When Sharing is Caring: A Parasite’s Perspecitve

We’ve all heard of Influenza A, or Flu. It’s a viral parasite that can and has caused massive pandemics in humans. However, Influenza A does not infect only humans. It also parasitizes birds, particularly waterfowl, and other mammals. Many parasites have this diverse infection ability, i.e., many parasites infect multiple host species.

For such parasites, questions regarding their transmission dynamics become more interesting (and complicated). Instead of considering the interaction between only one parasite and one host species, one has to consider how different host species contribute to the parasite’s population. One host species may be the parasite’s main host, adding more parasites to the parasite population than any other host species. On the opposite side of the spectrum, the parasite may have no main host; each host species may contribute equally to parasite numbers in the community at large. In all likelihood, most parasites probably fall somewhere in the middle of this range; the parasites do not have a single main host, but the host species also contribute unequally to the parasite population.

In the cases where a parasite has a few (or one) main hosts, those hosts are called reservoirs. By producing most of the parasite population, reservoirs maintain that population. Reservoirs are thus important research areas for parasite population control, and therefore wildlife and human health. For example, many parasites that infect humans have non-human animal reservoir hosts. These parasites, ones that spillover into humans from other species, are called enzootic parasites. If one can determine their specific wild reservoirs, then one can eliminate parasite transmission to humans in a number of ways, including: limiting human-reservoir contact and preventing successful transmission among the reservoir species members (which itself can be accomplished through different mechanisms such as: vaccination, culling, habitat modification). However, such tools and goals are not simple, even when managing only a few host species.

More than the practical and financial considerations in appreciably affecting even a few host species, the different types of reservoir hosts complicate management efforts. Not all reservoir hosts are so for the same reasons. Streicker et al. (2013) argue three main mechanisms create reservoir host species: 1) The host species can be super abundant and have high population density; 2) The host species can be very susceptible to infection; 3) The host species can be highly infectious, producing numerous new parasites when infected. In the first mechanism, the host maintains the parasite simply by being so numerous. While a small percentage of the host population is infected, the host population is so large that it produces many parasites and maintains the parasite population. In the second mechanism, because hosts are so susceptible, a very high proportion of the host population is infected and can maintain the parasites at relatively lower host population sizes. In the third mechanism, hosts are highly infectious, so even though few hosts are infected, a single infected host produces many new parasites and has a high chance of infecting another host. These three situations are not mutually exclusive, but Streicker et al. argue that which one mostly applies to a particular reservoir species should affect management practices regarding that species. For example, the authors analyzed the effect of targeted removal of infected hosts and random removal of hosts from populations where a host species was either only super abundant, only super susceptible, or only super infectious. For super abundant hosts, random removal of hosts did not efficiently decrease the host’s contribution to parasite population because there was a small chance of removing an infected host. Targeted removal of infected hosts resulted in a moderate reduction, however. For super susceptible hosts, where a large proportion of the host population is infected, both random and targeted removal of hosts efficiently reduced the host’s contribution to parasite numbers. Because so much of the host population is infected, the chance that a random removal gets an infected host is not much less than targeted removal of infected hosts. For super infectious hosts, random removal finds infected hosts in proportion to their frequency in the population, and since each infected host contributes a large parasite number, each removal of an infected host will noticeably decrease the host’s contribution to the parasite population. Targeted removal of infected infectious hosts will decrease the host contribution to parasite population much more than random, although. This occurs because when there are few infected individuals, and each produces a large number of parasites, specific removal of the infected hosts will remove a large proportion of the parasite population pool. These results indicate that – for removal of hosts as a management tool – the different mechanisms creating reservoir hosts affect the outcomes of management actions.

Citations

Streicker, D.G., A. Fenton, and A.B. Pedersen, Differential sources of host species heterogeneity influence the transmission and control of multihost parasites. Ecol Lett, 2013. 16(8): p. 975-84.

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