Tick-borne transmission of bacterial pathogens in the order Rickettsiales is responsible for diverse infectious diseases, many of them severe, in humans and animals. Transmission dynamics differ among these pathogens and are reflected in the pathogenvector interaction. Anaplasma marginale has been shown to establish and maintain infectivity within Dermacentor spp. for weeks to months while escaping the complex network of vacuolar peptidases that are responsible for digestion of the tick blood meal. How this prolonged maintenance of infectivity in a potentially hostile environment is achieved has been unknown. Using the natural vector Dermacentor andersoni, we demonstrated that A. marginale-infected tick vacuoles (AmVs) concurrently recruit markers of the early endosome (Rab5), recycling endosome (Rab4 and Rab11), and late endosome (Rab7), are maintained near neutral pH, do not fuse with lysosomes, exclude the protease cathepsin L, and engage the endoplasmic reticulum and Golgi apparatus for up to 21 days postinfection. Maintenance of this safe vacuolar niche requires active A. marginale protein synthesis; in its absence, the AmVs mature into acidic, protease-active phagolysosomes. Identification of this bacterially directed modeling of the tick midgut endosome provides a mechanistic basis for examination of the differences in transmission efficiency observed among A. marginale strains and among vector populations. T icks are biological vectors of the obligate intracellular bacteria of the order Rickettsiales, including pathogens that cause severe diseases in humans and animals (1). Transmission dynamics determine the epidemiologic features of a given disease and vary among the pathogen-vector pairs. For example, Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever, is maintained in the tick vector Dermacentor andersoni through transovarial transmission, and thus each new generation of larval ticks is infected. Additionally, infection is maintained with each successive molt, circumventing the need for feeding on an infected mammalian host to maintain the pathogen through time. In contrast, Anaplasma marginale, responsible for anaplasmosis in ruminants, must be acquired and transstadially transmitted by each new generation of adult D. andersoni ticks (2). It is not transovarially transmitted, and the larvae and nymphs of D. andersoni are not exposed to A. marginale because they feed preferentially on small mammals. Adult male ticks feed preferentially on cattle, move between hosts while seeking mates, and thus are the epidemiologically important transmission vector. Consequently, A. marginale must persist in the tick for the period of time necessary for an adult tick to ingest two separate blood meals, which depending on conditions, may occur in separate seasons. Experimentally infected adult male D. andersoni ticks are able to transmit A. marginale for up to 26 days after acquisition feeding (2-4). The mechanisms by which pathogens maintain long-term infections within ticks remain po...

The ability to rapidly detect and report infectious diseases of domestic animals and wildlife is paramount to reducing the size and duration of an outbreak. There is currently a need in the United States livestock industry for a centralized animal disease surveillance platform, capable of collecting, integrating, and analyzing multiple data streams with dissemination to end-users. Such a system would be disease agnostic and establish baseline information on animal health and disease prevalence; it would alert health officials to anomalies potentially indicative of emerging and/or transboundary disease outbreaks, changes in the status of endemic disease, or detection of other causative agents (eg, toxins). As a part of its mission to accelerate and develop countermeasures against the introduction of emerging and/or transboundary animal diseases into the United States, the Department of Homeland Security is leading and investing in the development of an enhanced passive surveillance platform capable of establishing animal health baselines over time and alerting health officials to potential infectious disease outbreaks or other health anomalies earlier, allowing for more rapid response, improved animal health, and increased economic security.

A mathematical model for a two-pathogen, one-tick, one-host system is presented and explored. The goal of this model is to determine how long an invading pathogen persists within a tick population in which a resident pathogen is already established. The numerical simulations of the model demonstrate the parameter ranges that allow for coexistence of the two pathogens. Sensitivity analysis highlights the importance of vector-borne, tick-to-host, transmission rates on the invasion reproductive number and persistence of the pathogens over time. The model is then applied to a case study based on a reclaimed swampland field site in southeastern Virginia using field and laboratory data. The results pinpoint the thresholds required for persistence of both pathogens in the local tick population. However, the invading pathogen is not predicted to persist beyond three years. Understanding the persistence and coexistence of tick-borne pathogens will allow public health officials increased insight into tick-borne disease dynamics.

DNA testing of eschars represents an under-utilized diagnostic test and may aid in cases where the diagnosis is not made clinically.