Infectious diseases are an important cause of pregnancy loss in dogs and cats. They can cause early embryonic death, resorption or abortion through their effects on the dam, the fetus, or the placenta. Other than interrupting pregnancy, many of these pathogens cause minimal clinical signs of maternal illness. Bacteria reported to cause fetal death and abortion in bitches include Brucella canis, Escherichia coli, β-hemolytic Streptococcus, Leptospira, Campylobacter, Salmonella, Mycoplasma spp. and Brucella abortus. Experimental infection with Toxoplasma gondii has also been found to cause abortion in bitches and queens. Viral agents are the most commonly reported infectious cause of abortion in queens. Calici virus is one of the most important. In addition to calici and herpes viruses, parvo virus (panleukopenia), feline leukemia virus, feline immunodeficiency virus and feline infectious peritonitis have been implicated as causes of abortion in cats. Canine distemper is reported to cause bitches to abort.

Apparent luteal insufficiency is discussed as a cause of resorption and abortion, but it is rarely documented in bitches or queens. Certain drugs that may be used to treat or prevent maternal illness are also known to be toxic to pregnant females, to be teratogenic, to cause fetal death, or to cause abortion. Nutritional imbalances can cause pregnancy loss. Fetal anomalies and chromosomal aberrations are cause abortion. Most congenital fetal anomalies have no identifiable cause. Some are known to be heritable. Some are caused by environmental factors such as exposure to teratogens. When normal-appearing, full-term puppies or kittens are stillborn, the most likely cause is fetal distress during parturition. Subsequent pregnancies and labor should be monitored more closely for signs of fetal stress.

The diagnostic efforts are directed toward finding the cause of resorption and abortion so that (1) the dam and any remaining viable fetuses can be treated properly, (2) the problem can be avoided during the subsequent pregnancies of this particular female, and (3) the rest of the colony can be protected from similar occurrences. The dam should be thoroughly examined for signs of illness and for the presence of remaining fetuses. Bitches and queens may abort part of a litter and carry the rest to term. Therapy for the aborting female is supportive and symptomatic, unless a cause can be found. If viable fetuses remain, the pregnancy can be allowed to continue. If not, any remaining contents of the uterus should be removed by ovariohysterectomy or through the administration of ecbolic agents.