Case #268 – January, 2010

A woman residing in the eastern United States brought to her doctor a tick, which she claimed was removed from her husband after he had spent the weekend doing yard work. The tick was sent to the State Entomologist for identification. Figure A shows the tick. Figure B shows a close-up of the mouthparts. Figure C shows a close-up of one of the spiracular plates. What is your diagnosis? Based on what criteria? Also, what is the public health importance, if any, of this genus in North America?

Figure A

Figure B

Figure C

The tick depicted in this case was identified as Dermacentor variabilis, commonly known as the American dog tick. Diagnostic morphologic features included:

an ornate dorsal shield (DS, Figure A).
the presence of festoons (FS, Figure A).
mouthparts (MP, Figure B) short, in relation to the basis capituli (BC, Figure B).
slight prolongation of the spiracular plate (arrow, Figure C).

Figure A

Figure B

Figure C

In addition to identification, we asked about the public health importance of this genus in North America. Members of the genus Dermacentor are the primary vectors of Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever (RMSF). Dermacentor spp. can also transmit Colorado tick fever virus, Francisella tularensis (tularemia), and have been implicated in tick paralysis.