Okay, well, I got the number wrong from memory: 50% is right.  I'll post
the correction.
 
Here is a past post of Dr. Bruce Williams, who is at least one of the
world's top ferret expert veterinary pathologists (Some other experts
refer to him as the best; all are agreed that he is really up there.):
 
http://ferrethealth.org/archive/browse.php?msg=SG3097
 
>Adopting a female in heat means that you will have vet bills right
>away. The first thing to do is have a complete blood count done at
>your vet - to ascertain whether she is anemic and how bad the anemia
>is.  If she is weak, and pale when you get her, the anemia may be
>advanced.  However, also remember that only 50% of jills suffer from
>anemia as a result of persistent estrus.
 
>Regarding treatment - if her blood count is over 25%, don't worry
>about the shot - just have her spayed. A shot may be benefitical
>for ferrets with a hematocrit less than 25%, to buy some time until
>she is a stronger surgical candidate. IF the blood is less than
>15%, a transfusion and the shot will be necessary.
>
>With kindest regards,
>
>Bruce Williams, DVM
 
BTW, adrenal growths also sometimes cause estrogen toxicosis.
 
In the second edition of _Biology and Diseases of the Ferret_ you
will find it on pages 47 and 48.  [See correction nect post.  BIG]
 
Page 218 in _Biology and Diseases of the Ferret, 2nd edition_ but
notice the mention that in Black Footed Ferrets and Siberian Polecats
bone marrow depression had not been documented.
 
Pages 137-139 in _Essentials of Ferrets, a Guide for Practioners_
(the second edition, by Dr. Purcell)
 
Anyway, those are some places to begin looking.
 
The second ferret we ever had (a breeder retired after a male bit through
one of her eyes while mounting) had to have an emergency spay before
coming home with us due to the onset of marrow suppression.  That was a
long time ago now -- maybe 24 or 25 years ago.
 
-- Sukie (not a vet, and not speaking for any of the below in my
private posts)
Recommended health resources to help ferrets and the people who love
them:
Ferret Health List
http://www.smartgroups.com/groups/ferrethealth
FHL Archives
http://ferrethealth.org/archive/
AFIP Ferret Pathology
http://www.afip.org/ferrets/index.html
Miamiferrets
http://www.miamiferret.org/fhc/
International Ferret Congress Critical References
http://www.ferretcongress.org
[Posted in FML issue 5196]