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I got 99 problems but a bee ain't one: how does host condition affect parasite development?

Over the course of the past few weeks I've been fascinated by extreme life cycles and had planned on writing about the development of organisms that possessed some of those life cycles. I stumbled upon a few examples of parasites that had complex multi-host lifecycles which perked my interest and also was something we hadn't dwelled on in class. We've touched briefly on in utero development which I suppose is somewhat like a parasite developing in a host, but parasite development hasn't been covered. Furthermore, is parasite development subject to similar environmental stressors as the ones we've discussed?  In a paper (cited below) by Logan A. et al., the authors want to know if pollen starvation (low food abundance) in hosts can alter parasite abundance in hosts. 

The researchers did an experiment involving Crithidia bombi (gut parasite) in bumble bees. In this experiment, the researchers were curious about how pollen starvation effects parasite abundance in host systems. Essentially, they exposed 40 bees to 2 treatments (20 apiece); one treatment being "no pollen" and the other being "pollen". Immediately before trials began, bees were given a dose of  approximately 25,000 parasite cells in inoculum. The bees were all given sugar water during the experiments. Over the course of 15 days, feces were collected and analyzed for parasite loads. 

Generally speaking, low food abundance and host malnutrition can lead to increased susceptibility in hosts (Sprunt and Flanigan 1956). However, how does low food abundance affect already established parasites? The authors came up with three mechanisms by which they expected to see a change in parasite abundance: (i) nutritionally mediated changes in host defenses, (ii) malnutrition of the parasite, and (iii) nutritionally mediated changes in the environment of the parasite. The results of the study show that in the pollen starved treatments, parasite abundance was lower and developmental stage demography was different in comparison with the reference population (figure 2); the authors believe that this decreased abundance and altered developmental stage demography was an adaptive response to changing conditions in the gut. In this study, the authors do not seek to determine exactly which mechanism is responsible for their results, although they do believe that some mechanisms may be acting synergistically. Rather, they sought to quantify what effect pollen starvation had on parasite development. 

When thinking about host environment effects on parasites, I think about bioaccumulation and maternal environment effects on offspring. In the case of bioaccumulation, exposure of prey to a given contaminant has a cumulatively larger effect on the consumer a trophic level above. The effects of this accumulation can trickle down to offspring development. With maternal environment effects, exposure to a pollutant or stress can cause alteration of development in offspring. How do macro-level environmental effects trickle down to smaller systems of development, especially host-parasite systems? Are the effects felt "to scale" in a sort of Russian nesting doll framework? For instance, how would malnutrition of a deer affect the development of Babesia (protozoa) that lives inside of a tick, which is then parasitizing said deer? These are some of the things I thought about after reading this paper. What comparisons can be made between what we've learned so far concerning development and the results of this study? 


Sources:

Logan A. et al., 2004. The impact of host starvation on parasite development and population dynamics in an intestinal trypanosome parasite of bumble bees. Parasitology (2005), 130, 637–642.

Sprunt D.H., and C.C. Flanigan. 1956. The effect of malnutrition on the susceptibility of the host to viral infection. University of Tennessee, 1956. 

Comments

  1. Jared BennettMarch 30, 2021 at 8:49 AM

    This is a really interesting concept. I am wondering about how applicable the results of this study would be to the deer-tick-protozoa example and think it would be fun to dive into. Bee's would be a very small host and the parasite would be super draining on its host as it likely has major fitness consequences. However with a deer tick, I would imagine that even a malnourished deer could provide a pretty decent blood meal for a tick and by extension its protozoa. With enough ticks those negative effects could compound but since deer are so large comparatively, I'm not sure what effects would be visible in the tick. Maybe the quality of the blood meal would be degraded by host malnutrition however and still negatively affect the parasite. Nice post!

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  2. miredelkindApril 13, 2021 at 6:30 PM

    It seems as though a key finding of the study is the correlation of pollen starvation and low parasite load. I know some agricultural scenarios use commercial bumble bee colonies for crop pollination, so I'd be interested to know if pollen-starving the colonies is used a a management option for parasitic load. This also leads me to question whether quality of floral resource or pollen has an effect on the parasite load. Perhaps supplying more nutritious pollen at a lower quantity to still induce starvation could facilitate a similar parasite-reducing effect without stressing the bee too much. Thank you for reviewing this paper!

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