Tularemia

Thumb with skin ulcer of tularemia.
Tularemia is caused by the bacterium, Francisella tularensis. Symptoms vary depending on how the person was exposed to the disease, and as is shown here, can include skin ulcers.
Lesion on the dorsal skin of right hand
This Vermont muskrat trapper contracted tularemia, which manifested as cutaneous lesions on the dorsum of his right hand.
Cutaneous lesions on the dorsum of his right hand.
Find more information on this substance at: PubChem, PubMed