Headway on dengue surveillance and outbreak response

TDR news item
23 September 2015

A new model for dengue surveillance and outbreak in the Americas progressed at a recent meeting of an international group of experts. The World Health Organization and its regional office in the Americas, the International Consortium for Dengue Risk Assessment, Management and Surveillance (IDAMS) and TDR brought experts together in August at the Institute Pedro Kouri in Havana, Cuba.

EFE / File

The main aspect of the meeting was to come up with more effective approaches for dealing with outbreaks, notably better coordination and improved alert systems. Given that the conference was held at the same time as the re-opening of the United States embassy in the Cuban capital, some saw this as symbolizing an opening to the world for new ideas. It also underlined the possibility of the Instituto Pedro Kouri becoming an important regional centre for TDR activities given its experience with dengue and other mosquito-borne diseases.

Dengue affects urban populations, often causing massive hospital overflows during the rainy seasons when the virus is transmitted by the Aedes mosquito. The experts are looking at how to better plan for these outbreaks, which includes early detection of outbreaks through alarm signals.

Coming up with better contingency planning

“The problem is always about report systems and how they work,” noted TDR’s Axel Kroeger. “There seems to be quite an under-reporting factor, notably how many actual cases are there in a particular country or region from an outbreak. These extension factors may well be 10 to 15 times higher than stated.”

“The idea is to identify alarms systems and to predict when an outbreak is coming.”

Axel Kroeger, TDR, Geneva

The group investigated the development of a more focused model for dengue contingency planning. TDR is currently preparing a handbook on this. “This is based on what is already published, notably what is known and what is not, as well as what works, and what does not,” added Kroeger.

Detecting early alarm signals is crucial

While much progress has been made through the Global Strategy for Dengue Prevention and Control coupled with better disease estimates, outbreaks are usually detected too late and response mechanisms are often inadequate. Almost no country has managed to use dengue outbreak alerts as a means for initiating early response.

Dengue tends to come in waves, and the number of cases in big cities may increase from 50 cases a week up to 3 000 cases on a peak day. “The idea is to develop alarm systems and to predict when an outbreak is coming,” said Kroeger.

Data from Brazil, Mexico, Dominican Republic, Vietnam and Malaysia are being compiled to develop this new model that can be adapted to individual country requirements and capacities. “By looking at the past ten years we wanted to see which outbreaks were preceded by these alarm indicators,” said Kroeger.

Defining a dengue outbreak

Defining outbreaks can prove exceptionally arbitrary. As a result, the conference considered the 1.25 standard deviation threshold derived from the 5-county study. This means that an outbreak is identified when the number of cases are 1.25 times the standard deviation over the historical mean (moving average) for three weeks. The outbreak ends when it is below this threshold for three weeks.

Alarm indicators for dengue outbreaks include temperature, humidity and rainfall. According to Kroeger, temperature can prove a highly critical indicator. “If we get increased temperature for three weeks of 1.25 standard deviations above the mean, then there is an up to 50 per cent likelihood of having a subsequent outbreak,” he said. Other potential alarm indicators ̶ such as increased number of fever cases, positive blood tests and mosquito breeding ̶ may be valid but depend on the reporting systems as well as on laboratory testing.

The initiative is testing these definitions and approaches in Brazil, Malaysia and Mexico to see if the alarm signals work. Results are expected by the end of 2016. A small follow-up meeting involving the five original countries will be held in Freiburg, Germany in October.

More effective community response needed

Best practises on surveillance are being collected in the countries. The economic impact of outbreaks, including the rising burden on existing health facilities, is also being assessed to obtain a better picture of the disease worldwide, but particularly in the Americas.

There is also work underway to find more effective participatory approaches by ministries of health, international organizations, vector control services and other key stakeholders. The International Red Cross Red Crescent Climate Centre in The Hague has contributed materials for community mobilization at early stages of outbreaks.

For more information, contact:
Jamie Guth
TDR Communications Manager
Telephone: +41 79 441 2289
E-mail: guthj@who.int.