Ringworm (Proceedings)


Ringworm (Proceedings)

Apr 01, 2009

Ringworm in animal shelters can lead to almost 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: Microsporum canis, Trichophyton mentagrophytes and Microsporum gypseum.

Factors that increase risk of ringworm

  • Age: Animals of any age are susceptible, but young animals (<1 yr old) & geriatric animals are at the highest risk.
  • Species and breed: Cats are at greater risk than dogs. Persian cats and Yorkshire Terriers are at relatively high risk, as are long haired cats in general.
  • Immune status: Conditions that compromise the immune system such as FIV, FeLV, pregnancy/lactation, malnutrition, or anti-inflammatory drugs, stress.
  • Preexisting Conditions: Animals with preexisting conditions that compromise grooming (such as URI) or skin integrity (such as flea allergies, overgrooming, and external parasites), weaning kittens as queens are beginning to grooming them less are at increased risk.


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. Ringworm is very durable in the environment. Ringworm can persist in carriers, furniture, carpets, dust, heating vents, furnace filters and the like, and can infect animals housed in a contaminated environment months and even years later. Ringworm can be spread readily on grooming implements, contaminated toys and bedding, or by humans on clothing and hands. It can be found on the hair of animals from a contaminated environment even when the animal itself is not showing any signs. In nature, the incubation period for ringworm is between 4 days and 4 weeks.

Clinical appearance

The most common locations include the face, ears, feet and tail of cats. However, ringworm can present with a wide range of appearances, including large areas of hair loss with or without crusts and exudate. Ringworm can cause infection of the toe nails and nail beds. Ringworm may resemble or secondarily infect other conditions such as flea allergy dermatitis, "stud tail" and "chin acne" in cats. Especially consider ringworm as a possible cause of these conditions if the cat is known to have been recently exposed. Ringworm lesions may or may not be pruritic (itchy).


Accurate diagnosis of ringworm is very important. Although there is no definitive way to confirm absence of fungus except by careful fungal culture and microscopic examination there are tools that, if used correctly, can greatly help in identifying suspect cases. These "tools" include using common sense and risk assessment. For example, a classic ring shaped lesion in a kitten, especially if multiple members of a litter are affected, is very likely to be ringworm.

A - Wood's lamp
The Woods lamp is an ultraviolet light with a specific wave length of light that causes some strains of Microsporum canis to fluoresce. Although not a perfect diagnostic test, a Wood's lamp – when correctly used - can be a very 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, KMR milk and ointments (e.g. 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:

  • A true Wood's lamp should be used, as opposed to a generic UV light. Woods lamps fluoresce at a particular wave length (360 nm).
  • A plug-in, rather than battery model, is ideal as the stronger light is more likely to generate fluorescence.
  • Perform the exam in a completely dark room.
  • Allow the light to warm up for 5-10 minutes, and hold the lamp over the suspect areas for at least 5 minutes, as some strains take time to fluoresce.
  • Look the animal over carefully, especially on the face, feet, belly, and inside the ears

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.

B - Direct microscopic examination
Like the Woods lamp, positive findings in direct examination can diagnose ringworm, but negative findings do not rule it out. False negatives occur between 40% - 70% of the time. 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. The slide is then allowed to stand for 30 minutes at room temperature. 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.