Onychomycosis in Dogs

The most common fungal infection of the claw is caused by ringworm.
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Onychology, or the study of the claw, identifies a number of diseases and conditions that can affect your dog’s claws. One of the most frequently diagnosed conditions, onychomycosis, simply means a fungal infection of the claws. The most common cause of onychomycosis in dogs is ringworm infection, also known as dermatophytosis. Dermatophytosis is most commonly diagnosed in the skin and hair, but in advanced cases can spread to the claws, causing onychomycosis. Although ringworm is the most common cause, other fungal infections of the claw are reported in rare instances.

Onychomycosis Caused by Dermatophytosis

Dermatophytosis is most commonly caused by the organisms Microsporum canis, Trichophyton mentagrophytes or Microsporum gypseum. The distribution of each fungal organism varies based on geography, so dogs in different parts of the country are affected by different types of fungus. Dogs who live in areas with warm temperatures and high humidity are at higher risk of ringworm infection. It is important to recognize that dermatophytosis is contagious to other dogs, cats and people. If your dog’s onychomycosis is due to ringworm infection, you will need to take proper precautions in order to prevent spread. Your veterinarian will discuss the necessary measures you need to take to prevent transmission to people or other animals.


Onychomycosis can be caused by any type of fungus that enters the claw bed, which often happens accidentally when the skin around the claw is cut or traumatized. Once the claw or claws become infected, your dog may show signs of pain, such as limping or chewing at the claws. You may notice swelling or redness around the base of the claws, loss of some claws or discoloration or deformity of the claws. If you notice such symptoms, have your dog examined by your veterinarian immediately.


The recommended test to confirm fungal infection of the claw is a fungal culture. Your veterinarian may submit one of the claws that has fallen off from your dog’s foot, or may harvest a claw if one is needed for testing. Growing, or culturing, the fungus will allow it to be identified. The most effective drug for each fungus type varies, so this test allows your veterinarian to choose the best treatment for your dog. Your veterinarian may also sample your dog’s skin and hair, since fungal infection of the claw is often due to spread of a fungus from the skin.


Onychomycosis can be a challenging condition to treat, because infection of the nail is very deep-seated. The treatment recommended by your veterinarian may include anti-fungal medications. These may be combined with foot soaks that contain anti-fungal medications. In extreme cases that do not respond to medications appropriately, claws may need to be removed. The course of treatment often lasts for several months to ensure elimination of the fungus. Treatment generally is not discontinued until repeat culture of the claws is negative.