Canine infectious disease update (Proceedings)
Bordetella Bronchiseptica Infection and Prevention
Closely related to Bordetella pertussis, the cause of "whooping cough" in humans, Bordetella bronchiseptica is a gram negative, aerobic coccobacillus particularly well adapted to colonize the ciliated respiratory epithelium of dogs and cats. Today, this organism is regarded as the principle etiologic agent of canine infectious tracheobronchitis (ITB). In the clinical setting, however, B. bronchiseptica infection should not be regarded as synonymous with ITB. Dogs infected with canine parainfluenza virus (CPiV) or canine adenovirus-2 (CAV-2) are expected to experience more severe respiratory disease when co-infected with B. bronchiseptica than with any these agents alone. Canine bordetellosis, i.e. B. bronchiseptica infection in the absence of either CPiV or CAV-2, is known to occur and can be associated with acute, fatal pneumonia in young dogs. B. bronchiseptica is transmitted through aerosolization of respiratory secretions. Bacteria can also be transmitted directly by contaminated dishware, human hands, and other fomites. Because B. bronchiseptica possesses several intrinsic mechanisms for evading host defenses, it is recognized for its role as a significant complicating factor in dogs with multiple-agent respiratory infections. The fact that outbreaks of canine ITB are common, despite widespread use of topical and parenteral vaccines in dogs for over 20 years, highlights the fact that current vaccines are not highly effective in preventing infection. On the other hand, our understanding of the role that B. bronchiseptica has in feline respiratory disease is only beginning to undergo scientific scrutiny.
Pathogenesis of InfectionMost of what is known about the pathogenesis of B. bronchiseptica is based on information derived from studies in dogs. In addition to dogs, infections have been documented in cats, pigs, various laboratory species, and humans. B. bronchiseptica rarely infects tissues outside the respiratory tract, a fact that supports the ease of transmissibility among dogs, particularly when housed in close quarters. Contributing to the ability of B. bronchiseptica to colonize respiratory epithelium is the fact that the bacterium possesses both fimbrial and non-fimbrial adhesins, that facilitate the attachment to host cells. Two non-fimbrial adhesins, filamentous hemagglutinin (FHA) and pertactin (Prn), are essential for the attachment of B. bronchiseptica to respiratory epithelial cells. Understanding the role of such proteins in the pathogenesis of B. bronchiseptica infection has been fundamental in investigations that may lead to the first acellular whooping cough vaccine. It is not unreasonable that such research may ultimately lead to improved vaccines for dogs and cats. Fimbriae, hair-like appendages extending from the cell membrane of B. bronchiseptica, recognize specific receptors within the respiratory tract. This allows B. bronchiseptica to colonize specific tissues where it then releases various exotoxins (such as adenylate cyclase-hemolysin and dermonecrotic toxin) and endotoxins that impair function of the respiratory epithelium (ciliastasis) and compromise the ability of the infected host to eliminate the infection. These potent toxins not only disrupt the protective action of the mucociliary apparatus, but also are believed to compromise phagocytosis and suppress both cellular and humoral immune responses. Additionally, B. bronchiseptica is regarded as an extracellular pathogen that has the unique ability to invade host cells. Once contained within the intracellular environment, bacteria are able to avoid immunologic defense mechanisms and establish a persistent infection (months) or carrier state.
Clinical signs of canine infectious tracheobronchitis (ITB) include paroxysmal coughing episodes, frequently associated with retching and expectoration, in an otherwise healthy, active dog. Swelling of the vocal folds, associated with laryngitis, can result in a loud, high-pitched cough often described as a "goose honk" or "seal honk." Expectoration of mucus following an episode of retching or hacking behavior may be misinterpreted by the owner as vomiting. Anorexia, fever, and lethargy may be observed among infected dogs during an outbreak. The onset of clinical signs typically ranges from 3 to 10 days following exposure. In most clinical cases, the onset of clinical signs can be associated with recent exposure to other dogs or general anesthesia and endotracheal intubation. The ability to elicit a cough on manipulation of the trachea is an inconsistent clinical finding that should not be used exclusively to rule canine ITB in or out.