Epidemiological characteristics of an urban plague epidemic in Madagascar, August-November, 2017: an outbreak report

Abstract

Background: Madagascar accounts for 75% of global plague cases reported to WHO, with an annual incidence of 200-700 suspected cases (mainly bubonic plague). In 2017, a pneumonic plague epidemic of unusual size occurred. The extent of this epidemic provides a unique opportunity to better understand the epidemiology of pneumonic plagues, particularly in urban settings.

Methods: Clinically suspected plague cases were notified to the Central Laboratory for Plague at Institut Pasteur de Madagascar (Antananarivo, Madagascar), where biological samples were tested. Based on cases recorded between Aug 1, and Nov 26, 2017, we assessed the epidemiological characteristics of this epidemic. Cases were classified as suspected, probable, or confirmed based on the results of three types of diagnostic tests (rapid diagnostic test, molecular methods, and culture) according to 2006 WHO recommendations.

Findings: 2414 clinically suspected plague cases were reported, including 1878 (78%) pneumonic plague cases, 395 (16%) bubonic plague cases, one (<1%) septicaemic case, and 140 (6%) cases with unspecified clinical form. 386 (21%) of 1878 notified pneumonic plague cases were probable and 32 (2%) were confirmed. 73 (18%) of 395 notified bubonic plague cases were probable and 66 (17%) were confirmed. The case fatality ratio was higher among confirmed cases (eight [25%] of 32 cases) than probable (27 [8%] of 360 cases) or suspected pneumonic plague cases (74 [5%] of 1358 cases) and a similar trend was seen for bubonic plague cases (16 [24%] of 66 confirmed cases, four [6%] of 68 probable cases, and six [2%] of 243 suspected cases). 351 (84%) of 418 confirmed or probable pneumonic plague cases were concentrated in Antananarivo, the capital city, and Toamasina, the main seaport. All 50 isolated Yersinia pestis strains were susceptible to the tested antibiotics.

Interpretation: This predominantly urban plague epidemic was characterised by a large number of notifications in two major urban areas and an unusually high proportion of pneumonic forms, with only 23% having one or more positive laboratory tests. Lessons about clinical and biological diagnosis, case definition, surveillance, and the logistical management of the response identified in this epidemic are crucial to improve the response to future plague outbreaks.

Funding: US Agency for International Development, WHO, Institut Pasteur, US Department of Health and Human Services, Laboratoire d'Excellence Integrative Biology of Emerging Infectious Diseases, Models of Infectious Disease Agent Study of the National Institute of General Medical Sciences, AXA Research Fund, and the INCEPTION programme.

Figure 1

Daily number of notified plague cases over time (onset date) by case classification Numbers of pneumonic plague (A) and bubonic plague (B) cases. Onset dates were imputed for 140 pneumonic plague and 22 bubonic plague cases. Seven pneumonic plague and two bubonic plague cases with missing onset, clinical examination, and sample collection dates are not shown.

Figure 2

CFRs by time period among confirmed, probable, and suspected pneumonic plague (A) and bubonic plague (B) cases, and probability of survival by days since symptom onset for confirmed or probable cases (C) Plots represent CFR and 95% CIs for confirmed, probable, and suspected cases by time period of the epidemic (initial, rapid growth, and control phase). CFR=case fatality ratio.

Figure 3

Spatial distribution of confirmed or probable plague cases Number of pneumonic plague cases per district (A). Number of bubonic plague cases per district (B). Red stars indicate districts with at least one confirmed case. The solid black line delimits the endemic districts.