Measuring Mosquito Mortality Christen Fornadel, PhD Global Health Mini-University 3/7/14.

Measuring Mosquito MortalityChristen Fornadel, PhD

Global Health Mini-University3/7/14

Outline1. Malaria mosquito basics2. Insecticide resistance monitoring3. Entomological monitoring in support of

LLIN and IRS programs4. Challenges and development of novel

monitoring tools

Plasmodium falciparum cycles exclusively between humans and female Anopheles mosquitoes

The Basics

Anopheles darlingi

Anopheles arabiensis

These are Anopheles mosquitoes

Malaria Mosquito Fun Facts

• Undergo complete metamorphosis: egg, larva, pupa, adult

• Males often emerge first and form swarms, cannot copulate until genitalia rotate 180o

• Females emerge, enter swarm, copulate in the air• Females may mate more than once• Sperm is stored in the spermatheca for lifetime• Males feed on nectar, females primarily on blood• Majority of anophelines have limited dispersal (1.5-2km)• Must survive a minimum of 15 days to deliver an infectious

bite with Plasmodium

Primary Vector Control Interventions

.

.

.

.

.. .... ........ ......Sugar feeding

MatingHost seeking

Oviposition

Indoor Residual Spraying

Insecticide Treated Bed Nets

How do we know if vector control is

working?

Resistance Monitoring

CDC Bottle Assay Results

Resistance intensityIn addition toResistance Frequency

How much resistance will affect control?

Intensity of Resistance

Su

rviv

al

Multiples of Diagnostic Dose

10X5X2X1X

50

100

0

25

75

22%9% 7%

• We are using CDC bottle bioassays to measure resistance intensity

• In Zambia, seeing operational impact of resistance when mosquitoes survive at higher concentrations of insecticide

• Survival at 5x and 10x the concentration of insecticide used to test for resistance is associated with blood fed mosquitoes in houses where ITNs were recently distributed

• Outbreak malaria in areas where mosquitoes survive after being exposed to 5x the diagnostic dose

• Quality Control• Residual Efficacy/Physical Durability• Impact

- Mosquito Behavior and Density- Mosquito Age- Malaria Infectivity/Transmission

Entomological monitoring in support of LLIN and IRS

programs

IRS: Quality Control and Residual Efficacy

• Good quality carbamate spray - 100% mortality 24 hours post IRS

• Residual efficacy dropped below 80% threshold at month 3

Cone Wall Bioassays

Months Post IRS

% M

orta

lity

0 1 2 3 4 50

20

40

60

80

100

CementMud

Tanguiéta

LLINs: Residual Efficacy and Physical Durability

What is being monitored?• Survivorship/attrition: the number of nets still in the

community being used as intended/the proportion of nets no longer in use as intended

• Physical durability: number, location and size of holes• Insecticidal activity: mortality or inhibition of blood feeding

Vector Control Impact: Mosquito Behavior and Density

• Used to determineHuman Biting Rate (the number of mosquitoes that bite a person during a night)

Human Landing Catch Collection

Vector Control Impact: Mosquito Age

The dissection of the abdomen and microscopic observation of the morphology of the ovaries can determine if:

• The female mosquito has laid eggs at least one time in her life – parous.• If the female mosquito has not yet laid eggs – nulliparous.

MR4 Chapter 5.5

This will allow estimating the parity rate of the mosquito population, i.e.

the proportion of parous females.

Older mosquito populations will have higher parity rates; IRS should reduce

parity

Older populations are more likely to transmit malariabecause they need to survive the time needed for the parasite to develop

inside the mosquito and to take at least two blood meals

Vector Control Impact: Malaria Infectivity/Transmission

Sporozoite Rate

Sporozoite rate = number of positive mosquitoes ÷ number analyzed

The presence of sporozoites in the salivary glands of the mosquito indicates that the mosquito is able to transmit malaria parasites to humans.

Determined by:• Dissection and microscopic examination of salivary glands • ELISA technique

Entomological Inoculation RateEIR = number of infectious bites / person / year

(human biting rate * sporozoite rate [1-20%])

Challenges and Development of Novel Monitoring Tools

• Cost• Only sampling portion of the population• Limited means of measuring outdoor

biting/resting• Current tools are difficult to standardize –

operator variability• Do not work well under all ecological

conditions - inadequate sensitivity at low densities

• Parity dissection is crude estimation of age and requires specialized training

Limitations of current tools:

Human Odor Lures for Sampling

Advantages: ―Would allow for efficient sampling without human “bait”―Reproducibility and objectiveness―Collect mosquito populations both indoors and outdoors

• Host-seeking mosquitoes guided by chemical cues: ammonia, lactic acid and carboxylic acids

• Blends are attractive to mosquitoes when combined with CO2

• Studies of human skin bacteria have identified 3-methyl-butanol as an attractant

―When blended with carboxylic acids shown to be as or more attractive than a human host for major African vectors

Efforts will be needed to standardize trap design and to determine comparability in different settings and with different vector species.

Plant-based Attractants for SamplingPlant sugars or “sugar meals” are an important source of energy for female mosquitoes and are the only source for males

Advantages: ―Attractive to both sexes―Catch females at all blood feeding stages and ages

More research needed to minimize catch of non-target insects and to design functional/inexpensive traps

Near Infrared Spectroscopy for Age Grading• Provides spectral data that correlates with mosquito age

―Quantitatively measures organic compounds

• Cuticular hydrocarbons change with age of mosquitoes

• Current methods 78%- 89% accurate in distinguishing <7 days old from ≥ 7 days old

Advantages: ―Non-destructive―Rapid―Minimum skill required

Equipment costs are high (~$40,000) and not readily field applicable, but promising area for more research

Questions?

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