See caption below.
The common myna, Acridotheres tristis.
Photo by Kim Bridges.

Natural History Highlight

Escape from Parasites:
Tracking the Distribution of Infectious Disease

Recent news reports tell of evidence of avian flu and the potential for the spread of disease.  A recent study involving Robert Fleischer, NMNH scientist with a joint appointment with the National Zoological Park, (along with the SI’s Farah Ishtiaq and Jon Beadell, and other colleagues), makes clear that bird introductions have the ability to move parasites around the World, a finding of some importance in this time of emerging infectious disease.

The study also questions the applicability of the well-known Enemy Release Hypothesis regarding invasive species (i.e. exotic species that are introduced outside their native ranges and that then increase and often disrupt native communities of organisms).

The paper, just published by Fleischer and his colleagues, describes a test of this hypothesis. The study assesses the prevalence and distribution of malarial parasite lineages in the common myna bird, using two well-known blood parasite genera.  According to the authors, “to our knowledge, this is the first comparative study conducted on blood parasites in an avian host that has been introduced to multiple regions from a common native region.” The original native habitat of the myna bird is India.

The study tests the enemy release hypothesis which would predict that a species that relocates leaves behind an "enemy" (parasites) and is therefore able to thrive in the new region without the parasites.

Using molecular methods, the authors explored whether overall parasite prevalence differs between introduced mynas and those in their native range, and whether mynas carry parasite lineages with them to their introduced region or obtain them once they get there.

avian malaria

Indian silvereye's red blood cells infected with avian malaria
(infected myna cells would look the same).
Photo by Eben Gering.

Conclusions show that “not all comparisons of introduced populations to the native population were consistent with expectations of the enemy release hypothesis”, i.e., some introduced populations (e.g., Australia, South Africa) had equivalent parasite loads to those found in India.  In addition, evidence suggests that while mynas brought parasites with them from India to their new homes in such places as Australia, South Africa and New Zealand, they also picked up new parasite types when they arrived. These findings are particularly relevant as they might apply to the spread of avian flu and other emerging infectious diseases.

The paper, The “Prevalence and evolutionary relationships of haematozoan parasites in native versus introduced populations of common myna Acridotheres tristis” is co-authored by Farah Ishtiaq, Jon S. Beadell (SI Fellows) and Robert C. Fleischer, (Head), all of the Smithsonian Genetics Program; along with collaborators Allan J. Baker (Royal Ontario Museum, Canada), Asad R. Rahmani (Bombay Natural History Society, India) and Yadvendradev V. Jhala (Wildlife Institute of India). The paper, just published online at Proceedings of the Royal Society B, thus represents a collaboration of SI scientists with colleagues in India and Canada.