Zika & Wolbachia III: Wolbachia transmission

    Is Wolbachia going to solve our Zika, dengue, chikungunya, and malaria problems? The short answer is maybe, the long answer is below. I have to be honest; this was a difficult post to write. The topic is technical and I found myself amassing a ridiculously long citation list (a tribute to how much foundational research plays into this story). My usual tack in this blog is to profile one of the citations and highlight how important that research is to the larger puzzle. I did that with the first Wolbachia installment but if I did that with everyone in this story we would end up with a book - which I could be working on. There are many unique and interesting players; I only skimmed the surface of Wolbach, and didn’t even get to Marshall Hertig or the countless others. Regardless of the level of difficulty, I did want to address the potential outcomes we could expect from using Wolbachia-infected mosquitoes to control disease transmission, and address the important question: How effective will Wolbachia-infected mosquitoes be at limiting the spread of human diseases? If you haven’t read the previous two posts in this series (I & II), I recommend you do so.

    It is probably clear by now that scientists are currently relying on decades of Wolbachia research (covering many different types of insects) to get us to this point. In order to take the next step it is necessary to develop experiments testing how Wolbachia will spread through an Aedes aegypti mosquito population once infected individuals are introduced into a disease afflicted area. Interestingly, one of the very intriguing things about Wolbachia that has been extensively studied is its mode of transmission. After years of research, hours of scientific brainpower, and 100+ publications, the true nature of Wolbachia as a genetic manipulator has been revealed. Wolbachia manipulates the reproductive genetics of its host to enable its spread through a population. How does it do this? The mechanisms of manipulation rely on the premise that Wolbachia is vertically transmitted, which means it is passed from parent to offspring. Specifically, Wolbachia is transferred from mother to offspring in the cytoplasm of the egg cell (the cytoplasm is the jelly-like matrix of a cell); Wolbachia cannot be passed from father to offspring in sperm cells. These are the four types of genetic manipulation that can occur in natural populations based on this type vertical transmission:

    1. Cytoplasmic incompatibility: If Wolbachia-infected males mate with uninfected females, or females infected with a different Wolbachia strain, the sperm will die before they can fertilize the egg^1,2. The outcome of cytoplasmic incompatibility = increased Wolbachia infection in the population because infected males can only produce infected offspring.

    2. Parthenogenesis: Females can produce offspring without mating with a male³. The outcome of parthenogenesis = increased number of females in a population spreading Wolbachia.

    3. Feminization of male offspring: Wolbachia infection in embryos causes male offspring to develop into females^4,5. The outcome = increased number of females in a population spreading Wolbachia.

    4. Male killing: Infected male embryos are not viable, leading to populations that are mostly female⁶. The outcome = increased number of females in a population spreading Wolbachia.

Image by Laurent Seroussi profiled by My Modern Met

    Notice a trend? Each of these mechanisms is often specific to a particular type of insect, so it is important to point out that the fruit fly strains of Wolbachia that prevent A. aegypti from spreading disease manipulate the genetics of their host through cytoplasmic incompatibility.

    Our first window into the transmission dynamics of Wolbachia is based on a study done in 1971 by Janice Harumi Yen and Allan Ralph Barr at the UCLA School of Public Health². Decades after S. Burt Wolbach and M. Hertig first discovered Wolbachia, Yen and Barr (studying the same species of mosquito, Culex pipiens) found that Wolbachia infection reduced the ability of certain C. pipiens mosquitoes from producing offspring. Through detailed studies, Yen and Barr found that when sperm were swimming toward the nucleus of the egg, they died after swimming through a field of Wolbachia bacterial cells. Yen and Barr called this phenomenon cytoplasmic incompatibility, a very syllabic phrase that essentially means: the environment of the egg is lethal to the sperm and the egg does not get fertilized. Because the sperm died after swimming through the Wolbachia field, Yen and Barr hypothesized cytoplasmic incompatibility had something to do with the Wolbachia cells present within the egg cell of the mosquito.

    Yen and Barr saw the implications of cytoplasmic incompatibility for controlling mosquito populations, but in a different way than is currently being considered. What we are thinking about here is not a way to control mosquito populations, but a way to release infected mosquitoes and then getting the infection to spread. In this scenario, you want uninfected mosquitoes (that can transmit human diseases) to die, but you want infected mosquitoes (that do not transmit human diseases) to reproduce and spread the Wolbachia infection. The beauty of Wolbachia as a genetic manipulator, especially when it comes to cytoplasmic incompatibility, is that it accomplishes both of these goals.

   

   We are building here. This is the foundation scientists are currently relying on to reduce the spread of Zika and other diseases transmitted by mosquitoes. There may be a voice in the back of your head saying, huh, I wonder if a politician at some point in history spoke out about wasteful spending on insects, their bacteria, or the fruit flies from which the effective Wolbachia strain came from? And the answer is yes, numerous times. Imagine if none of this research had been done.

    Now that we know Wolbachia infection has the inherent ability to spread through a population due to cytoplasmic incompatibility, we can plug this information into mathematical models (!) and make theoretical predictions. Don’t stop reading, we don’t have to get into the mathematics to understand what factors will affect how Wolbachia will spread through a mosquito population after infected individuals are released. The first factor to consider is the number of mosquitoes that currently exist in the target area. Jansen and colleagues⁷ found that the smaller the population of mosquitoes, the fewer infected individuals that need to be released; increasingly larger mosquito populations required the release of more infected adults and eggs. Thus, when planning a release it is important to consider the season, as we know that mosquito populations fluctuate throughout the year. It is also beneficial to have releases coincide with other population control methods (such as some of the biocontrol options talked about in the last post) to reduce population sizes before the release occurs.

    What are the associated risks? With the implementation of any control option there are risks to consider. Risk factors include logistical issues, like not being able to generate enough infected mosquitoes, and public opposition; there are also ecological and economic consequences to consider. Groups of individuals with expertise in different aspects of potential outcomes (biological, economic, and public) were convened to brainstorm and discuss widespread release of Wolbachia-infected mosquitoes⁸. Generally speaking, the risks were calculated as being very low. One of the biggest concerns was that households would reduce efforts to control mosquitoes (eliminate sources of standing water, etc.). Another concern was that dengue would evolve to overcome the resistance of Wolbachia-infected mosquitoes to dengue infection. This type of concern is persistent when dealing with pathogens that can evolve very rapidly (we have talked about this before). There is a reason a new flu vaccine is developed every year – it is called evolution. However, the panel of experts agreed that this risk was relatively low due to the efficiency with which Wolbachia-infected mosquitoes prevent the transmission of dengue.

    With all this in mind, release of Wolbachia-infected mosquitoes was recently done in Australia in dengue-impacted regions with successful results⁹. One month before the release of infected mosquitoes standing water sources (mosquito breeding grounds) were removed from the release sites. Over a period of 9-10 weeks, approximately 150,000 mosquitoes were released at each of two sites in Australia. With each subsequent release, the Wolbachia infection rate increased, and continued to increase up to five weeks after the last release of infected mosquitoes. At this point though, is too early to say how the release affected the transmission of dengue, and scientists are currently evaluating the best way to monitor these effects¹⁰.

    One important thing to note is that much of the research done on Wolbachia-infected mosquitoes was implemented with the goal of reducing the spread of dengue (primarily), not Zika. However, because these two diseases are similar, and transmitted by the same mosquito vector, this research can be applied to the recent Zika outbreak. This is important because it highlights the importance of supporting research during a time of crisis, as well in between crises. Our ability to respond to disease outbreaks is directly related to the research available at the time the outbreak occurs. This is especially pertinent as the number of infectious disease outbreaks is predicted to increase, in part due to climate change.

    Are you still with me? Have I answered the question? It may seem like the way my twenty-something self would have responded to the question of having a boyfriend: it’s complicated. It is complicated, but I think the bottom line is that Wolbachia-infected mosquitoes will be an important aspect of a multi-pronged approach to controlling human diseases vectored by mosquitoes. I think scientists have come a long way with this line of research and it has the potential to have a significant payoff. If you are still yearning for more I will steer you to the Eliminate Dengue website. This is an amazing and well-organized resource with an extensive publication section that provides PDFs of nearly all of the articles associated with the program (also includes current research on Zika).

¹Laven, H (1959) Speciation in mosquitoes: speciation by cytoplasmic isolation in the Culex pipiens-complex. Cold Spring Harbor Symposia on Quantitative Biology 24:166-173.

²Yen JH, Barr, AR (1971) New hypothesis of the cause of cytoplasmic incompatibility in Culex pipiens L. Nature 232:657-658.

³Stouthamer R, Breeuwer JA, Luck RF, Werren JA (1993) Molecular identification of microorganisms associated with parthenogenesis. Nature 361:66-68.

⁴Rousset F, Bouchon D, Pitureau B, Juchault P, Solignac M (1992) Wolbachia endosymbionts responsible for various alterations of sexuality in arthropods. Proceedings: Biological Sciences 250:91-98.

⁵Bouchon D, Rigaud T, Juchault P (1998) Evidence for widespread Wolbachia infection in isopod crustaceans: molecular identification and host feminization. Proceedings of the Royal Society London B 265:1081-1090.

⁶Hurst GDD, Jiggins FM, Hinrich Graf von der Schulenburg J, Bertrand D, West SA, Goriacheva II, Zakharov IA, Werren JH, Stouthamer R, Majerus MEN (1999) Male-killing Wolbachia in two insect species. Proceedings of the Royal Society London B 266:735-740.

⁷Jansen VAA, Turelli M, Godfray HCJ (2008) Stochastic spread of Wolbachia. Proceedings of the Royal Society London B 275:2769-2776.

⁸ Murray JV, Jansen CC, De Barro P (2016) Risk associated with the release of Wolbachia-infected Aedes aegypti mosquitoes into the environment in an effort to control dengue. Frontiers in Public Health 4:43.

⁹Hoffmann AA, Montgomery BL, Popovici J, Iturbe-Ormaetxe I, Johnson PH, Muzzi F, Greenfield M, Durkan M, Leong YS, Dong Y, Cook H, Axford J, Callahan AG, Kenny N, Omodei C, McGraw EA, Ryan PA, Ritchie SA, Turelli M, O’Neill SL (2011) Successful establishment of Wolbachia in Aedes populations to suppress dengue transmission. Nature 476:454-457.

¹⁰Lambrechts L, Ferguson NM, Harris E, Holmes EC, McGraw EA, O’Neill SL, Ooi EE, Ritchie SA, Ryan PA, Scott TW, Simmons CP, Weaver SC (2015) Assessing the epidemiological effect of Wolbachia for dengue control. The Lancet Infectious Diseases 15:862-866.