Can Malaria Parasites Control Mosquito Behavior? Are malaria parasites able to
control mosquito behavior to their advantage? A study \(^{43}\) investigated
this question by taking mosquitos and giving them the opportunity to have
their first "blood meal" from a mouse. The mosquitoes were randomized to
either eat from a mouse infected with malaria or an uninfected mouse. At
several time points after this, mosquitoes were put into a cage with a human
and it was recorded whether or not each mosquito approached the human
(presumably to bite, although mosquitoes were caught before biting). Once
infected, the malaria parasites in the mosquitoes go through two stages: the
Oocyst stage in which the mosquito has been infected but is not yet infectious
to others and then the Sporozoite stage in which the mosquito is infectious to
others. Malaria parasites would benefit if mosquitoes sought risky blood meals
(such as biting a human) less often in the Oocyst stage (because mosquitos are
often killed while attempting a blood meal) and more often in the Sporozoite
stage after becoming infectious (because this is one of the primary ways in
which malaria is transmitted). Does exposing mosquitoes to malaria actually
impact their behavior in this way?
(a) In the Oocyst stage (after eating from mouse but before becoming
infectious), 20 out of 113 mosquitoes in the group exposed to malaria
approached the human and 36 out of 117 mosquitoes in the group not exposed to
malaria approached the human. Calculate the Z-statistic.
(b) Calculate the p-value for testing whether this provides evidence that the
proportion of mosquitoes in the Oocyst stage approaching the human is lower in
the group exposed to malaria.
(c) In the Sporozoite stage (after becoming infectious), 37 out of 149
mosquitoes in the group exposed to malaria approached the human and 14 out of
144 mosquitoes in the group not exposed to malaria approached the human.
Calculate the z-statistic.
(d) Calculate the p-value for testing whether this provides evidence that the
proportion of mosquitoes in the Sporozoite stage approaching the human is
higher in the group exposed to malaria.
(e) Based on your p-values, make conclusions about what you have learned about
mosquito behavior, stage of infection, and exposure to malaria or not.
(f) Can we conclude that being exposed to malaria (as opposed to not being
exposed to malaria) causes these behavior changes in mosquitoes? Why or why
not?
