Professor Timothy Schowalter explains how deforestation – and increased urbanisation, as well as land use change – has an effect on vectors of human diseases such as mosquitos.
Disease and food availability have always been the greatest challenges to human survival. Diseases have plagued humans and their domesticated animals from earliest times, although the role of insects in disease transmission was not recognised until the late 1800s. The primary insect vectors of human diseases are mosquitoes, lice, fleas and sand flies. Ticks are non-insect arthropods that vector additional diseases. Of the most devastating human diseases (malaria, yellow fever, typhus, bubonic plague, smallpox, cholera and influenza), malaria and yellow fever are vectored by mosquitoes, typhus by human body lice, and bubonic plague by rat fleas.
Currently, mosquito-vectored diseases kill more than one million people per year and infect more than 500 million, mostly in poor, undeveloped countries. More than two billion people may be exposed to malaria, vectored by the mosquitoes in the genus Anopheles, each year. Controlling insect vectors of human diseases has been a major focus of entomological research and extension programmes. Ironically, however, anthropogenic changes in environmental conditions are increasing abundances of insect vectors and the incidence of human diseases.
Insects that vector diseases have particular habitat requirements and their abundances are regulated by food limitation and predation, just as are other organisms. Most mosquito vectors are associated with open habitats and proximity to standing water necessary for breeding, whereas lice and fleas are most associated with humans under crowded, impoverished and unsanitary conditions. Vector relationships are typically highly specific, requiring particular adaptations by the pathogen to permit survival in the vector and transmission into the primary (reservoir) host. For example, yellow fever is spread primarily by one mosquito species, Aedes aegypti, that, unfortunately, has been introduced around the globe via human transportation. However, some pathogens, such as the West Nile Virus, are more opportunistic and can be transmitted to a variety of bird species (reservoir hosts) by a variety of mosquito species, a factor that explains its rapid spread across the USA following its introduction in New York City in 1999.
The role of deforestation in disease abundance
Deforestation has been shown to be the primary factor contributing to increased abundance of mosquito vectors and incidence of human diseases. An analysis of data for 87 mosquito species from 12 countries showed that about half of the species (53%) were associated with deforested habitats, with 57% of those being confirmed vectors of human pathogens, compared to only 28% of species that were associated with forests. Species that vector multiple human pathogens were all favored by deforestation. This analysis demonstrates that the net effect of deforestation is increased abundances of mosquitoes that serve as vectors of human disease and reduced abundances of non-vector species.
A more recent review of more than 300 studies found that more than half (57%) documented increased human pathogen transmission by insects following anthropogenic changes in land use, including deforestation, fragmentation, or conversion to agricultural and urban land uses. The remaining studies showed reduced (10%), variable (30%), or no (2%) change in transmission rates. For example, deforestation associated with road construction and agricultural development in tropical Peru explained the subsequent increase in abundance of mosquitoes and resurgence of malaria in the region. Conversion of forests to grasslands also increased the likelihood that mosquito species characterising both habitats would acquire and transmit human diseases through a mingling of species at forest edges.
Fleas show similar responses to changes in land use. Conversion of forests to agricultural landscapes in Tanzania doubled the prevalence of flea-transmitted, plague-causing bacteria in rodent hosts, compared to adjacent conserved sites.
Cities often are premium habitats for disease vectors because of the abundance of standing water in ponds, fountains, gutters, drainage ditches, neglected garden containers, tires, etc., and the concentration of humans and domestic animals. Singapore has been recognised for its aggressive mosquito control program that includes mandates for citizens to empty water-holding containers that serve as breeding sites.
Deforestation and ecosystem conversion also are responsible for the loss of many species that require particular habitat conditions and may be important for the control of disease vectors and/or the dilution effect that results from disease transmission into resistant hosts in more diverse communities. For example, the complex interactions that limit tick abundance and spread of bacteria that cause Lyme disease depend on the diversity of small mammal species in the eastern USA. In years of low chipmunk abundance, ticks find white mice and transmit Lyme disease bacteria more easily to these primary reservoirs for this disease. This larger reservoir for disease results in greater human exposure and infection. By comparison, in years with high chipmunk abundance, fewer ticks find and transmit Lyme disease bacteria to mice, and humans experience lower exposure and infection rates.
Similarly, bird species diversity limits the abundance of reservoir hosts for West Nile virus and reduces the incidence of human cases. However, many of the bird species that are resistant to West Nile virus and dilute its transmission in diverse communities are absent from urban ecosystems. As a result, mosquito species that are the bridge vectors between birds and mammals are more likely to feed on crows, jays and cardinals that dominate urban ecosystems and are the primary reservoir hosts for this disease, resulting in greater exposure and infection of humans.
The changing climate
Climate change is likely to alter the distribution and epidemiology of insect-vectored diseases, although the effect of warming temperatures may be overshadowed by other factors. Many mosquito and tick species are already increasing in abundance and moving northward as global temperatures rise. Abundances of these vectors also are likely to increase in regions predicted to see higher precipitation or relative humidity.
Despite efforts to reduce vector abundances and incidence of human diseases, eradication of native mosquitoes is not a wise goal. Of the 3,000 species of mosquitos, relatively few feed on humans. Many do not feed on blood. Species in the genus Toxorhynchites, the largest mosquitoes, are predaceous on other mosquitoes. Mosquito larvae are important bacteriovores in standing bodies of water.
Furthermore, mosquitoes are the base of a food web that supports a variety of predaceous invertebrates and vertebrates, including many that provide biological control of other pests. Many popular insectivorous songbirds might disappear in the absence of mosquitos as a primary food source, at least during seasons when other insect prey may be unavailable. However, targeting invasive species would restore natural balances.
Pesticides and mosquito control
Non-specific, broadcast insecticides may do further harm to ecosystems. Mosquitoes in many areas have become resistant to the insecticides widely used for control, including DDT (DichloroDiphenyTrichloroethane) and pyrethroids. Continued use of broadcast insecticides is likely to increase the rate of resistance development in mosquitoes and other exposed insects. More targeted ways to combat mosquitoes include repellents, repellent-impregnated clothing, window and door screens, emptying water-holding containers near human habitations, and use of bacterial pathogens (for example, ‘mosquito dunks’) to control larvae in ponds or fountains.
Limiting further deforestation and human incursion into previously forested areas could greatly reduce conditions that are conducive to mosquito population growth and human exposure to disease agents. Obviously, finding the balance between minimising vector-borne diseases and sustaining ecosystem services necessary for human survival will be the key to protecting human and ecosystem well-being.
Professor Timothy D Schowalter
Department of Entomology
Louisiana State University
+1 225 578 1827
This article will appear in SciTech Europa Quarterly issue 28, which will be published in September, 2018.