Prolonged diapause and the stability of host-parasitoid interactions.

We investigated the effect on host-parasitoid dynamics of prolonged diapause, a feature of the life history of many animals living in unpredictable environments, by modifying the classical May (J. Anim. Ecol. 47 (1978) 833) host-parasitoid model. We considered three patterns of development of host and parasitoid: (a) prolonged parasitoid diapause controlled by host physiology, (b) parasitoid interference in host development, preventing parasitized hosts from prolonging diapause, and (c) host diapause independent of parasitoid attack. We found that single-year prolonged diapause shifted the boundaries of the May model towards a slight increase in stability. Longer periods of diapause prolongation had a stronger influence, but this influence remained modest if we considered realistic parameter values. In contrast to other recent studies, our results suggest that prolonged diapause does not necessarily compensate for the destabilizing effects of time lags on the influence of parasitoids on population dynamics.

El transporte público y la dinámica de la tuberculosis a nivel poblacional / The public transporte and the dinamic of the tuberculosis in population level

En este artículo hemos introducido, por primera vez, un modelo matemático para el estudio de la dinámica de enfermedades transmisibles, que toma en consideración el impacto de transmisión en dos ambientes diferentes para una población heterogénea. Calculamos el número reproductivo básico y determinamos el impacto de homotransmisiones y heterotransmisiones en el número de infecciones secundarias

El transporte público y la dinámica de la tuberculosis a nivel poblacional / The public transporte and the dinamic of the tuberculosis in population level

En este artículo hemos introducido, por primera vez, un modelo matemático para el estudio de la dinámica de enfermedades transmisibles, que toma en consideración el impacto de transmisión en dos ambientes diferentes para una población heterogénea. Calculamos el número reproductivo básico y determinamos el impacto de homotransmisiones y heterotransmisiones en el número de infecciones secundarias

Transmission and dynamics of tuberculosis on generalized households.

Tuberculosis (TB) transmission is enhanced by systematic exposure to an infectious individual. This enhancement usually takes place at either the home, workplace, and/or school (generalized household). Typical epidemiological models do not incorporate the impact of generalized households on the study of disease dynamics. Models that incorporate cluster (generalized household) effects and focus on their impact on TB's transmission dynamics are developed. Detailed models that consider the effect of casual infections, that is, those generated outside a cluster, are also presented. We find expressions for the Basic Reproductive Number as a function of cluster size. The formula for R0 separates the contributions of cluster and casual infections in the generation of secondary TB infections. Relationships between cluster and classical epidemic models are discussed as well as the concept of critical cluster size.

A model for tuberculosis with exogenous reinfection.

Following primary tuberculosis (TB) infection, only approximately 10% of individuals develop active T.B. Most people are assumed to mount an effective immune response to the initial infection that limits proliferation of the bacilli and leads to long-lasting partial immunity both to further infection and to reactivation of latent bacilli remaining from the original infection. Infected individuals may develop active TB as a consequence of exogenous reinfection, i.e., acquiring a new infection from another infectious individual. Our results in this paper suggest that exogenous reinfection has a drastic effect on the qualitative dynamics of TB. The incorporation of exogenous reinfection into our TB model allows the possibility of a subcritical bifurcation at the critical value of the basic reproductive number R(0)=1, and hence the existence of multiple endemic equilibria for R(0)<1 and the exogenous reinfection rate larger than a threshold. Our results suggest that reducing R(0) to be smaller than one may not be sufficient to eradicate the disease. An additional reduction in reinfection rate may be required. These results may also partially explain the recently observed resurgence of TB.

Combination therapy with hydroxyurea versus without hydroxyurea as first line treatment options for antiretroviral-naive patients.

PURPOSE: This study analyzed whether combination therapy with hydroxyurea (HU) could be considered as first line treatment for antiretroviral-naive patients. METHOD: The prospective open-label study was carried out from March 1996 to May 2000. The antiretroviral treatments were treatment 1-didanosine 400 mg/day, stavudine 60/80 mg/day, and HU 500 mg/day; treatment 2-two nucleosides plus a protease inhibitor; treatment 3-didanosine, indinavir, and HU (500-1,000 mg/day). The viral load (VL) and CD4 determinations were performed at weeks 24, 48, 72, and 96. RESULTS: The sample comprised 284 patients. The distribution of patients by levels of VL and CD4 were similar in the three treatment groups. At week 24, patients receiving T1 and T3 achieved higher percentages of undetectable VL (89% and 81%, respectively) with no significant differences (p =.127) between them. The T2 group showed a lower proportion (58%) of undetectable VL, which was significantly lower than T1 (p <.0001) and T3 (p <.0007). At week 48, the results were similar to week 24. At week 96, nearly all patients had undetectable viral load (UVL). The analysis of adverse effects showed that the T2 group at week 48 had a greater proportion of adverse effects that was significantly different from T1 (p =.0026); T3 had intermediate values with no significant difference from T2 (p =.45) and from T1 (p =.048). At week 48, T1 showed higher adherence level with significant difference from the other two treatments. CONCLUSION: Patients were followed for some 96 weeks and, with an intention-to-treat analysis, were found to do better virologically and Clinically in treatment groups containing HU. The combination of antiretroviral drugs with HU may be an excellent option as initial therapy because of its strong antiretroviral action, its lower rate of adverse effect, and the smaller cost as compared to other regimens.