How bad is surveillance for human rabies?

Two recent publications demonstrate widespread under-reporting of human rabies cases, but a new WHO initiative will help to improve the situation.

It is generally accepted that for most canine rabies endemic countries, surveillance data on human deaths are poor. Under-reporting is widespread because data collection occurs largely in urban hospitals, if at all. With rabies being a fatal disease of poor rural communities, many victims never report to healthcare facilities or return home to die when no treatment is available.

Without reliable and complete data on human cases it is difficult to argue for the resources needed to control canine rabies. To overcome this problem, modelling has been used to estimate the likely number of human deaths. However without accurate surveillance data, it is impossible to be sure if these estimates are correct, or monitor progress towards rabies control and elimination. With this in mind, two recent publications have attempted to collate data to give an indication of how extensive under-reporting is.

A study, published in Acta Tropica by GARC and partners, collated publically available data from online international health and rabies surveillance databases and the published literature. Two main approaches have been used – regular passive surveillance that relies on reporting of cases as they occur and much more intensive active surveillance that involves community-based searches for cases. The difference is profound, and active surveillance can reveal up to 100-times higher numbers of cases compared to passive surveillance. However, the intensive nature of active surveillance studies makes them very rare.

The study also cited published estimates of the human rabies burden in the literature. The two main sources of country estimates were 1) the Hampson and Partners for Rabies Prevention study from 2015 and 2) the most recent Global Burden of Disease model estimates from 2013. Whilst these modelled estimates varied due to their reliance on different methods and large extrapolations from limited data, both pointed to tens of thousands of human rabies deaths per year globally.

Estimates from these models for each country allow us to assess how poor the data for reported human rabies cases could be. Active surveillance data is close to these estimates (as they were inputs to the models), but the regular passive surveillance data falls well below, on average around 10% and sometimes only 1% of the estimated case numbers.

The WHO Weekly Epidemiological Record recently published an overview of human rabies surveillance data submitted to them by countries or confirmed through their networks, as well as more publically available data. Again these were shown next to estimated case numbers from the models. From this analysis, we can see that many countries have not recently reported data to WHO, and for those that do, the numbers are several-fold lower than the estimates.

In the near future, human rabies surveillance data will be included in the WHO’s Global Health Observatory. This will re-establish a global database of human rabies surveillance data for the first time since WHO’s RabNet was closed in 2011. It will provide a much needed overview of the global human rabies situation, which will become critical as plans for global elimination progress. Clearly, surveillance data will be inadequate for accurate monitoring in the first instance, and more active surveillance techniques are necessary to increase the completeness of surveillance data. However, surveillance data generally only improve once they become necessary for tracking progress towards control and elimination goals, and this encouraging step will bring us closer to reliable routine reporting of surveillance data for human rabies.

Written by Louise Taylor, based on the publications: Difficulties in estimating the human burden of canine rabies, published Dec 2015 in Acta Tropica, and Human rabies transmitted by dogs: current status of global data, 2015 published in January 2016 in WHO’s Weekly Epidemiological Record.