This project deals with the interactions of vector-borne microorganisms, arboviruses, rickettsiae and protozoans, and cells of hematophagus arthropods, mainly ticks. Model in vitro systems are developed and applied to studies of pathogen host-range, growth dynamics, persistence and interference. After prolonged growth in these systems, pathogens are examined for changes in invasiveness, virulence, or antigenic characteristics. By contract with collaborative laboratories overseas, we are studying critical factors influencing survival of ticks and of pathogens within ticks from a biochemical, histochemical, and physiological standpoints.