Chris,
 
This is a posting from my husband.  I try to keep him up to date on the mailing
list postings.
 
 
     I read the most recent posting regarding Tony, the ferret that died of
thymic lymphosarcoma, and a couple of inaccuracies were evident.
 
     I'm a small animal veterinarian with a strong interest in
exotics, most especially the ferret.  The practice I now work at has treated
ferrets for some 20 years.  We diagnosed some disease conditions
before anyone published their existence, such as estrogen toxicity/bone marrow
suppression, and ferret cardiomyopathy.  I currently own two ferrets.  My
interest in these animals has extended beyond the practice situation; the
following information is gathered from research literature published on ferrets
over the last 10 years.
     The first point regards Felv in the ferret.  No one has been able to
experimentally infect a ferret with feline leukemia.  Researchers are not 100%
convinced, but the current thinking is that ferrets are not susceptible (or so
rarely as to be clinically insignificant.)
     Felv belongs to a class of viruses called retroviruses.  The fact that
some ferrets test positive to the feline Felv test has been known for a long
time, and there has always been speculation about a ferret retrovirus (not
Felv) that might cross react with the Felv test, thereby explaining the
occasional positive test in ferrets.  There are several points to make here:
1. No retrovirus has yet been isolated from ferrets. 2. Even if a retrovirus
was isolated, it would remain to be proven that it can cause cancers, as the
Felv virus does in cats.  3. Lymphosarcomas are among the most common
spontaneous tumors in cats regardless of whether or not the Felv virus is
present; they are also the most common ferret tumor.  4. The feline leukemia
test is very sensitive but not extremely specific.  In other words, there are
very few false negative tests but there are some false positive tests, even in
the cat.  In the ferret, the species difference would increase the likelihood
of false positive test results.  Felv test results in ferrets cannot be
interpreted.   I feel that any assumption of a viral cause of thymic
lymphosarcoma in ferrets at this time is extremely speculative and should be
presented as such.  It is quite likely that the tumor that caused Toby's death
was a spontaneous neoplasm with no virus present.  The fact that the ferret led
a largely isolated life also makes this more probable.
     Thymic lymphosarcoma in ferrets occurs in solitary cases.  There have been
no reports of "clustering" of lymphosarcoma cases in multiple ferret
households; when one of the animals develops the tumor, the rest typically stay
healthy.  If a retrovirus was present, one should see multiple animals becoming
ill, as we do with feline leukemia cases.  This has not been reported to
happen.
     For the record, I saw a thymic lymphosarcoma case within the last few
weeks, and I ran the FeLV test out of curiosity.  It was negative.
     The second inaccuracy was more glaring.  Feline leukemia is
NOT a rare disease!  The company that makes the vaccine is not just tooting its
own horn to sell more vaccines.  Many reputable veterinary researchers have
verified the incidence of Felv.  We see more cat deaths from leukemia than all
other vaccinateable diseases combined (i.e. rabies, feline distemper, herpes,
etc.)  The Felv virus is present in cat populations everywhere in the U.S.
True, the virus dies easily outside the animal, but this only protects the cat
that is kept totally indoors.  An outdoor cat needs only to get in one cat
fight (with biting involved) to spread the disease.  The virus can survive for
3 days in moist areas such as food or water bowls.  A cat that drinks out of
a neighbor cat's water bowl runs a risk of contracting the disease.  Even
friendly grooming between cats is known to spread it.
     The incidence of clinical disease caused by Felv is estimated at 1% to 3%
of the entire cat population: up to 3 cats out of every 100 will die of Felv-
related disease, everywhere in the U.S.  In catteries or multiple cat
households the disease often runs rampant, and up to 30% of those animals will
die of Felv if it is present.  As an example, my in-laws had 12 cats, then took
in a stray that, unknown to them, had leukemia.  By the time their cats started
dying, 10 of the 12 tested positive for the virus.  That was several years ago,
and only one of those cats is still alive.
     These figures only deal with those cats that become clinically ill; the
exposure to the virus is estimated as high as 50% of the population
in some areas.  Fortunately, a lot of cats resist the permanent infection that
leads to death, at least for the first few exposures.  (A cat that fights off
the virus one time may succumb to it the next time it is exposed.)
To say that "the infectiousness is pretty low" is misleading; the virus is not
as contagious as, say, a flu virus, but one exposure can transmit it.  Any
cattery that does not test and vaccinate its animals for this disease is
asking for trouble.  The same goes for owners of outdoor cats.
 
        Sincerely,
 
        Mark Burgess, DVM
                                                                          
[Posted in FML 0022]