So let's first define a vector as an arthropod that being a mosquito, tick, flee or biting fly or other insect or other organism that transmit, send infectious pathogen from one infected person or animal to another. Here's a list of vector-borne diseases. So I'm sure you're familiar with all or many of these. So we have malaria, dengue, chikungunya, zika and yellow fever are all transmitted by the same vectors. West Nile, Lyme disease which we'll talk about, and then I won't read all the rest, but notice that I marked with a star that many of these are neglected tropical diseases. In fact, most of the neglected tropical diseases have also been more or less neglected with respect to the study of the relationship between climate change and these diseases, with the exception of dengue where there has been a lot of work. So the greatest burden of vector-borne diseases is found in developing nations and part of that is simply due to geography, that insect vectors tend to like warm climates, warm humid climates, and the developing nations tend to be located whether a tropical and subtropical climates. In addition, we have a lower level of socioeconomic development and public health infrastructure in developing nations that facilitates the spread and incidence of vector-borne diseases. Then poor persons also in general have a greater burden than more affluent persons, and that's due to a lower quality of housing for example that would allow mosquitoes to get into the housing more easily, tend to live closer to vector breeding sites and having less access to public health services. Then finally, children are more vulnerable to certain vector-borne diseases, and the best example of that is malaria. So if we look at this graph, this is showing incidents of malaria on a global scale. This is showing mortality. Here, we have the blue is males, red is females. They track about the same, and this axis is age, and the y-axis is is either incidence rate of mortality rate. You can see the concentration of high incidence mortality in the under five age group and then we have the five to nine, 10-14, but there's a great burden in the under five age group. Some good news is there's been progress against vector-borne diseases over the past one or two decades. So from 1990 to 2013, there was a 25 percent decrease in age-standardized mortality rates, and from 2005 to 2013, there was a 31 percent decrease in age-standardized disability-adjusted life years or DALYs. Now I know most of you probably know what DALYs are, but for those of you who don't let me define them briefly. So a DALYs summarizes the burden of premature mortality and disability due to a particular condition. So the DALY's for a particular condition are the sum of the years of potential life lost. So that is if a person has a life expectancy of 80 years and they die of that particular condition at age 50, then they've lost 30 years of potential life. Then the second component of DALYs is years lived with disability, but it's not the total years, it's adjusted for the severity of the disability. So for example, if a person lived for 30 years with a disability, that doesn't mean that we count 30 years lived with a disability. If it were a very mild disability, then we might count only three years of years lived with the disability. If it's a very severe disability, then we might count 25 of the 30 years as having lived with disability. Okay, so why has there been progress against vector borne diseases? So first has been socioeconomic development, and in general for various reasons that helps protect against vector-borne diseases. Increased access to health services, improved public health infrastructure, and finally more effective interventions. We'll get into some of that in more detail when we talk about some of the specific vector-borne diseases. So what about climate? So climate, and mainly we're talking about temperature, precipitation and humidity, can affect pathogens, including the rate of development and reproduction of the pathogen within the vector. So if that rate is fast, the faster that rate, in general the greater the transmission potential and the burden of the vector-borne disease. Climate can affect vectors including factors such as hosts seeking behaviors, egg laying behaviors, availability of breeding sites, the rate of egg larvae and pupae development, for mosquitoes for example, survival of the vector, abundance of vector predators and pathogens. Climate can also affect animal hosts for vectors. So such as affecting the availability of food and suitable habitat for the animal host as well as the abundance of animal hosts, predators and pathogens. Finally, climate can affect humans and human behavior, such as that would affect the degree of interaction of humans with the vectors. So for example, if the vectors are living in the forests but humans never enter the forest, then there's not going to be transmission of the disease to humans and it's not a problem. Also, climate affects migration which affects the incidence and spread of vector-borne diseases. So if a non-immune population of people to a particular vector-borne disease migrates to a place where the vector-borne disease is endemic, then those people will be very susceptible to that vector-borne diseases because they're not immune. Okay, so let's make a general statement about the role of climate in vector-borne diseases. So first there's no doubt that climate plays an important role in the spread, geographically distribution, and incidence of vector-borne diseases. However, climate is often a necessary but not a sufficient condition for the emergence or persistence of vector-borne diseases. So the potential geographic distribution of a particular vector-borne disease is determined by climate, and we could think of that as providing a climate envelope, so that the envelope shows the geographic areas where it's possible for the vector to live and survive. But non-climate factors are important in determining the actual distribution within the climate envelope. So those specific non-climate factors which we'll talk about in a minute could be determinative within the climate envelope of whether or not the vector-borne disease will actually exist in a particular geographic region. So what are the non-climate drivers? So let me just list those now and we'll go into some of these in more depth later when we talk about specific diseases. So first, pathogen evolution, including drug resistance. Humans susceptibility to infection such as herd immunity providing good protection. So that is if most people are immune or have at least a level of immunity to a disease, it helps to protect the entire population of that area. Ecosystem change. So vectors and animal hosts might survive better and some ecosystem than in others. Economic development. Water and land use, for example dams and deforestation affect habitat which affect the habitability of the particular region for vectors as well as animal hosts. Human behavior. International travel and trade has begun to become a big one, for example being responsible for the spread of Zika around the world. Public health infrastructure and progress. So for example if various effective malaria vaccine were developed that would totally changed the picture with regard to malaria. Vector and animal control measures, especially vector resistance to insecticides is a factor. Poverty and social inequality. War and famine help to flame vector-borne diseases. Migration, I mentioned that. Finally, governance. So good governance is a protective factor against the spread, distribution and incidents of vector-borne diseases. Okay, so I think it's becoming apparent that this is very complicated. So climate can affect the pathogen, the vector, animal host, human behavior. There are also this whole list of non-climate drivers. To make it even more complicated, climate change can affect the non-climate drivers, such as affecting pathogen evolution ecosystem change, economic development water and land use, war and famine, as well as migration. A key issue is how does and will climate change interact with non-climate drivers in the spread distribution and incidents of vector-borne diseases and what is the relative importance of climate change versus the non-climate drivers? It's important to recognize that there's a high degree of specificity here, we can't really necessarily make broad generalizations about how this is going to play out for vector-borne diseases as a whole. There's marked variation by disease and by geographic location.
