Disease is Sexy

Let’s stick with my last post’s focus on malaria, but talk about malaria infection in birds (Plasmodium relictum).  This comes from a 2013 Ecology Letters paper entitled “Malaria infection increases bird attractiveness to uninfected mosquitoes.” 

Very complicated diagram of a mosquito. Photo Credit: How Stuff Works.

One of my favorite “topics” in disease ecology is the manipulation of host behavior by parasites.  This is sometimes called the “extended host phenotype.”   And colloquially, manipulated hosts are often called “zombie” hosts (e.g., zombie ants, zombie snails).  As Cornet et al. (2013) pointed out, we usually think about this in terms of a parasite within a host manipulating that host’s behavior.  But here’s a different question:  can a parasite in one host affect a different host’s behavior?  For example, can Plasmodium relictum bacteria in birds affect mosquito behavior? 

“Zombie Snail.” Photo credit: Albus.

Cornet et al. (2013) put infected and uninfected canaries in cages, and let female mosquitoes bite their legs.  (They used some contraption to keep the rest of the bird’s body still, so that any bird defense mechanisms wouldn’t alter vector preference).  Then they used microsatellite genotyping of the blood meals that the mosquitoes took to figure out which mosquitoes fed on which birds.  They did this both during the initial, acute phase of bird infection, and during the later stage, when parasitemia was lower.

The majority of mosquitoes tended to bite one bird in the cage, and they preferred the bird with the most hematocrit.  (Hematocrit is the percent of red blood cells in the blood.)  Before birds were infected, their hematocrit was no different.  During the “acute” phase of infection, when parasitemia was very high, hematocrit was much lower in infected birds than in uninfected birds.  And finally, during the “chronic,” later stage of infection, hematocrit wasn’t different between infected and uninfected birds again.  (If you actually read the paper, you’ll see that hematocrit levels do look different on the graph, but p values were marginal except during the acute phase.)

Cornet et al.’s (2013) results in cartoon format. Malaria infection makes birds more attractive to mosquitoes, but only when infection is chronic and hematocrit isn’t reduced. 

So, mosquitoes preferred infected hosts, but only during the chronic stage of infection.  Cornet et al. (2013) suggest that during the acute phase, when red blood cell counts are lower, blood meals from infected birds wouldn’t be a good source of protein for female mosquitoes.  No matter how sexy the parasites make the birds, the low hematocrit should select for mosquitoes to stay away from infected birds during the acute stage.  But later, when parasitemia is lower and hematocrit increases again, the sexiness of the birds wins out.  Awesome!

Have you ever joked about how you like to go fishing/hiking/whatevering with your “sweet” friend because all of the mosquitoes bite him or her and leave you alone?  I’m going to start assuming all such “sweet” people have chronic malaria.     

Reference:

Cornet, S., A. Nicot, A. Rivero, and S. Gandon.  2013. Malaria infection increases bird attractiveness to uninfected mosquitoes. Ecology Letters 16: 323-329.