Inferring host-parasite relationships using stable isotopes: implications for disease transmission and host specificity.

Identifying the roles of different hosts and vectors is a major challenge in the study of the ecology of diseases caused by multi-host pathogens. Intensive field studies suggested that grasshopper mice (Onychomys leucogaster) help spread the bacterium that causes plague (Yersinia pestis) in prairie dog colonies by sharing fleas with prairie dogs (Cynomys ludovicianus); yet conclusive evidence that prairie dog fleas (Oropsylla hirsuta) feed on grasshopper mice is lacking. Using stable nitrogen isotope analysis, we determined that many blood-engorged O. hirsuta collected from wild grasshopper mice apparently contained blood meals of prairie dogs. These results suggest that grasshopper mice may be infected with Y. pestis via mechanisms other than flea feeding, e.g., early phase or mechanical transmission or scavenging carcasses, and raise questions about the ability of grasshopper mice to maintain Y. pestis in prairie dog colonies during years between plague outbreaks. They also indicate that caution may be warranted when inferring feeding relationships based purely on the occurrence of fleas or other haematophagous ectoparasites on hosts. Stable-isotope analysis may complement or provide a useful alternative to immunological or molecular techniques for identifying hosts of cryptically feeding ectoparasites, and for clarifying feeding relationships in studies of host-parasite interactions.