Ringworm is a zoonotic fungal infection that can wreak havoc on a shelter. Here is information about how to diagnose, treat and prevent this infectious disease.
Ringworm in an individual animal is a nuisance; however, ringworm in an animal shelter can lead to completely unmanageable outbreaks, thousands of dollars in diagnostic and medical costs, the possibility of spread to adopters and staff, and an intolerable blow to shelter status in the community. It is vital to have a consistent and effective strategy to prevent and manage this disease.
Ringworm, or "dermatophytosis", is a fungal infection affecting the skin, hair and occasionally nails of animals (and people). Three species of ringworm fungus most commonly affect cats and dogs:
The dermatophyte species that affect cats and dogs can be passed between these two species as well as to humans and other mammals.
M. canis is most often spread from contact with an infected animal or a contaminated environment, and therefore is by far the most likely to be a serious problem in a shelter. T. mentagrophytes is thought to be contracted mainly from exposure to rodent nests, and M. gypseum from contaminated soil, though the potential exists for spread from animal to animal in a contaminated environment.
Ringworm is very durable in the environment. Ringworm can persist on any surface and can infect animals housed in a contaminated environment months and even years later. (See information on environmental clean-up to learn more about determining sources of significant contamination.)
Ringworm can be spread readily on grooming implements, contaminated toys and bedding, or by humans on clothing and hands. In nature, the incubation period for ringworm is between 4 days and 4 weeks. Ringworm can be found on the hair of animals from a contaminated environment even when the animal itself is not showing any signs.
The name “ringworm” comes from the most common appearance of a circular area of hair loss and scaling. The most common locations include the face, ears, feet and tail. However, ringworm can present with a wide range of appearances, including large areas of hair loss with or without crusts and exudate. Ringworm can also cause infection of the toenails and nail beds. Ringworm lesions may or may not be pruritic (itchy).
There are other possible causes for all the types of lesions described above, so definitive diagnosis based on appearance alone is not possible. Not all areas of alopecia are due to ringworm. There typically is also evidence of inflammation - redness of the skin, swelling, warm to the touch, and itchy or painful. In a shelter study, the majority of cats with skin lesions were not infected with ringworm, AND a significant number of cats with no grossly apparent lesions were culture positive. Therefore, fungal culture should ideally be used to confirm or rule out ringworm in all cats, especially those at high risk due to presence of skin disease, breed or exposure risk (see below.) The most common differential diagnoses for hairloss to be considered include demodectic mange and staphylococcal folliculitis.
Accurate diagnosis of ringworm is very important: the consequences of a false positive test may be severe for the individual animal diagnosed, especially at agencies that euthanize ringworm positive animals. The consequences of a false negative are also significant, as it may allow an infected individual to spread ringworm within a facility or to a foster or adoptive home. Unfortunately, there is no rapid, reliable test for diagnosis of ringworm. There are ways to confirm its presence, but no definitive way to confirm absence except by careful fungal culture and microscopic examination.
If it is necessary due to practical considerations to make a “best guess” without performing a fungal culture, the animal’s history, clinical signs and results of Wood’s lamp examination should be considered together. For instance, a classic ring-shaped lesion in a kitten, especially if multiple members of a litter are affected, is very likely to be ringworm even if Wood’s lamp results are negative. On the other hand, ringworm is uncommon in adult dogs, so even a suspicious lesion is more likely to have another cause unless there is a highly suggestive history (i.e. known recent exposure, endemic area).
Recently (March 2015) IDEXX Laboratories introduced the Ringworm ReaPCR™ for diagnosing ringworm. From our experiences all suspect animals should still have cultures run to confirm the presence or absence of the fungus.
A Wood’s lamp is an ultraviolet light with a specific wavelength of light which causes some strains of M. canis (the most common feline ringworm species) to fluoresce. A Wood’s lamp - correctly used - can be a helpful and cost effective screening tool. Traditionally, it has been reported that 50% of M. canis strains will fluoresce under a Wood’s lamp. However, some people report that over 90% of M. canis strains will fluoresce. Bright apple green fluorescence coating the hair shafts is strongly suggestive of infection and warrants isolation and fungal culture.
False positive Wood’s lamp results are common, especially when the test is performed by untrained or inexperienced staff. Some medications will fluoresce if they are on the hair coat, including doxycycline, Clavamox, and eye ointments. Sebum may glow yellow and should not be mistaken for ringworm infection.
Fluorescence induced by M. canis infection can be distinguished from fluorescence due to contaminants by the fact that ringworm cannot be easily brushed off, and the hair shafts alone will fluoresce the characteristic apple green color rather than a large area of skin as would be seen with medication in the hair coat. Observation of known lesions will help develop proficiency in recognizing true fluorescence.
In order to maximize the usefulness of this test, it is important to correctly use the right equipment:
Although absence of Wood’s lamp fluorescence by no means rules out ringworm infection, a positive result is a good indicator to at least isolate the animal until fungal culture results can be determined.
Positive Wood's lamp findings and direct microscopic examination can diagnose ringworm, but negative findings do not rule it out. Direct microscopic examination is used to confirm the results of Wood’s lamp tests and may be especially useful if the Wood’s lamp test is equivocal. Results can be improved through practice.
Individual Wood’s lamp positive hairs may be carefully plucked from the hair follicle and placed in a drop of mineral oil and examined directly. Some people recommend clearing the sample of keratin by suspending it in 10-20% KOH or chlorphenolac prior to examination but we have not found this step to be necessary and these substances are corrosive to microscope lenses.
Infected hairs appear swollen, frayed, irregular or fuzzy in outline, and the normal structure of cuticle, cortex, and medulla is lost.
Recognition of affected hairs takes practice. To get experience in making a diagnosis by this method, examine known infected hairs from a Wood's lamp-positive lesion in conjunction with the results of culture from the same individual. Doubtful cases or discordant results of a Wood’s lamp and direct exam should be cultured. The decision to culture samples may be dependent on the shelter’s protocol regarding treatment or euthanasia of ringworm positive animals and staff’s confidence in evaluating a Wood’s lamp and direct microscopic examination of the hairs.
Although the gold standard for ringworm diagnosis remains positively identifying spores from growth on a fungal culture, Wood’s lamp test and direct microscopic exam in the hands of an experienced technician can be used as confirmation of infection. Fungal culture remains an important tool for discordant results, inexperienced staff, and for monitoring treatment and management.
Performance of fungal cultures test in-house where possible, as opposed to sending it out to a diagnostic laboratory, has several advantages. In order to properly manage an outbreak of ringworm affecting multiple animals, numerous cultures are often required. Cultures should be done for risk assessment in exposed cats, diagnosis of suspect lesions, and confirmation of cure after treatment. This can quickly become a prohibitive expense if cultures are not done in a cost-effective way.
Reading cultures in-house also permits a speedier diagnosis in positive cases as growth often occurs within a week, allowing earlier initiation of treatment. The amount of growth on a culture plate can help differentiate possible carriers from truly infected animals.
Foster parents can be instructed in the proper technique for toothbrush culture collection, and can simply bring samples in for ongoing monitoring purposes rather than bringing in all the infected animals each time.
Plate-style cultures are easier to inoculate via toothbrush than narrow jars or slants. They are also easier to collect samples from for microscopic examination. Plates that combine dermatophyte test medium on one side to give a red color change with most species of dermatophytes with rapid sporulating medium or Sabouraud's dextrose agar to aid in microscopic identification of colonies on the other side, are ideal. Plates can be ordered from Hardy Diagnostics.
Not all dermatophytes turn the dermatophyte test medium red, so false negatives are possible. Some other types of non-pathogenic fungi can cause the red color, so false positives are possible too. To be certain of a diagnosis of ringworm, it is imperative to microscopically examine and positively identify the fungus. The culture on rapid sporulating medium can be microscopically examined as well to positively identify the fungus present.
This is accomplished by microscopic examination of a “tape prep”:
Most culture media kits come with a guide to microscopic identification. Descriptions and photos for fungal identification can be found in Muller and Kirk's Small Animal Dermatology, 7th Edition (Saunders), chapter 2, Diagnostic Methods.
In most cases, pet animals spontaneously recover from ringworm within about 3 months. Awaiting self-cure is certainly a reasonable choice in a single animal household with no young children or immunocompromised adults where contagion is not a major concern. However, in a shelter environment or foster situation, a more rapid resolution is highly desirable. Reducing environmental contamination is just as important as treatment directed towards speeding recovery of affected animals.
Sandra Newbury, DVM and Karen A. Moriello, DVM, DACVD from the School of Veterinary Medicine at the University of Wisconsin-Madison have conducted an ongoing study of naturally occurring shelter ringworm to develop cost-effective systems for screening and treatment.
As a result of their findings Drs. Newbury and Moriello developed Dermatophyte Treatment in a Nutshell , a document that details diagnostic and treatment strategies they have found most effective in a shelter environment.
Clipping is often unnecessary in short and medium haired cats, and may worsen lesions through micro-trauma and mechanical spread of spores. However, clipping is indicated for very long haired cats and those that may be unable to groom due to conformation (e.g. Persians), concurrent severe upper respiratory congestion, or matted coat. Clipping may also be useful in cats whose coats simply become unmanageable after dipping.
Clipping should be performed gently with a #10 blade – taking great care to avoid clipper burn - and hair should be carefully disposed of. Clipping with a closer blade causes excessive trauma and increases the chance of worsening lesions. Clipping should be performed in a room that is easily cleaned since it causes heavy environmental contamination (see below), and instruments used should be carefully cleaned and dedicated only to that purpose, never used on healthy animals (i.e. do not use the surgery clippers for ringworm cats!).
It is important to wear protective clothing whenever handling a ringworm animal in a shelter, and especially when clipping. In a facility with an easily cleaned/disinfected euthanasia room, this might be a good place for clipping ringworm cats.
The most important component of treatment in a population is topical therapy.
This is critical in order to reduce immediate and ongoing environmental contamination. This is best accomplished through the use of topical therapy which sterilizes the coat and prevents the further growth of spores that can re-contaminate the environment.
Of all available topical therapies, lime sulfur solution is the most cost effective as it sterilizes the coat to prevent further growth of ringworm, is relatively easy to apply rapidly to a number of cats, and has been documented to work well in a shelter setting.
Miconazole shampoo in combination with chlorhexidine may also be effective; chlorhexidine alone is not. Enilconazole solution, although reportedly effective, is not available for this use in the United States (it is regulated by the EPA rather than the FDA, which does not permit off-label use in the same way). Some currently available topical treatments are ineffective or only partially effective; use caution if selecting another treatment other than lime sulfur solution. Chlorhexidine shampoos and locally applied topical ointments do not effectively sterilize the coat or provide residual activity against further growth. Topical terbinafine (Lamisil) and clotrimazole (Lotrimin), are not recommended for use in pets as they do not reduce further growth of ringworm lesions.
Pure Oxygen® Shampoo (accelerated hydrogen peroxide shampoo by Ogena Solutions) may eliminate ringworm spores from the coat. Ongoing studies are in progress to determine the efficacy of Pure Oxygen® Shampoo as a topical treatment for ringworm.
Please note: physically dipping cats in the lime sulfur solution is not recommended as this is stressful to the cats (and staff) and can lead to cross comtamination between cats being treated if the same lime sulfur dip solution is used on multiple cats.
Systemic treatment is an important adjunct to topical therapy, especially in a shelter where time-to-cure is an important consideration. Extended stays in a shelter or foster care increase the chance of spread, use precious isolation space, and may reduce the adoptability of the patient, especially if kittens are allowed to grow old in treatment.
The drawbacks of systemic treatment are the relatively high cost of the drugs and the possibility of toxic side effects. Itraconazole is a good choice due to its demonstrated efficacy, relative safety, and long half-life in the skin. We do not recommend compounded itraconazole as it has repeatedly been shown to be less effective and may result in treatment failure.
Fluconazole and terbinafine are also reportedly effective. Griseofulvin is effective, but more likely than itraconazole to cause toxic side effects. Ketoconazole should be avoided in cats if possible, as it can cause hepatotoxicity in this species. Lufenuron (Program) has been shown in repeated studies to be ineffective.
Animals on any systemic antifungal should be closely monitored, and all directions for administering the drugs carefully followed. These drugs should be avoided in pregnant animals.
Although many treatments are reportedly effective for individual animals, the frequency of self-cure makes these claims somewhat difficult to assess in the absence of supporting scientific evidence. In a population setting, treatment failure is more evident and common, and the requirements for highly effective treatment are rigorous. The following treatments have failed to demonstrate efficacy in controlled studies:
“Natural” treatments often suggested on well-meaning pet websites such as apple cider vinegar, coconut oil, tea tree oil, papaya, garlic, grapefruit seed extract, colloidal silver, betadine, and topical bleach are not effective and some of these home remedies are toxic to pets.
Pulse therapy is complicated and difficult to keep track of in a shelter setting. It requires a loading dose and ultimately does not reduce the amount of drug used in treatment. Therefore, pulse therapy is not recommended.
It is imperative to recognize that animals can still carry infective fungi even after signs appear to have completely resolved. Animals may be considered cured after 3 consecutive weekly fungal cultures are negative. (If twice weekly lime sulfur and systemic itraconazole are used, 2 cultures may be adequate.)
Fungal culture should be obtained, using the toothbrush collection method, starting at week 1 of treatment. Animals housed in a contaminated environment may become incidentally contaminated. If this is a concern, the animal should be kept in a clean cage or room for 5-10 days prior to obtaining the culture.
Because the fungal cultures take about two weeks to verify negative results, this means that treatment and documentation of cure will take a minimum of 6-8 weeks, and may take as long as 3-4 months. Housing and socialization of the animal during this period are extremely important considerations when deciding whether treatment in the shelter is practical.
There is currently no vaccine available that is protective against ringworm.
The best protection, as with many infectious conditions, is to practice excellent husbandry. Kittens, which are at highest risk, should be screened with a Wood's lamp and careful visual examination at intake and prior to being placed in foster homes.
Remember that some cases will not be detected with the Wood's lamp. For areas with high levels of ringworm, all kittens or litters should be housed separately in an easy to disinfect area for at least two weeks and observed before being allowed the run of a house (recalling that the incubation period is 4 days to 4 weeks, so this quarantine will catch some but not all cases).
Isolate affected or suspected animals and institute cleaning protocols to prevent further spreading. It is not necessary to completely isolate suspect or infected animals in a separate building as long as appropriate environmental cleaning is initiated.
The foundation of environmental decontamination is identification, removal and treatment (if a shelter has the appropriate housing/space and resources) of affected animals coupled with careful mechanical cleaning. Disinfection is an important part of the protocol but is adjunct to mechanical removal.
Clean all surfaces three times with any good detergent and clean rags. Follow this mechanical removal with a disinfectant. Concentrated bleach or even diluted at 1:10 is too harsh to be routinely used and thankfully is not necessary. Effective disinfectants include Accel® (Accelerated hydrogen peroxide 1:16), Accel® TB (hydrogen peroxide 0.5%.), Enilconazole, bleach diluted 1:32 with prolonged contact time (at least 10 minutes), 2% Potassium Peroxymonosulfate, Formula 409® (quaternary ammonium 0.3%,) and Clorox Clean-Up® (sodium hypochlorite 1.84%,). These have all been shown to be very effective on pre-cleaned surfaces where all organic matter has been removed. For additional specific information regarding the efficacy of various cleaning compounds against ringworm spores, see this 2013 study in Veterinary Dermatology.
High heat (> 110 degrees) is also effective. This temperature can be attained by commercial dishwashers, some commercial steam applicators (but not necessarily home steam carpet cleaners) and clothes dryers.
Environmental decontamination is usually straightforward in a typical shelter with easily disinfected and mechanically cleaned cages. It can present a much greater problem in a home or more home-like environment such as a group cat room.
Application of harsh disinfectants to every contaminated surface is an impossible goal in such environments. Fortunately, much can be accomplished through identification, removal and treatment of carrier animals, followed by repeated application of elbow grease.
Fungal cultures from the environment can take the guesswork out of decontamination, and spare a lot of unnecessary work and worry. First clean carefully and disinfect where possible, then take cultures of possibly contaminated areas to evaluate success (see below for Swiffer swab method.) If the culture is negative, the area can probably be safely re-opened to feline inhabitants. If positive, at least you know for sure you have to go back and try again. This can save a lot of agony over whether to replace carpets, furniture etc. The five D’s of ringworm decontamination are:
When any animal from a population is diagnosed with ringworm, the question arises: what do you do about the other animals in the environment? Do they all need to be cultured? Must they all be isolated while awaiting culture results? Will they all need treatment?
The answers to these questions are dependent on several factors. Not all animals in the same house or even the same room as a ringworm-infected animals will become infected themselves. The risk of infection depends on the animal’s individual immune status and grooming habits, the overall cleanliness of the environment, and the level of proximity between the exposed and infected animals. Some questions to ask include:
What is the baseline sanitation level? Is this a highly cleanable environment such as a bank of stainless steel cages in an otherwise empty room? Is this a home with lots of scratching posts, furniture and carpeting to collect spores? Somewhere in between, such as a bank of cages in a messy room, with lots of junk piled about? Is bleach or Accel used on a routine basis for disinfection?
How closely exposed were the animals? Were they each in separate cages, with minimal handling by staff likely to be carrying infection on their clothing? Was there some shared space such as an exercise area or “get acquainted room” where the animals comingle or spend time without cleaning between occupants? Are cats allowed to wander loose during cleaning but caged separately otherwise?
Is there evidence of spread? Has more than one animals been affected? Are all affected cats from one area of the shelter, or has it shown up in more than one room? Are animals that have been in the shelter long term (> 2-4 weeks) affected? (This suggests acquisition of infection in the shelter, as opposed to coming in already infected.)
If the environment is basically clean, animals are generally kept reasonably separated, and overall animal health is good, it is not uncommon for animals to survive a minor exposure without becoming infected. Ideally, all exposed animals will be toothbrush-cultured, but this is often impractical and may not be necessary in a reasonably well-run shelter.
On the other hand, toothbrush cultures all around are generally required in a foster home where there are extensive opportunities for contact, in a cage-free cat shelter or group cat room, or any time there is evidence of significant spread (multiple cats affected). For more information on interpretation of fungal cultures ("pathogen scores") and management based on culture results, please see https://www.giveshelter.org/dermatophyte-treatment-in-a-nutshell.html.
In the event of a true outbreak, many shelters will find it impossible to shut down intake for the duration of treatment. In such cases, it will be necessary to create a clean, separate area for new incoming cats. If this is impossible, a single dip in lime sulfur at intake can reduce the chances of infection if cats must be admitted to a contaminated environment.
As is always the case, prevention of an infectious disease outbreak is far cheaper and easier on all concerned than management of an outbreak. After an outbreak of any kind, all personnel should meet to ensure that all staff and volunteers understand how the situation came about and what methods of control were most effective in ending the outbreak. From this informational meeting a plan to prevent outbreaks in the future should be, developed, written down, posted and distributed, so that all involved are aware of their role in prevention of future outbreaks.
Daily cleaning will minimize the amount of environmental contamination that builds up while affected animals are housed there. In addition, thorough cleaning will be needed after the affected animals have left the environment (or recovered). This will be much easier if animals are limited to a small area of the house that has minimal furniture and carpeting (e.g. a large dog crate, bathroom). This is particularly critical until the first two weeks of effective topical treatment have been completed, and ideally until at least one negative fungal culture has been obtained.
Cleaning the home environment in the event of a ringworm infection:
After affected animals have been cleared out, follow the five Ds as described above. The extent to which extensive cleaning will be necessary depends on the level of environmental contamination. If kittens romped through the house for some time before diagnosis, or an effective topical treatment was not used consistently, extensive efforts may be required. If, on the other hand, the cat was kept reasonably confined to a cleanable area and effectively treated from the outside, basic cleaning may be adequate.
Some steps to consider include:
For additional information please see the Beating Ringworm Webinar Series on ASPCApro.