Cholera Outbreak in Haiti May Reach Almost Double Predicted Cases

Dr. Jaime Ferrán Inoculating for Cholera in Spain, 1885. Image courtesy National Library of Medicine.In October 2010, cholera broke out in Haiti for the first time in decades, devastating the country while it was still recovering from the 7.0 magnitude earthquake that killed hundreds of thousands and left millions homeless just nine months earlier. In typical conditions, cholera can be treated easily with an oral rehydration solution or, in severe cases, via intravenous fluids to replace what is lost to vomiting and diarrhea. With quick treatment, nearly all patients recover. Left untreated, however, the dehydration and shock caused by the disease can kill within a matter of hours.

In Haiti, the country’s already-poor infrastructure had been additionally damaged by the earthquake, leaving conditions ripe for water- and food-borne diseases; before the end of November, the cholera outbreak had spread across the country and killed almost 1,000 people. By the end of the year the death toll in Haiti had passed 3,000, and the Haitian government predicted that there would be more than 400,000 cases by the end of October 2011. Now, researchers at the Harvard School of Public Health and the University of California San Francisco have found that the number may reach almost twice that, predicting 779,000 cases of cholera and 11,100 deaths by the end of November.

Using mathematical models, the researchers predicted the number of cases based on the changing levels of immunity to the disease in the population, the duration of immunity provided by vaccination or previous infection (and the effectiveness of existing cholera vaccines, which is lower than other vaccines at between 50-67%), the proportion of cases that occur without symptoms, and other factors. These projections suggest that the number of cholera cases and deaths to come in the next eight months may far exceed previous estimates, which were based simply on a static attack rate.

However, the researchers also modeled the potential effect of intervention efforts on the epidemic. Their results suggest that efforts to provide clean water, antibiotics, and vaccination, whether alone or in combination, could significantly reduce the overall case count and save lives: they predict that reductions in the consumption of contaminated water alone could prevent 105,000 cases and 1,500 deaths from cholera. Vaccination of 10% of the population, they predict, could prevent 63,000 cases and 900 deaths.

There has been cautious optimism about the course of the outbreak after a drop in active cases in January of this year. But researcher Jason Andrews, MD, of Harvard and Massachusetts General Hospital, cautioned via email that the drop should not be used as an excuse to reduce or stop intervention efforts.

“When any infectious pathogen enters a completely susceptible population that has never seen it before, there will be a brisk upstroke in cases, [usually] followed by a fairly brisk downstroke,” he noted, explaining that this is exactly what was seen in the cholera outbreak, and is an expected result of more individuals gaining immunity after surviving the initial explosion of cases.

“Clean water, vaccination, and expanded case detection and antibiotic use can hasten the decline of cases and avert a significant burden of disease and death,” Andrews added. “Over the next year, cholera will likely become endemic in Haiti, where it reaches a steady state of continued cases. These interventions could lower that state and therefore avert cases and deaths.”

The modeling work done by Andrews and co-author Sanjay Basu, MD, in addition to providing updated estimates for the reach of the epidemic, also demonstrates the difficulty of assessing ongoing cholera outbreaks in comparison to other diseases. While a disease like measles is easier to track because of its distinctive and highly visible rash, cholera presents difficulties because of its high percentage of mild or completely asymptomatic cases; patients may have few or no symptoms, but can still spread the disease. (The high percentage of asymptomatic cases is also one of the largest difficulties in the efforts to eradicate polio.)

This characteristic of cholera makes it much more likely that cases are underreported, particularly in areas with poor health infrastructures (of course, these regions are generally the ones most at risk for cholera in the first place). In fact, since the researchers’ model depends on the percentage of cases admitted to the hospital—38% in their base model—their predicted number of total cases would be even higher if non-hospital cases were significantly underreported. They cite the example that if hospitalized cases accounted for only 10% of total cases, the number of expected total cases by the end of November would grow to 1,276,000.

In addition, the researchers note that the model cannot estimate the effect of the May-July rainy season, which may exacerbate the existing problems with the contaminated water supply. As a result, they write, their base estimate of 779,000 cases may actually be conservative.

In an accompanying editorial, David A. Sack, MD, of the Johns Hopkins Bloomberg School of Public Health, examines the challenges and benefits associated with each of the proposed intervention methods. He notes in particular that while rehydration can save many cholera patients’ lives without the help of antibiotics, antibiotics can not only shorten the length of a patient’s illness, but also allow for the treatment of more patients (by opening up more beds in hospitals) and reduce a patient’s period of being infectious, preventing him from spreading the disease and causing more cases.

Sack also notes that in “nearly all cholera outbreaks, more deaths are of those who do not reach a treatment centre.” These deaths, he points out, are typically among people in remote areas with no access to treatment. As such, these groups would be good targets for cholera vaccination in the case of an epidemic. While individuals in more developed areas have access to treatment should they fall ill, those in rural areas are more likely to die if they develop the disease.

Sack concludes his commentary with the hope that the epidemic will be under control within a year’s time, but agrees with Andrews’s assessment.

“The more realistic expectation,” he writes, “is for endemic cholera to continue for many years, as it has in sub-Saharan Africa since 1970, unless a coordinated effort is mounted with all available resources, including improved water and sanitation, improved case management with appropriate antibiotics, and the use of oral vaccines.”

Andrews JR, Basu S. Transmission dynamics and control of cholera in Haiti: an epidemic model. Lancet. 2011; Early Online Publication 16 March. Available at http://www.thelancet.com/journals/lancet/article/PIIS0140-6736%2811%2960273-0/fulltext

Sack DA. Comment: How many cholera deaths can be averted in Haiti? Lancet. 2011; Early Online Publication 16 March. Available at http://www.thelancet.com/journals/lancet/article/PIIS0140-6736%2811%2960356-5/fulltext


Additional reading

Centers for Disease Control and Prevention: Cholera
http://www.cdc.gov/cholera/

Centers for Disease Control and Prevention: 2010 Haiti Cholera Outbreak
http://www.cdc.gov/haiticholera/

Pan American Health Organization: Cholera Outbreak in Haiti
http://new.paho.org/hq/index.php?option=com_content&task=view&id=4500&Itemid=3527&lang=en

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