Observations on vaccine and post-infection immunity in contagious equine metritis.

The research study explores the effectiveness of a vaccine for contagious equine metritis (CEM), a sexually transmitted infection in horses. Despite showing high levels of antibodies, the vaccine didn’t completely prevent the infection but did reduce the severity of symptoms in the horses that contracted the illness.

Experiment Setup

• Four ponies were vaccinated twice with a formolised culture of Haemophilus equigenitalis, the bacterium responsible for contagious equine metritis (CEM). This resulted in a high circulating antibody titre indicating their immune systems responded to the vaccination.
• These vaccinated ponies, along with three unvaccinated ones, were subsequently exposed to CEM via a vaginal exudate containing the H equigenitalis bacterium.
• In addition, three ponies that had previously been infected with H equigenitalis and then recovered were rechallenged with the bacterium.

Observations

• Three of the four vaccinated ponies and all of the unvaccinated ones developed CEM after being challenged. However, the clinical and bacteriological symptoms were less severe in the vaccinated ponies.
• The ponies that had been previously infected and then reinfected showed symptoms of CEM again, reinforcing the notion that infection by H equigenitalis does not guarantee full immunity to the disease.
• Interestingly, H equigenitalis was recovered 17 days after challenge from the three vaccinated ponies which developed CEM, suggesting that even high levels of circulating antibodies could not entirely eliminate the bacterium.
• The pony that didn’t get infected post-vaccination didn’t have the highest circulating antibody titre of the vaccinates, indicating that factors beyond antibody levels may play a role in preventing the infection.

Conclusion

• The research highlights that while vaccination can increase antibody levels and reduce the severity of symptoms, it might not provide complete protection against CEM. This suggests a need for further investigation to improve the effectiveness of the vaccine and to better understand the immune response against H equigenitalis.