Immunization by intrabronchial administration to 1-week-old foals of an unmarked double gene disruption strain of Rhodococcus equi strain 103+.

This research is about a study that aimed to create a vaccine against the bacteria Rhodococcus equi, which causes fatal lung disease in young horses and immune-compromised animals and humans. The researchers genetically modified the bacteria, tested its effectiveness as a vaccine, and discovered that some very young foals did not respond to the vaccine due to weak immune systems.

Study Overview

• This research aimed to create a vaccine against the fatal pneumonia-causing bacteria, Rhodococcus equi. This bacteria is particular dangerous to one-week-old foals and other immune-compromised animals and humans. Given the absence of an effective vaccine against this infection, this study intended to genetically alter the bacteria to create a possible vaccine strain.

Methodology

• The researchers selected two specific genes of the bacteria, isocitrate lyase (icl) and cholesterol oxidase (choE), and modified them with a method called suicide plasmid, which they previously developed in their laboratory. This resulted in the creation of a genetically-altered “mutant” strain of the bacteria.
• The researchers then took five one-week-old foals and infected them with the genetically altered bacteria by introducing the bacteria directly into the bronchi of their lungs (intrabronchial administration).
• Two weeks after infection with the modified bacteria strain, all foals were exposed to the unaltered, disease-causing bacteria strain. This would test the effectiveness of the mutant bacteria serving as a vaccine, with the expectation that the previous exposure would help the foals’ immune system to fight off the infection.

Results

• Out of the five foals, three were successfully protected from developing pneumonia when later exposed to the unmodified, disease-causing strain. However, the other two foals developed pneumonia due to the genetically-altered bacteria that was initially used as the intended vaccine.
• These results demonstrate that a proportion of very young foals do not have immune systems mature enough to effectively fight off the Rhodococcus equi bacteria, regardless of whether it has been genetically modified or not.

Study Conclusion

• The researchers concluded that their vaccine strategy can work in a portion of foals, but some foals in their first week of life are too immune-compromised to respond effectively to the vaccine. They recommend further research to understand and address this immunity gap in very young foals.