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SarahandPelican

Sarah E. Bush
Research Assistant Professor - University of Utah Postdoctoral Researcher - University of Kansas

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Field Guide to Collecting Parasites

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Research - October 31, 2009
Parasites represent as many as half of all species of organisms on Earth (1).  This rich diversity is driven, in part, by host specificity.  Most parasites are host specific, and some parasites are so specific that they only infest only one host species.  Intriguingly, however, host specificity is in constant flux.  Parasites can, and do, switch to new host species.  In fact, one study estimates that as many as 61% of the parasites/pathogens affecting humans have zoonotic origins (2).  Furthermore, human introductions of plants and animals into new geographic regions have exposed wildlife to new parasites/pathogens that may ultimately threaten the conservation of global biodiversity (3).  My research focuses on the evolutionary ecology of host-parasite interactions.  In particular, I am interested in using macroevolutionary information for hosts and their co-evolving parasites to generate testable hypotheses about ecological factors determining host-specificity.

            For my Ph.D. I studied determinants of host specificity in an unusually tractable host-parasite system: pigeons/doves and their lice (Phthiraptera: Ischnocera).  I measured the ability of parasites to establish viable populations on novel hosts by experimentally dispersing feather lice to a series of host species (Fig. 1) (4,5).  I found that lice were able to establish on novel hosts if the novel host was similar in size to the native host.  Conversely, lice did not establish on novel hosts that differed in size  from the native host (by >25% in body mass).  This result is consistent with macro-evolutionary patterns in this system; host switching has occurred most frequently between similar sized hosts.  These results are interesting because parasite size correlates with host size in a diverse array of host-parasite systems, yet the adaptive basis of this correlation is poorly understood.  This research has been featured in two of the leading college textbooks on evolution (6).

            To understand the observed pattern, I also investigated several potential barriers to parasite establishment.  I found that preening, a bird’s primary defense against ectoparasites, is one barrier.  When preening was experimentally impaired, lice were able to establish viable breding populations on novel hosts an order of magnitude smaller than the native host.  These results were corroborated by proximal experiments showing that lice placed on small hosts were more susceptible to preening because they were physically unable to hide in the feather structures of smaller hosts.  In contrast, lice could not survive on larger novel host species even when preening was impaired.  This is surprising, in that additional experiments showed that lice could escape from preening on feathers of larger hosts, could eat feathers from larger hosts, and could remain attached to feathers of larger hosts (7,8).  It is still unclear why large novel hosts pose a barrier to louse establishment. Perhaps pheromonal cues are disrupted on larger hosts, making it difficult for the lice to find mates. I hope to pursue this question in the future.   

            As a postdoc, I am using a comparative approach to investigate ecological factors that influence the structure of parasite communities of terrestrial vertebrates in China. As the lead parasitologist on an NSF funded Biotic Surveys & Inventories project in southern China, I have processed over 2,000 individuals of more than 300 species of birds, mammals, frogs, and snakes.  From these vertebrates I have collected parasitic fleas, flies, lice, ticks, mites, pseudoscorpians, leeches, acanthocephalans, nematodes, cestodes, trematodes, coccidia and hematozoans.  To deal with this large and diverse collection of Chinese parasites, I have developed a network of collaborators from 15 different institutions who specialize on different parasite taxa.  In addition to collecting parasites, I have also sampled Chinese birds for avian influenza, including H5N1 (9).   These samples are being screened in collaboration with staff at the National Wildlife Health Center in Madison, WI.  I have already published work on several of these groups (lice, flies, ticks, acanthocephalans, digeneans, and avian influenza); additional work on these parasites and other groups (nematodes, and hematozoans) is in preparation.  

            As the Chinese parasites are prepared and further identified, it will soon be possible to compare populations among different field sites to test hypotheses about the structure of parasite communities.  Initial results from these comparisons indicate that parasite species richness is dependent on forest fragment size.  This result has important conservation implications.  Host specific parasites are known to have gone extinct with their hosts (10), a phenomenon called “co-extinction”.  The data from China suggest that local extinction of parasites may actually precede the extinction of their hosts.  This has direct implications for the conservation of parasite species, which are typically overlooked in conservation efforts.  Furthermore, these data suggest that the health of parasite communities can be used as an indication of the health of the host communities. 

            In the future I will continue to use a comparative approach to investigate ecological factors that influence the structure of parasite communities.  I am one of four Co-PIs on a new 5yr (2008-2013) NSF funded Biotic Surveys & Inventories project to survey the terrestrial vertebrates and parasites of the Philippines.  One of the things that I find most exciting about these rigorous biotic surveys is that comparative data from these studies can be used to generate relevant, testable, hypotheses about the structure of parasite communities.  For example, which parasites are most prone to local extinction?  Are the most host specific parasites lost first?  Are ectoparasites lost before endoparasites?  Are parasites with complex life cycles lost before those with simple life cycles?  How many parasites can coexist?  What eco-morphological or behavioral differences are required for parasite coexistence?   The question that I would most like to address in the near future is:  What happens when a new host invades the community?  Does the new host “share” its parasites?  Or does the native parasite community essentially protect its host from invasive parasites? (11) I intend to pursue these questions experimentally with captive host-parasite systems, in addition to my comparative work in the Philippines, and elsewhere.

1. Price, P.W. 1980. Evolutionary Biology of Parasites. Princeton University Press, Princeton, NJ.

2. Taylor, L.H., Latham, S.M., and Woolhouse, M.E. 2001. Risk factors for human disease emergence. Philos Trans R Soc Lond B Biol Sci. 356:983-989.

3. Daszak, P., Cunningham, A.A., and Hyatt, A.D. 2000. Emerging Infectious Diseases of Wildlife: Threats to Biodiversity and Human Health. Science 287:443-449.

4. Bush, S.E. and D.H. Clayton. 2006. The role of body size in host specificity: reciprocal transfer experiments with dove lice. Evolution 60: 2158-2167.

5. Clayton, D.H., S.E. Bush, B.M. Goates and Johnson, K.P.. 2003. Host defense reinforces host-parasite cospeciation. Proc. Nat. Acad. Sci. 100:15694-99.

6. Freeman, S. and Herron J. C., 2007, Evolutionary Analysis 4th ed., Prentice Hall, Upper Saddle River, NJ; Futuyma, D. J., 2009. Evolution 2nd ed., Sinauer, Sunderland, MA.

7. Bush, S. E., E. Sohn and Clayton, D.H. 2006. Ecomorphology of parasite attachment: experiments with feather lice. J. Parasitology 92:25-31.

8. Bush, S. E. 2009. Does microhabitat flexibility facilitate host switching by parasites? Funct. Ecol. 23: 578-586.

9. Peterson, A. T., S. E. Bush, E. Spackman, D. E. Swayne, and H. Ip. 2008. Influenza A Virus Infections in Land Birds, People’s Republic of China.  Emerg. Infect. Diseases 14: 1644-1646.

10. Koh, L.P.; Dunn, R.R.; Sodhi, N.S.; Colwell, R.K.; Proctor, H.C. & Smith, V.S. 2004. Species Coextinctions and the Biodiversity Crisis. Science 305:1632-1634.

11. This hypothesis assumes that parasites compete for host resources.  I have demonstrated that this is true among feather lice on Rock Pigeons; see Bush, S. E. and Malenke, J. R..  2008.   Host defense mediates interspecific competition in parasites.  J. Animal Ecol. 77: 558-564.