In article <Pine.SOL.3.91.950316222939.29567A-100000 at corona>,
Patrick O'Neil <patrick at corona> wrote:
>>>On 16 Mar 1995 bhjelle at unm.edu wrote:
>>>> For viruses, as opposed to certain parasites, there is quite
>> a bit of support for the notion of adaptation toward low
>> pathogenicity for the native host, while having significant
>> pathogenicity for non-native hosts.ri.
>>>>Not necessarily. The flu pandemic of 1918 is a nice case in which a
>previously standard flu strain in the United States quickly evolved into a
>major killer in the trenches of late World War I. The flu is easily
>transmissible as it is but get a handful of sick soldiers in a crowded
>trench where even if they are incapacitated by illness or combat injury
>they can STILL easily transmit to others, and you have the evolutionary
>pressure to select for particularly deadly and quick acting variants. The
Again, I don't understand the firm connection in your mind between
transmissability and virulence. These concepts should be dissociated.
Clearly the ease of transmission affects a the ecologic
success of viruses, but the role of virulence is less clearcut.
Does a dying soldier transmit influenza virus more effectively than one
who is destined to survive?
As for influenza, pandemic strains such as that from 1918 are thought
to involve reassortment in nature among viruses present in animal
reservoirs; the new segments are believe to be from avian viruses
(ducks). Since these viruses are "new", they don't necessarily help
us prove or disprove the hypothesis that viruses that are
ecologically adapted for a particular host are generally less
In my original post, a mention of three examples of viruses that
are harmless or nearly harmless for their own host species but
deadly for others got inadvertently deleted. These are SIVagm
(a non pathogen in African green monkeys, deadly to macaques),
hantaviruses (nonpathogenic to native rodent hosts, freqeuent severe
disease in man), and Herpesvirus saimiri (nonpathogenic to
host monkey, deadly to other monkeys and rabbits).
There is reason to believe that the "adaptation" is not so
much a function of viral evolution, but of selection for
hosts that are capable of resisting or withstanding effects
that are lethal for hosts with different genetic makeup.