UNIT 5B: SURVEILLANCE: GEOGRAPHIC ASPECTS, FOCI DEFINITION, DELIMITATION,

CLASSIFICATION

Allan SchapiraGMS Elimination course, Chiang Mai,

13/8/15

Geographic information systems

• Often equated with the choropleth maps, which are useful for presentation in the control phase, but not in the elimination phase.

In elimination phase, priority is to prepare and maintain maps as follows:

1. Large-scale maps of each malaria focus, showing:i. Residence where cases have been detectedii. Minimum features of the cases, e.g. contracted at residence or elsewhereiii. Mosquito breeding sites and/or potential breeding sitesiv. Boundaries and location of the focus

2. Small-scale maps of whole country or regions, showingi. The dynamics of malaria foci year by yearii. Landscape characteristics, roads, health facilities, altitude cut-offs

3. Delineation of the initially malarious area of the country and/or areas of ecological risk (see Unit 3, Stratification), to have

i. Overview of potential risk areasii. To include in the documentation for certification

The focus, and why do we need it? • Definition: “A circumscribed locality situated in a currently or

formerly malarious area and containing the continuous or intermittent epidemiological factors necessary for malaria transmission.” I.e. there is not necessarily transmission, but humans and mosquito vectors must be present.

• Purpose: To optimize targeting of a suitable package of interventions, which will interrupt or prevent transmission as rapidly as possible in a given area.

• Requirements: – (a) it can be assessed, whether there is transmission, cases without

transmission or no transmission; – (b) evidence-based geographic delimitation is possible.

The focus as an ecosystem

Note that we can talk about a focus even if there are no parasites at present. This will typically be a cleared up focus.

Purpose of working with foci• Where to search for cases

– which may have been the source of infection for a local case;– which may have been infected from a first detected case (index case), which was detected after

becoming infective ;– cases with a similar origin as the index case, whether local or imported (see diagram below);

• identifies area, which should be covered by vector control to prevent any forward transmission from mosquitoes, which may have become or will become infected from the cases in the area (detected or not);

• Identifies the human population, where malaria surveillance needs to be rigorous, so that (a) any new case is rapidly treated and (b) elimination from the focus can be documented.

a. Where to find case(s) which could be the source

Index case: the one detected first

c. Where to find cases that are the progeny of the index case

b. Where to find cases with similar source as Index

Focus in practice

1. One or more cases are detected and classified in a particular area

2. Geo-reference the cases. Place of detection and most likely exact place of infection. Use GPS (e.g. cell phone) and/or paper or electronic map.

3. Identify potential/actual breeding places present now or in rainy season. Use field observation and/or map.

4. Delimit the focus based on the location of:– the cases (index case and others found in the area since by PCD or ACD)– Breeding sites (potential or actual)– Distribution of the population– Local population movement

Re 3. Identifying breeding sites

• Based on knowledge of bionomics of local vectors• Consider that breeding sites may be absent at time

of initial search, but may appear after rains start• Look for breeding sites, if entomologically trained

person is available, otherwise potential breeding sites.

• Use field observations, if possible, otherwise maps (these do not of course show rain-water puddles)

Breeding places in forest malaria in GMS

• While the identification of breeding places in other ecotypes is challenging, it is usually impossible to identify all (or often even some) breeding places in forest malaria.

• The only solution is to make an educated guess based on ecology + likely place of exposure of cases. For example consider any forested area within a 1.5 km radius of likely site of exposure as potential breeding site.

Re 4. Delimit human habitations which should be included in the focus

• If breeding sites are ubiquitous, this can be done by applying a radius of 1.5 – 3 km around a detected case. Shorter, if there is one imported case, longer, if there is an outbreak or ongoing transmission with several cases per year.

• See, using field observation and/or maps, whether a village, a hamlet, a cluster of habitations will form a natural unit of human dwellings with breeding sites.

• Then draw a line around the focus on a map.• Avoid automatically equating focus with village. It may be, but

the focus may be smaller (when a subsection only is at risk) or larger, when several communities are epidemiologically connected.

Classification of fociCharacteristic Main

classificationStrategy

0 cases Cleared up Maintain 0

+ Cases, 0 transmission

Potential Prevent transmission

+ Transmission Active: Residual or new

Interrupt transmission

More on focus classification in practice

• An active focus is a focus with transmission during current calendar year and/or past calendar year. After 1 year with 0 local cases, it is cleared up (or rarely, potential). Always eligible for V.C.

• A potential focus is cleared up by the end of the calendar year, when the imported, induced or relapsing case was detected. Always eligible for V.C.

• Cleared up foci are classified as high risk (=eligible for V.C.) if highly vulnerable. Otherwise low risk (not eligible for V.C.)

Dried out streams, around which breeding sites could have formed. They are so widespread that they have not been delineated. Human habitation is concentrated and limited. White line: delineation of the focus made with the polygon tool.

Attempted delimitation of a focus around an index case at Khorixa, Namibia using Google Earth.

1 km

Geo-coordinates of Case 1, where cursor is placed

Elevation a.s.l. of Case

1

Zooming out, you can see that there are really no more habitations within 5-8 km

3 tools & 1 feature in GE

Placemark Polygon

Save Image

Shapes of foci1. The well-rounded

focus typically delimited by the distribution of human habitation

2. The elongated focus or chain of microfoci, typically determined by a river

Humans

Vectors

3. The heavily interconnected micro-foci. Metapopulation is a better concept. Typical of forested areas in Southeast Asia

Use of maps to guide operations