Section: Vaccinations and Deworming
Regular vaccinations and deworming programs should be continued throughout pregnancy, with some exceptions, following the routine schedule for the particular farm. These include the following:
Vaccines. Those used during pregnancy should always be of the killed or inactivated type, which is usually specified on the vaccine package or insert. Modified life vaccines can affect the fetus and induce abortion in pregnant mares.
Boostered vaccinations. Some vaccinations should be boostered specifically in pregnant mares. These include: Rhinopneumonitis (EHV-1), which should be given at 5, 7 and 9 months of pregnancy to prevent abortion; equine viral arteritis, prior to breeding, in farms or for breeds where the disease is a problem, or when breeding to a stallion that carries the virus; tetanus, encephalomyelitis (Eastern/Western) and sometimes also influenza, which should be given around 30 days prior to foaling; and, in certain geographic areas of the US (i.e. Maryland), a booster for botulism should also be given at 60 and 30 days prior to the due date. Vaccines given 30 days prior to foaling enhance concentrations of antibodies against these diseases in the colostrum (first milk), promoting adequate protection of the foal after nursing.
Deworming. In general, modern dewormers are safe for use during pregnancy, but if in doubt, this is usually specified in the package insert. There is some suggestion that deworming mares during the first three months of pregnancy can damage the embryo and induce pregnancy loss. Until more evidence is available, it is best to avoid any drugs during the first trimester of pregnancy.
In addition to the regular deworming schedule, pregnant mares are usually dewormed one month prior to the foaling date, and sometimes right after foaling. There is anecdotal evidence that deworming mares right after foaling can prevent foal heat diarrhea by killing certain parasites in milk called Strongyloides westerii, but this has not been scientifically proven.
In any case, you should always consult your veterinarian in regards to optimal vaccination and deworming treatments for your pregnant mare.
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