Claudia--
The recommendation is the standard one for those who have no choice but to
use endophyte infested pasture. But, as you experienced, it's still not a
fail-safe. The problem is that the effects of the toxins on estrogen and
progesterone are secondary--that is, they don't act directly on the tissues
which produce those two hormones. So it takes longer for those hormones to
respond to the removal of the endophyte from the diet. With the drug
treatment, the response is much quicker. My research showed an increase in
progesterone and a decrease in estrogen within about 7 days on average. The
prolactin responds within 24 hours--not surprising, since the toxins act at
the prolactin receptors in the pituitary gland. Block the receptors, and
you get prolactin release. The estrogen response takes a little longer,
because during mid-pregnancy the estrogen precursors are being produced by
fetal ovaries or testicles. Also, it's not so much the actual levels of
progesterone and estrogen in the system as it is the ratio of one to the
other. Therefore, if for some reason the ratio isn't back to normal you may
still experience problems like retained placentas.
My first study with mares was to test the effects of withdrawal from the
pasture on the mares, because they discovered by accident on the previous
study that when they started removing mares from the infested pasture to try
to stop the mare and foal deaths, the mares started developing udder tissue
and making milk. That led to my first mare project, and ultimately to the
domperidone (Equidone) research. We did still have some retained placentas
even on the controlled withdrawal study. Also, when I did a dose study on
the domperidone, when we started going higher that the initial dosage we
started seeing more retained placentas--which led me back to the above
conclusion, that it's the progesterone/estrogen ratios that cause those
particular problems. Now that I've had a few years to reflect on the work I
did then, I wonder if maybe that might be some sort of rebound effect of
removing the toxins from the mare's system, and a consequence of an altered
growth curve for the foals in utero.
You mentioned that the pasture was only 50% infected, and Bette just sent me
a private email wondering about levels, so I'll address that here--it
doesn't take much....a pasture doesn't have to be 100% infected to cause a
problem. However, once the drug becomes available commercially mare owners
can start using those pastures again as long as the mares are receiving the
Equidone. I know I did the research so I'm probably a little biased in
favor of it, but if the herd manager and the project vet were impressed with
the results of the drug, it has to be pretty close to a miracle
drug--neither man is ever easily impressed by anything.
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