Estimating Contact Process Saturation in Sylvatic Transmission of Trypanosoma cruzi in the United States

Although it has been known for nearly a century that strains of Trypanosoma cruzi, the etiological agent for Chagas' disease, are enzootic in the southern U.S., much remains unknown about the dynamics of its transmission in the sylvatic cycles that maintain it, including the relative importance of different transmission routes. Mathematical models can fill in gaps where field and lab data are difficult to collect, but they need as inputs the values of certain key demographic and epidemiological quantities which parametrize the models. In particular, they determine whether saturation occurs in the contact processes that communicate the infection between the two populations. Concentrating on raccoons, opossums, and woodrats as hosts in Texas and the southeastern U.S., and the vectors Triatoma sanguisuga and Triatoma gerstaeckeri, we use an exhaustive literature review to derive estimates for fundamental parameters, and use simple mathematical models to illustrate a method for estimating infection rates indirectly based on prevalence data. Results are used to draw conclusions about saturation and which population density drives each of the two contact-based infection processes (stercorarian/bloodborne and oral). Analysis suggests that the vector feeding process associated with stercorarian transmission to hosts and bloodborne transmission to vectors is limited by the population density of vectors when dealing with woodrats, but by that of hosts when dealing with raccoons and opossums, while the predation of hosts on vectors which drives oral transmission to hosts is limited by the population density of hosts. Confidence in these conclusions is limited by a severe paucity of data underlying associated parameter estimates, but the approaches developed here can also be applied to the study of other vector-borne infections

Delivery of an HIV Prevention Counseling Program in an Infectious Diseases Clinic: Implementation Process and Lessons Learned

Current national guidelines recommend that all HIV care providers routinely counsel their HIV-infected patients about reducing HIV transmission behaviors. In this article we identify the challenges and lessons learned from implementing a provider-delivered HIV transmission risk-reduction intervention for HIV-infected patients (Positive Steps). Based on a multi-site Centers for Disease Control and Prevention (CDC) initiative, we integrated the Positive Steps program into an infectious diseases clinic in North Carolina. Of the nearly 1200 HIV-infected patients, 59% were African American, 44% were white, 33% were women, and over 50% were between 25 and 44 years of age. We obtained feedback from a community advisory board, input from clinic staff, and conducted formative interviews with clinic patients and providers to achieve overall acceptance of the program within the clinic. Clinic providers underwent training to deliver standardized prevention counseling. During program implementation we conducted a quality assessment of program components, including reviewing whether patients were screened for HIV transmission risk behaviors and whether providers counseled their patients. Once Positive Steps was implemented, on average, 69% of patients were screened and 77% of screened patients were counseled during the first 12 months. In analyses of quarterly exit surveys of patients after their medical exams, on average, 73% of respondents reported being asked about safer sex and 51% reported having safer-sex discussions with their providers across six quarterly periods. Of those who had discussions, 91% reported that those discussions were “very” or “moderately helpful.” Providers reported time and competing medical priorities as barriers for discussing prevention with patients, however, provider-delivered counseling was routinely performed for 12 months. Overall, the findings indicate that the Positive Steps program was successfully integrated in an infectious diseases clinic and received well by patients