Dog - Ringworm (Dermatophytosis)

What is ringworm, and what causes it?

Ringworm is a skin disease caused by a microscopic fungal organism. It is zoonotic, i.e. it affects us as well as dogs, cats and other animals. Because in man the lesions are frequently circular, the name came from the fact that it was once thought to be caused by a worm curling up in the tissue.

There are several fungal species which can cause the disease that we call ringworm. These may also affect humans. The fungi live in hair follicles and cause the hair shafts to become brittle and break off at skin level which results in discrete patches of hair loss. As the fungus multiplies, the lesions may spread over the entire body.

What is the incubation period?

From first contact with the causative fungus it is about 10-12 days before any ringworm lesions are seen.

How is it diagnosed?

Diagnosis depends upon:

1. History and identification of suspicious lesions on the skin.
2. Wood’s Lamp examination. This involves the use of a special ultraviolet light which will cause some infected hairs to fluoresce. Unfortunately not all species of fungi do fluoresce under Wood’s Lamp.
3. Hair culture. This is the most accurate. It involves laboratory tests which may take up to 12 days to complete.

How is it transmitted?

Transmission occurs by direct contact between infected and non-infected individuals. It may be passed from dogs to cats and visa versa and also from infected dogs or cats to people (and visa versa!). If your child has ringworm, it may have been transmitted from your pet or from another child at school. Adult humans usually are resistant to infection unless there is a break in the skin (a scratch, etc.) but children are more susceptible.

If you or any members of your family have suspicious skin lesions, consult your GP without delay.

Transmission may also occur due to an infected environment, or infected utensils. This so-called 'fomite transmission' is not uncommon when grooming utensils are shared among multiple pets, one of which is infected.

Can the fungus be killed with disinfectants?

The fungal spores can be easily eradicated with a dilute solution of chlorine bleach or some of the DEFRA approved disinfectants that are effective against ringworm spores. Please contact us if you require advice.

What is the treatment?

Several types of treatment are available. The choice depends on individual circumstances, i.e. the severity of the infection, the number of pets are involved, if there are children in the household, and the difficulty of effective disinfection of your pet’s environment.

Treatment can involve:

1. Shaving the pet’s hair. This removes most of the infective hair and although a drastic procedure, has to be undertaken if the infection is extensive particularly in full coated dogs and cats.
2. Lime Suphur Dip. This is the traditional treatment for ringworm. The usual regime involves twice weekly dips/baths for the first two weeks then once a week for 4–6 weeks. Other in contact animals should also receive the treatment at least once. If any in contact animals subsequently develop lesions, this treatment often has to be combined with others, e.g. Griseofulvin. Lime sulphur does have certain disadvantages. It has an objectionable odour, it tarnishes jewellery and can be irritant to sensitive skin. Wear gloves if using lime sulphur.
3. Topical antifungal medication. These products are usually ointments, creams and lotions, applied daily to the affected areas for about ten days. They are useful for dogs with minimal lesions.
4. Antifungal shampoos. These all involve multiple baths, the frequency of which depends on the product. Unaffected pets should also be bathed in order to reduce the number of spores carried in the hair coats.
5. Griseofulvin. This is an oral drug administered usually as tablets. Treatment has to be given for a minimum of 30 days and often considerably longer and during medication the patient must have a certain minimal fat intake in the food in order to ensure absorption of the drug.

How soon will I see results?

None of the presently available treatments will produce immediate results. The areas of hair loss will get larger during treatment but within approximately 2 weeks, the hair loss should decrease and the crusty sore appearance of the skin should subside. Hair regrowth depends on the type of coat and is sometimes very slow, taking months in some cases. During recovery we will, of course, have to check your pet from time to time.

How long will my dog be contagious once treatment has commenced?

If treatment directions are diligently observed, most dogs can be considered no longer infectious after approximately 3–4 weeks. This however can be much longer if only minimal treatment measures are undertaken. It is prudent to minimise exposure of other dogs and cats and family members during the first two or three weeks of treatment.

Is it correct that sometimes ringworm cannot be cured?

If treatment advice is carefully observed most cases of ringworm in dogs are resolved satisfactory. However if the dog is elderly or has severe immune mediated problems, total cure may not be possible.