TWiV 429: Zika Experimental Science Team

Vincent meets with members of team ZEST at the University of Wisconsin Madison to discuss their macaque model for Zika virus pathogenesis.

You can find TWiV #429 at microbe.tv/twiv, or listen/watch right here.

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Viral RNA is not infectious virus!

Zika RNA and virusA study of sexual transmission of Zika virus among mice (link to paper) demonstrates beautifully that viral nucleic acid detected by polymerase chain reaction (PCR) is not the same as infectious virus.

Male mice were infected with Zika virus and then mated with female mice. Efficient sexual transmission of the virus from males to females was observed. This observation in itself is very interesting but is not the focus of  my comments.

To understand the dynamics of sexual transmission, the authors measured Zika virus shedding in seminal fluid – by both PCR, to detect viral RNA, and by plaque assay, to detect infectious virus. The results are surprising (see figure – drawn in my hotel room).

Zika virus RNA persisted in semen for up to 60 days – far longer than did infectious virus, which could not be detected after about three weeks.

Many laboratories choose to assay the presence of viral genomes by PCR. This is an acceptable technique as long as the limitations are understood – it detects nucleic acids, not infectious virus.

Despite the presence of Zika virus RNA in seminal fluid for at least 60 days after infection, these mice are not likely to transmit virus after a few weeks. There is a lower limit of detection of the plaque assay – approximately 10 plaque forming units/ml – whether that would be sufficient to transmit infection is a good question.

Why Zika viral RNA and not infectious virus would persist for so long is an important and unanswered question that should definitely be studied.

Recently many papers have been published which demonstrate that Zika virus and Ebolavirus can persist in a variety of human fluids for extended periods of time. These results have been interpreted with alarm, both by scientists and by science writers. However, in most cases the assays were done by PCR, not by plaque assay, and therefore we do not know if infectious virus is present. Viral RNA would not constitute a threat to transmission, while infectious virus would.

The lesson from this study is very clear – in novel experimental or epidemiological  studies it is important to prove that any viral nucleic acid detected by PCR is actually infectious virus. Failing to do so clouds the conclusions of the study.

There are few excuses for failing to measure viral infectivity by plaque assays. Please don’t tell me it’s too much work – that’s a poor excuse on which to base selection of an assay. Even if your virus doesn’t form plaques there are alternatives for measuring infectious virus.

If you are wondering how a plaque assay is done, check out my short video below.

TWiV 426: I’m Axl, and I’ll be your cervid today

The sages of TWiV explain how chronic wasting disease of cervids could be caused by spontaneous misfolding of prion protein, and the role of the membrane protein Axl in Zika virus entry into cells.

You can find TWiV #426 at microbe.tv/twiv, or listen below.

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TWiV 422: Watching the icosahedron drop

The TWiVestigators wrap up 2016 with a discussion of the year’s ten compelling virology stories.

You can find TWiV #422 at microbe.tv/twiv, or listen below.

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Zika virus in Nicaragua with Eva Harris

I spoke with Eva Harris of the University of California, Berkeley, on the state of Zika virus in Nicaragua.

Zika in the Guys

In this episode of Virus Watch, we explore the finding that Zika virus infects the testis of mice, causing damage to the organ, reduced sperm production, and less fertility. The important question: does the same happen in humans?

TWiV 414: Zika in the guys with Diamond

On episode #414 of the science show This Week in Virology, Michael Diamond visits the TWiV studio to talk about chikungunya virus and his laboratory’s work on a mouse model of Zika virus, including the recent finding of testicular damage caused by viral replication.

You can find TWiV #414 at microbe.tv/twiv, or listen below.

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TWiV 413: Partnerships not parachutes

From the EIDA2Z conference at Boston University, Vincent, Alan and Paul meet up with Ralph Baric, Felix Drexler, Marion Koopmans, and Stacey Schultz-Cherry to talk about discovering, understanding, protecting, and collaborating on emerging infectious diseases.

You can find TWiV #413 at microbe.tv/twiv, or watch or listen here.

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TWiV 410: Hurricane Zika

Sharon and Scott join the TWiV team to talk about their work on dengue antibody-dependent enhancement of Zika virus infection, and identifying the virus in mosquitoes from Miami.

You can find TWiV #410 at microbe.tv/twiv, or listen below.

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TWiV 408: Boston Quammens

Four years after filming ‘Threading the NEIDL’, Vincent and Alan return to the National Emerging Infectious Diseases Laboratory BSL4 facility at Boston University where they speak with science writer David Quammen.

You can find TWiV #408 at microbe.tv/twiv, or watch/listen here.

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