A New Look at the Dynamics of Human–Mosquito Contact
By John P. Roche, Ph.D.
Vector-borne diseases kill more than 700,000 people per year globally, with the majority of deaths being caused by mosquitoes, so understanding how mosquitoes spread disease into a human population can help us control mosquitoes and prevent disease. Unfortunately, our understanding of the transmission dynamics of mosquito-borne diseases has been limited. Mathematical models can be a powerful tool for identifying gaps in our understanding, and researchers at the California Academy of Sciences and Tulane University have developed a comprehensive new model of disease transmission dynamics that can help steer a new generation of research and mosquito control efforts.
Most research on transmission dynamics has focused on quantifying the number of mosquitoes in a given area (i.e., the vector density) as a primary measure of disease transmission risk, and the majority of modeling studies have assumed that mosquito bites are spread evenly through a host population. However, vector density as a variable (and the assumption of bites being spread evenly) are both unsatisfactory because disease transmission to humans could vary widely with the same vector density, and some hosts might be more susceptible to being bitten than others. For example, host protective behaviors, multiple mosquito broods per reproductive cycle, and variation in temperature could all affect disease transmission rate.
A new modeling study, published last week in the Annals of the Entomological Society of America, was led by Panpim Thongsripong, Ph.D., a postdoctoral fellow in microbiology at the California Academy of Sciences, joined by Academy colleagues Shannon Bennett, Ph.D., and Durrell Kapan, Ph.D., and Tulane’s James Hyman, Ph.D. In the study, they point out that, instead of using vector density, a far more accurate measure of transmission risk is the rate of contact between humans and mosquitoes. This can be quantified as what is called the vector–host contact rate: the number of times individuals of a vector species contact individuals of a host species in a certain amount of time. This measure is central to understanding transmission dynamics because it is the bites that spread a disease, and host factors, vector factors, and environmental factors can all influence the number of bites. “We maintain that a focus on host–vector contact,” Thongsripong says, “not vector abundance, leads to a more accurate depiction of vector-borne disease transmission dynamics.”
In their study, the investigators set out to generate a new, comprehensive model that could identify factors that influence human–mosquito contact and could thus guide future research to fill gaps in our biological knowledge. Variables related to the dynamics of vector–host disease transmission include the vector–host contact rate, the biting rate, the bite exposure rate, and the blood-feeding rate. The research literature often used these terms interchangeably, but Thongsripong and colleagues detailed that these terms have distinct meanings. The vector–host contact rate is the total number of contacts of individuals of a species of mosquito with individual humans. The biting rate is the number of bites one mosquito takes per unit of time. The bite exposure rate is the number of bites one host receives per unit of time; it is the bite exposure rate that determines disease transmission from the mosquito to the host. The blood-feeding rate is the number of blood meals per mosquito per unit of time. The blood-feeding rate determines disease transmission from the host to the mosquito.
Mosquito–human contact is a two-step process. The first step is biting, during which a mosquito probes into human tissues searching for a blood vessel and injects saliva containing anti-coagulants into the host. In the second step (if the mosquito is not disturbed by the host and has an opportunity to find a blood vessel), the mosquito pierces a blood vessel and siphons out blood in what is called a blood meal. A mosquito can spread a disease to a human in either step—by just biting, or through a bite that leads to a blood meal, which the female uses to develop her eggs. A human, on the other hand, can only spread a disease to a mosquito if the mosquito takes a blood meal.
The vector–host contact rate can be affected by a number of factors. For example, the contact rate can be decreased by defensive behavior by the host. Such behavior can include swatting mosquitoes away; it can also include highly effective measures such as bed nets and mosquito repellents. The number of blood meals a female mosquito takes per reproductive cycle can also affect contact rate; with more blood meals, all else being equal, the contact rate increases. It is often assumed that there is only one blood meal per reproductive cycle (known as the gonotrophic cycle in mosquitoes), but this is not always the case; several species of mosquito, including Anopheles mosquitoes that spread malaria and Aedes and Culex mosquitoes that spread a range of other viral diseases, have been observed to take multiple blood meals during one cycle. Temperature can also affect transmission: With low temperatures or windy conditions, mosquitoes seek blood meals less frequently. Precipitation can affect the contact rate: If no water is available for females to lay their eggs, they will be less likely to take blood meals and the contact rate will be lower. Contact rate can also be affected by host infection status; several studies have found that mosquitoes prefer biting individuals who are infected with pathogens over those not infected.
Thongsripong and colleagues provide a comprehensive model of mosquito–human contact dynamics; now, more field data are needed to test predictions of that model. “Despite its fundamental role in driving disease transmission,” says Thongsripong, “host–vector contact rates and patterns are rarely characterized in vector-borne disease research due to the lack of field quantification methods.”
When asked about what she would most like to see in future field research on human–mosquito contact, Thongsripong says, “The first step is to develop standardized ethical methods to quantify contact rates in various field settings (both the bite exposure rate in humans, and mosquito blood-feeding rate) that take into account the natural range of variation in host behavior and mosquito biology. Next, there is the need to feed these biological field data into new mathematical models developed specifically to test the importance of variation in human–mosquito contact dynamics in response to behavioral and environmental drivers.”
A critical ongoing factor of change affecting mosquito–human contact dynamics will be rising global surface temperatures. This will expand the range of mosquito habitat and thus increase the geographic range over which many mosquito-borne diseases are transmitted. This will make the study of mosquito transmission dynamics, and an application of its lessons, even more important going forward.
Annals of the Entomological Society of America
John P. Roche, Ph.D., is an author, biologist, and educator dedicated to making rigorous science clear and accessible. Director of Science View Productions and Adjunct Professor at the College of the Holy Cross, Dr. Roche has published over 200 articles and has written and taught extensively about science. For more information, visit https://authorjohnproche.com/.