Abortion in cattle is a significant cause of economic loss in the dairy and beef industries. The timing of the abortion can often provide a valuable clue for veterinarians trying to diagnose the causative agent.
Here are the primary infectious causes categorized by the agent type, along with the most common time of abortion:
Summary Table: Timing of Abortion by Causative Agent
| Causative Agent | Type | Most Common Time of Abortion | Key Notes |
|—|—|—|—|
| Brucella abortus | Bacteria | Last Trimester (5 months to term) | A major cause of “abortion storms.” Also causes retained placenta. It is a significant zoonotic disease (undulant fever in humans). |
| Leptospirosis | Bacteria | Last Trimester (6-9 months) | Often causes abortion storms. Associated with infected urine, contaminated water/feed. Can also cause stillbirths and weak calves. |
| Infectious Bovine Rhinotracheitis (IBR) | Virus | Second Half of Gestation (5 months to term) | Abortion can occur weeks to months after the cow’s respiratory infection. The fetus is often autolyzed (decomposed). |
| Bovine Viral Diarrhea Virus (BVDV) | Virus | Throughout Gestation | 1st Trimester: Early embryonic death. <br> ~40-125 days: Can result in a Persistently Infected (PI) calf. <br> Mid-to-Late Gestation: Abortion or congenital defects (e.g., cerebellar hypoplasia). |
| Neospora caninum | Protozoa | Mid-Gestation (4-7 months) | The leading cause of infectious abortion in many countries, including India. Can cause both sporadic abortions and abortion storms. Dogs are the definitive host. |
| Campylobacter fetus (Vibriosis) | Bacteria | Early Gestation (4-6 months) | Primarily causes early embryonic death and infertility, leading to irregular estrous cycles. Later-term abortions are less common but possible. |
| Listeria monocytogenes | Bacteria | Last Trimester | Sporadic cause of abortion. Often linked to feeding poorly fermented or moldy silage. The fetus is often septicemic. |
| Trichomonas foetus | Protozoa | First Trimester (within first 150 days) | Classic cause of early embryonic death and infertility, presenting as repeat breeding. Post-coital pyometra is a common sign. |
| Fungi (e.g., Aspergillus, Mucor) | Fungi | Last Two Trimesters | Mycotic abortions are sporadic and occur from ingesting or inhaling fungal spores from moldy feed or bedding. Often results in a leathery, thickened placenta (placentitis). |
| Trueperella pyogenes | Bacteria | Sporadic, any stage | An opportunistic bacterium that causes sporadic abortions, usually following a primary infection or septicemia in the dam. |
Detailed Breakdown
Bacterial Causes
* Brucellosis (Brucella abortus):
* Timing: Most classically causes abortions late in gestation, typically from the 5th month to term.
* Transmission: Ingestion of the organism from contaminated fetal fluids, placentas, or milk.
* Leptospirosis (Leptospira species):
* Timing: A common cause of late-term abortions, usually in the last trimester (6-9 months), often occurring in outbreaks.
* Transmission: Contact with infected urine, or contaminated water, feed, and bedding.
* Vibriosis (Campylobacter fetus venerealis):
* Timing: Primarily a cause of early embryonic death between 4 to 6 months. This often manifests as infertility and repeat breeding rather than observable late-term abortions.
* Transmission: Venereal (sexually transmitted).
* Listeriosis (Listeria monocytogenes):
* Timing: Sporadic cause of abortion, typically in the last trimester.
* Transmission: Ingestion of contaminated silage that has not been properly fermented (high pH).
Viral Causes
* Infectious Bovine Rhinotracheitis (IBR / Bovine Herpesvirus-1):
* Timing: Abortions are most common in the second half of gestation (from 5 months onwards), often peaking in the last trimester.
* Mechanism: The abortion event is often delayed, occurring several weeks after the cow recovers from the initial respiratory infection.
* Bovine Viral Diarrhea Virus (BVDV):
* Timing: BVDV is unique as it can impact pregnancy at any stage, with different outcomes:
* First Trimester: Early embryonic death and resorption, leading to repeat breeding.
* Days 40-125 of Gestation: Infection during this window can lead to the birth of a Persistently Infected (PI) calf, which appears normal but sheds the virus for life.
* Mid-to-Late Gestation: Can cause abortions or the birth of calves with severe congenital defects, most notably cerebellar hypoplasia (uncoordinated, wobbly calves).
Protozoal Causes
* Neosporosis (Neospora caninum):
* Timing: The most common time for abortion is in mid-gestation, from the 4th to the 7th month.
* Transmission: Primarily through ingestion of oocysts shed in the feces of canids (dogs, coyotes), the definitive hosts. It can also be transmitted from an infected dam to her calf (vertical transmission).
* Trichomoniasis (Trichomonas foetus):
* Timing: A classic cause of early embryonic death in the first trimester (usually before 150 days).
* Transmission: Venereal disease transmitted by infected bulls. It presents as infertility, repeat breeding, and occasionally pyometra.
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