The role of ecology in managing vector-borne diseases: Zika and beyond

fenton-aThe recent re-emergence and spread of the Zika virus, coupled with the link to a surge in microcephaly cases, has gripped the attention of the global health community, the general public, and professional golfers alike.  Of course Zika isn’t new – it was first discovered in 1947 – however the scale of the outbreak in 2015 was unprecedented.  Given that there are currently no effective vaccines or medicines against Zika, suggested management efforts have mainly focussed on vector control (e.g. through traditional insecticides, the use of microbes to control pathogens, or genetic manipulation or selective breeding of mosquitoes to reduce vector population sizes or otherwise prevent them from transmitting the virus).  To deploy these vector-targeted methods effectively it is clearly essential to understand vector ecology.  Indeed, recent attempts to explain the patterns of infection and predict the likely number of cases in the future highlight the importance of ecological processes such as: heterogeneities in transmission, the magnitude of herd immunity, seasonality in dynamics, seasonal forcing or other environmental drivers, and the potential for the virus to circulate within reservoir populations etc (see here and here).  Of course, these processes aren’t unique to Zika – they are fundamental aspects of the ecology of any vector-borne infection.  As such these ecological processes have been well studied in many vector-borne disease systems, whether they relate to human diseases or not.This breadth of ecological research across vector disease systems is reflected in a recent Virtual Issue compiled by Wiley including papers from Journal of Animal Ecology and other BES journals.

The unique complexities and challenges of vector-borne disease systems

An obvious, but crucial, aspect of vector-borne diseases is that they (by definition) involve multiple host species, and these species are often quite different – typically there would be at least one (and often potentially more) ‘definitive’ host species, such as a mammalian host, and at least one vector species, often an invertebrate, such as a mosquito, flea or tick.  The different life styles, physiologies and behaviours of these different species mean that there are potentially complex interactions between vector, host, pathogen, and the environment.  These interactions are often ‘nonlinear’ by nature, which means there isn’t a straightforward relationship between changes in one component and the effect on another.  For example a control approach that reduces the vector population by 90% may not lead to a 90% reduction in the incidence of disease – it could be greater, but it could be much less, depending on how exactly these nonlinearities arise.  Similarly, environmental temperatures are likely to have differential effects on the vector, host and pathogen, making the impacts of climate change for disease risk hard to predict for vector-borne pathogens.

What are the ecological options for vector control?

Traditionally, methods of vector control have involved the use of chemical insecticides, for example using insecticide-impregnated bednets to combat mosquitoes.  A major concern with these methods is the selection pressure these chemicals impose for the evolution of insecticide resistance in the vectors (see here and here).  As with managing the evolution of antibiotic resistance, the sustained and effective use of such insecticide-based methods requires a well thought-through, evidence-based approach that uses a sound understanding not only of the genetics of resistance, but also of the demography, behaviour and broader ecology of the vector species involved.

Given these concerns over insecticide use, alternative vector control approaches need to be, and are increasingly being, explored – and again these require a fundamental understanding of vector ecology in order to be effective.  As described above there are various potential methods for breeding or selecting for modified mosquitoes in order to reduce the risk of transmission.  Alternatively (or additionally), various biological control options are being explored.  In some cases these use a ‘standard’ biocontrol approach, replacing conventional chemical insecticides with natural enemies such as fungi to suppress vector populations.  Recently however alternative approaches have been explored, which use self-replicating symbiotic bacteria, such as Wolbachia, either to suppress vector populations or to reduce their capacity to carry infectious virus. These bacteria are transmitted vertically from females to their offspring, and often manipulate the vector’s sex ratio to increase their frequency in the vector population – as such these symbionts may be able to spread and maintain themselves in ways that traditional biocontrol agents usually cannot.  Initial trials have been very promising, with Wolbachia-infected mosquitoes increasing substantially among populations of Aedes aegypti, the vector of dengue virus, in trial areas in north-eastern Australia (see here and here).  However, there are still limitations to its success that need to be understood and overcome before it can be relied upon as an effective control strategy on a larger scale. More generally, these various biological control approaches involve the introduction of yet another species into the mix, and therefore the introduction of additional non-linearities and complexities that need to be understood before their effectiveness can be fully evaluated (see here and here).

The role of ecology in the management of vector-borne diseases into the future

Vector-borne diseases are one of the most concerning groups of pathogens, covering both major endemic diseases (e.g. malaria, dengue fever) and (re-)emerging diseases such as Zika.  While vaccines and other medical technologies will always be important in our battle against these diseases, they cannot and should not be relied upon alone; there is a clear need to develop a wider range of methods to augment these approaches, ranging from improved surveillance of vectors and pathogens in their reservoir communities, to improving our understanding of the non-linearities and thresholds underpinning vector and pathogen dynamics.

These issues are fundamentally ecological ones, and are best understood using classical ecological approaches, including the combination of conceptual development, empirical observation and experimentation, and mathematical modelling to handle the likely complexities that arise.  By using this knowledge, often obtained from studies on non-human disease systems, we will likely be better placed to develop more effective and sustainable strategies to combat vector-borne pathogens such as Zika, or malaria, or chikungunya, or dengue, or Lyme disease, or bubonic plague…

Andy Fenton

Associate Editor, Journal of Animal Ecology

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