Risks to a pregnant dog and her unborn pups increase considerably if the mother, with no reported prior medical history, is relinquished to a shelter.

The 2011 AAHA Canine Vaccination Guidelines address many issues and variables concerning vaccinations—including the issue of whether a pregnant dog should be vaccinated.

Traditionally, vaccination during pregnancy has not been recommended, due largely to the potential that some vaccines can cause fetal damage or death when administered to dogs that are pregnant.

Although the preferred recommendation is to avoid vaccinating during pregnancy, there are situations where the risk of maternal, fetal and neonatal infection with a potentially life-threatening virus must be weighed against the risk of vaccination, if the immunity of the dog is unknown.

“In a very high-risk environment, where you have a lot of dogs coming in and moving through a facility, high-density housing, high stress levels and a lot of dogs coming in with no prior immunization history, it can be argued that it is preferential to vaccinate even pregnant dogs to protect them in the face of the disease,” said Michael Moyer, VMD, AAHA president and Rosenthal director of Shelter Animal Medicine at the University of Pennsylvania School of Veterinary Medicine.

“The risk of them getting the disease and that having a negative effect on the litter is much greater than the risk from the vaccination, even though there are presumed to be some risks from the vaccine in a pregnant dog.”

According to the guidelines, the risks of serious illness or death to the mother and her fetus are possible should non-immune pregnant dogs be exposed to parvovirus or to distemper. The guidelines recommend that if a facility is unable to completely isolate pregnant dogs from other dogs, they should either be vaccinated or removed from the shelter.

Link Welborn, DVM, DABVP, AAHA Canine Vaccination Task Force chairperson, acknowledges that numerous viruses can be harmful, but canine distemper virus and canine parvovirus have the greatest potential to cause life-threatening illness. Coincidentally, the vaccines to protect dogs from these viruses also carry the most threat to pregnant dogs.

 “Modified live vaccines, like those commonly used for canine distemper virus, canine adenovirus and canine parvovirus, are generally considered to have the greatest potential to cause harm to fetuses; no vaccine can be considered completely safe for pregnant dogs,” said Welborn. “The fetuses have less risk from vaccination of the mother late in the pregnancy, since their development is more complete.”

Determining when a dog is pregnant and when she conceived can be problematic. Often, dogs are turned into shelters or picked up as strays in animal control environments and subsequently discovered to be pregnant. Moyer suggests it’s not uncommon for a high-intake shelter to receive at least one incoming pregnant dog every week.

“Sometimes they are discovered to be pregnant when they whelp in their cage,” said Moyer. “In other cases, they are moved into foster situations, and it becomes clear as their abdomen starts to enlarge and as they start to develop milk in the mammary glands. Then it becomes obvious they are pregnant.”

Oftentimes, the pet owner who relinquishes his dog to a shelter is unaware that the dog is even pregnant because there may be accidental or unattended meetings between an unaltered male and the pet owner’s intact female while she is allowed to roam around the owner’s yard.

The varying sizes and shapes of dogs, along with the different number of fetuses they can carry, could also mask the pregnancy to the naked eye. To make it even less detectable, a dog doesn’t necessarily conceive on the day of breeding. It may take 3–7 days following breeding for the female to conceive.

“Determination of early pregnancy generally requires advanced veterinary training, the use of imaging technology or blood tests,” said Welborn. “An experienced veterinarian can generally feel fetuses in the uterus at around days 25–30 of pregnancy. Ultrasound and blood tests can detect the fetuses slightly earlier, and X-rays somewhat later.”

Although shelter facilities may understand the inherited risks facing pregnant dogs in their care, there are times when they may not have the option to isolate the female dog nor place her in foster care. During these times, making the decision to vaccinate may be the only viable option.

“In general, it’s not a good idea to vaccinate a pregnant dog,” said Moyer. “If you know the dog is pregnant, you should delay until she has given birth, and you can vaccinate the day after she whelps.”