Monday, May 28, 2018


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. 2018 May 15. Post #18.

4. Parelaphostrongylus tenuis. Wikipedia Parelaphostrongylus_tenuis&oldid=835393860


Tuesday, May 15, 2018


Post #18

Donald A. Windsor

I hypothesize that other species may be involved in parasite-host interactions.

In my previous post #17, I speculated that probiotics may be involved. But after further research revealed that the terms “probiotic” and “contrabiotic” are already in widespread use, so I will not use them.

Instead I will use the term “biobroker”. A broker facilitates transactions between sellers and buyers. A biobroker facilitates interactions between parasites and hosts. Biobrokers are probably microorganisms, but could be any other organisms, including other parasites.

Just as a financial transaction can occur without a broker, a parasite can infect/infest a host without a biobroker. However, some transactions are much smoother with a broker, because the broker may be able to work out a good deal for both participants.

Parasites must convince hosts to accept them. It seems that this convincing is usually of the “offer they cannot refuse” type of bullying. Parasites are usually host specific, showing that most other possible hosts are not convinced to accept the parasite and meet such overtures with rejection.

Evolution is all about evading extinction. Parasites may benefit their hosts, at the species level, by helping them survive. When a host species survives, its parasite species stand a better chance at surviving.

When a host species goes extinct, its parasite species will also go extinct, unless it is already in other species of hosts. A parasite species that has only one host species will survive its host species extinction only if it can find another species of host.

Here is where biobrokers can save the parasite species, by helping find new host species.

Parasites carrying the appropriate bacteria would be more successful infecting hosts compared to parasites without these bacteria. These bacteria would be probiotic for the parasites and contrabiotic for their hosts. Likewise, if the hosts were harboring bacteria that would thwart the parasites, those bacteria would be probiotic for the hosts and contrabiotic for the parasites. Biobrokers are both probiotic and contrabiotic and all gradations in between.

Right now, this hypothesis is pure speculation. I am looking for some real-world examples.