Most Common Pathogens:
Dermatophytosis (Ringworm) is a superficial fungal infection of the skin and hair coat. In cats and dogs Microsporum Canis is the most prevalent in both species, with as many as 90% of cats with Dermatophytosis are infected by M. canis. Although zoonotic and highly contagious between animals, Dermatophytosis is treatable and not life-threatening. Treatment, although usually successful, can be costly, particularly in a shelter environment, where infected kittens must be isolated and withheld from adoption for prolonged periods of time until resolution is confirmed. Because of the highly contagious nature of this disease, variable clinical presentations, cost associated with treatment and higher occurrence in the most adoptable population of cats (i.e., kittens), an accurate and timely diagnosis is needed to facilitate disease identification and treatment.
Transmission and Pathogenesis:
The primary method of transmission is via direct contact with another infected animal. In some cases, the disease can be contracted via contact with contaminated fomites (including other exposed animals and human handlers). Key to establishment of the disease is damage to the skin, because healthy skin is a natural barrier to infection. Predisposing factors include but are not limited to the following: age extremes (very young or very old), geographic areas (more common in warm humid regions), concurrent systemic diseases, physiological stress, and overcrowding. The primary infective unit is the arthrospore, which is shed from infected hairs. Skin lesions develop when a critical mass of infective material contacts the skin, defeats natural host defenses, and starts to germinate. The superficial skin and hair are infected, and as the pathogen grows, it produces more infective spores. This results in hair loss, erythema (redness), and scaling of the skin. The incubation period from infection to visible lesions is 1–3 weeks, depending on where the lesions are located.
Dermatophytosis can present with variable clinical signs. Therefore, specific diagnostic tests are recommended to accurately diagnose or rule out Dermatophytosis in suspect animals. Screening by Wood’s lamp examination can be a rapid screening tool to detect infection, particularly when combined with direct examination of fluorescing hairs. However, out of the three dermatophyte species commonly identified in veterinary species, only Microsporum Canis produces detectable fluorescent metabolites. In addition, sebum and certain medications can also fluoresce, resulting in potential false-positive results.
The most widely used diagnostic or confirmatory test is a fungal culture and is considered the gold standard. In-clinic dermatophyte growth media culture plates, which include a color indicator, are available to help in identifying suspect colonies. However, many common nondermatophyte fungi may also cause color change, and if microscopic examination of fungal spores is not performed to confirm growth, there is a high risk of false-positive results.
The most significant limiting factor with fungal cultures as a diagnostic test is that it can take 7–21 days to determine if a specimen is culture positive or culture negative. This lag time may result in spread of the disease from an infected animal to another susceptible host, ongoing cost of isolating a suspect animal, or unnecessary treatment of an animal if the decision is made to treat pending results. Both situations can have serious health consequences. In people, this situation is further compounded by a higher rate of false-negative cultures as a result of daily hygiene practices. Because of the need for a faster and more accurate diagnostic test, diagnosis in people is increasingly being made via molecular testing with polymerase chain reaction (PCR). This technology is now widely available for use in veterinary patients.
IDEXX Panel (3565):
The Ringworm (Dermatophyte) RealPCR™ Panel is an accurate diagnostic tool for Dermatophytosis in cats and dogs, providing results in 1–3 working days, dramatically faster than traditional fungal culture. The panel includes Microsporum spp., Microsporum canis, and Trichophyton spp. real-time PCR tests and performs with greater than 95% sensitivity and 99% specificity.