The sex life of a mosquito and how it can affect us all (03/2004)
A new type of mosquitoes has been found among the United States population of Culex pipiex, a species known to transmit the West Nile virus, claim scientists in the journal Science1. This new population might help to explain why the United States has suffered several epidemics of the disease while only sporadic cases occur elsewhere in the world.
West Nile Virus (WNV) is a mosquito-borne disease and birds are the virus’s natural hosts. Occasionally, the disease can be transmitted to humans and other mammals. These animals, however, are incapable of spread the virus, as the level of WNV in their blood is too low to infect mosquitoes.
Since the identification of the WNV in a woman in Uganda in 1937, the disease has appeared in humans sporadically throughout the world. In 1999 however, when the first human case was registered in North America, an unusually high number of cases and deaths occurred in New York City. Since then, the WNV has spread throughout the United States causing regular epidemics and becoming a major health problem in this country; in 2003 alone, there were 9377 cases and 244 deaths.
Dina Fonseca, working at the National Museum of Natural History, Washington DC and the Department of Entomology at the Walter Reed Army Institute of Research, MD, USA, and Richard C. Wilkerson from the National Museum of Natural History, Washington DC, USA together with colleagues in the UK, France and Japan, trying to understand the reasons behind the U.S. epidemics decided to investigate Culex pipiex (Cx. pipex), a species of mosquito considered to be the primary carrier of WNV in United States.
Cx. pipex is a species known to be composed of individuals that although are physically indistinguishable, nevertheless, present distinct behaviours and physiologies. In terms of behaviour, two distinct populations of Cx. pipex seem to exist, the first living in colder temperatures and mainly feeding on birds, while the second group fed on humans and tended to live in warmer temperatures, either in warmer climates or underground in tunnels.
Fonseca, Wilkerson and colleagues thought that maybe these behavioural and habitat differences within Cx. pipex, could correlate with distinct patterns of disease. The team of investigators genetically analysed specimens collected from all around the world, and discovered that the two behaviourally distinct populations of Cx. pipex were also genetically different. This has not been detected before, probably because the separation of the two groups has occurred relatively recently (in the last 10.000 years), and so not only the morphology, but also much of the genetic of the two populations have not had time to diverge.
Also, and much more interestingly, Fonseca, Wilkerson and colleagues found that in the United States there existed a third group of Cx. pipex which seemed to be a hybrid of the two new populations just discovered. These hybrids feed on both birds and humans, contrary the majority of the ones found in countries with only sporadic cases of WNV that feed on only one type of animal. By feeding on both animals, this new population will act as a direct bridge for disease transmission from birds to humans According to the team of scientists, the high numbers of this hybrid population found in the U.S., could be the key, together with migration of infected birds and highly concentrated human populations, behind the unique American epidemics.
But other investigators have not been convinced by Fonseca and colleagues’ proposal and argue that because WNV was only recently introduced in North America, contrary to what happened in other parts of the world, it is possible that the high numbers of cases in this continent simply result from high susceptibility, in both birds and humans, to a new disease.
Fonseca however counters “ recent epidemics in Romania (1996) and Russia (1999), both countries where we know that WNV has existed for some time now, prove that the outbreaks can not only be explained by susceptibility to a new virus. Moreover, in these European outbreaks, Cx. pipex was the major type of mosquito involved in disease transmission contrary to what is usual in these countries, where other mosquito species tend to be the major WNV carriers”
What is clear however ever is that a higher number of hybrid mosquitoes biting both humans and birds will contribute to an increase in the disease among humans, and so be, most probably, one, if not a major cause behind the high number of cases in the U.S.
What is also very important in Fonseca, Wilkerson and colleagues’ work is that it shows that epidemic studies in the United States and consequently prevention and attitude to disease should not be directly extrapolated to other countries. Their work also alerts to the possible dangers of further epidemics in other parts of the world if these US hybrid populations were to migrate out of North America, and for the need to be prepared for such an eventuality
The team of scientists are now investigating the hypothesis that hybrid Cx. pipex were also involved in the occasional WNV epidemics that have occurred in Europe.
1 Science (2004); Vol. 203, pp. 1535-1538
link to the original paper - http://www.sciencemag.org/cgi/reprint/303/5663/1535.pdf