Cholera Epidemic in Guinea-Bissau (2008): The Importance of “Place”

As resources are limited when responding to cholera outbreaks, knowledge about where to orient interventions is crucial. We describe the cholera epidemic affecting Guinea-Bissau in 2008 focusing on the geographical spread in order to guide prevention and control activities

Number of cases, population, attack rate per 100 people (AR%), risk ratios (RR) and adjusted risk ratios (ARR) by age, sex and sanitary area.

<p>*Cholera treatment unit set up in this area.</p><p>**Cholera treatment centre set up in this area.</p><p>Sector Autónomo de Bissau, 2008.</p

Age and gender adjusted cholera attack rates (%) by Sanitary Area in Sector Autónomo de Bissau, 2008–2009.

<p>Coordinates expressed in sexagesimal degrees. * Sanitary area with a cholera treatment centre. + Sanitary area with a cholera treatment unit.</p

Weekly number of cholera cases and case fatality ratio (CFR%) in Guinea-Bissau 2008–2009.

<p>Weekly number of cholera cases and case fatality ratio (CFR%) in Guinea-Bissau 2008–2009.</p

Geographical distribution of the probability of finding a house with at least one cholera case in Bairro Bandim (Bissau) in percentage and areas over the 95% confidence interval.

<p>Coordinates expressed in sexagesimal degrees. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0019005#pone-0019005-g005" target="_blank">Figure 5a</a> shows the Google Earth™ picture and the overimposed image of the risk surface (probability of finding a house with at least one cholera case). The <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0019005#pone-0019005-g005" target="_blank">figure 5b</a> shows the risk surface and the two areas with statistically significant higher risk (black bold line). The same two clusters were detected (dashed blue circles) using the Kulldorff's spatial scan statistic (cluster A: Log likelihood ratio = 9.95, P = 0.029; cluster B: Log likelihood ratio = 8.81, P = 0.05).</p

Differences of K-functions and 95% confidence intervals between households with cholera cases and households without cases in Bairro Bandim (Bissau), 2008–2009.

<p>A homogeneous set of points in the plane is a set that is distributed such that approximately the same number of points occurs in any circular region of a given area. A set of points that lacks homogeneity is spatially clustered. The k-function is defined as the expected number of points within a distance <i>s</i> of an arbitrary point, divided by the overall density of the points. Due to variations in the spatial distribution of the population at risk, a k-function computed only for cases may not be informative. Instead, the k-function calculated for cases can be compared with the one calculated for non-cases, with the difference between the two functions representing a measure of the extra-aggregation of cases over and above the observed for the non-cases. This difference is represented in the figure above, showing extra-aggregation of cases.</p

Geographical distribution of the crude cholera attack rate by region and sub-regions in Guinea- Bissau, 2008–2009.

<p>Coordinates expressed in sexagesimal degrees.</p

Odds of internally caused case over time by area.

<p>Odds of a case being caused internally (i.e. as a result of other cases in that area) vs. externally for all areas throughout the epidemic, sorted by attack rate (top to bottom). Red represents those values in support of an internally driven epidemic and blue represents those supporting an externally driven epidemic. The observed epidemic curve is shown above in grey for reference.</p

Vaccination results by strategy and start time.

<p>Each plot shows the median (diamonds) and 95% predictive interval for the proportion of cases averted by vaccination start time for (A) attack rate-based, (B) population-based, and (C) connectivity-based targeting strategies. The colored lines represent the different number of areas vaccinated. Estimates made from simulations starting at the time of vaccination with 37,500 individuals vaccinated (75,000 doses). Purple lines (14 vaccination areas) are the same in each panel.</p