A vector excreta surveillance system (VESS) to support the rapid detection of vector-borne diseases

More than half of the global population is at risk for deadly and debilitating vector-borne diseases (VBD). VBDs, which include malaria, dengue, filariasis and many others, cause 1 million deaths each year and leave communities in economic hardship. While many efforts to control VBDs have been successful, we are working against the challenges of globalisation, insecticide resistance and climate change. The spread of Zika virus is a very relevant example of how these factors have fostered the rapid expansion of VBDs. The World Health Organization has called for more research to improve infectious disease surveillance so that we can prevent and control emerging and re-emerging diseases. Fortunately, there has been renewed political and financial momentum to provide interventions such as vector control and mass drug administration to combat these diseases. An early detection system would allow for a rapid response to help contain the spread, however these approaches can be costly and are out of reach of many developing nations.

We have developed a low cost, VBD surveillance tool which allows us to detect the presence of VBDs in a community. Mosquitoes pick up pathogens when they feed on a person. If the mosquito is a competent vector, the pathogen will go on to develop within the mosquito body and the mosquito becomes infective a week or two later. We have shown that when non-vector mosquitoes ingest infected blood, they excrete the pathogen DNA in their faeces over the next few days. When vectors ingest infected blood, they also excreta small amounts of pathogen DNA.

The vector excreta surveillance system (VESS) collects mosquito excreta for the purposes of screening for pathogen DNA. Mosquitoes are actively or passively collected using one of many commercial or home-made trap types for the purposes of disease-specific surveillance or research. We have made a modification to mosquito holding cups, where mosquitoes may be held prior to sorting and preservation. We have included a super hydrophobic lining below where mosquitoes rest which concentrates the excreta into a small reservoir or a funnel which deposits onto special preservative. This is a significant improvement over current monitoring methods because we are able to screen very large pools of mosquitoes at once. The mosquito samples are not used in the process so there are opportunities to integrate this method with other surveillance activities. The role of non-vectors is particularly useful since it would enable screening of other fly-borne illnesses without species-specific collections (including the parasites that cause leishmaniasis, river blindness, sleeping sickness.) There is scope for further development of excreta collection from a passive trap which would eliminate the need to actively collect the mosquitoes.

In our lab based studies we have successfully detected DNA from malaria, the filarial worm that causes lymphatic filariasis, and a trypanosome species that causes animal trypanosomiasis. This proposal aims to field-test this approach in a village in Ghana where we have already characterised disease prevalence, mosquito species and local ecology. We will use three different trap types so allow us to determine which species and life-stage (bloodfed resting mosquitoes, or gravid mosquitoes ready to lay eggs) is the most sensitive for detecting parasite DNA in the excreta. We will compare the detection from our pools of excreta with the detection from individual mosquito carcasses. We also aim to extend this methodology to the detection of viral particles from the mosquito-borne viruses West Nile, dengue, chikungunya and Zika.

This is an approach that disease control programmes in developing countries could implement in order to monitor the presence of a diverse range of blood-borne pathogens without having to collect blood from participants or process mosquitoes individually.

The emergence of outbreaks of preventable vector borne diseases has inspired us to develop an early response tool for the detection of human blood-borne pathogens from the mosquito population. We have developed a novel method to collect the excreta and faeces (E/F) from large pools of mosquitoes for the purposes of pathogen surveillance. We have successfully amplified DNA of filarial and protozoan parasites from the E/F of vectors and non-vectors exposed to moderate infections in blood. Benefits of the vector excreta surveillance system (VESS) include opportunities to integrate surveillance of many blood-borne pathogens through a non-invasive sampling technique which can be coupled with existing, disease-specific mosquito collections.

Our first aim it to field test the approach in our study site in western Ghana to detect malaria which is highly endemic and filariasis which is nearing elimination (2% prevalence) from a previous high endemicity. We will employ a Latin square design of three mosquito collection methods rotated among 8 households randomly chosen across four village habitats. In addition we will collect outdoor resting mosquitoes as they enter and leave the village from the breeding sites. We will compare Plasmodium spp and Wuchereria bancrofti infection prevalence from the E/F of pooled mosquitoes with the infection prevalence in mosquito carcasses. We will compare detection of pooled E/F between a field-friendly mini-PCR with test-strip visualisation to the standard quantitative PCR. Our second aim will be to determine the limits of detection to other important vector-borne pathogens: trypanosomes and the arboviruses West Nile, dengue, chikungunya and Zika. This will be done through lab-based exposures using vector and non-vector species to determine the optimum methods of detection post-exposure.

The foundation award will support a collaboration of scientists which represent the spectrum of translational research, enabling us to deliver a field-ready approach now and explore the promise to detect blood-borne pathogens of both humans and animals in the future. This project will have near term impacts on the research and development sector, with longer term impacts extending to disease control programmes, disease endemic communities, local economy, the general UK public and government.

Research and Development: Research outputs will enable the development of surveillance platforms that can be integrated across disease systems. Our field ready approach is designed to work with existing mosquito collection activities, however depending on our results, there is scope to develop a standalone passive trap system. If we find that detection is strongest in recently bloodfed mosquitoes, an adaptation of the resting box could be used to passively collect excreta, likewise if we find that detection is strongest in gravid females (3 days post-feeding), an adaptation of an oviposition trap would be best.

Businesses: Pharmaceutical companies (Johnson and Johnson, GlaxoSmithKline, Merck & Co) have pledged to donate drugs for the elimination of lymphatic filariasis and onchocerciasis. Drugs are currently being distributed annually to entire communities through mass drug administration. Sensitive tools to survey for the presence of NTDs such as LF and onchocerciasis would allow for the preservation and redirection of resources.

Disease endemic communities: Using LF as an example, the gold standard for disease surveillance involves direct collection of blood samples to detect the presence of antigen, or collection of blood samples at night to detect microfilaria. In communities that are nearing the endgame of elimination(1), prevalence is incredibly low however the risks or resurgence are high. Frequent sampling of the communities cannot be sustained over time and a new, non-invasive approach to xenomonitoring could be used longer term to sustain the gains made in LF elimination to date. Communities could take ownership of their own surveillance activities by setting and swabbing traps periodically, and partnering with District and Regional health offices to evaluate results.

Disease control programmes: An integrated surveillance approach would allow disease control programmes to use available resources to support surveillance activities and broaden the reach across multiple pathogens. This would allow valuable resources to be carefully allocated to at-risk communities without costly epidemiological surveillance. The approach may also enable more sensitive surveillance for diseases where expertise and resources are limited by providing preliminary information to leverage further funding for interventions. Results from this research will call for a shift from current disease surveillance and management practices that employ single tools for single targets. This will help to build capacity within national elimination programmes to monitor success and adapt strategies in response to specific needs.

Local economy - VESS will contribute to enhanced detection and targeted control which will boost tourism and provide opportunities to host international events.

UK government: Public Health England conducts fortnightly mosquito surveys at ports across the UK during the summer months to monitor mosquito abundance and identify invasive species(2). They use odour-baited commercial traps designed to capture large numbers of mosquitoes. With further research, VESS could be integrated into these mosquito surveys for the simultaneous screening for vector-borne diseases of both humans and animals. This would be a low cost method that easily fits within the goals and capabilities of Public Health England and would provide a model for vector-borne disease surveillance worldwide.