One of the most obvious causes to be considered is Rhinopneumonitis (EHV-1 virus). The timely use of "Pneumabort" or a similar innoculation has been shown to reduce this, BUT there has been evidence that improper storage of the vaccine at some stage may reduce it's efficacy, particularly exposure to excess heat or cold.
Placentitis (inflammation of the placenta) can be caused by infection by a bacteria, fungus or virus. The origins may be external or internal. The end result is commonly the death of the foal, and subsequent abortion.
Internal causes of placentitis can be bloodborne organisms, or even as a result of previous unresolved endometritis.
An external source of infection causing placentitis ("ascending placentitis") migrates through the cervix, and into, most commonly, the "cervical star" area of the placenta. This is more commonly associated with mares that have poor reproductive conformation ("tipped" uterus and/or sunken anus). Bacterial causes are similar to those isolated during pre-breeding uterine cultures - Streptococcus spp., E-coli, Klebsiella spp., Staphylococcus spp., Pseudomonas spp., and (less commonly in pre-breeding "work- ups") Actinobacillus spp., and Salmonella spp. Post mortem cultures should indicate the presence of these organisms.
Cervical incompetence may be a consideration, although one then gets into the perpetual question of whether the cervix became "incompetent" because of the impending abortion (due to other causes) or as a result of previous damage, or because of lowering of progestogen levels (which may also have been lowered as a result of the impending abortion due to other causes).
It is worth noting that the question of exogenous progesterone supplementation is a difficult one to address. Progesterone (specifically) levels drop significantly around day 120 of pregnancy, BUT a complex system of PROGESTOGENS becomes active. The progestogens are multiple and an analysis of blood samples for progesterone specifically will probably indicate low levels, but the progestogen complex levels (which it would be next to impossible to assay) would show "high". To compound this, even progesterone levels have been shown to vary greatly during the course of any given day, and up to 4 samples a day may be required to establish an accurate average level. Additionally, there has been no published documentation to support the concept that Altrenogest ("Regumate") or progesterone therapy has proved efficient in pregnancy maintenance in anything but ovariectomised mares. The main reason for this is the next-to-impossible chance of collecting enough suitable candidates for a sample study. HOWEVER(!!) there is much anecdotal evidence that such supplementation does aid in these situations. In late pregnancy habitual aborters, some veterinarians have prescribed the use of "Regumate" until as late as day 300 of pregnancy. One important note: If progesterone is given with the intention of preventing the abortion by keeping the cervix tight, it must be remembered that if there is another factor (such as an already dead fetus), that may well be retained as a result. Also, progesterone therapy has been shown to reduce the uterus' resistance to infection, which may or may not be a consideration.
Causes of late-term abortion that are not usually associated with, but may by incredible bad luck, be a cause of habitual (repeat) abortion include:
Twinning, which is a cause of abortion at any stage of pregnancy, but commonly either early in pregnancy or later on, when the placenta becomes unable to provide sufficient nutrition, and one or both fetus' dies. With the advent of ultrasound, and it's use as a pregnancy diagnostic tool, it is likely that we will see a reduction in the numbers of twin foals causing abortions during late pregnancy.
Umbilical torsion, whereby the fetus gets wrapped in it's own umbilical cord, most commonly a hind leg, and cut's off or severely reduces, it's oxygen/blood supply, causing it's death.
Equine Viral Arteritis may be a cause of late pregnancy abortions, but unlike the stallion, who can become a "shedder" of the virus in his semen, the mare when recovered from the clinical signs of the disease is usually resistant, and hence not likely to loose another foal as a result of it for some subsequent years until her titre levels drop below the point or protection.
Fetal deformity of course is another possibility, but unlikely as a repeat offender.
© 1998 Jos Mottershead and Equine-Reproduction.com
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