If your ferret has two blue dots tattooed in his right ear, chances
are he's from Marshall Farms, a large breeder located in Western New
York.  They tattoo one dot when the ferret is spayed or neutered and
the other when it's descented.  Several other breeders also mark dots
in their kits' ears, so a tattooed ferret may not be from MF.  Hagen,
a Canadian breeder, uses a red X (for females) or Y (for males).

Marshall Farms (MF) has been the subject of some controversy
because they sell ferrets to laboratories as well as for pets.  Some
people feel that MF's efforts to produce ferrets for lab use might
have resulted in their pets being genetically less healthy, but
there's no evidence to support that.  In fact, for many types of
research, genetically diverse animals are needed.

Dr. Bruce Williams, DVM, says:

    There have been a lot of rumors going around recently concerning
    Marshall Farms ferrets.  I'm not sure where they got started, but
    let's try to put this subject to bed.

    Sure, Marshall Farms ferrets develop tumors.  So do ALL ferrets
    [11.2.4]. We don't know why ferrets develop most tumors - we know
    that they are most likely to develop them between the ages of 4
    and 6, but not why.  But it is certainly not Marshall Farms'
    responsibility when a ferret that they sold two years ago develops
    a tumor...  To my knowledge - there are no inherent "defects" with
    Marshall Farms ferrets.  Don't get me wrong - I know that Marshall
    Farms is the biggest breeder of laboratory as well as pet ferrets.
    I don't condone laboratory research on ferrets, or other animals
    for that matter and I don't do any.  But I have never seen any
    problems with Marshall Farms ferrets that I could relate to
    Marshall Farms.

Jeff Johnston, an epidemiologist (though not a ferret vet), adds:

    The bigger risk for so-called "congenic" animals is not cancer,
    which seems to be the alleged association with MF ferrets, but
    infectious disease since a microbe that is seriously infectious to
    one animal, will be equally infectious to all.  And I haven't
    heard anyone report that MF ferrets are more susceptible to
    infectious disease than other ferrets.

    I don't believe that the evidence exists to convict Marshall Farms
    of breeding ferrets with defects.  And now that so many
    allegations have been lobbed against them, the information
    gathered about MF ferrets is almost certainly biased.  This
    happens all the time in the epidemiology of genetic diseases.  A
    particular defect occurs twice in a family--perhaps
    coincidentally--and the family and their doctors go out of their
    way to look for it.