In Southeast Asia, drug-resistant falciparum malaria may have evolved resistance to another frontline therapy and established itself in new territory in western Thailand, according to the World Health Organization. The new area in Thailand joins previous hot spots in Cambodia, Vietnam, and Myanmar, with the latter being badly equipped to stanch further spread. Despite containment efforts, the possibility this strain may spread to Africa, which has the most significant malaria burden, remains very real.

Twice before, drug resistance has appeared first in Southeast Asia then migrated to Africa (which bears a punishing 90 percent of the world's malaria burden). Malaria intensity at the Thai-Cambodia border is comparatively light, but somehow parasites at this location have managed to evolve a tolerance for some previous frontline drugs, chloroquine and sulfadoxine-pyrimethamine. From 1980 to 2004, malaria deaths increased three-fold to an estimated 1.8 million, according to a study published in The Lancet. Antimalarial drug resistance was "the likely driver" according to the study.

Artemisinin-based therapies now occupy the frontline of antimalarial therapy worldwide and have helped control the epidemic. Artemisinin is usually administered as part of combination therapies (ACTs), where it is paired with medicines that persist longer in the bloodstream to mop up residual parasites. By striking at parasites in two ways at once, the probability of mutations providing resistance to both drugs simultaneously becomes, in theory, infinitesimal.

ACT deployment is massive, with demand likely surpassing 250 million courses in 2011. The loss of ACTs as an effective therapy would have huge consequences. "Malaria control completely depends on ACTs," as Arjen Dondorp put it last October. Dondorp is Deputy Director of the Mahidol Oxford Tropical Medicine Research Unit in Thailand.

But the efficacy of artemisinin is weakening, despite its remarkable prowess. Swift action has been the drug's hallmark, but the parasites along the Thai-Cambodia border region have slowed the drug's pharmacodynamic rush. The time to clear parasites has grown longer in these regions. Worrying signs began appearing in 2002. Initially suspicion fell on the partner drug, mefloquine. But by 2006, artemisinin clearly lost a step. Some parasites, found in certain geographic areas, remained after the standard three-day treatment. Slowing clearance times are known harbingers of resistance.

And so it proved: treatment failures continued rising. As a result, Cambodia switched in 2008 to the most recently approved ACT. That pairs artemisinin with piperaquine, a chemical relative of chloroquine. As long as the partner drug continued to work, treatment would succeed. "Despite the changes observed in parasite sensitivity to artemisinins," the World Health Organization stated in late 2010, "the clinical and parasitological efficacy of ACTs is not yet compromised."

Initially, piperaquine restored the therapeutic punch of ACTs. But artemisinin continued to buckle. ACT treatment failures in some areas rose from just 8 percent in 2008 to 28 percent in 2010. As artemisinin left behind an increasing fraction of multi-billion parasite infections (that's how many parasites an infected, symptomatic person typically carries), pressure grew on the less capable partner drug. Within the noise of the clinical data on the new ACT came disturbing sounds of piperaquine cracking, but it was hard to be sure. WHO worried last November: "resistance against piperaquine has far reaching consequences and needs urgent confirmation." Today, WHO believes that increasing treatment failures are "most probably due to piperaquine resistance," according to Pascal Ringwald, coordinator of WHO's Drug Resistance and Containment program.

Saving artemisinin



Substantial efforts have gone into preserving the effectiveness of this critical malaria fighter. Having twice seen the drug resistance drama play out, the malaria research community and global health authorities have tried for a full script rewrite. "We are staying one step ahead of malaria," said WHO Director General Margaret Chan last October.

Over the last several years, with funding from the Gates Foundation, the WHO containment effort drew a noose around the "Tier 1" zones of confirmed resistance in Cambodia near the Thai border. Cases were choked off considerably, but resistance has not gone away. A low but steady number of cases have been reported since late 2009, despite well-coordinated, intensive screening and treatment of entire villages, including every man, woman, and child. If the example of Cambodia is indicative of the limits of intervention, eliminating drug resistant malaria may not succeed. And unless resistant parasites are completely eliminated, the eventual spread of resistance is considered inevitable.

WHO containment efforts drew a noose around the "Tier 1" zones of confirmed resistance in Cambodia near the Thai border. Cases were choked off considerably, but resistance has not gone away.

The limited spread and low levels of infection led Melinda Gates to say "data indicate that the resistance is being contained." But even as she spoke, the limits of containment were apparent. A new resistance hot spot is now suspected around Mae Sot in western Thailand, near the border with Myanmar. It's unclear if Mae Sot represents a spread of drug-dodging parasites or de novo development.

Thailand was already moving into a nationwide resistance control and elimination effort, and the country has been highly successful in suppressing malaria within its borders. However, containment efforts generally face nearly insurmountable difficulties. People can carry low levels of malaria parasites and not show symptoms, and these asymptomatic carriers can still be bitten by mosquitos and support the transmission of resistant parasites. Total elimination may require mass population screening and drug administration, but not all infections can be detected. Microscopy screening isn't sensitive enough. PCR-based methods are more discriminating, but they are also more expensive and require technical skills. And, although it's vastly more sensitive than microscopy, even PCR can miss very low levels of parasitemia believed capable of sustaining transmission. Thailand's roll-out of PCR-based surveillance has evidently been shelved in favor of an expanded microscopy network.

If finding all the resistant parasites looks daunting, so does killing them. As researchers recognized in 2009, the contest with drug resistance has a "last man standing" character. The difficulty of elimination escalates because "the last few infections to be cleared are almost all resistant."

The greater the frequency of resistant malaria, the greater the chance of further evolutionary evasion of drugs. And here, things also look discouraging. Surveillance continues to discover additional resistance foci. Mae Sot joins the known hot spots in Cambodia, one in Vietnam, and several confirmed and suspected regions in Myanmar. Chinese scientists in Yunnan province (across Myanmar's northern border) have published results showing increasing parasite clearance times. This likely means artemisinin resistance. Confirming resistance is tricky, as results hinge on the quality of the microscopy, which is unknown.

The disunited state of Myanmar



Resistant malaria in Myanmar is hugely worrisome in multiple respects. The Tier 1 population of people in areas with drug resistant strains in Cambodia totaled just 270,000. In Myanmar, Tier 1 encompasses a population nearly 20 times larger, 4.8 million people. Myanmar's border areas in the north and east are uncontrolled and coincide with areas of drug resistance. The central government, now apparently reforming, has continued to engage in clashes with ethnic-based militias in those areas. Villages sometimes burn to the ground in the conflict, while the civilian/combatant distinction essentially does not exist.

One of the largest narcotrafficking organizations in the world underpins Myanmar's most powerful, 20,000-man militia. Besides expansive poppy plantations for opium, Myanmar also continues to sow landmines—one of three countries in the world still doing so—with a predictable harvest of death and dismemberment.

"The difficulty of reaching the important areas goes up as the importance of doing so goes up," said researcher Nick White, of Mahidol Oxford Tropical Medicine Research Unit. Speaking at the Gates World Malaria Forum last October, White described Myanmar as "a black box."

However, according to WHO's Pascal Ringwald, "Myanmar has an excellent routine surveillance system and we have at least as many data from Myanmar as from Thailand." At the same time, however, there is a scramble to set up additional monitoring sites.

Unrest in Myanmar may cause surges of migration. Refugees might flow over into Bangladesh, for instance, a neighbor to India. It isn't certain, but past drug resistance is believed to have transited from India to Kenya.

Migration and genetics



That past spread may provide lessons for the present. Africa once switched from chloroquine because of resistance originating in Southeast Asia. But researchers realized the alleles for resistance to the new drug, sulfadoxine- pyrimethamine (SP), were already sprinkled across the continent at low frequency. Crucially, there is no fitness cost for SP resistance, enabling its random spread in the parasite population. When SP moved to the malaria frontline, resistance grew very rapidly because the genetic basis was already present. Just add drug selection pressure and resistant alleles quickly come to dominate the parasite population.

Artemisinin resistance is far harder to detect. Instead of comparatively easy assays, a kind of small scale clinical trial is required

Resistance to SP is easy to uncover: point mutations in the dihydrofolate reductase and dihydropteroate synthase genes provide reliable biomarkers. Artemisinin resistance is far harder to detect. Instead of comparatively easy assays, a kind of small scale clinical trial is required in which parasite clearance times must be carefully measured over a three-day course of treatment with an adequately sized cohort of patients.

Continent-wide surveillance is infeasible with such methods. Selective monitoring would perhaps survey places in Africa like South Sudan where malaria conditions similar to Cambodia prevail. But South Sudan may be the most violent place on earth, beset from the outside through vaguely defined new borders and lacerated within by internecine tribal violence.

The absence of biomarkers not only makes artemisinin resistance hard to find, it makes distinguishing the spread of existing mutations from the appearance of new ones impossible. If the blooms of resistance in Southeast Asia are independent, resistant strains might not need to be transferred for resistance to develop roots in Africa.

Asked whether elimination of drug resistant malaria was now impossible, WHO's Pascal Ringwald answered indirectly. "The Prime Minister of Cambodia has declared elimination of malaria as a priority and activities will continue " Other experts at the Centers for Disease Control and Prevention and the Gates Foundation also declined to comment on the question. Ringwald points out that "resistance of chloroquine emerged from 7-8 different foci," when it jumped to Africa. Fewer foci have been confirmed so far for artemisinin resistance, but the list is growing and none have been struck off.