After the first phase of Leptospira infection in cattle, the bacteria localise in the urogenital tract. Early symptoms are usually mild and transient and are often not noticed.
In cows, the first symptom is often a sudden decrease in milk yield. This can be accompanied by transient fever, mastitis-like changes in the milk and sudden loss of all milk with flaccid udder.
Abortions usually occur 6-12 weeks after the initial infection. If the infection occurs in the late gestation, an infected calf may be born. Diagnosis of leptospiral abortion is difficult and based on maternal and foetal serology, as no obvious lesions are associated with the infection.
Abortion rates range from up to 30% in herds not previously infected to 5% in herds where Leptospirosis is endemic.
The greatest effects of infection on fertility are low pregnancy rates and increased culling due to low fertility.
The manifestation of these symptoms varies depending on the infection status of the herd. In a chronic inactive state of infection, few signs of poor fertility are seen. During initial infection of the herd or an inactive state that becomes active, the symptoms are more apparent. What changes the health status of a herd from chronic inactive to acute active is not well understood.
A natural immunity is established in a herd after the initial infection phase, however all new animals that enter the herd are susceptible and suffer from an acute infection with the associated symptoms. This is also true in cases where animals that were not present in the infected herd during the initial infection but joined later (e.g. dry cows).
Leptospira hardjo infection can be responsible for abortions in pregnant cows and heifers.
Signs of leptospirosis include mastitis