What Treatment for Family Pet When Tot Has Tinea?

In his “Dermclinic” discussion of tinea corporis (CONSULTANT, June 2008, page 517), Dr David Kaplan stated that “the family was advised to have their dog examined by a veterinarian because the pet was the suspected source of the dermatophyte.” Examination of the family dog is probably not necessary and can be an expensive recommendation. It is true that ringworm is a zoonotic infection that can be passed to and from both dogs and cats. However, the causative organism is also ubiquitous in our environment. Other people as well as surface contact can also be sources of infection. Moreover, the zoonotic type and the one that most often causes ringworm in dogs and cats—Microsporum canis—is a relatively uncommon cause of human infection in the United States.¹ M canis accounts for only about 3% to 4% of cases of ringworm in humans. Instead, most of these cases are caused by Trichophyton rubrum (41% to 55%), Trichophyton tonsurans (31% to 45%), or Trichophyton mentagrophytes (6% to 9%).^2-6 Finally, with the exception of Yorkshire terriers and long-haired cats, such as Persians, very few dogs and cats are asymptomatic carriers of ringworm.^7,8

Unfortunately, treating pets for ringworm is expensive, burdensome, and often ineffective. Treatment usually requires multiple lime-sulfur dips or other harsh chemicals for periods of 21 to 126 days.⁹ The cost of evaluation and treatment can be considerable, and infections often persist despite treatment, especially in long-haired cats and in households with multiple pets.¹⁰

We would suggest a more conservative approach. With a single episode of ringworm in a pet owner (dog owner or cat owner), we recommend veterinary evaluation of a symptomatic pet only. Symptomatic pets are those with skin lesions such as patches of alopecia (Figure). Alternatively, the physician can obtain a fungal culture and refer those pets whose owners are positive for M canis. Frequent recurrence of ringworm in a pet owner and owning a high-risk pet (such as a Yorkshire terrier or long-haired cat) are also indications for a veterinary evaluation of the pet.

——Robert Ellis, MD
Assistant Professor of Family Medicine
University of Cincinnati

——Carrie Ellis, DVM, MS
Associate Veterinarian
Animal Hospital on Mt Lookout Square
Cincinnati

Drs Robert and Carrie Ellis raise valid points about the necessity of treating household pets. Although the literature suggests that household pets are a reservoir of infection,^1,2 there are no good studies that seek to identify whether the dermatophyte found on an affected child is the same one that can be isolated on an asymptomatic pet. Further complicating matters is the tendency of most researchers either to group children and adults together, as seen in the articles cited by the Ellises, or to not identify the specific dermatophyte that is associated with a specific anatomical location. These tendencies make it more difficult to determine whether infections in animals correlate with infections in children.

I have taken a less aggressive—and less expensive and less labor-intensive—approach to treating pets based on my knowledge of the treatment of scalp infections in humans. Other family members, including asymptomatic parents, have been shown to be a reservoir for dermatophytes in children with tinea capitis.³ In cases where I suspect that a pet may be contributing to the infection in a child with tinea faciei or tinea corporis, I recommend washing the pet with either a selenium shampoo or an over-the-counter ketoconazole shampoo to decrease the potential dermatophyte and spore counts. The pets do not appear to suffer any ill effects, and none of my patients have had a recurrence of their dermatophyte infection. This approach is also reassuring to parents who want to do something proactive to prevent further infections. Although it is not known whether such a practice provides any benefit beyond a psychological one, this possibility should not be overlooked.

——David L. Kaplan, MD
Clinical Assistant Professor of Dermatology
University of Missouri Kansas City School of Medicine
University of Kansas School of Medicine