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Transmission of malaria and measures of transmission intensity ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
Bi-regional Malaria Elimination Training. Mount Malayarat Country Club, Lipa City, Philippines 10 – 18th February 2014 .
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 ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
OutlineOutline
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 ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
Malaria life cycleMalaria life cycle
Transmission of malaria and measures of transmission intensity ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
Characteristics of malaria vectorsCharacteristics 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 ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
Main malaria vectors of the Asia Pacific Region
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 ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
Transmission of malaria and measures of transmission intensity ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
Main malaria vectors of the Asia Pacific Region
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 ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
Environmental factorsEnvironmental factors
Ambient temperature
Rainfall
Altitude
Relative humidity
Transmission of malaria and measures of transmission intensity ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
Incidence rateIncidence 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 ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
PrevalencePrevalence
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 ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
Transmission of malaria and measures of transmission intensity ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
Measuring intensity of transmissionMeasuring intensity of transmission
Empirically estimating transmission intensity– Entomological Inoculation Rate (EIR)– Incidence Rate– Prevalence
Basic concepts– Vectorial Capacity (C)– Basic Reproduction Ratio (R0)
Transmission of malaria and measures of transmission intensity ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
Entomological Inoculation RateEntomological 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 ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
Vectorial CapacityVectorial 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 ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
Vectorial capacityVectorial 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 = ma2bce-ng
g
Transmission of malaria and measures of transmission intensity ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
VC is also defined asVC is also defined as
Vectorial capacity is also defined as
C = ma2bcpn
- loge p
where p = e-g is the probability that the mosquito survives one day.
Transmission of malaria and measures of transmission intensity ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
Transmission of malaria and measures of transmission intensity ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
Basic Reproductive Ratio (R0)Basic Reproductive Ratio (R0)
The basic reproduction ratio, R0, 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.
R0 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 ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
Basic Reproductive Ratio (R0)Basic Reproductive Ratio (R0)
R0 forms a threshold condition: when R0 < 1 the steady state without disease is stable and malaria cannot sustain itself in the population; and when R0 > 1, the steady state with disease is unstable and malaria can sustain itself in the population.
Transmission of malaria and measures of transmission intensity ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
Basic Reproductive ratio and vectorial capacity
Basic Reproductive ratio and vectorial capacity
The basic reproductive ratio is the product of the vectorial capacity and the duration of infectivity of humans:
R0 = C X 1/r
= C/r
Transmission of malaria and measures of transmission intensity ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
Basic reproductive ratio and vivax infections
Basic reproductive ratio and vivax infections
Each infection of P. vivax can be considered as multiple (k) episodes, each with recovery rate, rv. For P. vivax, the basic reproductive ratio is:
R0 = C x k x 1/rv
= Ck/rv
Transmission of malaria and measures of transmission intensity ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
Equilibrium Relationship Between prevalence and vectorial capacity
Equilibrium Relationship Between prevalence and vectorial capacity
Transmission of malaria and measures of transmission intensity ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
Effects of control interventionsEffects 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 ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
Effects of Parameters on C and R0Effects of Parameters on C and R0
Reduction of m reduces C (and R0) linearly.
Reduction of 1/r reduces R0 linearly.
Reduction of a result in a squared reduction in C (and R0).
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 = ma2bc e-ng R0 = ma2bc e-ng
g rg
Transmission of malaria and measures of transmission intensity ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
ImportantImportant
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 ,10th – 18th February 2014, Mount Malarayat Country Club, Lipa City, Philippines.
Thank you
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