It’s rare that we stop and reflect on the sheer horror of life just 100 years ago before antibiotics, vaccines and sterile technique were commonplace. In those days, every meal was a game of Russian roulette with E.Coli and every sniffle from a child could spell their imminent demise. The 20th century has been a relatively safe haven from pathogens, but that might all come to an end soon, as drug-resistant diseases become more and more common, thanks to our misuse of medicine.
Over the years, multiple attempts have been made to eradicate Malaria, a disease responsible for millions of deaths. All attempts to date have failed, and malaria still affects an estimated 200 million people each year. This disease is becoming alarmingly adept at developing resistance to whatever drugs are thrown at it. Why is this, and why can all the drug resistant strains of malaria be traced back to the same point of origin – remote jungle regions of western Cambodia?
Protozoa, drugs and death
Malaria is not caused by a virus or bacteria, but by microorganisms called protozoa. The protozoa are spread by mosquitoes – transferred into new host’s bloodstreams in their saliva. Once in the bloodstream, the protozoa travel to the liver where they multiply. They feed on haemoglobin, the molecule within red blood cells that transports oxygen around the body. Haemoglobin is broken down into a delicious protein group which is ‘eaten’ by the protozoa, and a cytotoxic heme group which is released back into the bloodstream. This heme element is what causes the pathological symptoms of malaria: it damages the host’s cells, causing fever, vomiting, seizures and often death. It is estimated that of the 400,000-800,000 malaria cases result in death every year. The majority of these are attributable to P. Falciparum, the most severe strain of malaria – carried by female Anopheles mosquitoes.
We have a range of anti-malarial drugs available to us. There are already malaria strains that have partial or complete resistance to some of these drugs, so several are often used in combination. The most effective drug is Artemisinin (originally found in quinine, now mass produced in genetically engineered yeast). We don’t actually know how the drug works against malaria, but suspect that it has something to do with the molecule’s uncommon peroxide bridge structure. Many anti-malarial drugs work by targeting the protozoa’s food vacuole, where they accumulate to prevent it from breaking down haemoglobin.
One of the major factors that contribute to selective pressures in disease is whether people take their prescribed drugs correctly: Tens of millions of individual microorganisms may be present in a person infected with a disease. Some, by chance, are slightly less affected by Drug A than others. If the infected person takes most of their medicine it will kill most of the microorganisms, and those weakest to the drug are first to die. If the person, feeling better decides not to take the rest of their drug course, the hardiest bugs might just about survive. These go on to infect another person, but they all start with the same improved drug resistance baseline. With each course of drugs that get abused, a slightly hardier bug survives.
Drug resistant malaria was first reported in the 1950’s, as Chloroquine became less and less effective. Like all subsequent drug resistant malaria strains, it was first seen in Cambodia and spread to the rest of the world from there. A study carried out on the DNA of Falciparum malaria, from samples taken from affected people in Myanmar between 2007 and 2009; found that mutations conferring drug resistance spread expeditiously through the region. They sequenced several genes known to be involved in resistance. In just one of these, five mutations were known to increase drug resistance. In 2007 they found it was common for protozoa to inherit one or two of these mutations on the same chromosome. By 2009 it was more common to see protozoa with four or all five of the drug resistant mutations.
Why is Cambodia a breeding ground for drug resistance?
Another study looked at how drug resistance genes spread around the world. They looked at the genetic makeup of the disease from different populations all over south East Asia and Western Africa. Their first finding was what you would expect: the biggest differences in genetic makeup were seen between really distant populations – between western Africa and Southeast Asia, >1000km apart and separated by an ocean. Their next finding was surprising: the second biggest differences in population makeup were seen in different mosquito populations within Cambodia – some less than 200km from one another. While the malaria population of most of Cambodia was a pretty homogenous mix of different strains seen in the surrounding areas, three locations had very little in common with the main population, or with one another.
These three regions were, despite being physically close to one another, extremely geographically isolated. They were separated by dense jungle and poor roads. Very few people (who spread the disease to different mosquito populations) travel between these areas. It seems that ‘founder effects’ have come into play here: the three unique populations must have originally been started by very few individuals. As hardly any others have come in and contributed to the gene pool since then, mosquitoes in this region have inbred and inbred until they all have one very uniform genotype. The study reported there was so little genetic diversity in these populations they were practically clones. It is this lack of admixture with other populations that makes these three groups appear highly genetically distinct:
The World Health Organisation (WHO) attempted to eradicate malaria in Cambodia in the 1950’s and 1960’s, but as conflicts with in Vietnam and with the Khmer Rouge raged on, the effort was abandoned. As we know, the most crucial thing in wiping out disease – in a sick person or a whole country – is to finish the job. The WHO threw Chloroquine at Cambodia and then left before the job was done. Suddenly, thanks to this huge selective pressure, Chloroquine resistant malaria was widespread in the region. Since then, Pyrimethamine and Sulfadoxine have followed suit. The theory now proposed a is that the founders of the three isolated Cambodian populations were all drug resistant. However, they didn’t just have mutations that made them resistant to Chloroquine; they had mutations that made them more likely to develop resistance to all antimalarial drugs. In these three regions, cases of malaria already take far longer to respond to our newest drug treatments. The nature this causative gene is unknown, but could work by increasing the mutability of another gene, such as one that controls the shape of the plasmodium food vacuole – changes to which could prevent malarial drugs being able to bind to it.
The advantage of having a much inbred population is that the effects of recessive genes are seen more quickly. The majority of mutations are detrimental, so many inbred mutants will die, but when a positive mutation does occur, particularly in a population already under selective pressure, it can become ‘fixed (present in all individuals) rapidly. Drug resistance could hardly be cooked up more quickly than here in a Bond villain’s evil laboratory. When people do travel out of this area, they bring the new, improved malaria strains with them, where they admix into wider plasmodium population, spreading their selective advantage as they go.
Some unique circumstances and lucky mutations have made drug resistant malaria a rapidly growing issue of global importance. It is likely that we will soon face similar situations with other disease, over which we previously had control. It’s an arms race now – who will win: humans and intelligent drug design, or disease and the billion-year-honed process of natural selection?