Abortion in a thoroughbred mare associated with an infection with avirulent Rhodococcus equi.

This research is about an eight-year-old thoroughbred mare that aborted a fetus due to an infection with avirulent Rhodococcus equi, a type of bacteria. The study investigated the affected tissues, the mare’s feces, and intrauterine fluid to identify and confirm the presence of the bacteria.

Study Observations and Methodology

• The study began due to an eight-year-old thoroughbred mare aborting a 196 day old fetus. The mare had no prior history of illness, making the sudden abortion a point of investigation.
• For this study, the researchers examined internal tissues of the aborted fetus using immunohistochemistry and bacteriological techniques. While the placenta was not examined, the intrauterine fluids and the dam’s feces were collected fours days post abortion for bacteriological examination.
• It was observed that the main abnormalities found were oedema (swelling) and petechial haemorrhages (small, pinpoint red/purple spots) in the amnion (innermost layer of the fetal membranes) and the umbilical cord. However, no significant histological lesions were found in the fetus.

Identification of Rhodococcus equi

• The causative agent, Rhodococcus equi, was isolated from the lung, heart and stomach contents of the fetus, as well as from the intrauterine specimen and the mare’s feces. This bacterium is commonly found in the soil and is a well-known cause of respiratory diseases in young foals, though it can infect adult horses and other animals.
• Following the abortion, the mare was observed to have a high anti-Rhodococcus equi antibody titre, indicating that her immune system had been responding to the infection.
• The presence of Rhodococcus equi was confirmed in the fetus’ lung by immunohistochemical staining with antibodies that specifically bind to this bacteria.

Determining the Virulence of the Bacterium

• The bacterial samples collected from both fetus and mare were tested via polymerase chain reaction (PCR) and a mouse pathogenicity test. Both these tests confirmed that the Rhodococcus equi strains were avirulent, meaning they were not exceptionally harmful or disease-causing.
• To further characterise these bacteria, the researchers performed pulsed-field gel electrophoresis, a technique used to separate large DNA molecules by applying an electric field that periodically changes direction. All the samples derived from both mare and fetus showed only one VspI profile, indicating that it was the same strain of avirulent Rhodococcus equi leading to the abortion.