Guinea Pigs Show Resistance to Rhodococcus equi Infection

Rhodococcus equi, a facultative intracellular bacterium, is a common cause of pneumonia in 3- to 6-month old foals. While researchers often use small laboratory animals to study certain equine pathogens, such as the guinea pig model for Mycobacterium tuberculosis, attempts to establish a rodent model for R equi research have largely been unsuccessful.

Researchers at Texas A&M University recently performed a study to determine the usefulness of the guinea pig as an R equi laboratory model. Interestingly, they discovered that guinea pigs were remarkably resistant to even high doses of the bacterium.

Analyses

Thirty-four guinea pigs aged 2 to 10 weeks were challenged experimentally with 1 of 2 virulent R equi strains (33701+ and 5-331) via either low-dose aerosol infection (10¹-10⁴ CFU) or high-dose intratracheal infection (10⁶-10⁷ CFU). Low-dose infection typically induces pyogranulomatous pneumonia in foals, whereas high-dose infection causes diffuse, miliary pneumonia.

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Inoculated animals were euthanized on day 1, 3, 7, or 35 postinfection, at which time the lung, spleen, and liver were collected for histology and immunohistochemistry. Bacterial concentrations were determined from lung homogenates, and bronchoalveolar lavage fluid (BALF) samples were examined to determine retention of bacteria in airway tissues.

Results

R equi bacteria were not recovered from the lung, liver, or spleen of any animal euthanized 35 days after low-dose aerosol inoculation (10¹-10⁴ CFU) with R equi strain 33701+. Similarly, guinea pigs that received a 10³- to 10⁴‑CFU aerosol dose of R. equi strain 5-331 showed no clinical or histologic signs of pneumonia. However, all lung samples collected from the 5-331 group cultured positive for R equi on days 1, 3, and 7 postinfection. BALF samples, collected 24 hours after inoculation, did not contain R equi, suggesting no retention of bacteria in the airway.

The higher intratracheal dose of 10⁶ to 10⁷ CFU of strain 5-331 also failed to cause clinical signs in inoculated guinea pigs through day 7 postinfection; however, lung cultures were positive in 4 of 6 animals that received the 10⁶‑CFU dose and in all animals that received the 10⁷‑CFU dose. Interestingly, bacterial loads within lung tissue decreased from day 3 to day 7 postinfection, suggesting that guinea pigs were clearing the infection.

No clinical or histologic signs of disease, such as abscesses, inflammation, and development of bronchoalveolar lymphoid tissue, were observed in any of the guinea pigs in the study, even those administered the highest inoculum dose. Immunohistochemistry revealed similar numbers and distribution patterns for T cells, B cells, and interstitial macrophages in all animals regardless of inoculum dose. The authors concluded that an even higher inoculation dose or longer observation period may be necessary to induce clinical disease in guinea pigs.

Implications

Guinea pigs appear to be resistant to pulmonary infection with R equi, even at high doses equivalent to those that initiate pneumonia in foals. Despite their apparent resistance to infection, the guinea pig may still serve as a useful model for understanding R. equi pathogenesis and the components of a successful immune response to infection.

Dr. Stilwell is a medical writer and aquatic animal veterinarian in Athens, Georgia. After receiving her DVM from Auburn University, she completed an MS degree in Fisheries and Aquatic Sciences, followed by a PhD degree in Veterinary Medical Sciences, at the University of Florida.