Hans
(J.A.P.) Heesterbeek, Faculty of Veterinary Medicine, University of
Utrecht, The Netherlands Hans
(J.A.P.) Heesterbeek, Faculty of Veterinary Medicine, University of
Utrecht, The Netherlands
The consequences of infections in wildlife fall into a number of categories
that we could call: ecological risks, veterinary risks and zoonotic risks. Ecological
risks are the effects of infectious agents on wildlife populations themselves,
i.e. influences on the dynamics of populations and ultimately threatening survival
of rare species. Veterinary risks arise by the threat of important livestock
infections persisting in wildlife reservoirs. Zoonotic risks are threats of
wildlife infections finding their way into humans (often as a dead-end host,
but with severe consequences nevertheless).
Infectious agents in wildlife populations persist or do not persist for a host
of different reasons. As mechanisms one should think of (not an exhaustive list):
alternative hosts, host demography, reactivation, host density, alternative
transmission routes, environmental reservoir, spatial heterogeneity, long infectious
period, long incubation period.
The key quantity in studying the persistence and dynamics of infectious agents
and the effects of measures is the basis reproduction ratio R0: the average
number of new cases of an infection caused by one case in a population of susceptibles.
If R0 < 1, the agent cannot persist in the population. Taking into account
the essentials of host demography, contact structure between hosts, transmission
routes of the agent, effects of the agent on the host (e.g. increased mortality,
decreased fertility), infectivity and susceptibility and relevant heterogeneity
(such as genetical differences, age-structure, spatial heterogeneity), one can
calculate reproduction ratios under various control scenarios. One can in this
way make an honest comparison of the effects of different strategies. Dynamics
of infections in wildlife are complex since many of the factors mentioned give
rise to non-linear effects. These effects also arise because species interact
with other species and their environment in an ecosystem. Several of these factors
play a lesser role when studying infections in livestock or human populations.
The many feedbacks in the system make it very difficult to see the possible
consequences of conservation or control measures. It is there that mathematical
models can help. These models use the same ingredients as described above to
explore long-term dynamics of the agent and interaction with control, including
increasingly questions related to the evolution of virulence. This lecture will
use examples from all three risk categories to focuss on how the use of mathematical
models can contribute to understanding dynamics and persistence of infectious
agents in wildlife and how models are used to gauge effects of control measures
and conservation.
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