Let’s continue on our ringworm series of posts! Today, we’ll focus on ringworm cultures. Many of the shelters I visited last year were doing their own. And I was told once by a technician that “when the medium turns red, it means that the animal is infected.” Must admit I was usually outsourcing those tests to an external reference lab when I was at the vet faculty in Paris, but I thought: “Ok, let’s have a look in the literature and see what they say about these tests.”
Here is what I found:
- These cultures are usually realized on a Dermatophyte Test Medium (DTM). It consists of a nutrient medium plus inhibitors of bacterial growth and phenol red as a pH indicator.
About the medium
- Several variants are available, but despite some claim to speed growth the culture, this has not been found clinically significant.
- DTM containers should be loosely covered and placed in a plastic bag , to prevent dessication, cross-contamination and mite infestation.
- Incubation should be done at room temperature, between 21 to 23.8ºC.
- No need to incubate cultures in the dark: light exposure will not adversely affect fungal growth.
About the interpretation
- Dermatophyte colonies may appear as soon as 5 to 7 days post-inoculation. They almost develop within 14 days, assuming the animal has not been treated. For an animal that has been treated, it’s better to wait up to 21 days.
- Most often, large number of colonies will appear on the plate if the animal is truly infected.
- Plates should be inspected daily for a color change of the medium to red and growth of a white fungal colony. Change of color confirms that a fungi is growing but…
- … all fungal growth, including non pathogens, will lead to this red color change! Dermatophyte colonies are however never green, gray, brown or black.
- An immediate red color change is NOT definitive for pathogenic fungi. Non pathogen fundi can indeed appear grossly identical to those of dermatophyte colonies.
- Suspect colonies MUST be examined microscopically. After 7-10 days of growth, most colonies will begin to produce spores, whose morphology will allow specific identification of pathogens (see here)
- If spores are not visible, the procedure should be repeated 4 to 7 days later.
- A suspect colony that fails to produce spores or is difficult to identify should be sent to a qualified diagnostic lab.
So RED does not mean positive and it is important to confirm this finding under the microscope. As you can understand then, a definitive diagnosis of ringworm is not something easy to reach. Recently however, a new DNA test became available, and might make the procedure much easier. Wait and see then!
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