Microorganisms: Viruses Questions
Bring on the tough stuff
1. Which of the three types of vaccines described in this unit are least likely to reactivate as an infectious virus?
2. You are replacing Dr. Strange for a day in the virus clinic. A lab technician reports that Patient X has an HIV virus titre of 1 × 108 pfu/mL in his blood. What does this result mean?
3. Patient X tells you that he has been on anti-reverse transcriptase drugs, and he is surprised that he has so much HIV in his blood. What do you think happened, and how could you prevent this occurrence from happening again?
4. Patient X took an experimental subunit vaccine against HIV and is surprised that his body hasn’t developed immunity against HIV. What are some reasons that this vaccine did not work?
5. How do you think scientists first proved that prion disease was not of virus origin?
Microorganisms: Viruses Answers
1. Live attenuated viruses are basically viable viruses that have been grown in cell culture to weaken them. Therefore, they are the most likely to mutate and become functional viruses, as seen with the Sabin vaccine. Killed viruses are dead viruses and cannot reactivate, though a possibility exists that the genome of the killed virus could recombine with an active floating pool of viruses. The least likely vaccine to reactivate as an infectious virus is the subunit vaccine because there is only one protein and not the whole virus in the vaccine preparation. Both the killed and subunit vaccines have little chance of becoming infectious viruses; however, while the subunit vaccine has a zero chance or reactivating, the killed vaccine has a slightly more-than-zero chance. Scary.
2. A virus titre is the amount of infectious virus in a given sample. A titre of 1 × 108 pfu/mL means that in every milliliter of blood, there is 1 × 108 plaque forming units, or infectious virus particles. This amount is generally a high number to have in your blood, and Patient X is probably very sick.
3. The HIV in Patient X has mutated and become resistant to the anti-reverse transcriptase drugs. This mutation caused the virus to rapidly expand and spread. You can prescribe a combination of anti-reverse transcriptase, anti-protease, and anti-membrane fusion glycoprotein drugs to try to control the HIV infection. There is not an HIV vaccine that currently exists.
4. A subunit vaccine would not be likely to maintain immunity against HIV because HIV rapidly mutates, and whatever protein you use to generate immunity against, such as a capsid or envelope glycoprotein, will quickly be antigenically different from the administered subunit. Also, because HIV becomes latent, generating an immune response that will completely clear the virus is difficult, if not impossible.
5. The difference between prions and viruses is that prions do not have associated genetic material that they spread with. Therefore, one should be able to transmit prion disease from one animal to the next, and if you take the same infected tissue and treat it with DNAses, RNAses, and proteases that degrade DNA, RNA, and proteins, respectively, you should only see a loss of prion disease transmission upon protease treatment.
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