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.
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