Use of a sentinel host system to study the questing behavior of Ixodes spinipalpis and its role in the transmission of Borrelia bissettii, human granulocytic ehrlichiosis, and Babesia microti

Ixodes spinipalpis maintains Borrelia bissettii spirochetes in Colorado in a cycle involving wood rats and deer mice. This tick has been described as nidicolous, remaining either attached to its rodent hosts or in the rodent nest. Nidicolous ticks pose little risk of pathogen transmission to humans if they do not actively quest for hosts. To investigate the questing potent ial of I. spinipalpis, sentinel mice were placed in an area where I. spinip alpis had been commonly found on wood rats and deer mice. Concurrently, wil d rodent populations were trapped and analyzed for Lyme disease spirochetes , the agent of human granulocytic ehrlichiosis (aoHGE), and Babesia microti . A total of 122 I. spinipalpis larvae and 10 nymphs were found on 19% of 2 44 sentinel mice. In addition, 4 sentinel mice became infested with Malarae us telchinus or Orchopeas neotomae fleas. Questing I. spinipalpis were posi tively associated with woody shrubs and negatively associated with sunny an d grassy areas. Four sentinel mice became infected with aoHGE after having been fed upon only by I. spinipalpis larvae. One sentinel mouse became infe cted with B. bissettii after having an I. spinipalpis nymph feed on it, and one sentinel mouse became coinfected with aoHGE and B. bissettii after it was fed upon by a single I. spinipalpis nymph. These sentinel mouse convers ions suggest the possibility that the aoHGE is transovarially transmitted b y I. spinipalpis, and that I. spinipalpis is capable of simultaneously tran smitting B. bissettii and the aoHGE. The findings that I. spinipalpis quest away from rodent nests and will attach to and infect sentinel mice may be of public health importance. It suggests the potential transmission of the agents of human granulocytic ehrlichiosis and Lyme disease to other hosts b y I. spinipalpis, in regions of the western United States where Ixodes paci ficus is not found.