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Applied and Environmental Microbiology, April 2007, p. 2398, Vol. 73, No. 7
0099-2240/07/$08.00+0     doi:10.1128/AEM.02767-06

LETTER TO THE EDITOR

Multiply Infected Vectors


   LETTER
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References
 
Wielinga et al. (10) provide an important reminder that ticks and other insect vectors are sometimes microbial "zoos" rather than single-pathogen vehicles. A single tick may simultaneously transmit Borrelia and Babesia species (7) or, perhaps, coinfect humans with Borrelia, Babesia, and Ehrlichia species (1, 3).

Wielinga et al. report that the frequency of Ixodes ricinus ticks coinfected with Borrelia and Ehrlichia or Anaplasma species is higher than would be expected on the basis of independence between the pathogens, that this frequency increases with tick age, and that the degree of the statistical surplus of coinfections varies with habitat type. Given that the striking surpluses over the 2 to 3 years in the park, forest, and heather habitats (P < 0.01 for each by {chi}2 and Fisher's exact tests) appeared to be absent over the 5 years in the dune habitat (P > 0.1), might these differences between the habitats have been mediated by differences between the years in the age structures of the tick populations in the field or the assay samples? Since an individual vector rarely clears an infection, infections accumulate with age, and in a vector population, frequencies of multiply infected vectors typically track seasonal and other environmental influences on age structure (4, 5).

Wielinga et al. also point out that pathogens sharing a vector may interact within that vector. Among mosquito-borne pathogens, for instance, transmission of the Rift Valley fever and eastern equine encephalitis viruses by Aedes mosquitos coinfected with Brugia species is enhanced (8, 9); Plasmodium and Wuchereria species coinfect Anopheles punctulatus at higher-than-expected frequencies (2) but suppress each other's devel-opment in Anopheles gambiae (6). Wielinga et al. have drawn attention to an important but neglected set of topics for research, the results of which should improve our understanding and control of vector-borne diseases.


   FOOTNOTES
 
Ed. Note: The authors of the published article declined to respond.


   REFERENCES
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 Letter
 References
 

  1. Belongia, E. A. 2002. Epidemiology and impact of coinfections acquired from Ixodes ticks. Vector Borne Zoonotic Dis. 2:265-273.[CrossRef][Medline]
  2. Burkot, T. R., L. Molineauz, P. M. Graves, R. Paru, D. Battistutta, H. Dagoro, A. Barnes, R. A. Wirtz, and P. Garner. 1990. The prevalence of naturally acquired multiple infections of Wuchereria bancrofti and human malarias in anophelines. Parasitology 100:369-375.
  3. Magnarelli, L. A., J. W. Ijdo, J. F. Anderson, S. J. Padula, R. A. Flavell, and E. Fikrig. 1998. Human exposure to a granulocytic ehrlichia and other tick-borne agents in Connecticut. J. Clin. Microbiol. 36:2823-2827.[Abstract/Free Full Text]
  4. McKenzie, F. E., and W. H. Bossert. 1997. Mixed-species Plasmodium infections of Anopheles (Diptera: Culicidae). J. Med. Entomol. 34:417-425.[Medline]
  5. McKenzie, F. E., G. F. Killeen, J. C. Beier, and W. H. Bossert. 2001. Seasonality, parasite diversity and local extinctions in Plasmodium falciparum malaria. Ecology 82:2673-2681.
  6. Muirhead-Thomson, R. C. 1953. Inter-relations between filarial and malarial infection in Anopheles gambiae. Nature 172:352-353.[CrossRef][Medline]
  7. Piesman, J., T. C. Hicks, R. J. Sinsky, and G. Obiri. 1987. Simultaneous transmission of Borrelia burgdorferi and Babesia microti by individual nymphal Ixodes dammini ticks. J. Clin. Microbiol. 25:2012-2013.[Abstract/Free Full Text]
  8. Turell, M. J., P. A. Rossignol, A. Spielman, C. A. Rossi, and C. L. Bailey. 1984. Enhanced arboviral transmission by mosquitoes that concurrently ingested microfilariae. Science 225:1039-1041.[Abstract/Free Full Text]
  9. Vaughan, J. A., and M. J. Turell. 1996. Dual host infections: enhanced infectivity of eastern equine encephalitis virus to Aedes mosquitoes mediated by Brugia microfilariae. Am. J. Trop. Med. Hyg. 54:105-109.[Abstract/Free Full Text]
  10. Wielinga, P. R., C. Gaasenbeek, M. Fonville, A. de Boer, A. de Vries, W. Dimmers, G. A. O. Jagers, L. M. Schouls, F. Borgsteede, and J. W. van der Giessen. 2006. Longitudinal analysis of tick densities and Borrelia, Anaplasma, and Ehrlichia infections of Ixodes ricinus ticks in different habitat areas in The Netherlands. Appl. Environ. Microbiol. 72:7594-7601.[Abstract/Free Full Text]
F. E. McKenzie
Fogarty International Center
National Institutes of Health
Bethesda, Maryland

Applied and Environmental Microbiology, April 2007, p. 2398, Vol. 73, No. 7
0099-2240/07/$08.00+0     doi:10.1128/AEM.02767-06





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