Sunday, December 30, 2018

Answer to Case 525

Answer: Amblyomma sp. nymph

Everyone did a great job describing the diagnostic features of this tick. There were a few that were trying to determine the gender of the tick, but as Blaine reminded us, it is not possible to determine the gender of a nymph - both male and females will have a short dorsal shield. Below are some of the key morphologic features that are useful in this case.

  1. The tick has 8 legs, thus indicating that it is either a nymph or adult (vs. a larva).
  2. There is no porose area or genital aperture, indicating that it is a nymph.
  3. The anal groove is posterior to the anus (some call the groove "chalice-shaped"), thus eliminating Ixodes spp. 
  4. The basis capituli does not have lateral projections (i.e. it is rectangular rather than hexagonal), thus ruling out Rhipicephalus spp.
  5. The palpi are longer than the basis capituli, thus ruling out Dermacentor and Haemaphysalis species. 
  6. It has eyes, thus ruling out Haemaphysalis and Ixodes species.
  7. There are festoons present, although they are difficult to see. These are seen with Amblyomma, Dermacentor, and Haemaphysalis species, but not Ixodes spp. 

This constellation of features is consistent with an Amblyomma species. You would need to have a better view of the hypostome (absent in this case) and coxal spurs in order to make it to a species-level identification.

Old One made an interesting comment which I hadn't heard before: "The broken hypostome and chelicera hints at the possibility that this tick has long mouthparts that were cemented deep in the host's tissue. Ticks with shorter mouth parts (e.g. Dermacentor) cement at the surface of the skin, often leaving a skirt of cement attached to the capituli (may be confused as host skin) and often retain their mouthparts." He also noted "The high activity level of this tick is a characteristic seen in Amblyomma and even more so in its cousin Hyalomma. They are sometimes described as almost predatory. Literally one can hear them running through grass to get to their host (prey)." This is behavior I have witnessed first hand, and am dreading the day that Amblyomma americanum makes its way up into Minnesota!

If you are looking for resources on tick identification, you can start with some easy guides such as the ones that Blaine and I published (CAP arthropod bench reference guide, and our CMR article "Laboratory Identification of Arthropod Ectoparasites"). These contain references for species-specific keys as well. There is also a nice key in Lynn Garcia's text and the Manual of Clinical Microbiology chapter on arthropods. Georgia Southern University also has a fun interactive key that walks you though some of the main decision points for tick identification. Happy tick hunting!

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