129: Arbovirus evolution with Greg Ebel

How do arboviruses evolve as they pass between different hosts? Greg Ebel discusses his research on West Nile virus evolution and what it means for viral diversity. He also talks about using mosquitos’ most recent blood meal to survey human health in a process called xenosurveillance.

Julie’s Biggest Takeaways:

Mosquitoes and other arthropods have limited means of immune defense against infection. One major defense mechanism is RNA interference (RNAi). RNAi uses pieces of the West Nile viral genome to select against the viral genome, which helps select for broadly diverse viral sequences. The more rare a viral genotype, the more likely it is to escape negative selection inside the mosquito host, allowing this viral sequence to increase in frequency. 

West Nile virus passes largely between birds and mosquitos. Culex mosquitos tend to prefer birds, and this leads to an enzootic cycle for the virus passing between birds and mosquitos. The viral life cycle inside the mosquito has several important steps: 

• The virus first enters as part of the mosquito blood meal. 
• The virus infects epithelial cells of the mosquito midgut.
• After 3-5 days, the virus leaves the midgut (midgut escape) to enter the mosquito hemolymph.
• In the next mosquito blood meal, virus is expelled with saliva, which has anticoagulant activity.

West Nile virus selection undergoes cycles of selection as it passes from vertebrates (mostly birds) to invertebrates (mosquitos):

• In vertebrates, the virus must escape to cause viremia in a short period of time for replication to occur before the immune system recognizes and eliminates the virus.
  • This leads to purifying selection, or elimination of amino acid variation that decreases viral protein function.
• In mosquitos, the virus spends several days in the midgut epithelial cells and then hemolymph, leading to a longer selection time.
• This leads to more viral diversity in the mosquito host. RNAi further drives population diversity. Through stochasticity, a single viral population will often come to dominate a single infected mosquito.

How do scientists know which virus replicates best? Competitive fitness tests measure which virus grows to a higher population in a given environment. A manipulated virus (one passaged in a mosquito or selectively mutated at distinct sequences) and its non-manipulated parent sequence are inoculated at known proportions, and given a certain amount of time to replicate. By measuring the final proportions, Greg and his team can determine which sequence was more fit in that given environment. 

Xenosurveillance uses mosquitoes to detect a wide array of pathogens at clinically relevant levels. Testing began with in vitro blood-bag feeding, and was validated with studies in Liberia and Senegal. The microorganism sequences are so diverse that the information was used to identify novel human viruses. These studies also provide insight into mosquito feeding habits, which helps in disease modeling.

Links for this Episode: 

• Greg Ebel Lab Website
• Rückert C. et al. Small RNA Responses of Culex Mosquitoes and Cell Lines during Acute and Persistent Virus Infection. Insect Biochemistry and Molecular Biology. 2019.
• Grubaugh N.D. et al. Mosquitoes Transmit Unique West Nile Virus Populations during Each Feeding Episode. Cell Reports. 2017.
• Grubaugh N.D. and Ebel G.D. Dynamics of West Nile Virus Evolution in Mosquito Vectors. Current Opinion in Virology. 2016.
• Fauver J.R. et al. Xenosurveillance Reflects Traditional Sampling Techniques for the Identification of Human Pathogens: A Comparative Study in West Africa. PLoS Neglected Tropical Diseases. 2018.
• Fauver J.R. The Use of Xenosurveillance to Detect Human Bacteria, Parasites, and Viruses in Mosquito Bloodmeals. American Journal of Tropical Medicine and Hygiene. 2017.
• Tracey McNamera: Canaries in the Coal Mine TEDxUCLA
• New York Times: Encephalitis Outbreak Teaches an Old Lesson. 1999.
• ASM Article: The One Health of Animals, Humans, and Our Planet: It’s All Microbially Connected