This article is from the Fall 1999 AFRMA Rat & Mouse Tales news-magazine.
By Carmen Jane Booth, D.V.M.
Case scenario: A person gets a rat from a pet store (rat appears fine), the rat then has babies a week later (rat still appears fine), when the babies are 4 weeks old (everyone still appears fine) the mom and babies are taken to a vet because two of the babies have megacecum (those two are euthanized) and all left there for a day for the vet to check them out; everyone goes home; 5 weeks of age everyone looks fine; then when they are 7 weeks old a couple of the babies look scruffy with patches of fur missing and scabs on the skin. Did they get the ringworm from the mother rat who never showed any symptoms and the babies never developed anything until they were 7 weeks of age or possibly from being at the vet’s office where an infected cat or dog may have been there on the premises?? The owner was given an oral medicine to treat all of her rats for 30 days. The owner did get the ringworm too. She had a couple of spots show up on her right before she noticed them on her rats. Her doctor prescribed an oral medicine for her.
Ringworm or Dermatomycosis is caused by a fungus. The group of fungi causing “Ringworm” are from the class Dueteromycetes (Fungi imperfecti). Depending upon the species, the etiologic agent is different. In rats and mice, the most common cause is Tricopyton mentragrophytes although they can become infected with other types more common to other species of animals, such as cats and dogs. Infected rats and mice are then contagious to any other animals/people. Humans are susceptible to fungal infections, but most cases come from cats.
These fungi are ubiquitous in the environment and infections are usually subclinical (no apparent symptoms or lesions). Even though it is in the environment, most cases are spread by direct contact. Animals can be infectious, but never show symptoms. Clinical lesions vary from mild areas of alopecia (hair loss) to marked hair loss with thick scaly skin. They are not thought to represent a significant zoonosis to humans. Definitive diagnosis is made by fungal culture of hair or skin scrapings. Some strains of fungi show green under UV light but you have to be very careful because UV light is very damaging to the animal’s or human’s eyes.
I would suspect from the outbreak history that someone touched a cat that had the disease and spread it to the rats. It is very uncommon in domestic rodents housed in indoor cages. In most cases, simple topical antibiotic/steroid creams or antifungal medications will clear it up. In severe cases an oral drug is used, but I shy away from it unless the problem is severe. The treatment time varies with the individual animal. Some cases resolve in as early as 2 weeks, some take a month or more. When my cat was a kitten, she gave it to me. In most cases, the fungi cannot invade the skin unless there is a scrape or other break in the skin. This disease is not fatal nor is it considered to be very significant. As a veterinarian, I have seen this most frequently in kittens from shelters.