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 must euthanize ringworm positive animals. The consequences of a false negative may be even more severe, 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, then history, clinical signs, and results of a 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 RealPCR™ for diagnosing ringworm. From our experiences all suspect animals should still have cultures run to confirm the presence or absence of the fungus.
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. For more information, refer to Who gets Ringworm? in Chapter 1. The most common differential diagnoses for hair loss to be considered include demodectic mange and staphylococcal folliculitis.
A Wood’s lamp is an ultraviolet light with a specific wave length of light which causes some strains of Microsporum canis to flouresce. Although not a perfect diagnostic test due to the relatively high frequency of false negative results, a Wood’s lamp - correctly used - can be a helpful and cost effective screening tool. It has been estimated that somewhere between 30-80% of M. canis strains will fluoresce; the actual frequency in ringworm-infected cats has not been documented but may be higher than the 50% commonly quoted. Bright apple green fluorescence coating the hair shafts is strongly suggestive of infection and warrants isolation and fungal culture. A negative Wood’s lamp exam does not rule out infection and suspicious lesions should always be cultured. Some drugs and other products, notably tetracycline drugs and ointments (e.g. doxycycline, terramycin), will also fluoresce. Fluorescence induced by dermatophyte infection can be distinguished from fluorescence due to contaminants by the fact that ringworm can not be easily rinsed off. 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 use the right equipment, correctly:
Although absence of Woods 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.
Hair may be suspended in 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. Arthroconidia (beaded chains of small rounded cells) and hyphae can sometimes be seen. Hyphae are uniform in diameter, septate and variable in length and degree of branching. Dermatophytes do not form macroconidia in tissue, so any macroconidia seen represent other species of fungus.
Recognition of affected hairs takes practice. To get experience in making a diagnosis by this method, examine known infected hairs from a Woods lamp-positive lesion. Doubtful cases should be cultured.
Ultimately the only truly reliable way to diagnose ringworm is via fungal culture. Performance of this 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 cats, numerous cultures are required: 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 – 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 cats. 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 cats 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.
Always confirm species microscopically - some non-pathogenic fungi can look at lot like ringworm to the naked eye. Ringworm species will develop colony growth at the same time as color change develops in the medium. In addition, ringworm species have a characteristic white, fluffy colony appearance, whereas other fungal species will often appear yellow, green, or slimy.
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.
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.
No matter what species of ringworm you identified on the culture plate, your first step after identifying the species is to count the number of colonies growing on the plate. This is called a pathogen-score or “p-score.”
The number of colonies growing can be an indication of whether the cat is truly infected or not.
A positive culture means that spores were present on the fur when you cultured the cat, but it is possible the cat is not truly infected. Cats with a low p-score and no visual lesions are called “dust mops,”, and they are not a high risk for becoming infected or infecting other animals/people.
A P3 culture generally indicates a true infection. P1 or P2 may mean a true infection or may mean the cat was a “dust mop.”
After assigning a p-score to the culture plate, the next step is to repeat your visual exam to look for suspicious lesions on the suspect cat. If you identified M. canis on the culture plate, you should also repeat a Wood’s exam.
Use any clean plain plastic bin with a lid.
Place on top of a low-heat heating pad wrapped in a towel (the ones without timed auto-shutoff are ideal, such as the ones that come with some dog beds). Place a fish tank thermometer and a damp paper towel in the bin and monitor a 2-3 times per day before using it for cultures to determine what the temperature range in the bin is. The ideal temperature range is 80-86 °F (27-30 °C).
If the temperature is too warm trying placing additional layers between the bin and the heating pad. If the temperature is too cool reduce the layers or try wrapping the entire bin in layers to help hold in the heat.