Near-Infrared Spectroscopy: A One-Stop-Shop for Mosquito Epidemiological Monitoring?

蚊子传播的疾病，如疟疾、登革热、基孔肯雅热和寨卡病毒，在世界热带地区造成巨大痛苦。控制这些疾病的主要方法之一是使用杀虫剂杀死蚊子，防止它们在人与人之间传播感染。每年在控制蚊子上花费数百万英镑，但令人惊讶的是，没有简单的方法来评估这些干预措施杀灭蚊子或防止它们传播疾病的能力。由于当地的天气模式，一个地区叮咬蚊子的数量每天都会有很大的波动，所以诱捕蚊子的数量不能很好地预测人口的规模。更重要的是，蚊子的数量本身并不是一个很好的风险预测指标，因为许多疾病需要几天的时间才能在蚊子体内发展并到达口器。这意味着只有年龄较大的蚊子才能传播感染。因此，蚊龄对于评估灭蚊干预措施的效果非常重要，但目前还没有一种简单、准确的方法来评估蚊子种群的年龄。近红外光谱(NIRS)是实验室发展起来的一种新的年龄分级和物种识别技术。它通过测量随着蚊子年龄的增长，一束光与蚊子身体的反射方式的不同来预测蚊子的年龄。与其他方法不同，近红外光谱不需要昂贵的化学品或程序，任何人只要最少的培训就可以进行。这使得它可以作为现场监测蚊子年龄的常规方法。目前，近红外光谱仪无法非常准确地预测单个蚊子的年龄，而且只对实验室饲养的蚊子进行了测试，这些蚊子可能比野外捕获的蚊子更均匀(因此得出更准确的结果)。然而，对于疾病控制来说，了解蚊子种群的平均年龄比了解个体的年龄更重要。我们的初步工作表明，如果我们改变分析近红外光谱结果的方式，我们可以对蚊子种群的平均年龄做出高精度的预测。该项目打算将近红外光谱从实验室带到野外，并测试它是否足够好，能够用于蚊子种群的常规监测。该项目将使用半现场和现场数据来实施这项技术，并概述需要捕获多少蚊子(以及在多少天内)才能产生足够准确的估计，以指导蚊子控制措施的部署。工作将集中在两种最重要的蚊子传播感染上：疟疾(2015年导致43.8万人死亡)和登革热(每年感染4亿人)。然而，这里开发的技术可以应用于其他疾病和蚊子物种。近红外光谱还可以用来区分肉眼无法区分的密切相关的蚊子物种。这一点很重要，因为并非所有这些蚊子都具有相同的传播疾病的能力，并且受到控制干预措施的影响不同。与年龄分级类似，近红外光谱区分物种的能力需要在实地进行更严格的测试。也有证据表明，近红外光谱可能能够检测到蚊子是否感染了导致登革热的病毒。这将首先在布基纳法索的实验室进行疟疾测试，然后在实地进行测试。目前，蚊子的种类、年龄和感染状况是使用各种费力和昂贵的程序来估计的，这使得它们无法在世界上较贫穷的地区用作常规监测工具。一种同时进行这三种检测的廉价单一方法将对公共健康产生重大影响：我们可以更廉价、更快速地描述疾病传播的风险，并评估控制计划的效果。

Mosquito borne diseases such as malaria, dengue, chikungunya and zika cause huge suffering in tropical regions of the world. One of the main approaches to controlling these diseases is to use insecticides to kill mosquitoes and prevent them from transmitting the infection from person to person. Millions of pounds are spent on mosquito control each year though surprisingly there is no simple method for evaluating the ability of these interventions to kill mosquitoes or prevent them from transmitting disease. The number of biting mosquitoes in an area fluctuates substantially from day to day due to local weather patterns so the number of mosquitoes caught in traps is a poor predictor of the size of the population. More importantly the number of mosquitoes in itself is not a good predictor of risk as many diseases take several days to develop inside the mosquito and find their way to the mouthparts. This means that only older mosquitoes can pass on the infection. Mosquito age is therefore very important for assessing the effectiveness of anti-mosquito interventions but currently, there is no easy, accurate way of assessing the age of a mosquito population.Near-Infrared Spectroscopy (NIRS) is a new age-grading and species identification technique that has been developed in the laboratory. It predicts the age of the mosquito by measuring how a beam of light is reflected differently from the bodies of mosquitoes as they get older. Unlike other methods NIRS doesn't require costly chemicals or procedures and it can be carried out by anybody with minimal training. This makes it feasible for use as a routine method for monitoring mosquito age in the field. Currently NIRS cannot predict the age of an individual mosquito very accurately and tests have only been done on mosquitoes reared in the laboratory which are likely to be more uniform (and therefore give more accurate results) than those caught in the wild. However, for disease control it is more important to know the average age of the mosquito population than the age of individuals. Our preliminary work suggests that if we change the way we analyse NIRS outputs we can generate highly precise predictions of the average age of the mosquito population. The project intends to take NIRS from the laboratory to the field and test whether it is good enough to be able to be used in the routine monitoring of mosquito populations. The project will use semi-field and field data to operationalise the technique and outline how many mosquitoes need to be caught (and over how many days) to generate estimates accurate enough to guide the deployment of mosquito control.The work will concentrate on the two most important mosquito borne infections: malaria (which kills 438,000 people in 2015) and dengue (which infects 400 million people annually). However the technique developed here can be applied to other diseases and mosquito species. NIRS can also be used to differentiate closely-related mosquito species that are indistinguishable by eye. That is important, as not all of these mosquitoes have the same ability to transmit disease and are affected by control interventions differently. Similarly to age-grading, the capacity of NIRS to differentiate species needs to be more rigorously tested in the field. There is also evidence to suggest that NIRS might be able to detect whether a mosquito is infected with the virus that causes dengue disease. This will be tested for malaria, first in the laboratory in Burkina Faso and then in the field. Currently mosquito species, age and infection status are estimated using a variety of laborious and costly procedures that preclude their use as routine monitoring tools in poorer parts of the world. A single, inexpensive method for doing all three tests simultaneously would have significant public health impact: we could describe the risks of disease transmission and evaluate the efficacy of control programs far more cheaply and quickly.