1. Introduction
Modern poultry production frequently involves the raising of
large numbers of birds on built-up litter at high stocking densities
in houses specifically designed for that purpose (Bell and Weaver,
2002). Thus, a typical broiler house may contain as many as
20–50,000 chickens reared beneath one roof at a stocking density
of one bird per 0.08 m2. Production is usually based upon the ‘‘all
in–all out’’ principle in which birds are placed on litter upon arrival
from the hatchery, remain in the house for the duration of the rearing
period, and then are removed either for slaughter (broilers) or
transferred to cages (some egg layers). After a brief period of a few
weeks, known as the ‘‘down-time’’ a fresh flock is introduced. The
litter may be replaced after every flock or after several successive
flocks, in the USA it is common for six flocks to be raised each year
with no change of litter. Thus, for the period of production, the
flock represents a closed population. This has important implications
for those organisms that parasitize poultry and the development
of preventive control strategies.