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My name is Bill Reisen.
I'm a research entomologist
with the Center
for Vectorborne Diseases within
the School of Veterinary Medicine
at the University of
California, Davis.
My Henry Stewart talk
today is entitled
Parasite Transmission by Arthropods.
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The objectives of my talk will
be to review problems of parasite
acquisition by arthropods,
compare modes of replication
and transmission by
different parasites,
and present some of
the factors affecting
the efficiency of this transmission.
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Before we get too far
into our lecture today,
I'd like to go over
some basic definitions
to make sure we're
all on the same plane.
A parasite is any organism
that requires another organism
or host for its requisites for life.
In this definition, then, not only
the microorganisms transmitted
by arthropods are parasites,
but also the arthropods
themselves, which take nutrients
and sustenance from their host.
Disease is caused by the interaction
of the parasite infection
and the response of the host.
Parasites that cause
disease are pathogens.
A definitive host is the host
in which sexual reproduction
by the parasite occurs,
whereas the intermediate host
is where asexual
reproduction occurs.
In general, most arthropod vectors
serve as intermediate hosts,
but there are exceptions.
In arthropod-borne
parasite transmission,
the arthropod vector provides
the mechanism of transmitting
or sending the parasite
from one host to another.
Transmission of the parasite can
be nonpropagative or mechanical,
and this is usually by contaminated
mouth parts of the vector.
In contrast, when the injected
parasite develops or reproduces
or propagates within the arthropod,
transmission is biological.
Biological transmission between
the vector and vertebrate host
is termed horizontal
transmission, in contrast
to vertical transmission that occurs
only within the arthropod species.
Within the arthropod,
transmission can
be transstadial-- across stadia,
or stages of the arthropod--
or transgenerational-- from
one generation to the next.
A stadium is a stage of the
arthropod between molts,
and there are instars,
or molts within stages.
Intrinsic incubation is the
period within the vertebrate host
between infection and disease onset.
The extrinsic incubation
period is within the vector,
and it is the time between its
infection and transmission.
