MadSci Network: Virology Query:

That is an interesting and important question. There are two parts to your question and an answer to each might take a full chapter or more if handled at the level a virologist might like. On the other hand, a simple answer might be the best. I will try to keep it simple but I cannot promise this.

How do viruses mutate?

A virus is a bit of genetic information (RNA or DNA) packaged in an envelope of proteins and/or lipids sometimes including sugars. Viruses cannot live by themselves but must be able to quickly get into eukaryotic (Plant or Animal) cells to survive. They use the energy metabolism and biosynthetic machinery of the cell to replicate themselves. During the phase of replication inside the eukaryotic cell, a virus makes a copy of its RNA or DNA and from that copy duplicates itself. The RNA or DNA in a virus usually encodes enzymes involved in this process in addition to gene sequences that encode the envelop proteins.

Sometimes during the process of copying the RNA or DNA of the virus, small errors (substitutions in nucleotide base pairs) occur in the copy. These errors are replicated into subsequent copies. If the change isn't fatal to the virus and causes it to stop replicating, then the virus has resulted in a mutation. If that mutation results in a changed protein that enables the virus to survive, infect or replicate better the virus will become more infectious.

In general, most viruses don't mutate that often. Notable exceptions are the Human Immunodeficiency Virus (HIV) and the Influenza Virus (Flu). HIV is a retrovirus (it has a "reverse transcriptase" enzyme that enables it to convert its RNA into a complimentary DNA that can "integrate" into the host genes. This is often a situation that results in frequent mutations due to gene sequence "reading errors." The Flu virus also mutates frequently. However, virologists and epidemiologists have found that this RNA virus changes by two mechanisms. One mechanism is by reassorting or recombining. There are many flu viruses and each has a host range specificity. That there are flu viruses that only infect animals of a given species like pigs, ducks and humans. However, passage of a duck flu virus through a pig or vice versa sometimes results in genetic adaptation through reassortment or recombination genes so it becomes infectious in humans. Therefore, monitoring duck and pig flu viruses in addition to human cases are some of the things the CDC does to determine what antigens should be in the next vaccine. In addition to these kinds of big changes in flu virus genes, there are also minor point mutation changes that cause "antigenic drift" so the virus of the same type is slightly different antigenically and can escape elimination by the body's immune response to that type's vaccine.

Now we are ready to talk about the airborne part of your question. In the movie outbreak, the discussion was about the fact that the influenza virus is very capable of surviving in aerosols. Aerosols are generated when you blow air over moist surfaces. For example, a natural aerosol we all enjoy is a "sea breeze" what you don't know about sea breezes is the fact that there is 300 times more ocean bacteria in a cubic meter of sea breeze than there is in a cubic meter of the sea water the breeze blows over. The reason for this is that the sugars on the surface of bacteria that live in the sea reduce the surface tension at the air water interface. When air blows over the water to produce a fine spray, the places where those droplets form are where the surface tension is weakest. Therefore, the bacteria literally jump up into the air to become part of a sea breeze.

The same thing is true for flu viruses. The sugars and proteins on the virus surface sometimes produce an optimal condition for the virus to be included in the water droplets that form when you sneeze or cough. A mutation that makes the ability of a virus to "jump" into the air and survive in an air droplet (ie preventing the droplet from completely drying out) would be part of the process of becoming an airborne pathogen. Another part of that process might be a mutation that allows the virus to attach to and infect the cells that line the respiratory tract.

Fortunately, the chance is very tiny of such mutations occurring in a virus that has no history (in animals or man) of being able to be spread in the air. In the movie outbreak there is discussion of this kind of mutation occurring to ebola virus. The intent of this is to involve the audience in the terror that the storyline is developing. In fact, ebola viruses that infect man aren't spread very well in air. Most of the secondary human cases that are reported during ebola epidemics occur because of direct contamination of cuts and mucosal surfaces with blood or secretions and not due to airborne spread. The mourning practices in African villages involve extensive touching the dead family member and this contributed to the devastating spread of the virus among families. The health care givers were also at risk because of the ways they have to touch patients in the course of providing care.

Since ebola virus is not normally capable of being spread by aerosol a mutation that allowed that would indeed be terrifying. Fortunately, no such mutation exists and we can all rest easy for a while.

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