Late-term abortions, defined as abortions occurring at four months of gestation or greater, can be a major frustration to dairy producers and result in large financial losses. Each abortion can cost producers as much as $1,900, depending on the stage of gestation when the abortion occurs, cow performance, current feed and milk prices, and the value of replacement animals.
Diagnosing the cause of late-term abortions provides valuable information to both dairy producers and their veterinarians. This knowledge allows them to implement management, nutritional and/or vaccination protocols, which can ultimately improve the reproductive health of the herd.
Determining the cause
The best way to find out the cause of late-term abortions is for dairy producers to work with their herd veterinarian and submit samples to a veterinary diagnostic laboratory. Tissue samples will be cultured and examined under a microscope to look for potentially infectious organisms. Depending on the pathogen, your veterinarian may also need to submit samples from the dam such as feces, serum, milk or vaginal swabs. Additional samples such as feed or water may be needed to help determine noninfectious causes of late-term abortions. The approximate cost of an abortion workup at a veterinary diagnostic laboratory is $50-$400.
If the dairy producer is located near the laboratory, the entire fetus and complete placenta are the best samples to submit for an abortion workup. However, if that’s not possible, your veterinarian can collect appropriate samples and ship them to the lab.
These tissues include heart, lung, liver, kidneys, intestine, blood and skeletal muscle from the fetus, plus large pieces of placenta. Some less commonly submitted samples that can be critical for diagnosis include contents of the abomasum, thymus, spleen, lymph nodes and brain. Submitting all these samples will help maximize the chances of identifying the cause of the abortion. Eyeballs should be submitted if nitrate/nitrite poisoning is suspected. One of the most crucial samples to submit is the placenta. Not submitting the placenta will severely decrease the lab’s chances of determining the cause of the abortion.
In addition to submitting proper samples, dairy producers should also supply the lab with a thorough health history of their farm. Important pieces of information to include are herd size, number of abortions, age of aborting animals, trimester that abortions are occurring in, vaccination history, newly purchased animals, recent vaccinations including modified live vaccines and clinical illnesses in the herd.
Diagnostic challenges
On average, the cause of an abortion is identified by laboratories in less than 50% of cases. While this number is frustratingly low, there are things producers and veterinarians can do to help improve their odds.
Aborted fetuses and placentas often are found in less-than-ideal shape. They can be decomposed, mummified, frozen, rotten from extreme heat, covered in manure or somewhat eaten. Manure and mud should be washed from the fetal skin and placenta promptly. If the tissues are in good condition, they should be kept cool (but not frozen) from the time they are located until they arrive at the laboratory. Tissues that are too decomposed for testing should not be submitted.
When abortion storms occur, producers should submit fetal tissues and placentas from multiple cases. Although it will cost more in diagnostic fees, this approach will significantly increase the likelihood of determining the reason for the abortions, thereby allowing the producer to implement strategies to help prevent them from happening in the future.
Noninfectious causes of abortion
When we first learn that a cow has aborted, our minds commonly jump to pathogens as the cause of the abortion. However, nonpathogenic factors can also cause cows to slip their late-term calves.
Congenital defects, toxins, nutritional abnormalities, hormonal fluctuations and fetal stress can all occasionally cause cows to abort their calves. Many of these cases occur late in pregnancy because this is when fetal demands on the dam are the greatest.
Some congenital defects may result in obvious changes in the appearance of the fetus and some may not. Abortions due to congenital defects usually occur as an individual cow problem and not as a herd outbreak. Cows exposed to high levels of nitrates/nitrites and some mycotoxins in their feed or water can abort their calves, as can cows fed rations that contain deficiencies and/or toxicities of some minerals and vitamins. On occasion, a cow that develops a very high fever due to an infection may abort her calf. Cows carrying twins may lose their calves late in gestation, especially if both are located in the same uterine horn.
Physical trauma to the cow such as pushing, squeezing, slipping, fighting or falling does not commonly result in fetal loss. This is because the fluids surrounding the calf are protective and absorb much of the force. Rarely, the umbilical cord will wrap around parts of the fetus, leading to abortion.
Infectious causes of abortion
Late-term abortion can be caused by several infectious agents including bacterial, viral, fungal and protozoal organisms, including:
- Bacteria: Salmonella spp., Leptospira spp., Listeria spp., Brucella abortus and other “opportunistic bacteria”
- Viruses: Infectious bovine rhinotracheitis (IBR, bovine herpes virus type 1) and bovine viral diarrhea (BVD) virus
- Molds: Aspergillus fumigatus
- Protozoa: Neospora caninum
Some infectious causes of abortion may occur more commonly in certain parts of the country (regionally) or at times of the year. Additionally, a handful of bacteria that cause late-term abortions are zoonotic, meaning they can be transmitted from animals to people. These bacteria include salmonella, leptospira, listeria and brucella. Thus, farm personnel should wear appropriate protective gear and practice proper hand washing after handling aborted calves and placentas.
Below are a few pathogens dairy producers should be on the lookout for when abortions occur late in pregnancy.
Salmonella
All serotypes, or kinds, of salmonella can cause abortions in dairy cows. However, some serotypes are found more commonly in abortion workups than others. These include Salmonella Dublin, S. Cerro, S. Montevideo, S. Typhimurium and S. Newport.
Salmonella organisms are shed in very large numbers in feces. They are spread from one cow to another when cows consume the manure of an animal that is shedding the bacteria. They usually ingest this infected manure in their feed or water.
After the bacteria enters the cow’s digestive tract, it moves into her bloodstream and makes its way to the uterus. From there, salmonella infects both the placenta and fetus, spreading to many of the fetus’s organs. Salmonella can cause the fetus to die at any stage of pregnancy, but it is usually slipped at five to nine months of gestation. After it is aborted, the calf may look decomposed and have gas trapped under its skin while the placenta appears thickened and inflamed.
With salmonella, dairy producers may experience sporadic individual cow abortions or abortion storms (i.e., when several cows abort in a short period of time). Abortion storms often occur in the face of other stressors such as overcrowding, heat stress, dry-off, diet changes, pen moves and recent transportation.
Cows that abort due to salmonella may be sick (high fever, diarrhea, not eating and depressed) or appear completely healthy. In some of the author’s investigations of abortion storms, the only symptom of salmonella infection in the herd was late-term abortions.
'Other' bacterial pathogens
A diverse group of bacteria that can cause late-term abortions in dairy cows are referred to as opportunistic pathogens. These bacteria are not contagious and do not spread from cow to cow. Rather, they are bacteria that are commonly found in the cow or her environment. Most cows on dairy farms are exposed to these bacteria on a continual basis but only a few ever experience problems. Cows that develop infections are thought to have suppressed immune systems.
These bacteria enter the cow’s bloodstream and then cause an infection in the placenta, which eventually leads to the death of the fetus. Infections with these bacteria typically result in sporadic abortions, not abortion storms, and the dams are generally not ill when they abort. Examples of bacteria in this group are Trueperella pyogenes, Bacillus spp., Escherichia coli, Streptococcus spp. and Pasteurella multocida.
Molds
Mycotic abortions usually occur during the third trimester of gestation, and cows typically appear healthy both before and after the abortion occurs, although some cows may develop a retained placenta. Mycotic abortions occur sporadically and usually affect only one or two animals in the herd at a time. They are reported to occur more commonly during the winter months.
Aspergillus fumigatus is the most common cause of mycotic abortions and is isolated from approximately 75% of abortions caused by molds. A. fumigatus is ubiquitous in the environment and commonly found in soil and plant matter. It can also produce airborne spores. A. fumigatus is unique in that it can both infect tissues and produce mycotoxins, some of which are very immunosuppressive to dairy cows.
If the cow has a competent immune system and the mold is present in low numbers, the cow should be able to kill it before it causes issues. However, if the cow’s immune system is suppressed and/or if the mold is present at very high levels, it can cause health problems, including abortions.
The aborted fetus may appear normal. However, in 25% of cases, the fetus will have dry, scaly, raised lesions on its skin. The placenta is usually inflamed, thickened and necrotic.
Prevention strategies
There are effective vaccines available for some causes of late-term abortions. When pathogens such as salmonella, IBR, BVD or leptospira are found, dairy producers should work with their herd veterinarian to determine if vaccination is right for their farm, or if they are currently vaccinating for these pathogens to see if they need to amend their current protocol. Additionally, your veterinarian will be able to help implement management strategies that lessen the risk of late-term abortions.
Although determining the cause of an abortion can be a difficult and costly process, it can yield very valuable information for dairy producers and their veterinarians. Providing a detailed herd health history and appropriate samples (including placenta) that are in good diagnostic condition will help improve the odds of the laboratory finding the reason for the abortions. Only when we know what is causing late-term abortions in a herd can we implement strategies to help prevent them from occurring again in the future.
