There were only a few percentages we were told to “memorize for boards” in veterinary school, a dozen years ago now. These included odds of malignancy in dog/cat mammary masses (50 percent/90 percent); canine splenic mass diagnoses (33 percent hematoma/hemangioma/ hemangiosarcoma); failure rates of TPLO versus TPP versus. lateral band in canine cruciate disease (now debunked); and, in dermatology, the sensitivity of the Wood’s lamp in diagnosing dermatophytosis: less than 50 percent.
Where did “50 percent” come from, and why has it been lodged so firmly in our heads? So firmly that I recently ran across this same number in the crisp 2022 edition of a small animal textbook. That number is wrong, and my veterinary friends, colleagues, students still quote it. It is also possible the first percentages quoted here need updating—they probably do (See: “When in doubt cut it out! But by how much?” by Brennen McKenzie, MA, MSc, VMD, cVMA on VPN Plus+).
A closer look at M. canis
Dermatophytosis in small animals is generally caused by one of three fungal organisms: Microsporum canis, M. gypseum, and Trichophyton mentagrophytes.
M. canis is the only one of these that produces pteridine when metabolizing the hair, a phosphor that fluoresces under the narrow spectrum of UV light (around 320-380 nm) emitted by the Wood’s lamp. Lucky, M. canis is responsible for the vast majority of the dermatophytosis we encounter in cats and dogs.1
The fungal organism M. canis is primarily spread through close contact, and animals frequently develop infections when there are breaks in the skin barrier (fleas, scratches), their immune system is mildly or moderately depressed (youth, stress, disease), and the environment is contaminated and conducive. It takes two to four weeks to develop symptoms after exposure, when all these conditions are met. The animals most commonly affected are kittens, puppies, and cats housed in crowded environments (catteries, hoarding situations) in warm seasons or environments.1–3 Like all diseases, a proportion of animals do not develop dermatophytosis when exposed. Competent immune systems, healthy skin barriers, and good grooming habits likely prevent a significant number of infections, even in contaminated environments. However, fungal spores can be carried on the haircoat, and carried from one environment to another, by perfectly healthy animals.
Diagnosis of dermatophytosis usually involves a few different tactics. Hairs that glow green under Wood’s lamp are characteristic, though there can be false negatives (you have to look at the right spot on the animal with a good lamp in the dark) and false positives (medications, urine, feces, lint, squeeze cheese, etc., also glow).
Trichogram examinations are definitive, but challenging without practice or proper magnification. The mainstay of dermatophytosis diagnosis has been fungal culture, which can be performed at a clinic or sent to a lab.4
Recently, PCR testing for common dermatophytes has been offered by an American pet healthcare corporation that offers a faster turnaround, but is not perfectly sensitive or specific either.5,6 None of these tests are considered a “gold standard.”1
Prognosis and treatment
Given infection after exposure is not certain even when a cat or kitten comes from a contaminated environment, distinguishing between fomite carriers and truly infected cats is important. Context matters: around 75 to 80 percent of kittens and cats without detectable fluorescence that are part of an infected family group will actually be infected,3 while in endemic outbreak situations, around 25 to 30 percent of cats typically test positive.2
We can determine fomite versus infected status for high-risk animals in a few ways: the first is careful examination for lesions and fluorescence. When neither are found, a full-body toothbrush culture can be performed (before treating topically, just in case). Full-body cultures are semi-quantitative: fewer than five colonies typically indicates fomite carriage rather than infection.7
The time to colony growth on culture for fomite cats is longer, typically more than a week or ten days.3 Subsequent culture will be negative, and no lesions will be detected on follow-up examination.
Treatment, usually lime sulfur and an oral antifungal (itraconazole or terbinafine), may interfere with the ability to diagnose a cat with dermatophytosis on culture or PCR. (Note: Recent reports of lime sulfur being banned by the U.S. Environmental Protection Agency (EPA) are incorrect, and many distributors are carrying and selling the product. No other topical treatment has been found to be comparable in terms of speed and success, but studies are ongoing.8)
Generally, patients “cure” faster than the pteridine diminishes, and the presence of pteridine does not predict the presence of spores in treated cats, but the glow softens and can be more difficult to appreciate. Treatment also interferes with the ability of fungi to grow on culture—it takes longer, and you will have fewer colonies, and PCR may also pick up nonviable fungal nucleic acids.
The long and short of Wood’s lamp
Understanding this now, consider the history behind that “less than 50 percent.”
In the 1950s, a series of four studies were conducted on dog and cat hair samples sent to clinical laboratories, which were examined with the Wood’s lamp and grew M. canis on culture. The range of sensitivities in these reports was 38 to 54 percent, lower for dogs, and generally without details about patient age, presence of other lesions, previous Wood’s lamp examination, treatment, or number of colonies on the plate. At least 20 of the patients in two of the papers overlapped; many of these were Persian cats cured within one to three topical treatments, suggesting they may have been fomite carriers.1 I am not sure which textbook picked this number up first, but it made a lasting impact.
Recently, data from 57 additional papers correlating Wood’s lamp findings with M. canis infection were pooled by authors of the World Association for Veterinary Dermatology Clinical Consensus Guidelines for Diagnosis of Dermatophytosis in Dogs and Cats. They found Wood’s lamp sensitivity in clinical settings to be closer to 72 percent, and when examining experimentally infected cats, sensitivity was 100 percent.1
The difference between clinical and experimental populations suggests:
1) We still struggle to identify (read: misclassify) which cats are fomites and which are infected
2) Two to four weeks is a long incubation period with host- and environment-related influences and variable presentation,
3) It can be difficult to appreciate fluorescence without a good lamp, a dark room, or allowing your eyes to adjust, even when lesions are visible in plain light.
Repeating incorrect statistics is not without consequences. I frequently hear from practitioners and shelters that Wood’s lamp examinations are “so unhelpful” that they don’t use them, don’t have them, and opt for costly or delayed diagnostic options instead. Who will that cat expose and infect during the three to five days between sampling and PCR results? A child? A whole shelter of cats? Alternatively, how long will that kitten in their prime socialization period sit alone in isolation, receiving antifungal tablets and lime sulfur baths before a culture proves they were negative all along?
My final comment about Wood’s lamps is to remind vets about the predictive value of tests. Dermatophytosis is a rare diagnosis, even among cats and dogs with skin disease. A low prevalence increases the value of a negative Wood’s lamp exam in ruling out M. canis. This property can be especially useful when trying to investigate the source of infection (i.e. was it the family cat?) in human ringworm situations.
Likewise, the ability to look at a whole population, as with a litter of kittens, further improves the accuracy of the Wood’s lamp for infected and uninfected cats. Wood’s lamps are inexpensive, easy, non-invasive, welfare-supportive, and fairly accurate for cats actually infected with M. canis dermatophytosis. l
Lena DeTar, DVM, DACVPM, DABVP, is the interim director of Maddie’s Shelter Medicine Program at Cornell University’s College of Veterinary Medicine, where she lectures in the classroom, teaches vet students and interns at the Tompkins County SPCA, provides consultation services, and conducts research. Her interests include infectious disease management and prevention (especially dermatophytosis), population management, shelter surgery, and the welfare impacts of shelter design. She serves on the editorial board for the Journal of Shelter Medicine and Community Animal Health, and as vice president of the Association of Shelter Veterinarians. Dr. DeTar is a member of the Evidence-Based Veterinary Medical Association (EBVMA), with different members writing this column. While all articles are reviewed for content, the opinions and conclusions of the author(s) do not necessarily reflect the views of the EBVMA. For information about the associate, or to join, visit https://www.ebvma.org.
- Moriello KA, Coyner K, Paterson S, Mignon B. Diagnosis and treatment of dermatophytosis in dogs and cats. Vet Dermatol. 2017;28(3):266-e68. doi:10.1111/vde.12440 https://pubmed.ncbi.nlm.nih.gov/28516493/
- Gordon E, Idle A, DeTar LG. Descriptive epidemiology of companion animal dermatophytosis in a Canadian Pacific Northwest animal shelter system. Can Vet J. 2020;61(July):763-770. https://pubmed.ncbi.nlm.nih.gov/32655161/
- DeTar LG, Dubrovsky V, Scarlett JM. Descriptive epidemiology and test characteristics of cats diagnosed with Microsporum canis dermatophytosis in a Northwestern US animal shelter. J Feline Med Surg. 2019:1-9. doi:10.1177/1098612X19825519 https://pubmed.ncbi.nlm.nih.gov/30776947/
- Newbury S, Moriello KA. Skin diseases of animals in shelters: Triage strategy and treatment recommendations for common diseases. Vet Clin North Am-Small Anim Pract. 2006;36(1):59-88. doi:10.1016/j.cvsm.2005.09.007 https://pubmed.ncbi.nlm.nih.gov/16364778/
- Jacobson LS, McIntyre L, Mykusz J. Comparison of real-time PCR with fungal culture for the diagnosis of Microsporum canis dermatophytosis in shelter cats: a field study. J Feline Med Surg. 2017;20(2):1-5. doi:10.1177/1098612X17695899 https://pubmed.ncbi.nlm.nih.gov/29172910
- Moriello KA, Leutenegger CM. Use of a commercial qPCR assay in 52 high risk shelter cats for disease identification of dermatophytosis and mycological cure. Vet Dermatol. 2017;(March 2015):1-5. doi:10.1111/vde.12485 https://pubmed.ncbi.nlm.nih.gov/28833730/
- Moriello KA, Newbury S. Recommendations for the management and treatment of dermatophytosis in animal shelters. Vet Clin North Am – Small Anim Pract. 2006;36(1):89-114. doi:10.1016/j.cvsm.2005.09.006 https://pubmed.ncbi.nlm.nih.gov/16364779/
- https://blogs.cornell.edu/cornellsheltermedicine/2021/09/30/whats-going-on-with-lime-sulfur-updates-from-dr-detar-and-the-asv/ (Accessed November 2022)