Bug of the Month: Actinobacillus equuli
Actinobacillus equuli is a common Gram-negative (i.e. looks red/pink with Gram stain) bacterium, the natural host of which is (not surprisingly, based on the name "equuli",) the horse. This organism can be found quite commonly in or on healthy horses, for example, as part of the normal bacterial flora of the upper respiratory tract. However, A. equuli can cause disease in certain situations.
Horses that most often become infected and sick from A. equuli are foals. This bacterium can cause a wide range of infections in these young animals, including septicemia (bloodstream infection), meningitis, pneumonia, omphalophlebitis (umbilical infection) and septic arthritis (joint infection). Foals can get infected from contact with other horses or even their own mares because the bacterium is so common. However, it typically only becomes a problem in foals that do not receive adequate colostrum immediately after birth, or that do not receive proper umbilical care.
Disease in adult horses due to A. equuli is uncommon. As in foals, the bacterium can cause a variety of infections in adults, including pneumonia, wound infections and guttural pouch infections. These infections are usually opportunistic - they occur following another problem that affects the animal's normal defense mechanisms, such as viral pneumonia or lacerations. Primary infections with A. equuli, including peritonitis, pleuritis and septicemia, can occur in healthy adult horses, but are very rare.
Infection with A. equuli is diagnosed by culturing the bacterium from the infected body site. Culture also permits identification of what antibiotics may or may not be effective for treating the infection. Actinobacillus equuli is usually susceptible to a variety of routinely-used antibiotics.
