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Can cats be asymptomatic carriers of ringworm?

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Date:
Authors: Dr. Christina Barrett
Document Type: FAQs
Topics: Infectious Disease
Species: Feline

Dr. Barrett answers the question of whether a cat can be an asymptomatic carrier of ringworm, discusses the effectiveness of screening tools like the Wood’s lamp, and explains how to determine whether a positive fungal culture indicates true infection.

Question:

Hi! We adopted a cat that we believe might be an asymptomatic ringworm carrier because my son and I both have contracted the fungus. We took the cat to the vet hoping to get more info but she said all the tests used to check for ringworm are inaccurate at best. She didn’t take any cultures or scan him with a Wood’s lamp, but she did check him over and say he showed no obvious signs of an infection but recommended we give him lime sulfur dips “just in case.” We are wanting to bring him into the house, but I’m worried that I’ll never know if he’s clear of the fungus (or if he ever had it!). Can cats be asymptomatic carriers of ringworm?

 

Answer:

Hi and thank you for your question! Feel free to share this reply with your veterinarian as some of the information we’ll go into will be more relevant to a practitioner. 

 

The most commonly isolated pathogen causing ringworm is Microsporum canis. While infection with M. gypseumM. persicolor and Trichophyton spp. can occur, they are of little to no clinical concern. M. gypseum is contracted mainly through contaminated soil, and M. persicolor and Trichophyton spp. mainly through exposure of rodent nests; these fungi are rarely associated with transmission to people and not known to cause outbreaks.

There are several tests that can be used to determine if a cat has ringworm; these are very reliable when used correctly. An easy screening test to perform is a Wood’s Lamp exam and if done properly it has a 90-100% chance of detecting M. canis positive cats.1 More information on the usefulness of the Wood’s lamp as a screening tool can be found here and details on how to perform a Wood’s Lamp exam can be found here. A direct microscopic exam can also be performed; this is usually used to confirm results of a Wood’s Lamp exam. This test looks for the presence of infected hairs (swollen, frayed, irregular or fuzzy outline, etc.) and sometimes fungal hyphae can also be seen. A positive Wood’s Lamp and direct microscopic exam can confirm the presence of ringworm, but negative results do not rule it out. The most reliable way to diagnose ringworm is by quantitative fungal culture. If you suspect ringworm (i.e. have lesions on physical exam or consistent history, etc.) then a culture should be completed even if Wood’s Lamp and direct microscopic exams are negative. Without the proper exam and diagnostics, ringworm can be missed.

 

We try not to use the term carrier when describing cats with ringworm as the term carrier implies that the cat “harbors a specific infectious agent without discernible clinical disease and serves as a potential source of infection.” Ringworm is not a disease that cats will harbor for long periods of time, nor is it a disease that will lay dormant in a cat and resurface during periods of its lifetime. If a cat’s fungal culture is “positive” for M. canis, we need to determine whether they are truly infected with ringworm or if they are a “dust mop.” The term “dust mop” is used to describe a cat that has spores on their fur from the environment but is not actually infected. These cats will be free of lesions, have a negative Wood’s Lamp exam, and have minimal colony growth on fungal culture. To quantify culture results, a Pathogen Score (P-score) is used when examining the colony growth on the culture plate. A P1 score is categorized as 1-4 colonies, P2 is 5-9 colonies and P3 is greater than 10 colonies. A P3 score generally indicates a true infection while a P1 and P2 may mean the cat is a “dust mop” and not actually infected. For cats with a P1 or P2 score, a physical exam and thorough Wood’s Lamp exam should be repeated; if still no lesions and Wood’s Lamp negative than it would be considered a “dust mop” and not actually infected with ringworm. Next steps at that point are dependent on the individual situation. Some veterinarians may feel comfortable adopting the cat out at that time, knowing that any residual spores have likely been groomed off over the last 7-14 days while the culture was incubating. Others may choose to apply lime sulfur once (to eliminate any spores that may still be present on the coat) prior to placing up for adoption.

 

With the above information in mind; we would suggest if you are worried your cat may have ringworm or may be a “dust mop” to have your veterinarian start with screening tests and determine next steps after those results. I hope you found this helpful and if you would like to read more on ringworm please follow the links below.  

 

1. Diagnosis and treatment of dermatophytosis in dogs and cats. Clinical Consensus Guidelines of the World Association for Veterinary Dermatology

2. UW SMP Ringworm Guidebook 

 

Please let us know if you have any further questions or concerns!

 

University of Wisconsin Shelter Medicine Program

Christina Barrett, DVM
Maddie’s Shelter Medicine Intern 
Shelter Medicine Program
University of Wisconsin – School of Veterinary Medicine
www.uwsheltermedicine.com
www.facebook.com/UWShelterMedicine