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发表于 2010-5-29 15:20:49
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Retained Fetal Membranes
Etiology
RFM or retained placenta is a very common condition in
dairy cattle. Fetal membranes should be expelled in less
than 8 hours following normal parturition; therefore
retention for longer than 8 to 12 hours is considered
abnormal. Abortion, either infectious or sporadic, occurring
during the last half, of pregnancy frequently results
in RFM. Hydrops, uterine torsion, twinning, and dystocia
in general result in increased incidence of RFM when
compared with normal parturitions. Heat stress and periparturient
hypocalcemia also predispose to the condition.
Cows induced to calve by pharmacologic means
such as exogenous corticosteroid administration should
be anticipated to have RFM. Nutritional causes such
as overconditioning of dry cows and carotene and selenium
defi ciencies also have been incriminated. Low
le vels of vitamin A as occur in hyperkeratosis and polybrominated
biphenyls toxicity are associated with RFM,
metritis, and abortion. In selenium-defi cient areas, cattle
that have low selenium values may have an increased
incidence of RFM, metritis, and cystic ovaries. Vitamin E,
which has been shown to enhance neutrophil function,
also may be involved. Cattle fed stored feeds from areas
that are selenium defi cient should be monitored for selenium
status and supplemented routinely. Selenium
and vitamin E could be related to RFM either as a result
of pure defi ciency or altered neutrophil function.
Cattle that have RFM following parturition may be at
greater risk of the condition in subsequent years. Perhaps
more importantly, epidemiologic studies show that cows
with RFM have a higher incidence of metabolic diseases,
mastitis, metritis, and subsequent abortion. Therefore
despite the fact that many cows with RFM remain asymptomatic
as regards immediate uterine health, associated
diseases are a defi nite risk. Decreased resistance to uterine
and other infections in cattle with RFM is partially explained
by proven neutrophil dysfunction associated
with the condition in periparturient cows. In addition to
reduced neutrophil function, cattle with acute metritis
associated with RFM could have a depletion of neutrophils
in the peripheral blood as a result of acute recruitment
of neutrophils to the infected uterus as evidenced
by the degenerative left shift in the leukogram observed
in some septic metritis patients. Although septic metritis
or chronic endometritis does not occur in most cattle
with RFM, the urge to treat RFM is based primarily on the
inability to predict which cows will develop clinically
signifi cant sequelae.
Recent evidence strengthens the hypothesis that RFM is
mediated by impaired neutrophil function beginning in
the late dry period. Reduced neutrophil migration toward
tissue extracts of placentomes can be detected as long
as 2 weeks before calving in cows that go on to develop
RFM. Other neutrophil functions, such as oxidative burst |
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