This article addresses only one aspect of prevention of Zika virus caused disease; it is the fifth in a series. For previous writings on the subject and to contextualize this writing, please refer to my blog.
I had previously written on the vector control benefits of attempting 'functional' extinction of the Aedes aegypti species that likely carries Yellow fever, Dengue fever, Chikungunya, and Zika virus, illnesses that lead to morbidity or mortality in the many thousands every year. The arguments were many, but the main point was that Zika virus was absolutely going to impact the health of Americans via local transmission and that many would suffer needlessly. I went on to urge that the time was right for the planned 'functional' extinction of Aedes aegypti but at this point that approach would no longer be sufficient. As expected, we have since seen the first infant death in the U.S., due to Zika virus and have the first U.S. birth of a child with Zika virus caused microcephaly. More that 2,722 people in the U.S. have been infected, with 35 from local transmission, according to the CDC Case Counts in the U.S. Sadly, more will likely follow.
In the face of this tragedy, there are essentially three possible strategies: precautionary avoidance of being bitten, immunizations, and vector control. Much work, money, energy, and press is given to the topic of immunization. Many of us believe that the vector control side of the equation has been mostly relegated to the sidelines. I would like to highlight a more rational and reasonable view of vector control, for the time that we live in now, with the extent of mosquito populations there are now, in the beginning of September, 2016.
The second part of the title of this writing is partially a quote, often attributed to William Gibson, and goes right to the heart of the matter. The nature of vector control, of how to control the mosquitoes, is at a time of significant transition. We can briefly separate vector control into two prevailing forces: targeted vector control versus widespread vector control. Historically, a great deal of vector control is widespread in nature, a blunt instrument, not choosing a single species as a target. One common control example is spraying insecticides for mosquitoes. The hope is that this will knock down the overall mosquito population and thereby limit transmission of disease, here the Zika virus. While this may work, it also kills many species of mosquitoes that do not bite people. And worse, it kills many other insects, aside from mosquitoes, such as bees. As well, we have a century of experience to understand that pesticides accelerate adaptation pressures towards the toxin, making insects immune. There is a better way: targeted controls!
I would like to start with an illustration from another field for a moment. Historically, the most common form of chemotherapy is a widespread attempt to interrupt the life cycle of fast growing cells, such as cancers. Unfortunately there are other fast growing cells in the body, such as hair, and mucus membranes, and this blunted approach causes ulceration, mucositis, and hair loss as side effects to these agents. Those types of chemotherapies cannot differentiate between 'good' cells or 'bad' cells, working rather on all fast growing cells. More recently, though, we have developed, in precision medicine, targeted therapies; treatments aimed at the cancer cells while sparing many non-cancer cells, targeting specific genes or proteins that are highly associated with that cancer. Monoclonal antibodies are one such example.
It may be that, as compared to vector control, the future is here in cancer care. It may be that when working on vector controls we might become better stewards of the environment by targeting the threatening organism rather than wholesale destruction.
In the last piece I highlighted one targeted therapy aimed at Aedes aegypti that may have worked well at the time it was written, in April 2016, trying to stem the tide, limit the number of species that may carry the Zika virus and at the same time sparing all other mosquito and insect species. I would like to highlight a different method that may work at this time.
I think it is time to get into a bacterial warfare with the Zika virus, using Wolbachia species as the main tool, and here are the reasons why:
1. There are numerous studies showing that Wolbachia in insects impacts the likelihood of transmission of diseases, including limiting the spread of Zika virus.
2. Wolbachia is an endosymbiont in arthropods, living in over half of the insects alive, so it is already present in nature.
3. Introducing Wolbachia into a new species of insect is not a difficult task, and has been accomplished in the lab easily enough.
4. Introduction of Wolbachia into a new species is essentially a one-time event as the process self propagates, making this form of control relatively inexpensive.
5. As we watch Wolbachia spread from insect species to insect species, it seems plausible to believe that this will happen in Aedes aegypti, sooner or later, and that all we are doing is speeding up that time.
6. It is always better to work with nature, instead of fighting it. Here this introduction of bacteria into the mosquito is nature evolving not us trying to stop nature from doing what nature wants to do. A much more likely formula for success.
7. In fact, this form of vector control has begun in several countries, and seems to be succeeding. Overall Zika virus transmission is down AND we are not killing other insect species at the same time.
It does not take a great stretch of the imagination to develop a targeted vector control program, that includes a combination of biological introduction of other organisms to control the viral transmission, here I am offering up Wolbachia, in combination with targeted mosquito killing methods, here I offer the Sterile Insect Technique (SIT) or RIDL (Release of Insects Carrying Dominant Lethal), both described last time. This type of program targets the one insect while leaving others relatively unharmed, as opposed to spraying insecticides. In many ways WHO agrees with this concept, though it is months delayed in execution.
This future has begun to take hold, even in the US. Clovis, California has begun to release these mosquitoes that have had Wolbachia introduced. In a way, this experiment is underway, though slow to take hold. In the balance of risk of leaving the current state as it is and introducing these types of targeted applications, I think the scales are easily tipped by the screaming of infants and parents. If we do not take advantage of these tools and strategies, the only choices left will be immunizations which have a measured though incomplete success, and avoidance which has shown to be problematic as a lone strategy.