From:
"Church, Robert Ray (UMC-Student)"
Date: 2002-07-12 18:38:38 UTC
Subject: Bob C: Vitamin A and CD-2
To: <ferrethealth@smartgroups.com>
Cc: "Church, Robert Ray (UMC-Student)" <rrc961@mizzou.edu>
Message-ID: <43C5FBB91EB9CC468627C2BCB23391D32B9C9F@TIG-MAIL2.tig.mizzou.edu>
2. Are you sure measles is closely related to canine distemper? Measles do=
esn=92t seem to cause the same problems in children as distemper does in fe=
rrets.
Actually, the two viruses attack victims in nearly the same way, impact the=
same basic systems, have the same basic symptoms (with some exceptions), a=
nd cause the same sort of problems (again, with exceptions). The biggest d=
ifference is not in how it works or what it does, but in the degree of expr=
ession of symptoms. Genetically, there is a tremendous amount of similarit=
y between canine distemper and measles, causing a great deal of debate. Al=
l agree distemper and measles belong to a Morbillivirus complex that includ=
es rinderpest (closely allied viruses include peste des petits ruminants, p=
hocid (seal) distemper, and dolphin and porpoise Morbilliviruses). The deb=
ate is centered about the evolutionary history of the disease complex.
The most recent hypothesis is that the ancestral disease was in early cattl=
e first selected for domestication, then chance mutations allowed it to cro=
ss into early domestic dogs and stone-age humans. This is thought to have =
occurred somewhere in the Mediterranean, probably centered in the Fertile C=
rescent extending from Egypt to Babylon. One argument suggests these early=
agricultural people associated contracting measles from diseased dogs (not=
called measles then, but lumped into various skin diseases collectively ca=
lled leprosy), which might explain why dogs were considered such pariahs in=
the Old Testament. These hypotheses are far from proved, but they are int=
eresting, and do explain some archaeological and historic records. Regardl=
ess of the how, when, and where of the measles-canine distemper association=
, the fact NOT in dispute is that the two are subtle variants of the other.=
While defined as separate species, the two diseases are much closer genet=
ically than most people.
If you compare the morbidity and mortality of modern dogs suffering distemp=
er to that of Medieval Era humans with measles, you find similar numbers. =
Canine distemper is much like measles in that it is a =91hot=92 disease, sp=
reading rapidly, but burning itself out if population levels are insufficie=
nt to maintain an infectious population. Canine distemper is so hot in mus=
telids that it is self-limiting, exploding into being, killing scores of an=
imals, then dying out just as rapidly. There are two prevailing reasons of=
fered to explain why canine distemper is so deadly to ferrets. First, dist=
emper has so recently been introduced into mustelids that they haven=92t ha=
d time to evolve resistance. Second, polecat progenitors of the domesticat=
ed ferret lead a solitary existence that prevents the widespread spread of =
disease that would spur the rapid development of viral resistance. The rea=
l reason is probably a combination of the two. Introduced canine distemper=
, while devastating to local populations, would be limited in time because =
most polecats would die before passing on the disease to second-generation =
victims. Because normal polecat populations are too dispersed to support w=
idespread =91hot=92 infections, the disease would be geographically (space)=
limited. The combination of the two reduces (or eliminates) the pressure =
needed to rapidly evolve disease resistance. It is also a clue to the rapi=
d disappearance of black-footed ferrets; a disease that would normally be l=
imited in time and space suddenly found freedom by the widespread introduct=
ion of distemper-carrying dogs, negating the protection against hot disease=
s granted by a widely dispersed population.
The amount of research that shows the extreme similarity of these diseases =
is tremendous. All three Morbillivirus infections (rinderpest, canine dist=
emper, measles) cause acute systemic disease that involves the respiratory =
tract, lymphoid system, and central nervous system. IN GENERAL, Morbillivi=
ruses first attack epithelial cells, including the skin, and the layer of c=
ells lining the respiratory and digestive systems. In skin, Morbilliviruse=
s cause maculopapular eruptions (small, raised spots), cracking or splittin=
g of oronasal epithelium, and gross desquamation (the loss of skin in spots=
, resulting in crusty, draining lesions). In the respiratory tract, Morbil=
liviruses generally cause spotting of the buccal mucosa, oronasal discharge=
, coughs, pseudo-asthma, and frequently results in pneumonia. In the gastr=
ointestinal tract, Morbilliviruses cause stomatitis and diarrhea. In the g=
enitourinary tract, it causes general irritation, or burning on urination. =
Morbilliviruses attacking the epithelial cells of the middle ear generally=
result in otitis media and sometimes deafness. Infection of the eye resul=
ts in various lesions that may produce permanent vision problems or blindne=
ss. In the central nervous system, it can cause encephalomyelitis, and can=
cause the myelin sheathing of cranial and spinal nerve cells to deteriorat=
e, resulting in loss of muscular control, local sensation, vision, or heari=
ng. In advanced cases, generalized viremia (systemic viral infection) is t=
he cause of pervasive atrophy (wasting), hyperemia (increased blood in vari=
ous parts of the body), multiple organ system problems, including reduced h=
epatic protein synthesis, fever, and reduced immunocompetance leading to se=
condary bacterial and viral infections. The exact symptoms and degree of e=
xpression vary from individual to individual, and from species to species.
Many virologists argue that because the pathogenesis and clinical course of=
canine distemper infections in ferrets is so similar to all other Morbilli=
virus infections, ferrets infected with distemper could be used to study th=
e course of ALL Morbillivirus infections in ANY species, including measles =
(West 2000 is an exception, but not because of a lack of similarity). In o=
ther words, canine distemper in ferrets is so similar to measles in humans =
that infected ferrets can and are used to model childhood measles.
Bob