This may seem strange as it is a blood-borne parasite, but there is a simple explanation. Before these microfilaria can develop into heartworms, they must be ingested by a mosquito and complete several stages of development within that intermediate host. Even if microfilaria crossed the placenta into the puppy's circulation, heartworm disease would not occur in the puppy. Two infectious diseases that can cause reproductive failure in dogs include brucellosis and mycoplasmosis. It has been shown that puppies can be infected with mycoplasmas while passing through the birth canal.
Toxoplasmosis is a protozoal disease that can infect kittens via the transplacental route.
Feline leukemia virus can also be transmitted via the milk or placenta. As you can see, many different disease can be a risk to the unborn or newborn puppy or kitten. Parasitic, bacterial, and viral diseases can all be transmitted across the placenta or through the mother's milk. In addition, the mother's overall physical health, nutritional status, and genetics greatly influence the health of her offspring. We offer a spay and neuter page that covers every objection to spaying or neutering that I have heard in 10 years of practice.