Tenacious Endemic Typhoid Fever in Samoa

Abstract

Background: Typhoid fever has been endemic on the island nation of Samoa (2016 population, 195 979) since the 1960s and has persisted through 2019, despite economic development and improvements in water supply and sanitation.

Methods: Salmonella enterica serovar Typhi isolates from the 2 hospitals with blood culture capability and matched patient demographic and clinical data from January 2008 through December 2019 were analyzed. Denominators to calculate incidence by island, region, and district came from 2011 and 2016 censuses and from 2017-2019 projections from Samoa's Bureau of Statistics. Data were analyzed to describe typhoid case burden and incidence from 2008 to 2019 by time, place, and person.

Results: In sum, 53-193 blood culture-confirmed typhoid cases occurred annually from 2008 to 2019, without apparent seasonality. Typhoid incidence was low among children age < 48 months (17.6-27.8/105), rose progressively in ages 5-9 years (54.0/105), 10-19 years (60.7-63.4/105), and 20-34 years (61.0-79.3/105), and then tapered off; 93.6% of cases occurred among Samoans < 50 years of age. Most typhoid cases and the highest incidence occurred in Northwest Upolu, but Apia Urban Area (served by treated water supplies) also exhibited moderate incidence. The proportion of cases from short-cycle versus long-cycle transmission is unknown. Samoan S. Typhi are pansusceptible to traditional first-line antibiotics. Nevertheless, enhanced surveillance in 2019 detected 4 (2.9%) deaths among 140 cases.

Conclusions: Typhoid has been endemic in Samoa in the period 2008-2019. Interventions, including mass vaccination with a Vi-conjugate vaccine coadministered with measles vaccine are planned.

Keywords: Salmonella Typhi; Oceania; Samoa; epidemiology; typhoid fever.

Figure 1.

Left: Annual number of blood culture-confirmed typhoid fever cases in Samoa from 2008 to 2019. Right: Annual incidence (blood culture-confirmed cases per 10⁵ persons per year) from 2008 to 2019.

Figure 2.

Left: Mean number of blood culture-confirmed typhoid fever cases per month based on aggregated data from 2008 through 2019; error bars represent the 95% confidence interval. Right: Mean monthly precipitation in Samoa from 1909 to 2009 (Source: The World Bank Climate Knowledge Portal).

Figure 3.

Mean annual incidence of typhoid fever by 5-year age groups, reported as blood culture-confirmed cases per 10⁵ persons per year in Samoa from 2008 to 2019. Insert: The same data from 2013 to 2019 when single-age resolution was available are presented to show incidence by each year of age for the first 5 years of life. Error bars represent the 95% confidence interval.

Figure 4.

Map showing the incidence and geographical distribution of households of cases of blood culture-confirmed typhoid fever occurring in 2019 in Samoa. Incidence (cases per 10⁵ population) is represented by variable shading from white to dark blue, with darker hues representing higher incidence ranges. Each yellow dot approximates index case(s) of typhoid within a household to portray the density of cases. To maintain patient confidentiality, a mapping program randomly displaced the marker slightly on the map. Households of all cases were visited by a Samoa Typhoid Epidemiologic SWAT Team that recorded GPS coordinates with a Garmin GPSMAP 64SC. The insert shows the Apia Urban Area and 3 rural regions—Northwest Upolu, Rest of Upolu, and Savaii—with the percent of the 2019 population in parentheses. Upolu’s population of 156 077 comprises 77.7% of the total population of Samoa, whereas Savaii’s population of 44 797 comprises 22.3% of the total population. Mapping and point displacement were performed using QGIS 3.8.0-Zanzibar (QGIS Development Team, QGIS Geographic Information System, Open Source Geospatial Foundation Project, http://qgis.osgeo.org).