INTERMEDIATE
HOSTS AND BIOBROKERS −
DEER BRAINWORM
Post # 19
Donald
A. Windsor
Having
intermediate hosts in a parasite’s life cycle must confer some
survival advantage. But what? Some parasites have them; some do
not, and some have two. Having or not having intermediate hosts
seems to depend on particular situations. This subject has been well
covered. It was even modeled (1). A recent review was published in
2015 (2).
My
interest is in the participation of other species in the
parasite-host interactions, the biobrokers (3). Life is more
complicated than we realize and species interactions seem to involve
more than merely two species. The other species are not readily
seen.
A
good example is the deer brainworm Parelaphostrongylus tenuis.
This nematode has the white-tailed deer (Odocoileus virginianus)
as its definitive host and several species of snails and slugs as its
intermediate hosts. Worm eggs hatch in the deer’s bloodstream and
the larvae travel to the lungs, get coughed out, swallowed, and
defecated onto the ground vegetation. Snails and slugs (gastropods)
eat the mucus coating on the scat pellets which contain the larvae.
When the deer graze they inadvertently ingest the gastropods. The
larvae from the gastropods travel from the deer’s gut to the
central nervous system, where they mature, breed, and lay eggs in the
blood stream (4).
The
biobrokers in this interaction between the deer and the gastropods
are the many species of ground vegetation. These plants have to be
palatable, or at least not repulsive, both to the deer and to the
gastropods. The point is that other species are involved in the
interaction between the deer and the gastropods. The biobrokers
would not be needed in a lab setting, but my concern is what happens
in nature. The participation of the gastropod intermediate hosts
offers an additional conjecture.
The
brainworm is further involved in a complex network of other host
species besides the white-tailed deer. In moose (Alces alces)
it is usually fatal, unlike in deer where it usually is not.
Consequently, overlap of these species is inhibited.
It
could be argued that a biobroker is no different from a
non-biological thing. Indeed. I frequently confront that thought.
Inert, synthetic Astroturf could be substituted for plants, and
gastropods would probably eat deer scat placed on it. But deer would
probably not eat snail/slugs on Astroturf or on anything else.
References
cited:
1.
Coisy, Marc ; Brown, Sam P., ; Lafferty, Kevin D. ; Thomas Frederic.
Evolution of trophic transmission in parasites: why add intermediate
hosts? The American Naturalist 2003 August; 162(2): 172-181.
2.
Auld, S.K.J.R. ; Tinsley, M.C. Review. The evolutionary ecology of
complex lifecycle parasites: linking phenomena with mechanisms.
Heredity 2015; 114: 125-132.
3.
Windsor, Donald A. Biobrokers in parasite-host interactions.
parasitesdominate.blogspot.com 2018 May 15. Post #18.
4.
Parelaphostrongylus tenuis.
Wikipedia https://en.wikipedia.org/w/index.php?title=
Parelaphostrongylus_tenuis&oldid=835393860
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