Answer to Case 723

Answer to the Parasite Case of the Week 723: mite, not scabies. Given that this mite was found in a fungal culture from skin scrapings, it could very well be a dust mite (Dermatophagoides sp.) as many of you suggested. However, mites are challenging to identify, so genus/species level identification is best left to the acarologists (people who study mites and ticks). 

The biggest concern here is its location - on a fungal culture in the mycology laboratory. Mites are a terror in the mycology lab. They crawl from plate to plate, contaminating and destroying cultures from patient samples. In most cases, the culture plates need to be destroyed and the lab decontaminated. This is the reason why my colleague, Dr. Wengenack, was so upset to find these! However, she knew of my interest in mites and was kind enough to donate this case to the blog.

Some readers questioned if these were scabies mites. Fortunately scabies mites (Sarcoptes scabei) can be easily differentiated from most zoonotic and environmental mites by their rounded bodies and short legs:

Case of the Week 723

This week's case was donated by Dr. Nancy Wengenack and the Mycology lab at Mayo Clinic. These objects were found in a fungal culture for dermatophytes (lactophenol blue stain). What are they, and what is their significance?

Answer to Case 819

 Answer to the Parasite Case of the Week 819: Mite, not a human pathogen. 

Nice job to everyone who recognized it! As several of you noted, its presence in a urine specimen is almost certainly incidental and represents environmental contamination rather than a true infection. There are many different types of mites in the environment, and identification to genus or species can be quite challenging and generally requires examination of specific morphologic features by someone with expertise in acarology. Fortunately, we don't need to do that here. From a clinical laboratory perspective, the important thing is recognizing this as a mite and determining whether it could be medically significant.

Several readers correctly emphasized that this is not the scabies mite, Sarcoptes scabiei. Idzi nicely pointed out that S. scabiei has a much more rounded body with short, stubby legs. Compare our mite with the classic appearance of S. scabiei, and the difference is quite striking. The very long setae ("hairs") and long legs on our mite are also a useful clue that we are dealing with something else. As Florida Fan so aptly put it: “What so hairy! Yet so small…” — a mighty mite indeed! Florida Fan also noticed that we've encountered similar visitors before in Cases 548 and 634. Here is a nice image from Case 723:

While this is not a human parasite, it doesn't necessarily mean it is incapable of bothering humans. Some mites that normally parasitize birds, rodents, or other animals can opportunistically bite humans. Bird mites are a good example, particularly when their usual avian hosts disappear or abandon a nest. They do not establish a Sarcoptes-like infestation in humans, but their bites may cause pruritic papules and can be quite bothersome. Therefore, the clinical history is important. An incidental mite found in a specimen from an asymptomatic patient is quite different from mites repeatedly recovered in the setting of an ongoing unexplained dermatitis. In the latter situation, it may be worthwhile to have specimens formally identified by an entomologist or other appropriate expert and to investigate the home or workplace for a possible source. Pest-control professionals may also be helpful in identifying and eliminating an environmental infestation. 

So, what should the laboratory report? Something simple such as “Mite identified; not consistent with Sarcoptes scabiei” would be reasonable. Further identification is unnecessary unless there is a specific clinical or epidemiologic reason for doing so.

Thanks again to everyone for the excellent discussion! And many thanks to Dr. Vasundhara (Vasu) Rangaswamy for sharing this case with us. As I mentioned previously, Dr. Rangaswamy practices in rural India and has generously shared the following photographs of herself performing a slit-skin smear on a patient with leprosy. In one of the images, she is using a separate light source for microscopy, as reliable electricity is not always available in her rural setting.

Dr. Rangaswamy has kindly given me permission to share these photographs and hopes that they will help raise awareness of the realities of practicing medicine in rural, resource-limited settings. They are also a wonderful reminder of the ingenuity and dedication required to provide essential diagnostic services when many of the resources we routinely take for granted are not readily available.