virology blog

About viruses and viral disease

Pop quiz

Is circovirus DNA infectious?

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Circovirus genomeThe US Food and Drug Administration does not want Rotarix, the rotavirus vaccine, to be used because it contains porcine circovirus 1 DNA. If complete copies of the circovirus genome were present, would they constitute a potential threat to recipients? Put another way, is circovirus DNA infectious?

Here is the information you need to answer this question.

  • The circovirus genome is a circular, single stranded DNA molecule (pictured).
  • To infect a cell, the two viral proteins encoded in the DNA must be produced.
  • To produce proteins, mRNA must be synthesized from the viral DNA.
  • Single-stranded DNA cannot be copied  into mRNA; the DNA must be double-stranded.
  • The circovirus particle consists of a protein shell surrounding the viral DNA. There are no other components in the virion.
  • During infection of cells by circoviruses, the particles enter the nucleus where the viral DNA is released
  • If naked DNA is added to cells, a good fraction ends up in the nucleus.

Knowing these facts, can you determine whether introduction of circovirus DNA into cells would lead to viral replication?

Virology pop quiz: Answers

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BaculovirusA few weeks ago I asked readers to find the errors in the following statement concerning an experimental influenza vaccine produced by Protein Sciences which involves synthesis of the viral HA protein in insect cells.

They warned that the virus could mutate during the southern hemisphere’s flu season before returning north in a more lethal form in autumn, in a pattern similar to that seen in the deadly 1918 flu pandemic, which claimed an estimated 20 to 50 million lives around the globe.

The CDC (Centers for Disease Control and Prevention) sent us a dead virus, which is perfectly safe, and then we extracted genetic information from that virus.

The statement ‘in a pattern similar to that seen in the deadly 1918 flu pandemic’ is wrong. There is no evidence that mutation led to the emergence of a ‘more virulent’ virus that caused more severe disease in the fall of 1918. The only virus available to study was reconstructed from material obtained in November 1918. The first influenza virus was not isolated until 1933. The idea that a more virulent virus emerged in the fall has nevertheless become firmly established – without any scientific evidence to support the hypothesis. See “Riding the influenza pandemic wave” for more information.

The second problem is the statement that CDC sent the company a dead virus. Viruses are not living, so they cannot be killed. What the company received is an inactivated virus which cannot replicate in cells. There are many ways to inactivate viral infectivity, including heat, ultraviolet radiation, or treatment with chemicals such as formalin.

For extra credit I asked readers to critique the following statement:

Protein Sciences’ technology is also safer “because these caterpillars don’t have any association with man or other animals, so there’s no chance for their cells to learn how to propagate human viruses,” Adams told AFP.

What exactly was the spokesman trying to say? That there is no chance that influenza virus will replicate in insect cells? That’s impossible to say. The fact that ‘caterpillars don’t have any association with man’ is irrelevant (I’m not an entomologist, but I don’t believe that statement is correct). It’s possible that the virus could be adapted to grow in insect cells in the laboratory. And of course, cells don’t ‘learn’ how to propagate viruses. When viruses are selected for growth in new cells – a process that we call expanding the tropism of the virus – changes in the viral genome are usually responsible.

Parenthetically, there is a better way to make an influenza virus vaccine in insect cells – by synthesizing virus-like particles. When the influenza viral HA, NA, and M1 proteins are made in insect cells, virus-like particles are produced that lack the viral genome. These have been shown to be immunogenic in ferrets, and are capable of inducing a protective immune response.

Ross, T., Mahmood, K., Crevar, C., Schneider-Ohrum, K., Heaton, P., & Bright, R. (2009). A Trivalent Virus-Like Particle Vaccine Elicits Protective Immune Responses against Seasonal Influenza Strains in Mice and Ferrets PLoS ONE, 4 (6) DOI: 10.1371/journal.pone.0006032

Virology pop quiz

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BaculovirusThis week’s pop quiz involves analysis of an AFP news article entitled “US company makes first batch of swine flu vaccine“. The article reports that Protein Sciences has been awarded a contract from the US Department of Health and Human Services to produce a vaccine by synthesizing the viral HA protein in insect cells. Here are two paragraphs from the article:

They warned that the virus could mutate during the southern hemisphere’s flu season before returning north in a more lethal form in autumn, in a pattern similar to that seen in the deadly 1918 flu pandemic, which claimed an estimated 20 to 50 million lives around the globe.

The CDC (Centers for Disease Control and Prevention) sent us a dead virus, which is perfectly safe, and then we extracted genetic information from that virus.

What is wrong with these statements? Post your answers in the comments section.

For extra credit, critique this statement from the same article:

Protein Sciences’ technology is also safer “because these caterpillars don’t have any association with man or other animals, so there’s no chance for their cells to learn how to propagate human viruses,” Adams told AFP.

Pandemic quiz

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The World Health Organization has declared that the world is now at the start of an influenza pandemic. WHO Director-General Dr. Margaret Chan said today:

I have conferred with leading influenza experts, virologists, and public health officials. In line with procedures set out in the International Health Regulations, I have sought guidance and advice from an Emergency Committee established for this purpose.

On the basis of available evidence, and these expert assessments of the evidence, the scientific criteria for an influenza pandemic have been met.

I have therefore decided to raise the level of influenza pandemic alert from phase 5 to phase 6.

The world is now at the start of the 2009 influenza pandemic.

The illustration below will need to be modified for the next edition of my textbook. This week’s pop quiz: What should be the origin of the 8 RNA segments of the 2009 H1N1 pandemic strain? Here is a larger version of the image if you subscribe to the adage that a picture is worth a thousand words.

influenza-pandemics

Virology pop quiz

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cfn_header1The following is from a story posted 2 June 2009 in Central Florida News 13:

OCOEE — Grief counselors will be available at Ocoee High School Tuesday after a student died suddenly from an unidentified virus Sunday.

Here are my questions.

1. How can it be stated that the student died ‘from an unidentified virus’ if no virus was identified?

2. Assuming that a virus was responsible for the student’s death, how would you identify it?

Post your answers in the comments section.