foxrabiesblueprint.org

5.5.11 What sample size do I need for rabies surveillance?

Testing healthy animals is likely to give negative results, and is therefore of no value, as the presence of rabies virus can only be confirmed in the late stage of the disease and there is no “carrier state” or asymptomatic sub-clinical infection. Testing healthy animals will therefore bias the overall results considerably.

In contrast to sample sizes recommended by OIE, those recommended by the WHO and EU for rabies surveillance are 4 foxes/100 km² and 8 foxes/100 km² a year, respectively. OIE, on the other hand, only states that an adequate system of surveillance should be in place in all countries, whatever the rabies status (rabies-free or infected countries).

In fact, experience in Europe has shown that defining a sample size for rabies surveillance is meaningless, as European countries experienced extreme difficulties obtaining enough animals for rabies testing. An apparently pragmatic approach was to supplement the routine sampling with (healthy) hunted or trapped animals (hunting bag/fur harvest) samples in order to meet WHO or EU sample size requirements. Rabies pathogenesis in terrestrial mammals is distinctive, as infected animals will eventually die from the disease.

Taking this concept into consideration, a sample size cannot be defined for rabies surveillance in order to prove the absence or presence of rabies in wildlife, regardless of the reservoir species. Instead, rabies surveillance should focus on indicator animals (see sections 5.2.1; 5.2.2), and numbers of these animals cannot be predetermined (read more here). In any case, surveillance efforts should be adequate in space and time.


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Version 1 - Last updated November 2012