Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. Bi-regional Malaria Elimination Training. Mount Malayarat Country Club, Lipa City, Philippines 10 – 18 th February Transmission of malaria and measures of transmission intensity Dr Rabindra Abeyasinghe (MVP) Regional Office for the Western Pacific Transmission of malaria and measures of transmission intensity Dr Rabindra Abeyasinghe (MVP) Regional Office for the Western Pacific

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. Outline Malaria life cycle Malaria vector characteristics Main Malaria Vectors in Asia Pacific Transmission dynamics and its relevance to elimination

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. Malaria life cycle

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. Characteristics of malaria vectors Anthropophilic Endophilic/exophilic Endophagic/exophagic Breeding in close proximity to humans Longevity Susceptibility status to insecticides

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. Main malaria vectors of the Asia Pacific Region Philippines –An. flavirostris, An. balabacencis Solomon Islands & Vanuatu –An. farauti, An. koiliensis, An. punctulatus China & Republic of Korea –An. sinensis, An. lesteri Malaysia –An. balabacencis, An. barbirostris, An. leucosphyrus

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. Main malaria vectors of the Asia Pacific Region Nepal & Bhutan –An. culicifacies, An. fluviatilis Sri Lanka –An. culicifacies, An. subpictus Thailand –An. dirus, An. minimus

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. Environmental factors Ambient temperature Rainfall Altitude Relative humidity

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. Incidence rate The incidence rate is number of new infections occurring in a given population unit in a given time period. Not all inoculations lead to new infections due to a variety of factors: –The inoculum's intrinsic factors –The human host's intrinsic factors –Interaction between parasite diversity and host diversity –Prophylactic interventions. The traditional API (annual parasite index) is an incidence rate.

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. Prevalence Prevalence is the number of existing cases, new and old, in a defined population during a specified period (period prevalence) or at a given point in time (point prevalence). The traditional SPR (slide prevalence ratio) is the prevalence of malaria among fever cases. Prevalence is expressed as a proportion.

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. Measuring intensity of transmission Empirically estimating transmission intensity –Entomological Inoculation Rate (EIR) –Incidence Rate –Prevalence Basic concepts –Vectorial Capacity (C) –Basic Reproduction Ratio (R 0 )

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. Entomological Inoculation Rate The entomological inoculation rate is the number of mosquito bites (inoculations) containing sporozoites received by the population unit in a given period of time (day, month or year).

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. Vectorial Capacity Vectorial capacity (C) is the potential number of secondary cases originating from one primary case in one day, assuming that the human population is, and remains, fully susceptible. Vectorial capacity measures the potential of the mosquito population to transmit malaria in a region. This can be affected by human factors (such as living in screened houses or using insecticide treated nets). Vectorial capacity does not measure the amount of malaria in a population.

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. Vectorial capacity The expression for the definition of vectorial capacity can be divided into 5 stages: –The primary, infective, case is bitten by a certain number of mosquitoes per day (ma) or the man-biting rate (bites per man per night by vector population) –Man biting habit (a) –Some of these mosquitoes get infected (c). –Some of these mosquitoes survive the extrinsic incubation period so they are alive and infectious (e -ng ). –They bite a certain number of humans before they die (a/g). –Some of these humans get infected (b). C = ma 2 bce -ng g

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. VC is also defined as Vectorial capacity is also defined as C = ma 2 bcp n - log e p where p = e -g is the probability that the mosquito survives one day.

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. Basic Reproductive Ratio (R 0 ) The basic reproduction ratio, R 0, is the expected number of secondary cases that would originate from one primary case during the whole period of the patient's infectivity, assuming that the community is, and remains, fully susceptible to malaria infection. R 0 does not give the actual number of new infections at any time (which depends on the susceptibility of the population) but the potential for new infections.

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. Basic Reproductive Ratio (R 0 ) R 0 forms a threshold condition: when R 0 1, the steady state with disease is unstable and malaria can sustain itself in the population.

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. Basic Reproductive ratio and vectorial capacity The basic reproductive ratio is the product of the vectorial capacity and the duration of infectivity of humans: R 0 = C X 1/r = C/r

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. Basic reproductive ratio and vivax infections Each infection of P. vivax can be considered as multiple (k) episodes, each with recovery rate, r v. For P. vivax, the basic reproductive ratio is: R 0 = C x k x 1/r v = Ck/r v

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. Equilibrium Relationship Between prevalence and vectorial capacity

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. Effects of control interventions Long lasting insecticidal nets reduce m, a, and 1/g (increase g) Indoor Residual Spraying reduces m and 1/g (increases g), it may also reduce a if the insecticide has a repellent effect; Space spraying, source reduction and larviciding reduce m; Reduction of man-vector contact reduces a; Treatment of cases increases r (decreases 1/r).

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. Effects of Parameters on C and R 0 Reduction of m reduces C (and R 0 ) linearly. Reduction of 1/r reduces R 0 linearly. Reduction of a result in a squared reduction in C (and R 0 ). Increase of g is amplified much more by the exponential form of the survival of the mosquito through the extrinsic incubation period, e -gn, in addition to the reduction of the longevity 1/g. C = ma 2 bc e -ng R 0 = ma 2 bc e -ng g rg

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. Important Not all interventions have equivalent effects on the parameters. The effectiveness of interventions programmes also depends on: – Coverage of the interventions attainable due their cost – Quality of operations, and their maintenance – Acceptability of the interventions and their sustainability. Vector control interventions will also lose effectiveness due to the development of physiological and behavioural resistance in mosquitoes.

Transmission of malaria and measures of transmission intensity, 10 th – 18 th February 2014, Mount Malarayat Country Club, Lipa City, Philippines. Thank you