what is malaria? - sfu.ca · what is malaria? •an infective disease caused by protozoan parasites...
TRANSCRIPT
WHAT IS MALARIA?
• An infective disease caused by protozoanparasites that are transmitted through thebite of an infected Anopheles mosquito;marked by paroxysms of chills and fever
A LITTLE ETYMOLOGY
• The term malaria dates back to the mid18’th century
• It is credited to the Italian physicianFrancesco Torti
• The name means “bad air” -mala = bad and aria = air
MALARIA FACTS
• Causal agents are Plasmodium sp.• Mosquito vectored (Anopheles sp.)• 300,000 new human cases/year• Kills > 700,000/yr• Most victims are children
MALARIA LIFE CYCLE
HOW WE MODEL MALARIA
• Classic approach is to build S.I.R.models:–Susceptible– Infected–Recovered
IN S.I.R. MODELS
• We classify individuals according totheir state and then use mathematicalfunctions to determine transition ratesbetween states (e.g. S ➔ I )
IN S.I.R. MALARIA MODELS
• There are 5 classes, 3 for humans(S.I.R.) and for mosquitoes (S.I.)
MALARIA MODEL VARIABLES
X X Healthy but susceptibleY Y InfectedZ Recovered and immuneb b Natural mortality rateα α Diseased mortality rateν Recovery rateβ Biting rateγ Immunity loss rate N = X + Y + Z
CLASSIC MALARIATHEORY
… dXdt
= !"YNX + b + #( )Y + b + $( )Z
CLASSIC MALARIATHEORY
… dXdt
= !"YNX + b + #( )Y + b + $( )Z
dYdt
= "YNX ! b + # + %( )Y
CLASSIC MALARIATHEORY
… dXdt
= !"YNX + b + #( )Y + b + $( )Z
dYdt
= "YNX ! b + # + %( )Y
dZdt
= %Y ! b + $( )Z
CLASSIC MALARIATHEORY
• …dXdt
= !"YNX + b + #( )Y
dYdt
= "YNX ! b + #( )Y
BASIC REPRODUCTIVE RATE
R0 =! 2NN
b + "( )+ b + " + #( )
VECTORIAL CAPACITY
Where: m = mosquito/human ratio, β = bitingfrequency, p = survival, n = length of cycle
C =m! 2"n
# ln "
VECTORIAL CAPACITY
Where: m = mosquito/human ratio, β = bitingfrequency, p = survival, n = length of cycle
C =m! 2"n
# ln "
ANOPHELES BIOLOGY
PREDIURESIS MOSQUITO NUTRITIONALECOLOGY
• What mosquitoes eat and why
SOME “FACTS”
• Mosquitoes feed on sugar to maintainsomatic (body) function
• Mosquitoes feed on blood for gametic(reproduction) function
• Sugar feeding trades off with bloodfeeding
• Sugar feeding impacts survivorshipand blood feeding
WHAT ABOUT SUGAR?
• The word on the street:– An. gambiae can allocate energy
from human blood to soma– Nectar feeding is rarely observed– Fructose-positive samples are not
common (e.g. 20% Beier 1996)
WHAT ABOUT SUGAR?
• The conclusion:• Plant feeding is rare and trivial for An.
gambiae (and other mosquitoes closelyassociated with humans)
BUT• There are lots of reasons to believe
otherwise
WHAT ABOUT SUGAR?
• BUT:• Without knowing sugar digestion rates,
frequency of fructose positivity ismeaningless
• Behaviors that are not frequentlyexpressed are not necessarily trivial
• Feeding on humans is dangerous
AND!!!
• Nearly every mosquito lab that I know ofprovides sugar to adult mosquitoes. WHY?
VECTORIAL CAPACITY
Where: m = mosquito/human ratio, β = bitingfrequency, p = survival, n = length of cycle
C =m! 2"n
# ln "
ENERGY PROFILES INNATURE LIFE IS TOUGH
• Energy is in short supply forAnopheles.
Res
erve
s
TWO MORE FACTS
• Under nutrient stress, femalemosquitoes reallocate resources awayfrom survival and towardsreproduction
• Nutrition state can affect the impact ofPlasmodium on female survival
THE SMOKING GUNS
• Observations• Crop collections• Digestion rates• Frequency estimates
CONTROLLED EXPERIMENT
• Give female mosquitoeschoice of blood host andnectar host odors
A CLEAR PREFERENCE
CLASSIC ASSUMPTION
• Mosquito behavior is hardwired andinvariant
• Recall, we assumed that beta was aconstant but if sugar alters beta then itisn’t
AN OBSERVATION
• Everybody is plastic
VARIABILITY IN NATURE
• …
HOW TO EXPLOIT ENERGYVARIABILITY IN NATURE
• Employ bednets in the context ofmosquito energy budgetconstraints….
A MINI VILLAGE
• …
HOW TO EXPLOIT ENERGYVARIABILITY IN NATURE
• Employ bednets in the context of mosquitoenergy budget constraints
TESTING THE THEORY
•Our mini domicile• ….
TESTING THE THEORY
Energy state effects
!
TESTING THE THEORY
• Semi field conditions
TESTING THE THEORY
• Semi field conditions