Sunday, January 13, 2008

Answer to Case 4

Identification? Trombiculid mite.
The larvae are referred to as 'chiggers'. Not to be confused with 'jiggers' caused by the 'jigger flea' Tunga penetrans. My entomology professor gave me the following memory aid to distinguish them: Jiggers Jump (they're fleas), whereas mites don't.

Name a disease it spreads: This little mite spreads scrub typhus in its larval form.

There weren't a lot of responses to this case, and I admit that I'm not surprised. It's a challenging arthropod to identify. I put it in here to point out the main differences between mites and ticks. Both are arachnids with 6 legs in their larval form and 8 in their adult form. The bodies are not segmented which allows differentiation from insects. Mites are usually smaller than ticks, but this is not the most reliable feature to distinguish them. Both tick and mite larvae can be microscopic.

Here are two useful features for identification.
First, mites usually appear 'hairy' (due to presence of setae) whereas ticks do not. Note the fine hair-like structures on the above example. Second, ticks have a central piercing mouthpart called a toothed hypostome (arrow below).

Top: Example of a tick larva. The arrow points to the toothed hypostome (click on image to enlarge).

Note that this is absent in the mites. Also, note that the tick larva does not demonstrate any 'hairs'/setae. Otherwise, the 2 organisms are very similar.

mite mouth parts (note no toothed hypostome)

Other mites of medical importance include the scabies mite Sarcoptes scabiei, and the house-dust mite.

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