Ringworm is a skin disease caused by a fungus (plural is fungi).  Because the lesions are often circular, ringworm was once thought to be caused by a worm curling up in the tissue.  We now know ringworm has nothing to do with worms but rather a skin disease caused by a fungal infection.

The causes of ringworm

The correct word for a ringworm infection is dermatophytosis.  There are four species of fungi that will cause dermatophytosis in cats and dogs.  In cats, the most common species causing the problem is Microsporum canis.  In fact, M. canis is so well adapted to cats that it is thought up to 20% of cats can be asymptomatic (not showing signs) carriers. However, dogs and cats could get any one of the four species and thus cause the dermatophytosis.

Any of the four fungal species can be transmitted from dog to cat, cat to dog, and dog/cat to human.  The fungi live in hair follicles and cause the hair shafts to break off at the skin line.  This usually results in round patches of hair loss, but it doesn't necessarily have to be a round lesion.  As the fungus multiplies, the lesions can become irregularly shaped and spread over the entire body.  Ringworm is actually an infection in the dead layer of the skin, hair and nails.  The fungus can utilize this dead tissue in the skin as a source of nutrition.

The incubation time of ringworm before signs appear

The incubation period is 10-14 days.  This means that following exposure to the fungus, it takes 10-14 days before any lesions occur.

How ringworm is transmitted

Transmission can be by direct contact between an infected individual to a non infected individual.  It can be passed from cats to dogs and vice versa.  Humans can also get ringworm from their pet.  If a child gets ringworm, it could have come from a pet or from another child at school.  Adult humans are fairly resistant to infection unless there is a break in the skin or the individual is immunosuppressed (AIDS, chemotherapy, etc.).  Children are more susceptible than adults.  Consult your family physician if any suspicious lesions occur on you or any family members.  Bring your pet to your veterinarian if any hair loss patches are seen.  Transmission can also occur from an infected environment.  The fungal spores have the potential to live in bedding or carpeting for up to 2 years.  The spores can be killed with a diluted chlorine bleach solution (1 cup chlorine bleach in a galloon of water) where it is feasible to use this type of solution.  On carpeting, it takes extremely hot water to kill the spores and most, if not all, steam cleaners are incapable of delivering that hot of water to the carpet area.  Cleaning the best you can is what needed.

Clinical signs of ringworm

The clinical signs are going to vary from case to case.  The biggest complaint the owner has is that the pet has patches of hair loss.  The patches are typically round but can be irregularly shaped.  These patches may be associated with scaling and crusting of the skin.  The animal may or may not be pruritic (itchy).

How to diagnose ringworm

Diagnosis is based on a positive fungal culture.  Your veterinarian will pluck a few hairs at the edges of the lesions and place the hair on a special culture media that will allow fungus to grow if present.  However, the are many fungal spores in the environment on a normal basis that do not cause ringworm.  Even if there is growth on the culture media, the veterinarian should identify the individual spores underneath the microscope to confirm if it is one of the four fungal species that cause ringworm versus a contaminant.  Once the ringworm has been identified, your veterinarian will discuss treatment.

How to treat ringworm

Treatment varies from case to case.  In animals with very few lesions (e.g. patches of hair loss), the veterinarian may just treat those areas topically versus treating the entire body of the pet.  In pets where the lesions are over most of the body, then your have to treat the entire body.  This can be done several ways or a combination of treatments.

Treating with Griseofulvin

Griseofulvin is a tablet that you give your pet orally.  It will reach the deep hair follicle and kill the fungus.  It has to be given every day for a minimum of 4-6 weeks.  At the end of 4-6 weeks, a repeat fungal culture should be done.  If there is no growth, the medication can be discontinued.  If growth occurs on the second culture, treatment should continue.  The pills are not absorbed well from the stomach unless there is fat in the stomach at the time the pill is given.  This can be accomplished by feeding a high fat diet, such as a rich canned cat food or a small amount of fat trimmings from meats.  Consult with your vet to see if your pet can have this.  Griseofulvin is the only drug approved for systemic (oral) treatment of feline and  canine ringworm.  There are other antifungal medications that are used in cats and dogs that are not FDA approved for use in animals, yet we use them because they have been shown to be effective also.  These medications are itraconazole and ketoconazole.  These are overall considered safe for cats and dogs but like any drug, can have side effects.  These medications are discussed below.

Griseofulvin can also have side effects.  If any side effects occur, to please stop all medications and call your vet.  Griseofulvin can cause vomiting and diarrhea.  It can cause birth defects and should not be used on pregnant animals as well as any animal going to be used for reproduction.  Griseofulvin can cause bone marrow suppression.  This is rare but potentially fatal.  Because of this, griseofulvin should not be used in cats with immune suppression diseases such as FeLV or FIV.  Any cat or dog with an existing potential bone marrow problem already should avoid griseofulvin.  Any animal going to go on griseofulvin or itraconazole should have full blood work done to make sure they are ok prior to starting medication.  Animals then should have periodic blood checks after starting medication to monitor their organs while on the drug.  Your veterinarian will discuss this with you.

Treating with Itraconaole

Itraconazole is a new anti-fungal drug.  It is very costly for large dogs.  It is not approved for cats or dogs but many animals have done quite well on the drug.  It offers the advantage that animals who cannot or should not be on griseofulvin can do well on itraconazole.

Treating with antifungal shampoo

Antifungal shampoos can be used and if used, need to be done weekly.  Antifungal shampoos are recommended for exposed but unaffected pets living in the household with the positive pet.  The shampoo gets the spores off the hairs so the spores don't drop into the environment and prevent reexposure.  Shampoos alone will not cure ringworm.  Shampoos are only used in conjunction with other treatments or for those pets exposed to the infected pet but not showing signs.

It is highly recommended that all animals in contact with the positive pet be treated.  Your veterinarian will discuss this with you and what options you have.

Treating with lime sulfur dip

Lime sulfur dips are also a nice, if not smelly, treatment.  It is typically applied to the entire animal once a week for 4-6 weeks then you repeat a fungal culture.  You can dip all the animals in the house to prevent exposed animals from getting ringworm.  The product your vet carries will vary and the directions will be given to you by your veterinarian.  You should always wear gloves while using it because it will make your hands smell plus stain/damage any jewelry, especially silver.  White dogs and cats could have a change in their hair color during treatment but that resolves once treatment is stopped.

Shaving long hair dogs and cats is recommended in veterinary textbooks.  It will vary from case to case but do not be surprised if your veterinarian recommends it.  Clipper blades can spread infection if you are clipping several cats at the same time so extra care is needed to prevent cross contamination.

Any treatment that is started will take time.  In fact, in the areas of hair loss, those areas may worsen before getting better.  The average time of treatment if 4-6 weeks but it can take longer.  If a cat is thought to be the carrier, your veterinarian can test the cat with a fungal culture even though the cat does not have clinical signs.  Animals that are taking a long time to cure should be checked for underlying disease.

Your environment should be cleaned weekly and done well to prevent recontamination.  Family members should wash hands well after handling the pet(s) with ringworm.