Page 478 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Reproductive system: 2.1 The female reproductive tr act 453

VetBooks.ir be non-pregnant at the end of the gestational period Aetiology/pathophysiology
The virus is transmitted by aerosol via the respira-
after being diagnosed pregnant in early pregnancy,
yet no abortion has been detected. Premonitory
contact horses. Abortion material, membranes and
signs are uncommon, but in late-gestation placen- tory route and usually spreads slowly between in-
titis cases premature mammary gland development fetal fluids are also highly infectious.
and lactation may be detected. Mares are rarely
observed aborting, but if they are, they may show Clinical presentation
some signs of parturition. More frequently, the The timing of abortion is usually around 9 months
aborted fetus is discovered in the stable or field. (>5 months to term), 4–14 weeks post infection,
Dystocia is extremely rare. Following the abortion and without premonitory signs. The foal is usually
there are rarely any complications because the com- aborted wrapped in its membranes. In some cases
plete fetoplacental unit is usually delivered, but this the foal may be born alive, but it invariably dies
should not be assumed. The fetal membranes should within 24 hours.
be assessed and an internal examination of the mare
performed to confirm complete delivery as occasion- Diagnosis
ally, part of, or the entire placenta is retained, which Diagnosis is based on history, fetal and placental
can lead to significant medical illness if not treated. pathological changes and virus isolation. The aborted
Once the mare has aborted the objective is to material can have quite specific lesions such as exces-
return her to health as soon as possible and re- establish sive serosal fluids, small (1 mm) white areas of necrosis
normal reproductive function, but this must all be in the liver, an enlarged spleen and pneumonic lung
coordinated with the biosecurity measures that exist. lesions, pleuritis and exudate in the airways. The cho-
Prophylaxis may not be possible with some causes rion may be oedematous, with the chorionic side out
of abortion, but with others it can be very effec- (due to rapid expulsion of the fetus). Histologically,
tive. The reduction in the incidence of twins by the there will be foci of necrosis in the liver, lungs and
routine use of early-pregnancy ultrasonography has spleen and intranuclear inclusion bodies within hepato-
considerably decreased twinning as a cause. The cytes and other cell types. PCR testing on the aborted
identification of endometrial disease can help predict material can be performed for a rapid result. It has
the chances of abortion and may help focus specific been suggested that abortion can be mediated by the
treatment pre-conception. Vaccination of preg- production of a vasculitis involving the endometrium
nant mares for EHV-1, EHV-4 and EVA is helpful and/or the chorion, with antibody–antigen complexes
under certain circumstances. Identifying mares with having been found within affected blood vessels.
poor vulval conformation or cervical abnormalities
(lacerations, incompetency) prior to covering and Management
correcting them at an early stage by a Caslick’s pro- There is no treatment for the aborting mare or
cedure (vulval abnormality) or cervical repair or pro- aborted fetus. The mare will develop natural immu-
gestagen treatment will help reduce the incidence of nity following recovery from the abortion; however,
ascending placentitis/endometritis in the mid-/late- this lasts only 4–6 months and therefore repeat
term pregnant mare. abortions can occur in consecutive pregnancies.
Vaccination of the pregnant mare with a killed
INFECTIOUS CAUSES OF ABORTION vaccine at 5, 7 and 9 months of gestation can substan-
tially decrease the incidence of abortion in groups
EQUINE HERPESVIRUS TYPE 1 of mares and in epidemic storms. Other preventive
Definition/overview control measures include quarantine of all mares
In many parts of the world this is the single most that enter a stud farm until considered suitable to
important infectious cause of abortion. It may be expose to other stock on the farm, batching of preg-
sporadic or more extensive (termed ‘storms’). This nant mares of similar gestational age, minimising
virus also causes various respiratory and neurological the mixing of stock and isolation of pregnant mares
disorders (see pp. 676 and 1067). from young stock.