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Ringworm on Dogs and Cats
May 7, 2023
By simply mentioning ringworm, the stress level quickly rises among cat people and shelters. Difficult to treat with a long incubation period, ringworm is one of the most complex infectious diseases affecting shelters. With the constant influx of animals, ringworm is often brought in with an animal, either purchased or returning from a show.
What is Ringworm?
Both cats and dogs can get ringworm, or dermatophytosis, but cats give us more trouble when we try to eliminate it. Even though we know how to treat fungal infections, cats can carry ringworm spores a long time. They’ll break when their immune system is down, and often during queening. There have been increased reports of ringworm cases in long-haired cats and Yorkies.
There are thirty fungal species that can affect dogs, birds, and cats, but we usually only see the three main ones: Microsporum canis, M. gypseum and Trichophyton spp. M. Canis causes 90 percent of the infections in cats, while dogs may have any of the above species. Ringworm is uncommon for dogs that live in colder climates. Though fungal infections thrive in warm, moist climates, animals can unfortunately get ringworm in any climate.
Fungal spores are the size of dust and they bury themselves in your pet’s hair. An infected animal can shed spores that can live in the environment for up to two years. In an animal with ringworm, the shedding hair is infective and is easily spread while grooming or medicating. Some people feel cats get subclinical infections, but it’s more likely they are just mechanically carrying the spores while resisting the actual infection themselves.
Fungal spores can only get a foothold on the skin if there is some trauma or abrasion – even small abrasions will do. External parasites also play a role because scratching trauma sets the skin up for fungal infection. Once attached, the spores grow slowly and take two to four weeks to show clinical signs.
What Does Ringworm Look Like on Cats and Dogs?
Dog and cat ringworm shows up as a small round lesion that causes patchy hair loss. It is not usually itchy, and most animals feel little discomfort. Young animals have more generalized infections, while older animals have more focal spot infections. Nursing kittens often get it on their face, ears, and front legs. Ringworm infections vary on dogs—puppies have typical ringworm spot lesions, but it’s less common with adult dogs. Most lesions on adult dogs are typically mange or bacteria.
How to Diagnosis Ringworm in Dogs and Cats
There are four main ways to diagnose cat and dog ringworm. Fungassay® Ringworm test kits require a hair sample from the infected area. Place the infected hair on the test medium. The test kit provides the proper environment for the ringworm spores to continue growing. Within one to two weeks the color will change to indicate diagnosis.
The other option is fluorescence, or a Portable Woods Lamp. Unfortunately, only M. canis will show under black light, and topical treatments like iodine will also fluoresce. Because of this, untreated animals are the best candidates for black lights. Leave the lamp on for five minutes to obtain a stable, reliable wavelength. Infections should be a bright, apple-green color – if not, it’s a false fluorescence.
The third way to diagnose ringworm is a PCR test on the skin and hair. The fourth, more invasive way is with a surgical skin biopsy.
If you don’t feel comfortable diagnosing ringworm yourself, your veterinarian can collect scales and crust from the animal’s skin and perform a fungal culture or examine hairs under a microscope.
Treating Ringworm on Dogs and Cats
Ringworm treatment for dogs and cats must be safe, effective, available, and easy to use. Vet Basics® Lime Sulfur Dip is an effective topical treatment both for eliminating infection and reducing the shedding of spores that lead to environmental contamination. Lime sulfur has a strong odor, but it dissipates soon after dipping.
Cats or dogs should be bathed first, then dip them. You do not need to immerse the animal in the solution, just be sure to wet the fur to the skin. The correct dilution is 8 oz/gallon of water. You can use a sink, a pump sprayer, or an ortho sprayer, depending on the size of animal dipped. Troublesome cats can be put in a wire cage and treated with a sprayer.
Lime Sulfur Dip can be used on nursing queens and kittens as young as four weeks. Be sure to keep the animals warm until they are dry, especially kittens and puppies that can’t regulate their own body temperature. Wipe the dip off of the mammary glands before returning a queen to her kittens.
New ringworm spots should be treated immediately with Vet Basics® ChlorConazole™ Spray to stop further transmission. It is also effective to treat current infections after shampooing or dipping. You can also apply All Purpose Nu-Stock to infected areas on dogs over 12 weeks of age to provide fast and effective relief from ringworm.
A newborn’s immune system is suppressed at birth, so that is the time they are most susceptible. Pregnant moms can be bathed with Vet Basics® Chlor 4 Shampoo or Vet Basics® ChlorConazole™ Shampoo before birth to stop the transmission of spores to the babies. Humane groups will find it helpful to bathe pregnant females that have an unknown history or are not in the best condition. Lime Sulfur can be used if necessary, but it is stressful for the pregnant mom. Most important, most oral therapy is not safe for pregnant animals!
Oral Treatment of Ringworm
A combination of topical and systemic treatment is the most successful at eliminating fungal infections in catteries. Itraconazole is given orally once daily at five mg/lb for 21 days. Toxicity is rare in cats and dogs, so this drug is much safer than drugs used in the past. Itraconazole accumulates in the keratin of the hair and skin, so it treats from the inside out and supports the immune response.
Terbinafine is also a safe drug for dogs and cats with few side effects. This is dosed at 20–30 mg/kg PO q 24 hr until cure.
Ketoconazole is best reserved for dogs. It is not recommended orally for cats because of safety, plus it is not very effective orally. It is very effective topically and safe in both species. Dogs are dosed orally at five mg/lb/daily – once a response is seen, you can give every other day until it’s cured.
Griseofulvin has a narrow margin of safety with significant side effects of bone marrow suppression in cats, so it should be avoided. It is safer and considerably less expensive in large dogs. Griseofulvin is used at 25 mg/lb/daily dose, divided between AM and PM, and it’s often given for 50 days. Griseofulvin is no longer used often for this.
Can Ringworm Live in the Environment?
When it comes to the environment, remember that ringworm spores can and will be anywhere. Vacuum your carpets, furniture, and other fabrics frequently, and throw the bag away. Whatever disinfectant you use, be sure it kills fungal spores! Disinfect all bedding, laundry, cages, and equipment with a disinfectant that has residual properties, such as Health Guard™ Laundry Additive & Disinfectant. Airborne transmission is minimal if your disinfecting practices are adequate, though spores will still float in the environment.
In the nursery, Chlorhexidine disinfectant is very good on ringworm, plus it’s safe and can be used in and around the kittens or puppies while nursing. If the mom was bathed before queening and you use Chlorhexidine in the nest box, you’ll leave few places for the ringworm spores to establish themselves.
Oxine, Rescue, and Virkon all are reasonable against fungal infections as well. Fogging with Oxine helps us get a handle on the spores. It settles where we do not get spray disinfectant and hopefully get the spores in those tiny spaces. It’s safe to do without an activator where the animals are.
Ringworm is contagious to humans, so it’s even more important to keep fungal infections under control! When we send a kitten or puppy home, it’s important the fungal spores don’t go with them. Bathe the kitten or puppy shortly prior to moving them into a new home. With the right disinfection program, we can prevent ringworm spores from colonizing your facility.
If you have more questions on how to treat ringworm on dogs and cats, call us at 800.786.4751.
Updated by Marty Greer, DVM, Revival’s Director of Veterinary Services
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Written by: Donald Bramlage, DVM
Donald Bramlage, Doctor of Veterinary Medicine, practiced veterinary medicine for 30+ years and is known for his work in managing parvovirus. He received his Doctor of Veterinary Medicine from Kansas State University in 1985. He served as Revival’s Director of Veterinary Services from 2011 until his retirement in 2019.