A malarial survey is conducted in 50 villages having a population of one lakh. Out of 20000 slides examined, 500 turned out to be malaria positive. The annual parasite incidence is –
**Core Concept**
The annual parasite incidence (API) in a given area is a measure of the number of new malaria cases occurring in that area over a one-year period, expressed as a rate per 1,000 population. It is an essential parameter for assessing the burden of malaria in a region.
**Why the Correct Answer is Right**
The API is calculated by dividing the number of new malaria cases (parasite-positive slides) by the total population of the area and then multiplying by 1,000. In this scenario, the number of parasite-positive slides (500) represents the number of new malaria cases. The total population of the villages is 100,000 (one lakh). Therefore, the API is calculated as (500 ÷ 100,000) × 1,000 = 5.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it represents the parasite rate, which is the proportion of slides that are positive for malaria parasites, not the annual parasite incidence.
**Option B:** This option is incorrect because it represents the parasite prevalence, which is the total number of malaria cases (both new and existing) in a given area, not the annual parasite incidence.
**Option C:** This option is incorrect because it represents the number of malaria cases per 1,000 population, but it does not account for the fact that the cases are new and occurring over a one-year period.
**Clinical Pearl / High-Yield Fact**
When calculating the API, it is essential to use the number of new malaria cases (parasite-positive slides) and the total population of the area to avoid overestimating or underestimating the malaria burden.
**Correct Answer:** D. 5.