Ringworm!: the truth in your cats and dogs

What is ringworm?

“Ringworm” aka dermatophytosis is a skin disease that is not caused by worms at all, but rather a fungus. It is fairly common in companion animals such as dogs and especially cats, but other mammals are susceptible including: humans and domestic animals (cattle, sheep, etc.).

A number of different fungi species can cause ringworm, but most common are:

• Microsporum canis: nearly 100% of cat ringworm; 60-70% of dog; highly contagious & zoonotic
• Microsporum gypseum: environmental contaminant; common cause of infection in immune-suppressed people
• Trichophyton mentagrophytes: ringworm organism in a number of different species (rodents, guinea pigs, rabbits); rarely causes disease in humans

Microsporum canis is the most common organism causing ringworm in cats. Up to 20% of cats may be asymptomatic carriers (showing no signs of infection) of the fungus.

The fungi that cause ringworm feed on the outer layer of skin (keratin), hair and nails. In general, ringworm is not a life-threatening or horrible condition and is usually managed easily with anti-fungal medications and environmental treatments.

Where does ringworm come from?

As mentioned, there are a number of different species of fungi that can cause ringworm. Some species are found mostly on living mammals whilst some are more prevalent in the environment (dirt, dust). Thus, ringworm (dermatophytosis) can be transmitted by direct contact involving an infected animal or via a contaminated environment.

Heat and moisture help fungi grow, and skin lesions often occur where there has been a previous break in the skin's natural defense barrier (for example: areas where scratches or abrasions have occurred).

Animal predisposition for ringworm infections is seen in: multiple pet households/environments, young animals and older or immunocompromised animals. Certain breeds such as long-haired cats (*especially persians!) also have an increased risk.

Clinical signs

In animals:

• May have no obvious clinical signs!
• Hair loss – both dramatic and subtle can be seen
• Mild scaling of skin
• Reddening of skin
• Fine crusting
• Scratching

(Ringworm on a cat's belly)

      (Ringworm on the face)

 

(Ringworm on a cow)

                                                                                                 (Ringworm on a dog)

In people:

• Ringworm or 'tinea' can appear in a number of areas on people including but not limited to: the face or neck, scalp, groin, feet, hands, nails or body (pretty much anywhere!)
• Rounded area of redness and swelling on the skin. Crusting may also be evident.
• Itchiness
• People handling animals will often present with lesions on the face, neck and arms (areas that are in close contact when cuddling animals)

 

Veterinary care

Because ringworm is a disease that can pass to humans (zoonosis), it is always a good idea to seek vet attention when an animal has suspicious skin problems. A general work-up for an animal with possible ringworm lesions might include:

• Trichogram – involves plucking a number of hairs & examining the hairs under microscope for evidence of fungal spores. This isn's the most accurate test for ringworm but it can give an indication of how contagious an animal might be (high number of spores = more contagious)
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

(Infected hair and a healthy hair)

• Wood's Lamp examination – involves using a special light to examine the skin. Approximately 50% of cases of Microsporum canis will fluoresce a bright apple green colour under the lamp. Unfortunately, this is not the most reliable test as not all species will glow under the light and other debris and chemicals may look similar to a positive result. A positive test result is only suggestive of a dermatophyte infection. It may be a useful tool to monitor the effectiveness of therapy, but a fungal culture is still needed for a definitive diagnosis.
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  (Positive Wood's Lamp result)
  
  
• Fungal culture – is the gold standard in diagnosing ringworm and is required to confirm a diagnosis. This involves collecting hair and skin crusts and allowing the samples to grow on a sterile fungal culture medium for at least 21 days. Cultures from untreated infections usually show a positive result within 5-10 days.
• Skin biopsy- Not as commonly used to diagnose ringworm, but will confirm a diagnosis in 80% of cases.

Treatment

In pets, the clinical signs of ringworm are usually self-limiting and will resolve spontaneously with time (2-3 months). However, to speed up recovery and prevent further transmission of the disease to other animals or people (!), treatment is recommended.

Topical antifungal therapy is generally the treatment of choice for mild cases with localised skin lesions. These may include antifungal shampoo or rinses & antifungal creams such as Miconazole or Imaverol. Treatment with topical creams or lotions is usually administered daily to the skin lesions for 3-4 weeks.

Systemic antifungal therapy may be recommended (antifungal tablets) when a pet is severely affected or there are a number affected animals. These might include but are not limited to: Griseofulvin & Intraconazole.

Systemic treatment is usually continued until 4weeks past the resolution of skin lesions (or beyond the 2^nd negative fungal culture test).

Clipping: promotes faster healing and is highly recommended in long-haired cats and dogs.

Environmental decontamination: to reduce the load of fungus in the environment and lower risk of zoonosis . This requires repeated cleaning to remove organic debris as well as applications of an antifungal disinfectant. Household bleach at 50% dilution is ideal to treat all in-contact surfaces weekly until resolution of the disease – although some publications suggest a 1:10 dilution is adequate. The surface should stay wet for at least 10 minutes to kill the ringworm spores. Bleach will not kill spores in the presence of dirt, so it is important that the surface be properly cleaned before bleaching.  Ideally, treat all at-risk animals in the household (particularly in cattery-type situations). When in doubt, seek a fungal culture test for all cats in the household (even if they have no signs of ringworm!). Antifungal shampoo weekly to reduce environmental contamination.

Human risk: zoonotic potential!

A 'Zoonosis' is a disease or infection which can be naturally transmitted between vertebrate animals and people (World Health Organization defintion). Ringworm (dermatophytosis) is considered a zoonotic disease because it can be transmitted to humans from animals. However, there are other sources of the disease including: the environment and other humans!

Children and immune-suppressed people tend to be the most at-risk groups for contracting dermatophytosis from animals. Please seek advice from a physician regarding prevention and treatment of human cases of ringworm!