Katrin White

Effect of seasonal host reproduction on host-macroparasite dynamics

The impact of seasonal host reproduction on the population dynamics of host-macroparasite interactions is considered. We modify the classic Anderson and May model so that parameters associated with host reproduction are periodic functions of time with a period corresponding to a year. This allows us to compare our findings with those already well documented. If, in the absence of any seasonality, a stable steady-state solution exists annual reproduction gives rise to stable annual population cycles. Moreover, the parameter domain for which there is stability is increased by the seasonality. However, if the life span of the free-living stages is reasonably long, and the continuous model has limit cycle solutions, complex behavior can be observed in the seasonally forced case. Results also indicate that if seasonal effects are ignored, regulation of the hosts by the parasite population is overestimated.

Wolf-deer interactions: a mathematical model

Field studies done in northeastern Minnesota indicate that stable wolf (Canis lupus) territories exist separated by buffer regions in which white-tailed deer (Odocoileus virginianus) density is greatest. We present a mathematical model to describe this interaction based on simple movement rules for the wolf population and basic predation dynamics. Solutions suggest that pack social requirements to care for the young set up differential predation rates causing segregation of high deer and wolf densities. More interestingly, they also suggest that the predator-prey interaction may play an important role in segregating and maintaining the territories.

The HPV vaccination strategy: could male vaccination have a significant impact?

We investigate the potential success of the human papilloma virus (HPV) vaccine, taking into consideration possible waning immunity and the influence of behavioural parameters. We use a compartmental, population-level ordinary differential equation (ODE) model. We find the effective reproductive value for HPV, Re0, which measures the threshold for infection outbreak in a population that is not entirely susceptible, together with infection prevalence. We study the effects of different parameters on both of these quantities. Results show that waning immunity plays a large part in allowing infection to persist. The proportion of the population not sexually active when vaccination occurs affects Re0, as does the rate at which individuals become sexually active. In several cases, infection persists as a result of an infection reservoir in the male cohort. To explore this further, we introduce male vaccination and find the conditions for which vaccination of males could be considered appropriate.

The effects of spatial movement and group interactions on disease dynamics of social animals

The effects of spatial movements of infected and susceptible individuals on disease dynamics is not well understood. Empirical studies on the spatial spread of disease and behaviour of infected individuals are few and theoretical studies may be useful to explore different scenarios. Hence due to lack of detail in empirical studies, theoretical models have become necessary tools in investigating the disease influence in host-pathogen systems.In this paper we developed and analysed a spatially explicit model of two interacting social groups of animals of the same species. We investigated how the movement scenarios of susceptible and infected individuals together with the between-group contact parameter affect the survival rate of susceptible individuals in each group. This work can easily be applied to various host-pathogen systems.We define bounds on the number of susceptibles which avoid infection once the disease has died out as a function of the initial conditions and other model parameters. For example, once disease has passed through the populations, a larger diffusion coefficient for each group can result in higher population levels when there is no between-group interaction but in lower levels when there is between-group interaction. Numerical simulations are used to demonstrate these bounds and behaviours and to describe the different outcomes in ecological terms.

Exploring Short-Term Responses to Changes in the Control Strategy for Chlamydia trachomatis

Chlamydia has a significant impact on public health provision in the developed world. Using pair approximation equations we investigate the efficacy of control programmes for chlamydia on short time scales that are relevant to policy makers. We use output from the model to estimate critical measures, namely, prevalence, incidence, and positivity in those screened and their partners. We combine these measures with a costing tool to estimate the economic impact of different public health strategies. Increasing screening coverage significantly increases the annual programme costs whereas an increase in tracing efficiency initially increases annual costs but over time reduces costs below baseline, with tracing accounting for around 10% of intervention costs. We found that partner positivity is insensitive to changes in prevalence due to screening, remaining at around 33%. Whether increases occur in screening or tracing levels, the cost per treated infection increases from the baseline because of reduced prevalence.