Dick Bossart asked about the types of distemper vaccines.  There seems to be
some concern over the use of "live" viruses.  Maybe I can assuage some
fears.  Vaccines can come in several forms: live, modified or attenuated
live, killed, or subunit vaccines.  There are advantages and disadvantages
of each.  Actual "live" viruses are seldom used unless the virus is altered
in such a way that it doesn't cause the disease it's supposed to prevent.
 
In the case of distemper, the virus is specific for a few species including
dogs and ferrets.  Humans can be infected with canine distemper but the
virus doesn't go far, and the infection may not even cause any symptoms in
people.  We're not naturally "immune" to the virus; humans are considered an
incompetent host.
 
To make the distemper vaccine, the virus is cultured in cells or in live
host animals other than dogs or ferrets.  Using selected strains of virus
over many generations, vaccine makers create a strain of the virus that will
infect and grow in another species' cells.  As the virus has mutated or
adapted to its new host, it loses its ability to grow well in the original
host species.  The virus becomes adapted for chick embryos (in the case of
Fervac-D) and not for dogs or ferrets.  The virus can additionally be
treated with chemicals or other means to make it even less harmful.
 
>Can unvaccinated ferrets gain any protection by "catching" the modified
>virus?
 
No.  In a sense, your vaccinated ferrets all "catch" the modified virus, but
because it's adapted to infect avian cells, it doesn't cause lethal disease
in ferrets.  Your unvaccinated ferrets are unlikely to catch the modifed
virus from the vaccinated ferrets, because the infection shouldn't ever
reach the point where your vaccinated ferret sheds the modified viral
particles.
 
>...I am wondering if there would be any advantage with putting [unvaccinated
>ferrets] with recently vaccinated ones in the hope of some immunity being
>transferred?
 
Not possible, however, your previously vaccinated ferret may retain some
residual immunity, even if it can't be revaccinated each year.  I don't know
if this would be enough to prevent infection with wild-type distemper or
keep such infection from being deadly.  Have any of the vets on the FML
heard how well ferrets or dogs with an old distemper vaccinations fare when
exposed to known distemper?
                                *****SUMMARY*****
Advantages of live vaccines: because a very low-level infection does occur,
the immune system produces a very strong and long-lasting immunity, also,
the vaccinated animal makes antibodies to many, many pieces of the virus,
boosting the odds that one or more of those antibodies will neutralize the
wild-type virus that would otherwise be lethal.
 
Disadvantages of live vaccines: sometimes the virus doesn't lose all of its
ability to infect the original host or it picks up what it needs to cause a
virulent infection once in the body. This doesn't happen often, but it does
happen. Fortunately, canine distemper is a DNA virus, and so it mutates less
often than RNA viruses like influenza, but, it can still happen--rarely.
 
Killed vaccines and subunit vaccines produce immunity that is relatively
weaker than modified live virus vaccines, and the immunity often doesn't
last as long.  More shots are often required than with modified live
vaccines.  Subunit vaccines pose essentially no chance of causing infection
from the vaccine itself, but have the greatest chance of allowing the
wild-type virus to evade the immunity those vaccines induce.  This means
that a small mutation in the wild virus could make it instantly lethal to an
entire population of animals vaccinated with a vaccine made from a small
subunit of the virus.
 
--Jeff Johnston ([log in to unmask])
[Posted in FML issue 1542]