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Date:
May 5, 2009
INTRODUCTION
The study of factors affecting the distribution and
maintenance of disease agents in the environment is
called epidemiology.
Understanding the epidemiology of parasites
provides the foundation upon which we design
effective prevention and control programs.
Without this information, we cannot use all
the tools available to us to control both the adult
worms in the host and the larvae on pasture.
As a result, we tend to become dependent
strictly on deworming, which becomes a matter of
conven-ience and usually has little impact on the
parasite population and little economic value.
We are
fortunate when it comes to South American camelids
because, with a few exceptions, their
gastrointestinal parasites are substantially the
same as those that live in other North American
livestock.
Consequently, the numerous studies on the
epidemiology of cattle and sheep parasites done in
the
>70s and
>80s can give us
insights into design of appropriate control
programs for these parasites in
llamas/alpacas.
The life
cycle patterns of the gastrointestinal nematodes we
deal with are generally similar.
They are short and direct (do not require an
intermediate host).
Eggs passed in the feces usually hatch in 24
hours under optimal environmental conditions.
The first-stage larva molts to a second-stage
larva which molts to the third-stage larva
(infective stage).
Once the infective larva is ingested by the
animal, the time for development to sexually mature
adult nematodes is 2 to 4 weeks for most genera.
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In a broad sense, the factors
dictating the level and extent of parasitism are
climate, management conditions of pasture and
animals, and the population dynamics of the
parasites within the host and in the external
environment.
For the purpose of describing the effect of
seasonal climatic differences and management
conditions, parasite populations are divided into
three components.
The largest component, numerically, is the
population of free-living stages on the pasture.
The
next largest component is the number of
infective larvae on pasture that are available to
the host.
The smallest component is the number of
parasites actually present in the host.
Pasture contamination with parasite eggs is a
continuous process throughout the year, but hatching
of eggs, development of larvae through the
free-living to the infective stage, distribution
onto herbage and survival on pasture differs during
the course of a year.
The prevailing weather conditions are the
primary factors influencing these differences.
Changing weather patterns results in
fluctuations and discontinuities in the numbers of
infective larvae available to the grazing animal in
different seasons.
The effect of extreme weather conditions also
has an effect on the animals themselves as well as
forage growth and quality.
This, in turn, will also influence the
general health of the animals and their
susceptibility to parasitism.
Temperature is the primary
factor regulating the hatching of eggs and
development of larvae.
All stages can be killed by extremely low
temperatures as well as exposure to direct sunlight.
Moisture also influences the .... |
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