Monday, December 1, 2014

Ringworm infection in cats - it's not really a worm!


Ringworm lesion
Generalized ringworm lesions
"Ringworm" is the common name for the skin infection caused by certain types of fungi. Despite the name, ringworm is not caused by a worm at all. The fungi feed upon the dead cells of skin, nail beds and hair. In people, this causes a classic round, red lesion with a ring of scale around the edges and normal recovering skin in the center. Because the ring of irritated, itchy skin looked like a worm, the infection was erroneously named. The fungi responsible are called "dermatophytes," meaning "plants that live on the skin" thus the more correct term for ringworm is "dermatophytosis." The characteristic "ring" appearance is primarily a human phenomenon. In animals, ringworm frequently looks like a dry, gray, scaly patch but can also mimic any other skin lesion and have any appearance. The fungi live in the hair follicles and cause the hairs to break off at the surface of the skin. As the fungus multiplies, small round lesions can become large, irregularly shaped patches of hair loss and spread across the whole body. Most commonly, ringworm is seen in kittens, likely due to their weaker immune system. Ringworm most commonly affects the face, head and ears. Less commonly, the feet, tail and nails are affected, and even less commonly do we see generalized infections covering the entire body. High humidity and large populations of cats tend to increase the risk of ringworm infection, and certain breeds like Himalayans and Persians are at higher risk, as well. You can read about Samson and his battle with ringworm on one of our other blog posts. This may be due to increased difficulty in grooming all the way down to the skin due to large amounts of long thick fur. FIV positive or otherwise immunocompromised cats are also at higher risk. Regular or frequent bathing of cats may remove protective skin secretions that help protect the skin from fungal infection.

There are about 40 different species of dermatophyte fungi. The most common species to infect humans is the Tinea family of fungi. Cats tend to become infected by Microsporum canis, Microsporum gypseum, and Trichophyton spp. Microsporum canis is responsible for up to 98% of dermatophyte infections and is considered "true" ringworm.
Different types of dermatophytes look different on culture medium
The spores of dermatophyte fungi are extremely hardy in the environment; they can live for years. All it takes is skin contact with a spore to cause infection. Infected animals are continuously dropping hairs covered with thousands to millions of spores, because infected hairs become fragile and easily break off and shed into the environment. Some animals (about 20%) are carriers and never show signs of skin irritation themselves, but can infect others readily. Ringworm can be transmitted between people, dogs and cats by direct contact, as well as being picked up from the environment. Environmental “hot spots” for contracting ringworm are schools or daycares, gyms, and the out-of-doors in general. Spores may live in bedding or carpet for 18 months or more. Different species of fungi come from different kinds of animals or even from soil, so determining the dermatophyte species can help determine the source of the fungal infection.

Ringworm lesion in a human
Yes, ringworm is contagious to people; however, some people are at greater risk than others. The fungus takes advantage of skin belonging to those with reduced immune capacity. This puts young animals and children, elderly people and pets, those who are HIV+, people receiving chemotherapy or taking medication after blood transfusion or organ transplant and highly stressed people and animals at high risk. If you have cuts or abrasions to the skin, that allows the fungal spores a better foothold, as normal, unbroken and healthy skin is resistant to fungal infection. In general, if you do not already have ringworm at the time your pet is diagnosed, you probably will not get it, since lesions appear about 10-12 days after infection. If you find suspicious lesions on your body, check with your family physician.

In some cases, we know for sure that dermatophyte fungi are present while in other cases we are only highly suspicious. Lesions on animal skin are rarely the classic ring-shaped as in people (in fact, in animals, lesions are often not even itchy) thus some testing is usually necessary. On many animals, ringworm presents as roughly circular areas of broken hairs or bare skin, often with scaly, inflamed skin. Sometimes, ringworm infections can look almost identical to flea allergy dermatitis or other skin diseases. Hair loss is usually involved, but the degree of itchiness and inflammation is highly variable.

Hair shaft fluorescence under Wood's Lamp
Microsporum canis, the most common ringworm fungus, will fluoresce apple green in approximately 50% of cases. Fluorescence is an easy test to perform and may provide a strong clue that dermatophytes are present. Further testing is usually needed, however, to absolutely confirm diagnosis. It is important not to mistake scabrous material, dust particles and free-floating skin flakes as fluorescence. If the fluorescence is present, it will light up along the hair shafts at the edge of the lesion.

Your veterinarian may wish to examine some hairs for microscopic spores. If spores can be seen on damaged hairs then the diagnosis of ringworm is confirmed; however, as spores are very difficult to see, many veterinarians skip this step.

For a fungal culture, some hairs and skin scales are placed on a special culture medium in an attempt to grow the ringworm fungi. The advantage of this test is that it not only can confirm ringworm but also can tell exactly which species of fungus is present. Knowing the identity of the fungus may help determine the source of infection. The disadvantage, however, is that fungi require about 10 days to mature to the phase where they can be identified by their special "fruiting bodies" and spores.
Also, this is the only test that is helpful in determining if animal is an asymptomatic carrier. The other tests require an apparent skin lesion to test. A pet with no apparent lesions can be combed over its whole body and the fur and skin that are removed can be cultured. Carrier animals are usually cats living with several other cats.
Characteristic growth and media color change on fungal culture

Sometimes the lesions on the skin are so uncharacteristic that a skin biopsy is necessary to obtain a diagnosis. Fungal spores are quite clear in these samples and the diagnosis may be ruled in or out. Depending on the outcome of preliminary tests, your veterinarian may begin ringworm treatment right away or postpone it until after more definitive results are available.

Commitment is the key to success especially if you have more than one pet. Infected animals are constantly shedding spores into the environment (your house) thus disinfection is just as important as treatment of the affected pet. Areas of hair loss will likely get bigger before they get smaller. Within 1-2 weeks, hair loss should stop, there should be no new areas of hair loss, and the crusting on the skin should subside. If any of these do not occur within 2 weeks, we should see your pet again. Infected pets generally remain contagious for about 3 weeks after treatment begins.

There are two primary medications being used to treat ringworm: Griseofulvin and Itraconazole (brand name "Sporonox"). Veterinary dermatologists disagree as to which is better. Both medications are relatively expensive, must be given with food, and have significant potential to cause birth defects in pregnant pets.
Treatment with either medication typically is continued for 1-2 months and should not be discontinued until the pet cultures negative. Stopping when the pet simply looks well visually frequently leads to recurrence of the disease.
Characteristic Microsporum fungal macroconidia and spores

GRISEOFULVIN (brand name Fulvicin)
This medication must be given with a fatty meal in order for an effective dose to be absorbed by the pet. Persian cats and young kittens are felt to be sensitive to its side effects which usually are limited to nausea but can include liver disease and serious white blood cell changes. Cats infected with the Feline Immunodeficiency Virus commonly develop life-threatening blood cell changes and should never be exposed to this medication. Despite the side effects, which can be severe for some individuals, Griseofulvin is still the traditional medication for the treatment of ringworm and is usually somewhat less expensive than Itraconazole.

This medication is highly effective in the treatment of ringworm but is available in capsules far too large to be useful to most small animals. This means that a special compounding pharmacy must reformulate the medication into a more useful size. Nausea is a potential side effect for this medication, but otherwise, side-effects are fewer than with Griseofulvin. The primary reason it is sometimes passed by in favor of griseofulvin is expense. Itraconazole is also effective in treating many life-threatening fungal infections whereas Griseofulvin only treats ringworm.
By increasing the amount of Itraconazole in the environment, we may be creating resistance in more dangerous fungi which could become a problem over the years. On the average, cats treated with Itraconazole and nothing else were able to achieve cure two weeks sooner than cats treated with Griseofulvin.

In the late fall of 2000, the Journal of the American Veterinary Medical Association published a study of over 200 cases indicating that Lufenuron (the active ingredient of Program) could be an effective treatment for Ringworm. This began as an observation that animals using Program or Sentinel for flea control did not get ringworm. Questions about this work have come up from the public since the release of this work. The fact is that the study is still under scrutiny by members of the American College of Veterinary Dermatology and “the jury is still out.” It is clear that the doses purported to be helpful with Ringworm should not be harmful. At this time the use of Lufenuron for Ringworm is reasonable if other more conventional treatments are used at the same time, but its efficacy is controversial and it should not be considered as the only method of treatment.

Both the above medications work by interfering with fungal reproduction rather than by directly killing the fungus. This is fine from the pet's perspective as either medication should be able to clear the fungus without further therapy; however, we also would like to reduce contamination of the environment. This means actually killing the fungus on the pet so that the hairs dropped will not be infectious but applying topical therapy. For many years cats with ringworm lesions were shaved to allow for easier topical treatment. We now know that shaving may be spreading the fungus by creating microabrasions on the skin and allowing the spores to take hold in new locations, or simply by spreading spores across the body as the clippers are moved from one area to another. Because of this, shaving is not always recommended (depending on the number of cats present in the home and the length and thickness of the hair).
Ringworm lesions on an ear

Dips are recommended twice a week and can be performed either by the hospital or at home. If you attempt this kind of dipping at home, you should expect:
  • Lime Sulfur will stain clothing and jewelry
  • Lime Sulfur will cause temporary yellowing of white fur
  • Lime Sulfur smells strongly of rotten eggs.
The dip is mixed according the the label and is not rinsed off at the end of the bath. The pet should be towel dried. Shampooing is not necessary.

The problem with decontaminating the environment is that very few products are effective. Bleach diluted 1:10 will kill 80% of fungal spores with one application and any surface that can be bleached, should be bleached. Vigorous vacuuming and steam cleaning of carpets will help remove spores and, of course, vacuum bags should be discarded. To reduce environmental contamination, infected cats should be confined to one room until they have cultured negative. The rest of the house can be disinfected during this confinement period.

In 1994, Fort Dodge released a vaccine which could be used either in the prevention of ringworm infection or in its treatment. Our hospital does not recommend the use of this product as, while it may prevent the development of obvious lesions in a cat, it probably will not prevent infection. This means that vaccinated cats could more easily become carriers of infection. The vaccine does not appear to be available any longer (since 2003), and was meant to be used in a treatment situation where many cats were infected. It was designed as a supplement to the treatments described above rather than as a preventive for cats in general.

There have been several studies which showed that this fungal infection should eventually resolve on its own. Typically, this takes four months, a long time in a home environment for contamination to be occurring continuously. We recommend treatment for this infection rather than waiting for it to go away.


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