New research from Oklahoma State University helps explain why horses with the hormonal disorder pituitary pars intermedia dysfunction (PPID, also known as Cushing’s disease) can be more susceptible to infection.
In a two-phase study, the researchers analyzed blood samples from 23 horses with PPID and 39 who were healthy. They specifically investigated the processes that immune cells called neutrophils use to neutralize infectious or inflammatory agents and help the body heal.
“Neutrophils migrate to areas of infection or inflammation in a process called chemotaxis. To exit the blood vessels, neutrophils first stick onto the vessel walls, a process known as adhesion. Without adhesion and chemotaxis the neutrophils can’t get to the site of infection or inflammation,” explains Dianne McFarlane, DVM, PhD. “Once they arrive [at an area of inflammation] neutrophils can engulf foreign or necrotic material including bacteria---a process called phagocytosis. They are able to kill bacteria by releasing enzymes and chemicals, a process known as oxidative burst. Together, these four actions of neutrophils are necessary for a robust immune system.”
McFarlane and her team discovered that horses with PPID had significantly reduced oxidative burst activity and adhesion than did healthy horses. None of the study horses had active infections, and the reduction in these functions was not correlated to severity of PPID.
McFarlane says this study underscores the importance of managing PPID horses with medication to balance their hormone concentrations and avoid secondary infections. “In older horses with infections that are not responding well to treatment, it is important to determine their PPID status,” she says. “Even when giving the correct antibiotics, it can be very difficult to cure infections in horses with high concentrations of immunosuppressive hormones such as occurs in horses with PPID.”
Reference: “Neutrophil function in healthy aged horses and horses with pituitary dysfunction,” Veterinary Immunology and Immunopathology, June 2015
This article first appeared in EQUUS issue #457, October 2015.