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The role of disease in mammals can be generalized to all vertebrates (Yuill 1987).

Parasites can be expected in all wildlife species in every ecosystem. Death of the host

is unusual and occurs only if (i) serious illness facilitates transmission, as in rabies;

(ii) the parasite does not depend on the infected host for survival and can complete

its life cycle after the host dies; and (iii) the pathogen moves through host popula-

tions over a wide geographic area and over a long period of time. Disease may have

a drastic effect on survival of wildlife but more commonly its effects are subtle. It

can adversely affect natality or normal movement. Brucellosis in caribou has both

effects. A caribou cow infected with brucellosis may abort her fetus, and the same

disease may also cause lameness from degenerative arthritis in the leg joints.

Infective agents can also affect the host’s energy balance by reducing energy intake

or increasing energy costs through higher body temperature and metabolic rate.

Simple models for describing the way a disease establishes and spreads through a

population start by assuming a constant host population size. This assumption

allows us to understand transmission processes over short time intervals. More com-

plex models can also account for changes in parasite and host populations.

For directly transmitted infections of microparasites such as rinderpest we can divide

the host population (N) into three groups: susceptibles(S), infected(I), and recov-

ered(R). The dynamic relationships are illustrated in a simple compartment model,

called the SIR model (Fig. 11.1) (Anderson and May 1979). Host population size is

determined by birth and death rates. Death rates arise from disease and other causes.

The effects of disease are described by: (i) the per capita rate of mortality due to

disease (α); (ii) the per capita rate of recovery (γ); (iii) the transmission rate or

coefficient (β); and (iv) the per capita rate of loss of immunity.

The rate of change of the susceptible population is given by the rate of transmis-

sion of disease from infected to susceptibles. Thus:

=− (11.1)

where Nand βare assumed to be constant.

The rate of change of the infected population is given by the rate of transmission

from infected to susceptibles minus the rate of recovery of infected animals. Thus:

βSI

N

dS

dt

180 Chapter 11

11.3 The basic parameters of epidemiology

11.3.1Simple
compartment models
of parasite–host
interactions

Births

SUSCEPTIBLES RECOVERED

INFECTED

Deaths

Fig. 11.1The SIR
model showing the
relationships between
susceptibles, infected,
and recovered
components of the
population.