RINGWORM

WHERE WOULD MY PET PICK UP THIS INFECTION?

(Photocredit: Morguefile.com)

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; however, the skin must be abraded as the fungus cannot infect healthy intact skin. This means that freshly shaved, scraped, or scratched skin is especially vulnerable. Infection can come from direct contact with an infected symptomatic animal, direct contact with an asymptomatic carrier, or contact with spores in the environment. Infected symptomatic animals have skin lesions rife with fungal spores. Carriers may be infected animals who do not have obvious lesions (a common scenario towards the end of a treatment protocol) or they may be animals who are not actually infected per se but simply have spores on their hairs just as a non-living object might have spores on its surface. Infection is transmitted when spores bind to abraded skin. Skin lesions typically appear 1-3 weeks after exposure.

There are several species of dermatophyte fungi. Different species of fungi come from different kinds of animals or even from soil thus determining the ringworm species can help determine the source of the fungal infection. Predisposing factors towards infection include age (puppies and kittens are at higher risk than adult animals), life-style (free-roaming or hunting animals being predisposed), and local climate (pets living in warmer more humid climates being predisposed). Immune suppression from the FIV or Feline Leukemia Virus turns out not to be a predisposing factor as one might expect, especially since immune suppression is a human risk, but there are two breed predispositions of note: Persian cats and Yorkshire terrier dogs. Infection rates are higher in these breeds as are treatment failures.

Persian Cat (Photocredit: Morguefile.com)

Yorkshire terrier (Photocredit: Morguefile.com)

HOW DOES THE DOCTOR KNOW THIS IS REALLY RINGWORM?

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 as we will describe.

Most veterinary hospitals are equipped with Wood's lights and employ them for the screening of pets for ringworm lesions. Unfortunately, fluorescence may be difficult to find and, complicating matters, many topical products and non-infectious debris will also fluoresce. Further testing is often needed. 

PCR TESTING

The newest testing method involves testing hairs for dermatophyte fungus DNA. The benefit of this testing is that it is much faster than the culture but is still able to confirm the infection as well as determine the species of ringworm fungus involved. This makes PCR testing an excellent way to make the diagnosis of ringworm initially but can pose a problem for determining the end of treatment. The downside of PCR testing is that it tests for fungal DNA, not for live viable fungus. When the pet is first diagnosed, if fungal DNA is present on a skin lesion, we can assume the fungus is causing infection. After treatment, however, the fungus is killed or damaged to a point of being harmless but its DNA will still be present creating a positive PCR test. For this reason, PCR is best used for detecting fungus in an untreated patient but culture is probably best at determining when treatment can be discontinued.

TREATMENT

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. The infected pet will require isolation while the environment is disinfected and should not be allowed back into the "clean" area until a culture is negative. Ideally all pets should be tested and isolated until they have been deemed clear of infection at which point they can be allowed back into the "clean" area.

Infected pets generally require oral medication which may be supplemented with topical treatment (dipping, lotion, or both). Very localized lesions might get away with topical treatment only.

ORAL MEDICATION FOR INFECTED PETS

Oral medication provides the foundation for the treatment of ringworm as it is oral medication that renders the fungus unable to reproduce and spread. With the spread of infection controlled, only the pre-existing fungus remains and generally can be removed with topical therapy as described later on.

There are presently primarily two medications recommended to treat ringworm: Itraconazole and terbinafine. (Griseofulvin is also available and has been the traditional anti-ringworm oral medication for decades. While griseofulvin is still as effective as the other medications but the newer products appear to be safer and griseofulvin is rapidly becoming a historical note only.)

Treatment with oral medication typically should not be discontinued until the pet cultures negative. Stopping when the pet simply looks well visually frequently invites recurrence of the disease.

ITRACONAZOLE

This medication is highly effective in the treatment of ringworm. Recently, it has become available in an oral suspension approved for cats which is most likely going to be the form your veterinarian prescribes. Itraconazole is also available as a human product, either capsules or liquid. The human product is not very practical for pet use as the capsules are too strong and the liquid too weak. If human product is to be utilized, it is important to have it compounded professionally into appropriately sized capsules using brand name Sporonox®, rather than from generic. The reason for this is bioavailability (how much of the consumed drug actually makes it into the body after oral consumption). Generics and bulk product simply have poor bioavailability and are not recommended. Compounded itraconazole is expensive and compounded itraconazole from brand name product is even more expensive but investing in a medicine that is not bioavailable is even worse so it is important to get either brand name Itrafungol® made for cats or brand name Sporonox® made for humans (and reformatted into a pet sized dose). Nausea is a potential side effect for this medication but probably the main reason it is passed by in favor of griseofulvin is expense. On the average, cats treated with itraconazole and nothing else were able to achieve cure two weeks sooner than cats treated with griseofulvin. Once the issue of the form of medication is resolved, there are several dosing regimens that have been used: daily, one week on/one week off, two weeks on/two weeks off and the list goes on. The bottom line is that itraconazole is effective against ringworm in any of the protocols. Nausea can be a side effect. The main reason it is passed by in favor of griseofulvin is expense.

(original graphic by marvistavet.com)

For more information on Itraconazole, click here.

TERBINAFINE

This is a newer antifungal on the scene and seems to be effective against ringworm fungi. While originally expensive, the generic form is currently relatively inexpensive. Terbinafine is best given with food and cannot be used during pregnancy or nursing.

For more information about Terbinafine click here.

GRISEOFULVIN

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. Treatment typically takes 1-2 months.

For more information on Griseofulvin, click here.

LUFENURON - NOT EFFECTIVE AGAINST RINGWORM

Lufenuron is an oral product used in flea control. It works by inhibiting the insect's ability to make chitin, an important component of its exoskeleton. It turns out that dermatophyte fungi also have chitin in their cell walls and some initial research suggested that lufenuron was a helpful adjunct to other more conventional treatments. This has not panned out in the long term and its use has been largely abandoned.

For more information about Lufenuron - click here.

TOPICAL TREATMENT FOR INFECTED PETS

While the oral products suppress the infection on the host, they do not kill the spores. Topical treatment acts by directly killing fungal spores. This is not only valuable in preventing environmental contamination by the infected animal but also is important in preventing infection in animals in contact with the infected animals. Topically treated hairs will not be infectious when they drop into the environment. In situations where it is difficult to confine the infected animals away from the non-infected ones, topical therapy becomes especially important. So what sort of options are available?

MICONAZOLE-CHLORHEXIDENE RINSE OR SHAMPOO

Miconazole (an antifungal) and chlorhexidene (a disinfectant) synergize each other when combatting ringworm. They are available as a combination rinse as well as shampoo. The rinse, which is allowed to dry on the pet, is effective in killing ringworm spores though in the field lime sulfur seemed associated with faster cure (median 48 days vs. 30 days with lime sulfur). Allow a 10 minute contact time for miconazole-chlorhexidine shampoo. Twice weekly application of either rinse or shampoo is the currently recommended frequency of use.

There are also products where chlorhexidene and miconazole are used as single agents. Chlorhexidene alone not effective and miconazole alone is effective but is vastly more effective when synergized by chlorhexidene. It is best not to use these products separately.

TOPICAL LOTIONS AND OINTMENTS

There are numerous antifungal products available for treatment of isolated lesions. Miconazole, clotrimazole and other anti-fungal topicals can be applied in this way but these treatments should be considered adjuncts to other therapies.

ENVIRONMENTAL TREATMENT

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. It should be noted, however, that bleach cannot disinfect anything if there is dirt or grime present. General cleaning should always precede disinfection. Vigorous vacuuming and steam cleaning of carpets will help remove spores and, of course, vacuum bags should be discarded. Wood floors can be decontaminated with daily swiffer use (or similar product) and twice weekly wood soap cleaning. Laundry can be decontaminated by running it through a washing machine twice; bleach is optional.

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. Be sure to clean areas with a detergent or soap to remove organic debris as disinfection will not work if the surface is not clean first. Cultures of the pet are done monthly during the course of treatment.

Once a cat cultures negative and is removed from the contaminated room, decontamination should be achieved in 1 to 3 cleanings.

The ringworm fungus can remain infective in the environment up to 18 months, maybe longer.

IDENTIFYING CARRIERS

When there is a pet with ringworm in the home, all other pets should be tested. A carrier of ringworm is one that is infected but not showing lesions (usually this will be the pet that has been on treatment for a while and appears visually to be cured but in fact is still infected) or one that is simply carrying the fungus on its fur in the same way an inanimate object might have fungal spores on its surface. Both types of "carriers" must be identified as they are both capable of spreading the infection.

The MacKenzie Toothbrush Test is the best approach to the pet with no obvious lesions. Here the pet is combed with a clean toothbrush and the hair that comes off is cultured for ringworm. This allows sampling of the whole cat when no lesions are visible either with the naked eye or with the Wood's lamp.

WILL RINGWORM GO AWAY BY ITSELF?

There have been several studies that showed that this fungal infection should eventually resolve on its own. Typically, this takes 4 months, a long time in a home environment for contamination to be occurring continuously. Actively treating the infection is considered a better approach that simply waiting for it to go away while environmental contamination progresses.

WHAT TO CHANGE IF THE OUTBREAK SEEMS TO GO ON FOREVER (LIKE OVER 100 DAYS)

After a couple of months of medication and dipping, the outbreak is generally over. If the outbreak is still going strong, then it is time to look for corners that may have been cut and holes in the program that need patching:

• If one is using visual lesions as the endpoint for treatment, it is important to change to fungal culture as the standard.
• Dipping is labor intensive and people tend not to do it twice a week as is optimal. Twice a week dipping should be instituted if there is trouble eradicating the infection.
• The environment must be properly decontaminated and this includes not just identification but confinement of affected pets. If infected pets are not confined, they will contaminate the environment and keep getting re-infected.
• Consider whether the pet has a defective immune system. If a second disease is present, it must be controlled if the pet is to recover.
• Itraconazole compounded from bulk product does not have the same bioavailability as itraconazole compounded from prescription product. This means, in short, that it does not work as well. Changing to compounded prescription product or to terbinafine may make a big difference.
  
• Lastly, it is important to consider that the diagnosis may be wrong if only visualization was used to make the diagnosis. Proper testing as outlined above is crucial to the diagnosis of dermatophytosis. A biopsy may be needed.

If you yourself become infected, contact your doctor to receive treatment. Veterinarian are not able to make recommendations for human disease or infection, even if the infection came from a pet.

Page last updated: 5/21/2021