Yellow fever in Ghana: Predicting emergence and ecology from historical outbreaks

Abstract

Understanding the epidemiology and ecology of yellow fever in endemic regions is critical for preventing future outbreaks. Ghana is a high-risk country for yellow fever. In this study we estimate the epidemiology, ecological cycles, and areas at risk for yellow fever in Ghana based on historical outbreaks. We identify 2371 cases and 887 deaths (case fatality rate 37.4%) from yellow fever reported in Ghana from 1910 to 2022. Since implementation of routine childhood vaccination in 1992, the estimated mean annual number of cases decreased by 81% and the geographic distribution of yellow fever cases also changed. While there have been multiple large historical outbreaks of yellow fever in Ghana from the urban cycle, recent outbreaks have originated among unvaccinated nomadic groups in rural areas with the sylvatic/savanna cycles. Using machine learning and an ecological niche modeling framework, we predict areas in Ghana that are similar to where prior yellow fever outbreaks have originated based on temperature, precipitation, landcover, elevation, and human population density. We find differences in predictions depending on the ecological cycles of outbreaks. Ultimately, these findings and methods could be used to inform further subnational risk assessments for yellow fever in Ghana and other high-risk countries.

Fig 1.. Yellow Fever Cases Reported by Region in Ghana

Reported YF cases that could be located on a regional level in Ghana are shown for the periods before and after routine childhood YF vaccination, 1960-1992 and 1993-2022. The regions of historical cases have been updated to match the current terminology for the 16 regions in Ghana.

Fig 2.. Yellow Fever Outbreak Origins by District in Ghana

The districts where reported YF outbreaks were suspected to originate in Ghana based on primary/index cases are shown for the periods before and after routine childhood YF vaccination, 1960-1992 and 1993-2022. Also shown is Mole National Park and the West Gonja District where the 2021 and 1983 outbreaks were suspected to originate. For the purposes of this figure, outbreaks are defined as more than one case of YF.

Fig 3.. Predicted Habitat Suitability for YF in Ghana

The left panel shows the average Maxent cumulative output for models including 21 YF occurrences in Ghana and the following covariates: human population density, elevation, landcover, precipitation of driest quarter (BIO17), and annual mean temperature (BIO 1). The right panel shows the average output of the models excluding 6 confirmed urban YF occurrences and includes the following covariates: precipitation of driest quarter (BIO17), precipitation of wettest quarter (BIO16), landcover, and mean temperature of driest quarter (BIO9).

Fig 4.. Predicted Habitat Suitability for YF in Ghana based on the Minimum Training Presence Threshold

These binary habitat suitability maps represent areas greater or equal to the minimum training presence threshold (MTP) for each group of YF models. The MTP is defined as the lowest predicted habitat suitability value for the occurrences in the training data. Therefore, the highlighted areas above the MTP represent locations with ecological conditions at least as favorable as the location with the least suitable known YF occurrence in Ghana.