if in doubt do consult your veterinary surgeon.
How Is A Ringworm Diagnosis Confirmed?
Diagnosis of ringworm is largely made on clinical examination and the visual appearance of the
affected areas of skin. Clinical suspicion may increase depending upon the history provided, as to how the skin problem initially presented. Ringworm does not usually cause itchiness (pruritus), however, occasionally it can, which sometimes confuses matters!
Tests which may be performed to confirm or rule out ringworm include:
1. Hair pluck samples
2. Skin scrape samples
3. Skin biopsy samples
How can these samples be tested?
1. Microscopy – this involves looking under a microscope for fungal spores in a hair pluck or skin
2. PCR testing– used to check for the presence of ringworm DNA in hair or skin samples
3. Culture – this can take up to 3 - 4 weeks, so has become less useful since quicker PCR testing became available, but can identify the species of ringworm involved in a particular case should this information be required.
4. Histology – the analysis of a tissue biopsy under a microscope to look for fungal spores within hair follicles
How Is Ringworm Spread?
It can be spread either via direct contact (for example between two horses who are sharing the same field or barn, whose bodies will touch each other when they are interacting with each other) or indirectly through the environment or via fomites (objects that can carry infection) – fencing, vehicles used for equine transport, stable doors, haynets, haybars, buckets, rags, mucking out equipment, tack, saddle cloths, girths, grooming brushes, sponges, clipper blades, contaminated clothing, etc. This indirect spread can happen easily because fungal spores are very resistant and may survive for long periods in the environment or on contaminated surfaces.
Microsporum equinum and Microsporum gypseum can also be transmitted between horses, ponies, and
donkeys by biting flies.
When Should We Be Particularly On The Lookout For Ringworm?
Your horse is at greater risk of ringworm if they have mixed with unknown equines who they would not normally have contact with, for example at a competition or other equine gathering. Another common time for ringworm to appear is with the arrival of new horses to the yard. As ever, a period of 2 - 3 weeks quarantine should be observed for all new horses or ponies arriving in a yard, as this is the minimum incubation period for ringworm; ringworm may not appear until the end of this time 2 - 3 weeks, and therefore the isolation period should not be shortened.
What Should Be Used To Treat Ringworm?
Nothing? This is not advised. As outlined above, although the infection can be self - limiting over 5 - 10 weeks, if an individual is knowingly left untreated, there may be considerable direct spread of ringworm spores to other equines or indirect spread via the environment. This can result in a high risk of an outbreak and an increased likelihood of spread to human handlers.
Topical antifungal agents, such as the VetGard product – these should be used to treat all affected areas and will reduce spread to other areas of the affected individual and reduce spread to any in - contact animals. These topical agents should be used as per the manufacturer's guidelines and owners should monitor the response – if the lesions do not resolve or any unexpected side effects occur owners should contact their veterinary surgeon for advice.
Systemic antifungal drugs – these are not commonly used in equines due to a lack of scientific evidence to support their use, their high cost, and slow effect.
Environmental decontamination – this is arguably the most important and most difficult part of managing a ringworm case or outbreak. All surfaces and other fomites as listed above need to be treated with a multi purpose disinfectant, such as the Aqueos product, or with a fogger, such as the VetGard system, known to be effective against ringworm to remove fungal spores.
Any skin material, scabs, or hair from affected individuals should be burnt if possible.
How Can The Risk Of Zoonosis Be Reduced?
Gloves should always be worn when treating an infected individual, or when cleaning or disinfecting the environment or fomites. People in close contact with affected equines should monitor themselves in case they develop any skin lesions suspicious of ringworm. If they are concerned, they should consult a doctor or pharmacist.
DR JESSICA PUTNAM BV MEDSCI(HONS) BVM BVS (HONS) MRCVS. AUGUST 2022
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