transmission of malaria and measures of transmission intensity, 10 th – 18 th february 2014, mount...

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


OutlineOutline

Malaria life cycle

Malaria vector characteristics

Main Malaria Vectors in Asia Pacific

Transmission dynamics and its relevance to elimination


Malaria life cycleMalaria life cycle


Characteristics of malaria vectorsCharacteristics of malaria vectors

Anthropophilic

Endophilic/exophilic

Endophagic/exophagic

Breeding in close proximity to humans

Longevity

Susceptibility status to insecticides


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




Nepal & Bhutan– An. culicifacies, An. fluviatilis

Sri Lanka– An. culicifacies, An. subpictus

Thailand– An. dirus, An. minimus


Environmental factorsEnvironmental factors

Ambient temperature

Rainfall

Altitude

Relative humidity


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.


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.


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)


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).


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.


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


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.


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.


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.


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


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


Equilibrium Relationship Between prevalence and vectorial capacity

Equilibrium Relationship Between prevalence and vectorial capacity


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).


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


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.


Thank you

transmission of malaria and measures of transmission intensity, 10 th – 18 th february 2014, mount...

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