A Crisis Beyond Haiti

The cholera outbreak in Haiti—and recent findings in bacterial genetics—are changing the way researchers look at vaccines. A mass immunization campaign in the country could not only check the spread of disease among Haitians, but also decrease the danger that the invading bacteria could mingle with other cholera strains in the region to form an even more harmful bug.

The risk of a swiftly expanding epidemic, which already has spread to the neighboring Dominican Republic, extends to other countries in the Caribbean, Central America, South America and beyond, argue John Mekalanos, chair of Microbiology and Molecular Genetics, and Matthew Waldor, a Howard Hughes investigator and the Edward H. Kass Professor of Medicine at HMS and Brigham and Women’s Hospital. A vaccination campaign in Haiti could weaken the introduced bacteria’s foothold in the region, they say, cutting the risk that the bugs could promote a more deadly strain by making potent genes available to other cholera bacteria through the continual DNA swapping that occurs in nature. These issues are weighing heavily on policymakers at the highest levels in global health, said Mekalanos.

Cholera is an ancient disease that has been tracked in a series of pandemics since the early 19th century. It typically has arisen in the Ganges delta of India and Bangladesh. Caused by consumption of food or water that carries the Vibrio cholerae bacterium— usually through fecal contamination—the disease entails a potent intestinal infection that results in severe diarrhea and dehydration. An estimated three to five million cases occur each year, resulting in 100,000 to 120,000 deaths. The most recent eruption, the seventh pandemic, originated in Indonesia in 1961 and spread to India, Bangladesh and other areas of the world. The pandemic exploded in Lima, Peru, in 1991, and since then has become established in much of Central and South America. The United States has been touched primarily by a different strain of cholera, with sporadic cases occurring along the Gulf Coast of Texas and Louisiana, largely associated with consumption of raw or undercooked seafood.

Remarkably, the seventh pandemic strain did not take root in Haiti, a reprieve perhaps explained by the country’s geographic isolation on the island of Hispaniola, which it shares with the Dominican Republic. Until the recent outbreak, the island had been cholera-free for about a century despite climatic risk factors such as El Niño and La Niña, identified by some as the cause of the 1991 Latin American epidemic and the current eruption in Haiti. Hispaniola’s charmed status apparently has changed with the recent outbreak, which emerged in Haiti nine months after the country’s potable water infrastructure was destroyed in the Jan. 12 earthquake.

The U.S. Centers for Disease Control and Prevention (CDC) responded to the first reports of cholera in Haiti, conducting pulsed-field gel electrophoresis (a relatively limited molecular fingerprinting technique) on copies of the bacterium. The test suggested that the bug was most closely related to seventh pandemic strains from South Asia. Given that the disease had broken out very close in time and place to the arrival of U.N. peacekeeping troops from South Asia, speculation surged that it may have been introduced by these troops. This possibility has spawned deadly demonstrations in Haiti.

In a paper appearing online Dec. 9 in The New England Journal of Medicine, Waldor; Mekalanos; Stephen Calderwood, the Morton N. Swartz, MD, Academy Professor of Medicine (Microbiology and Molecular Genetics) at HMS and Massachusetts General Hospital; Jason Harris, assistant professor of pediatrics at Mass General; Eric Schadt, chief scientific officer of the biotechnology company Pacific Biosciences; and colleagues substantiated for the first time that the strain of cholera erupting in Haiti matched bacterial samples from South Asia and not those from Latin America. They used a definitive third-generation, realtime, single-molecule DNA sequencing method developed by Pacific Biosciences. Thanks to a crash effort at the company, the scientists identified both single nucleotide variations and the presence and structure of certain chromosomal elements that indicate a kinship between the Haitian isolates and El Tor O1 variant strains isolated in Bangladesh in 2002 and 2008, which have been associated with more severe disease in that country. Even these results do not confirm the bug’s most likely path to Haiti, but simply support the conclusion that it was introduced by human activities rather than natural means. Waldor and colleagues also published a perspective online Nov. 24 in NEJM, advocating the establishment of a cholera vaccine stockpile in the United States to be used to quell outbreaks like the one in Haiti.

A mass vaccination campaign in Haiti would face significant challenges in restraining the rampant disease. The World Health Organization reports that three oral killed-virus vaccines are available against cholera, two of them nearly identical. All would require at least two doses, from one to six weeks apart. “Most people would argue that those vaccines are not ideal for squashing an established epidemic,” Mekalanos said, since the multiple-dose requirement delays immunity to an already pervasive infection and complicates delivery. “But they might actually still be useful in a place like Haiti if you were to take the highest-risk refugee camps and immunize people. Even if you didn’t have enough doses, you might be able to get some herd immunity effects by just getting the fraction of the at-risk population down a little bit. I am a firm believer that it’s cases that push the epidemic along, so the fewer cases you have, the less likely the epidemic will be more explosive.”

Several other vaccines are under development, and one of them, called Peru-15, originated in the Mekalanos lab. Based on a live, attenuated, genetically engineered bacterium, Peru-15 was shown to be safe in hundreds of volunteers in U.S. clinical trials. It is now licensed by Vaccine Technologies, Inc. (VTI) of Hainan, China, which is preparing to launch a clinical evaluation of the vaccine manufactured in China, where cholera still occasionally occurs.

“VTI is quite serious about producing the vaccine and hopefully getting it approved by the Chinese FDA,” said Mekalanos. If the regulatory hurdles in China are sufficient for acceptance in other countries, he said, “there’s a possibility that as a humanitarian move, the Chinese government might stockpile enough of the vaccine for an emergency on the scale of Haiti.”

The situation in Haiti and the region could get much worse, according to Mekalanos. Through the common mechanism of horizontal gene transfer, bacteria share genetic elements with bugs that are not their offspring. One mode of transfer is the bacterial virus, called bacteriophage or phage. When a phage infects a bacterium, it inserts its DNA into the bacterial genome. If the inserted DNA, or prophage, contains genes that increase the fitness and virulence of the targeted bug, it may spur the new strain to proliferate and become dominant in the surrounding ecosystem.

A study published in the Oct. 21 Nature by Mekalanos and collaborators in Bangladesh—Shah Faruque, Faizule Hassan and M. Kamruzzaman— reveals steps in the development of the seventh pandemic strain, which displaced the classical strain in India and Bangladesh and eventually moved to Africa, Central and South America, and now Haiti. The research centered on the origin of the most critical virulence genes, the two that encode cholera toxin. Mekalanos and Waldor had shown in 1996 that these genes are encoded by a phage dubbed CTX. Waldor later demonstrated that the CTX phage can only insert its DNA into the chromosome of the seventh pandemic precursor strain if that strain carries a certain gene sequence called dif, a site that enables integration of genetic elements. In the recent Nature paper, Mekalanos and colleagues added more to the story.

“And what we were surprised to find,” Mekalanos said, “is that the dif site encoded by the seventh pandemic strain appears to be encoded by another virus, the so-called TLC phage.” TLC has nothing to do with tender loving care. As a satellite phage, it forms a virus particle and mounts an infection by leaning on a helper phage. The researchers tracked down this helper virus and two other viruses that have conspired to produce the quantum leap in virulence shown by the seventh pandemic clone.

The “scary prospect,” said Mekalanos, is that the seventh pandemic clones now endemic to Central and South America have all of these prophages lined up in their DNA, making them highly successful in the regional ecosystem. There is now a chance that the South Asian strain in Haiti may introduce a portion of its more virulent cholera toxin gene, which came from the classical cholera strain once predominant in India and Bangladesh. Gene swapping among bacteria is remarkably quick, said Mekalanos. Genetic elements can be transferred in the lab within hours after mixing one strain with another, and in the environment, this transfer may take only weeks.

“One can predict that there’s a distinct possibility that this new variant form of cholera might go from Haiti to some site in Latin America and from there, further,” said Mekalanos. “And one can expect, if that happens, not only a displacement of the resident strains in whatever ecological niche they have, but also increased severity and maybe incidence of disease wherever the previous seventh pandemic strain has managed to establish endemicity. So this is a disaster beyond the shores of Hispaniola.”

It is this scenario that Mekalanos and Waldor believe could be addressed through mass vaccination. What the outbreak in Haiti is forcing officials to consider, said Mekalanos, is whether to “vaccinate Haiti to squash this epidemic as quickly as possible—before those genes get into the Latin American strains, and mortality changes from one percent to maybe 10 percent, which would be a horrific thing to happen.”

“So I think it’s changing the way we are viewing vaccines,” Mekalanos said. “Vaccines should be used not only to prevent disease, but also to prevent the transmission of fitness genes from one pathogen to another. These genes could render even existing pathogens capable of causing more severe disease in locales they have long established as home.”

For more information, students may contact John Mekalanos at john_mekalanos@hms.harvard.edu.

Conflict Disclosure: The authors declare no conflicts of interest.

Funding Sources: This research was funded in part by the National Institutes of Health; the authors are solely responsible for the content of this work.

Disclaimer: The researchers are unable to provide treatment recommendations for individual cases.