Scientists in Ghana recently unveiled a novel strategy that relies on cutting-edge Canadian biotechnology to fight the disease lymphatic filariasis (LF), more commonly known as elephantiasis.
LF spreads via mosquitoes that have fed on infected people, thereby transferring the parasitic worms that cause the disease to the next blood meal. The worms enter through the skin, colonize the bloodstream, and clog the lymphatic system. These clogged lymphs swell to painful proportions, often leaving victims permanently disfigured. Today over one billion people in over 80 countries are at risk of infection. A large proportion of these potential victims reside in West Africa.
In an effort to eradicate LF by 2020, health authorities are treating people in LF-affected communities annually with a drug cocktail that works to lower the density of worm larvae in human blood, a strategy designed to reduce the ability of mosquitoes to transfer LF.
The mosquito family is diverse and each species has different LF larvae transmission rates. The particularly dangerous species are those able to transfer larvae from individuals that have been treated with anti-LF drugs—thereby thwarting conventional methods of LF control.
This fact led scientists to speculate that the key to controlling the spread of the disease is in assessing the mosquito population of LF-affected communities. By determining the proportion of menacing mosquito species in a community, public health workers will be able to supplement drug treatment with pesticides targeting at-risk regions. But how do you design a fast and easy way to profile a community’s mosquito population?
Canadian scientists may have the answer.
In 2003, researchers at the University of Guelph, headed by Dr. Paul Hebert, showed that every organism can be identified on the basis of a simple DNA-based assay, even those that are very closely related and difficult to distinguish by other methods. The assay relies on the fact that all species encode evolutionarily related genes, with members of the same species possessing an almost identical genome. This provides scientists with a genetic “barcode” to catalogue earth’s biodiversity.
The mitochondrial genome presents an ideal source for this approach. In addition to being the “power plants” of a cell, mitochondria also encode their own genome, separate from that in the cell’s nucleus. Mitochondrial genes undergo a higher rate of mutation than nuclear genes and therefore display more genetic variability between species.
A number of foundations are currently dedicated to compiling encyclopedias of genetic barcodes.
Dr. Jim Woodgett, director of research at the Samuel Lunenfeld Research Institute, and senior investigator at the Ontario Cancer Institute at Mt. Sinai Hospital in Toronto, agrees that DNA barcoding “is a good way to differentiate between related species, and is relatively fast.”
“Variants [of species] can be correlated with behaviour […] and variant genes can be associated with [that] behaviour,” says Dr. Woodgett.
In the case of LF, barcoding can distinguish between the innocuous species of mosquito and those that are able to transmit LF larvae from an individual treated with anti-LF drugs.
Until now, DNA barcoding has been used by taxonomists and ecologists to catalogue the biodiversity of different environments. For the first time, this biotechnology is being employed in the war on a major global disease. Scientists at the University of Ghana, in collaboration with the JRS Biodiversity Foundation in Philadelphia, are using this system to lessen the transmission of LF.
DNA barcoding enables researchers to protect biodiversity while targeting disease. In communities affected by LF, researchers will be able to determine whether drug treatment is enough to contain LF infections or if pesticide treatment is also required. This approach will ensure that only communities inhabited by the most dangerous mosquito species will be sprayed, thereby maintaining as much biodiversity as possible and protecting fragile ecosystems from pesticides that are not mosquito-specific.
However, Dr. Sandra Smith, a professor within U of T’s Faculty of Forestry and an expert on insect ecology and management, isn’t so sure that they’ve found the silver bullet. She says that differentiating between species of mosquito is already possible on the basis of morphology, but she is quick to mention that barcoding can be useful in determining the identity of larvae, which may be more difficult.
To control the mosquito population public health workers can employ pesticides and biological agents, as well as reduce standing water to prevent mosquito breeding. However, Dr. Smith warns that pesticides can’t be used “endlessly and at high concentration,” or they will push mosquitoes to evolve resistance.
Whether DNA barcoding can be applied to other human diseases remains to be seen.
Dr. Woodgett is optimistic that DNA barcoding, and in the future, deep-sequencing (which can sample more regions of the genome) will aid in the fight against other human diseases. He predicts that barcoding may one day be used to investigate the effects of dietary change on the complex gut flora of patients with inflammatory bowel disease.
Dr. Smith suggests that the work on mosquitoes and LF may be translated to stemming the transmission other mosquito-borne diseases such as yellow fever, malaria and some forms of encephalitis that are also only transmitted by certain species of mosquito.
DNA barcoding alone may not eradicate LF by 2020, but as Dr. Smith says, “you don’t solve a problem with a silver bullet; you solve it with a toolbox.”
