Scientists have provided a rather grim prognosis for global health: the recent increase in nutrient enrichment due to human activities, such as nitrogen pollution through fossil fuel combustion, is likely contributing to several varieties of infectious diseases in humans and wildlife.
Lead author Pieter Johnson from the University of Colorado and colleagues referenced several studies showing the links—both direct and indirect—between nutrient enrichment and the emergence and resurgence of human and wildlife illnesses, such as limb malformations in amphibians and malaria, West Nile virus and schistosomiasis in humans.
As they describe in the latest issue of Ecological Applications, the negative side effects associated with fertilizer use need to be taken into account during agricultural planning, especially in regions of the world that are already struggling with environmental and public health challenges. Tropical and subtropical regions of Africa, Asia and South America are especially at risk, the researchers explain:
All three continents contain regions experiencing explosive growth and development, while still contending with rampant poverty, widespread environmental damage and a huge disease burden. Without question, increases in fertilizer application and food production in these regions will likely have substantially positive effects on human health by reducing malnutrition and improving quality of life. However, a significant concern lies with the unintended side effects of such efforts: will increases in the alteration of environmental nutrient concentrations incur an increased risk of disease?
They describe, for example, how nutrients can indirectly increase the prevalence of malaria and other mosquito-borne diseases. That is, nutrient enrichment in aquatic ecosystems boosts the abundance of plants in that habitat; these aquatic plants provide mosquitoes with protection from predators, such as fish, and support the bacteria that serve as food for the larval mosquitoes. These conditions create the perfect ecosystem in which mosquitoes, some carrying the malaria-causing protist Plasmodium, can thrive.
In another study referenced by Johnson and colleagues, female Culex restuans mosquitoes, which are known for transmitting West Nile virus, oviposited more than ten times the number of egg clutches in containers with elevated nutrient levels than in containers with normal levels.
The effects of nutrient enrichment are also being seen in wildlife populations where it is linked to limb malformations in amphibians. Specifically, an infection of the parasite Ribeiroia causes missing, misshapen and extra limbs in amphibians, limiting their survival rate. This parasite uses several hosts in its lifecycle, starting with herbivorous snails; therefore, an increase in algae growth due to nutrient runoff boosts both the snail and parasite populations.
In one study, the jump in snail population produced a three- to fivefold increase in amphibian infection. These findings, say the researchers, reach beyond aquatic ecosystems and into public health:
Considering the strong response of Ribeiroia infection to elevated nutrient conditions and the ecological parallels between the life cycles of Ribeiroia and Schistosoma, these results may have important epidemiological implications.
In other words, if the results of the Ribeiroia study are any indication of the potential prevalence of other parasites, then schistosomiasis–a parasitic worm infection that afflicts 200 million people primarily in Africa, Asia and South America —could possibly increase as well.
Johnson, P., Townsend, A., Cleveland, C., Glibert, P., Howarth, R., McKenzie, V., Rejmankova, E., & Ward, M. (2010). Linking environmental nutrient enrichment and disease emergence in humans and wildlife Ecological Applications, 20 (1), 16-29 DOI: 10.1890/08-0633.1