One billion people were on the move or have moved in the last year. The majority of this movement occurs within countries, but people are also moving across international boundaries. The scale of people moving raises the question: are we the most prevalent vectors of disease?
Restrictions to limit travel for essential purposes only has slowed down the movement of people within countries and between countries in an effort to curb the spread of COVID-19 and protect those most vulnerable to the pandemic. It is easy to see how human movement plays a role in the movement of new pathogens and historical movement has introduced malaria, leprosy, and smallpox across the globe. But what about diseases that are endemic?
Research has focused on emerging infections, but work also been done to show how human movement affects important viruses, such as measles and yellow fever virus. In this blog we’ll focus on the increasing capacity to understand how human movement affects parasitic diseases, when it is crucial to understand human movement, and how lessons can be applied to improve control of endemic disease. We will use extensive work on malaria to illustrate how human movement affects transmission, particularly close to elimination targets, and some new theoretical work on how these tools and concepts can be used in neglected tropical diseases, namely soil-transmitted helminths.
The reasons for movement are many: business, tourisms, labour migration (including seasonal work), forced migration, climate change refugees, and even human trafficking. The drivers of movement are complex and dynamic, but are essential to understand for the sustainable control of many infectious diseases.
The majority of research on human movement and parasitic diseases has been conducted on malaria. A previous post discussed a recent paper that investigated the role of mosquito movement in the long- distance dispersal of vectors and potentially disease. However, human movement leads to introduction of malaria to new places and has been shown to increase risk of infection with zoonotic malaria. For countries aiming for elimination, it is important to prevent introduction of new cases and differentiate this from ongoing transmission within resident populations. For example, a study examined this question across Southeast Asia and demonstrated that some malaria parasite transmission networks are isolated, suggesting that human movement is not that important in these areas. However many other parasite transmission networks occurred on international scales, suggesting treating migrants and/or travelers will be essential. Within Bangladesh, another study combined genetics of the parasites with surveys and mobile phone calling data to understand where transmission was occurring within a heterogenous region with large areas of forests but also cities. Combining these three datasets, they were able to show that parasites were being spread over large distances through human movement and that the clinical cases alone could not be used to design interventions in that part of the country. Movement between rural and urban environments, through human movement (for work, education, family visits, etc) has also been shown to be important for tuberculosis and other diseases.
Although there is not empirical data, yet, for the role of human movement in the spread and transmission of soil-transmitted helminths, some recent work has modelled the impact of movement on control of STHs. As countries move from elimination of STH as a public health problem to elimination of the diseases, the authors show that the role of human migration will become increasingly important. If a location is near achieving elimination, migration of infected humans into the control area can lead to a variety of outcomes, depending on how many are infected and how often they are moving in/out. Therefore, in places that have achieved control of a disease, it will be important to understand the role of human movement in subsequent infections.
Efforts are underway to help map population movement, but oftentimes the most vulnerable are left out of these metrics. Migration, rather than a challenge to health systems, can be an opportunity for improving health of migrants and residents. There are many organizations working to improve access to health services and in particular improve outbreak response. As more and more people move around in this world, it will be important to address health of migrants to improve health for all.