131: Powassan virus and tick biology with Marshall Bloom

How does tick biology influence their ability to transmit disease? Marshall Bloom explains the role of the tick salivary glands in Powassan virus transmission and the experiments that led to this discovery. He also provides a historical background for the Rocky Mountain Labs in Hamilton, Montana, and talks about the 3 elements to consider when working with potentially harmful biological agents.

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Julie’s Biggest Takeaways

There are 3 elements to consider when working with potentially harmful biological agents:

Biosafety: protecting the laboratory workers from the infectious agents in the lab.

Biocontainment: protecting the community by keeping the infectious agent contained within the facility.

Bioassurity: protecting the individual by ensuring those working with infectious agents are capable to do so.

You need 4 bites of an APPLE for full lab safety, for work in labs from high school level through biosafety level 4:

• A: Administration. Training, paperwork, etc.
• P: Personal protective equipment (PPE). Varies from gloves to positive pressure suits, depending on the microorganisms under study.
• PL: Laboratory procedures. Standard operating protocols.
• E: Engineering. Biosafety cabinets and labs that have protective features.

Most of the vector-borne flaviviruses, including Powassan virus, don’t cause overt disease in the people they infect, so many people never know they’ve been infected. Without serological surveys, it’s difficult to know the full range of infected individuals versus those that develop serious disease. Serious disease often manifests in neurological symptoms such as encephalitis, with 10-15% mortality rate; half of those suffering neurological disease will continue to have serious sequelae for years.

The Rocky Mountain Labs was once the world reference center for ticks: it held thousands of samples which represented the type species for the entire world.

The tick salivary glands look like a bunch of grapes: the stem of the grapes is a series of branching ducts. The “grapes” at the end of the ducts are the acini, which is Latin for ‘little sac.’ These acini play a major role in tick feeding, and different types of acini play different functional roles:

• Type 1 acini: cells have no granules. Acini involved with fluid exchange.
• Type 2 and type 3 acini: cells with granules. Cells degranulate to release vasoactive compounds into tick saliva during feeding.

Featured Quotes

“The first isolation of Powassan virus was from a little boy in Powassan, Canada in 1958. If you look at the cases over the years, the numbers are going up, but compared to Lyme disease, they’re pretty low: there’s been less than 200 cases, all told.”

“Amazingly, the Powassan virus can be transmitted in as little as 15 minutes….[and] a female tick can take days to get a full meal.”

“I take a tick-centric view. If I can anthropomorphize, as my old friend Stanley Falkow used to say, he’d say ‘think like the microbe.’ The microbe doesn’t really care if we get sick or not. The microbe is just trying to make a living and survive.”

“One of the really surprising things is that infected ticks can infect uninfected ticks, if they are feeding right next to each other. Ticks like to feed in groups: it’s called co-feeding. The virus can transferred really quickly, 15 min, which is way faster than the virus can go through a replication cycle. What that means to me is that the ticks are infecting each other….we want to investigate the role of co-feeding.”

“If something sounds like fun or sounds important, and especially if something sounds fun AND important, then you should do it.”

Links for this Episode:

• Paules CI et al. Tickborne Diseases--Confronting a Growing Threat. New England Journal of Medicine. August 2018.
• Amazon: Fighting Spotted Fever in the Rockies by Esther Gaskins Price 
• New York Times: Kay Hagan obituary
• Grabowski JM et al. Dissecting Flavivirus Biology in Salivary Gland Cultures from Fed and Unfed Ixodes scapularis (Black-Legged Tick). mBio. January 2019.
• ASM on Instagram
• Grabowski JM, Offerdahl DK, and Bloom ME. The Use of Ex Vivo Organ Cultures in Tick-Borne Virus Research. ACS Infectious Disease. Marhc 2018.
• Twitter thread from @BugQuestions: Rocky Mountain Spotted Fever and Howard Ricketts
• History of Microbiology Tidbit: A Short History of the Screwworm Program