Abortion in cats can be the result of maternal causes, fetal causes or related to medications.Maternal Causes
The most common maternal causes of abortion are infection with Brucella or herpes virus. Brucella is a bacteria that is contagious among cats and can be contagious to people. All cats in a breeding program should be tested for Brucellosis before ever being bred. Those cats testing positive should be removed immediately from the breeding program. There is no effective treatment nor vaccine to prevent the spread of Brucellosis.
Those infected with Brucella usually abort the kittens around 45 to 55 days of the pregnancy. If strict rules regarding the removal of Brucella-positive cats are not adhered to, Brucella infection can have a devastating effect on a breeding program.
Herpes virus is difficult to test for in the mother but can be detected in aborted kittens. Herpes can result in late stage abortions. It can also cause infertility and is a primary cause of fading puppy syndrome.
Other less common infectious causes of abortion include Campylobacter, Streptococcus, Leptospira, E. coli, toxoplasmosis, distemper and mycoplasma. The mother can be tested for some of these bacteria if abortion has occurred.
The fetus may have developmental abnormalities that are not consistent with life. Severely deformed or improperly developing kittens are usually aborted early in the pregnancy. A cause for these abortions is rarely found and may be related to genetic disease or birth defects.
Other causes of spontaneous abortion include administration of glucocorticoids or Chloramphenicol during pregnancy. For this reason, medication administration during pregnancy should be done with extreme caution and only under your veterinarian's advice.