Above: An illustration of the life cycle of Plasmodium, the parasite that causes malaria. For more information, visit http://www.dpd.cdc.gov/dpdx/HTML/Malaria.htm (Centres for Disease Control and Prevention)
Whether it was a story about swine flu on the evening news or a movie like Contagion, outbreaks of infectious diseases have probably caught your attention at some point. The scientists responsible for tracking the spread of these diseases in the human population are called epidemiologists. And DNA barcoding has provided epidemiologists with some exciting new tools in the fight against diseases like malaria, sleeping sickness, and avian influenza.
Did You Know? Infectious diseases are illnesses caused by microbes (like viruses, bacteria, and parasites) that can be transferred from person to person.The key to understanding how an infectious disease spreads is to know where it lives (its reservoir) and how it gets around (its vector). A reservoir is any habitat where a disease-causing microbe normally lives. It can be in humans, in other animals, or in the environment. A vector, on the other hand, is an organism (usually an insect) that spreads the infectious microbe from one host to another. Generally, the vector is not harmed by the microbe, which is just catching a ride between hosts.
Mosquitoes might seem like nothing more than a minor summertime annoyance. But they are actually a common disease vector. Mosquitoes’ deadliest ride-catcher is Plasmodium, the parasite that causes malaria. Mosquitoes acquire Plasmodium by feeding on the blood of an infected person. Plasmodium multiplies and matures while inside the mosquito, then travels to the mosquito’s salivary glands. When the mosquito feeds on its next human victim, the parasite sneaks into that person’s bloodstream.
The best way to prevent people from getting malaria is to control mosquito populations. Common methods include using window screens and insecticidal nets, as well as spraying standing water with insecticides. However, there are around 3500 different species of mosquitoes worldwide, and only certain species are capable of spreading malaria. Scientists therefore need to be able to identify which species of mosquitoes live in a given body of water to avoid unnecessarily damaging the environment by spraying insecticides where only harmless mosquitoes live.
Did You Know? In Africa, one child dies from malaria every minute.The Mosquito Barcoding Initiative was started in 2005, with the goal of creating a database of DNA barcodes that will allow scientists to easily identify more than 80% of the world’s known mosquito species. This will allow epidemiologists to determine where to apply insecticides in order to better protect the health of both humans and the environment.
Another disease vector is the tsetse fly, which is responsible for the spread of sleeping sickness. Sleeping sickness is caused by the parasite Trypanosoma brucei, which causes fever, headaches, and fatigue. If left untreated, the disease eventually leads to coma and death. Much like mosquitoes and malaria, tsetse flies acquire and transmit sleeping sickness by feeding on the blood of infected animals.
In several locations in Africa, epidemiologists have recently used DNA barcoding to identify which animals were the source of the infected blood ingested by tsetse flies. They found that, in some locations, tsetse flies fed mainly on wildlife (such as buffaloes, elephants, and giraffes). In other locations, the flies fed mostly on cattle. This knowledge will help epidemiologists design better sleeping sickness prevention strategies. For example, in areas where cattle are the main hosts, the cattle can be given insecticides to reduce the population of tsetse flies.
Did You Know? Sleeping sickness kills about 3 million cattle per year, costing African economies around $4.5 billion.Identifying reservoirs of infectious microbes is also important for preventing illness. For example, some strains of avian influenza virus (such as H5N1) can cause serious disease when they infect humans. So epidemiologists need to track the bird species that are natural reservoirs for this virus. A common surveillance technique for avian influenza is to collect feces from wild bird habitats and test the samples for the presence of the influenza virus. By adding DNA barcoding to their toolkit, epidemiologists can also precisely identify which bird species the infected feces came from. This is particularly important in the case of migratory birds, since they can quickly spread new strains of influenza between different parts of the world.
Whether they are hunting mosquitoes, tsetse flies, or the feces of migratory birds, epidemiologists can use DNA barcoding to zero in on those vectors and reservoirs most responsible for the spread of infectious disease. Not only does this knowledge allow epidemiologists to take more effective action to protect human health, it can also help limit negative side effects of activities like the spraying of pesticides. And it gives renewed hope to tropical countries battling endemic diseases, as well as countries worldwide wary of possible epidemics.
General news and science websites
Malaria: Global Killer (National Geographic) DNA barcoding on track to revolutionise malaria control (Natural History Museum UK) Name, rank and serial number: Biologists want to barcode half a million species in the next five years (The Economist) Paul Hebert: Evolution and Ecology (Science.ca)
Public health organization publications
Information on Avian Influenza (Centres for Disease Control and Prevention) Malaria (World Health Organization) Anopheles Mosquitoes (Centres for Disease Control and Prevention) Parasites – African Trypanosomiasis (also known as Sleeping Sickness) (Centres for Disease Control and Prevention)
Muturi et al. 2011. Tracking the feeding patterns of tsetse flies (Glossina genus) by analysis of bloodmeals using mitochondrial cytochromes genes. PLOS ONE. 6:e17284 Lee et al. 2010. DNA barcoding techniques for avian influenza virus surveillance in migratory bird habitats. Journal of Wildlife Diseases. 46:649-654