From the moment a newborn foal exits the warm fluid environment of its gestation and draws a first postpartum breath of air, it is exposed to millions of would-be invaders that would do it harm if not for the components of the foal’s immune system, which are myriad in number and elegant in their complex yet coordinated interactions. Foals are born precocious, meaning that they are designed and equipped to become ambulatory and travel in the protective custody of a herd within hours of being born (contrast this with the situation in humans, in which newborn infants are helpless and completely dependent on adult care for years). In many ways a foal’s immune system is similar to the foal itself, in that it is capable of recognizing and initiating protective responses against pathogens from the time of birth. In fact, equine fetuses can mount an immune response while still in utero. Nevertheless, some aspects of immunity in foals are not mature and fully operant in the neonatal period, and certain pathogens are particularly adept at finding loopholes in a neonatal foal’s protective mechanisms and causing disease despite all the cells and circulating antibodies that exist to prevent infection.

One such pathogen is Rhodococcus equi, a bacterium that is ubiquitous in environments where horses reside, that adult horses have no reason to fear, but which can cause a lethal abscess-forming infection in the lungs and other tissues in young foals.

What is It?

Rhodococcus equi is a facultative intracellular bacterium, meaning that it prefers to live inside of cells. But R. equi won’t select just any cell: what makes R. equi distinctive is that the cells it targets to infect are the very cells that exist to destroy it