Colleen J. Sedney Tracy L. Nicholson and Eric T. Harvill The Bordetella genus includes at least nine distinct species, five of which are known to infect and cause disease in a variety of animals, including dogs and other canid species, cats, rabbits, turkeys, sheep, pigs, horses, seals, rodents, and humans. Infections with certain members of the Bordetella genus in their specific host are often chronic with a range of clinical signs, from asymptomatic to severe bronchopneumonia. While animal infections with members of the Bordetella genus are not required to be reported to the Department of Agriculture in the United States, the high virulence of Bordetellae can result in rapid transmission among animals, making voluntary reporting an important factor in reducing the spread and burden of disease. Transmission events have previously resulted in outbreaks and some cases have led to fatal outcomes, particularly in young animals. Most veterinary relevant Bordetella species can infect multiple host animals, and several examples of zoonoses have been documented, with the most severe cases involving immunocompromised individuals. In this chapter, we explore the various Bordetella species from a veterinary perspective, their clinical presentation and identification, current vaccines and therapies, the host response to infection, and factors that contribute to colonization and disease. Gaps in knowledge regarding the efficacy of current vaccines and prevalence will also be addressed. Bordetellae are aerobic Gram‐negative coccobacilli that grow at an ideal temperature of 37°C. Isolation and growth of most Bordetella species can be achieved on blood or MacConkey agar, with mature colony formation in two to five days depending on the species. This agar is specialized, due to Bordetella sensitivity to detergents and unsaturated fatty acids. Colonies are round, whiteish‐gray, with a shiny top, and are hemolytic, creating a discoloration on the blood agar and a transparent ring around the colony. Species are indistinguishable from one another in mixed culture. Growth in liquid culture can also be achieved in specialized medium that includes glutamic acid, proline, cysteine, nicotinamide, and glutathione. Identification of Bordetella species can also be achieved by their biochemical characteristics, including hemolysis, catalase and oxidase and nitrate reductase activity. Bordetella species are traditionally divided into two groups, termed the “classical” and “non‐classical” Bordetella species (Table 17.1). The classical Bordetellae include B. bronchiseptica, B. parapertussis, and B. pertussis. Phylogenomic analysis suggests that members of this group evolved from a B. bronchiseptica‐like progenitor; specialization to a specialized life cycle allowed for dramatic gene loss from the two sequent species (B. parapertussis, B. pertussis; Parkhill et al. 2003). The genetic evolution of the non‐classical Bordetellae is less studied, but genome comparisons suggest that other species have specialized to a host and similarly underwent gene loss (Table 17.1). Table 17.1 Bordetella species involved in infections in animals and humans. Source: Modified after Rivera et al. (2020). The common ancestor of the Bordetella species is believed to have been an environmental organism. The implications of this discovery lead to the observation that B. bronchiseptica, the species with the largest genome, appears to have previously developed and retained the ability to survive interactions with environmental predatory amebae via exploitation of its lifecycle (Taylor‐Mulneix et al. 2017; Rivera et al. 2020). Other Bordetella species that are mammalian host specific and not found in the environment are unable to survive interactions with these amoebas (Taylor‐Mulneix et al. 2017; Table 17.1). B. bronchiseptica was first isolated and associated with respiratory disease in dogs in 1910 and subsequently found to cause respiratory disease in other mammals. B. bronchiseptica infects a broad range of wild and domesticated mammals, including both companion and livestock animal species, and is capable of surviving in the environment. Infection can result in a spectrum of disease severities, including long‐term chronic infections of the upper respiratory tract. B. bronchiseptica is both a causative and contributing agent of canine infectious respiratory disease (CIRD), also known as kennel cough. The 5.2 Mb genome of B. bronchiseptica is the largest of the Bordetella genus and appears to have functional genes expressing all the well‐characterized virulence factors that contribute to its pathology (Tables 17.1 and 17.2). Table 17.2 Expression of main known or suspected virulence factors in veterinary relevant Bordetella species. B. bronchiseptica is one of the most frequently isolated CIRD pathogens, but its prevalence varies geographically. A recent study involving animal shelters in the United States reported that 19.5% of dogs tested were positive for B. bronchiseptica, whereas a study from Poland reported 30% of dogs tested were positive for B. bronchiseptica, and one from Germany reported a 78% positivity rate in dogs (Lavan and Knesl 2015; Day et al. 2020). B. bronchiseptica is highly contagious and is transmitted by both direct and indirect mechanisms (Coutts et al. 1996; Nicholson et al. 2012, 2014). Several studies have reported transmission between dogs and between cats (Coutts et al. 1996; Dawson et al. 2000; Reagan and Sykes 2020). The incubation period of this species is recorded in dogs to be around 3–10 days (Reagan and Sykes 2020). The species B. avium is primarily an avian pathogen, most notably causing turkey coryza, though limited human infections have been reported. This species differs from others in the genus in that it is non‐hemolytic and has the smallest genome of the sequenced Bordetellae (Table 17.1). Despite the small genome size, B. avium contains over 1000 genes not found in B. bronchiseptica. These genes are predicted to encode surface or secreted proteins that are thought to enable this species to colonize and cause disease in avian rather than mammalian hosts (Sebaihia et al. 2006). A member of the classical Bordetellae, B. parapertussis comprises two distinct subgroups, a human‐adapted pathogen causing pertussis‐like illness, and another that causes chronic pneumonia in sheep. Whereas the human‐adapted B. parapertussis can colonize mice in laboratory settings, the ovine‐specific subgroup does not efficiently colonize murine hosts due to its susceptibility to murine complement‐mediated killing (Hester et al. 2015). B. hinzii was first described in 1994. Primarily a commensal in avian hosts, some strains of B. hinzii have been shown to be transmissible in young turkeys, causing asymptomatic infection (Register and Kunkle 2009). As is the case for B. bronchiseptica, immunocompromised humans can be susceptible to severe B. hinzii infections. The presence of B. hinzii isolates in laboratory mice was first reported in 2008 and similar colonization was subsequently reported in various areas of the world. Recent data, however, indicate that these isolates were more likely to be the newly described species Bordetella pseudohinzii (Ivanov et al. 2016). The recently identified Bordetella species, B. pseudohinzii, naturally colonizes multiple species of rodents, including mice and rats (Loong et al. 2018). Colonization in mice has been demonstrated to cause progressive chronic otitis media, leading to severe hearing loss (Dewan et al. 2019). In vivo studies show that B. pseudohinzii targets ciliated cells, decreasing ciliary movement and causing tracheal inflammation (Perniss et al. 2018). Unlike most other Bordetella species, B. pseudohinzii is non‐hemolytic on blood agar. B. pertussis, B. holmesii, B. trematum and B. petrii are not known to be of veterinary interest. Other isolates from environmental sources have been named as Bordetella species but not yet fully characterized or associated with animal diseases. Transmission of Bordetellae typically occurs through direct contact with infected animals or fomites, as well as airborne or indirect transmission (Nicholson et al. 2012). B. bronchiseptica is highly contagious and can result in widespread transmission events, particularly in large groups of animals in confined spaces. This includes, but is not limited to, long‐ and short‐term kennel facilities, flocks, herds, and industrial animal facilities. Underreporting and the lack of laboratory confirmation of B. bronchiseptica infections, as well as asymptomatic infections, makes the true impact of large‐scale transmission events difficult to quantify. However, experimental transmission studies have demonstrated 100% transmission efficiency via indirect or airborne transmission (Nicholson et al. 2012). Transmission between cats and dogs in animal shelters and residential homes have also been reported (Dawson et al. 2000). While animals of all ages can be infected, young animals are at particular risk for severe clinical disease. Several Bordetella species of veterinary interest can infect humans, including increasing case reports of immunocompromised humans being infected with B. bronchiseptica, B. avium, B. hinzii, and other species. These infections have been commonly reported in patients with cystic fibrosis, a community in which ownership of companion animals is popular. A report of cystic fibrosis patients described that approximately half (47%) of respondents reported dog ownership, with 28% reporting ownership of a cat, and 21% owning both a cat and a dog. A possible route of transmission are companion animal vaccines that use a live attenuated platform, which generally contain a warning label advising owners to socially distance themselves from their pet during the recommended period of shedding of the live vaccine strain. This period can last from 35 days to 11 weeks depending on the vaccine used. Therefore, any potential zoonotic infection or carriage of live veterinary vaccine strains, including B. bronchiseptica in pets, should be of interest to pet owners with cystic fibrosis or other immunodeficiencies. The clinical presentation associated with Bordetella infection vary based on the species and the infected host. Cats, dogs, and pigs with a B. bronchiseptica infection can exhibit clinical signs including coughing, sneezing, nasal discharge, fever, and/or lethargy. Similar symptoms can be observed in turkeys and sheep infected with B. avium and B. parapertussis, respectively. Despite the high morbidity associated with Bordetella infections, many animals can be colonized, but not show clinical signs, serving as asymptomatic carriers that continue to shed and transmit Bordetella to cohorts. Lesions are typically observed in the nasal cavity, trachea, and lungs of pigs, turkeys, and other animals, with the occurrence and severity of the lesions varying based on animal age, immunocompetence, strain characteristics, and housing conditions. The pathology of these lesions is due to the attenuation of epithelium and loss of cilia, as well as inflammatory cell infiltration (Taha‐Abdelaziz et al. 2016). In addition to superficial lesions at the nasal cavity, pneumonic lesions can also form and are characterized by necrosis, alveolar hemorrhage, neutrophil accumulation, and fibrosis (Brockmeier et al. 2019). The most notable lesion caused by a Bordetella species occurs in poultry and results in the modulation of tracheal rings, ultimately leading to tracheal collapse (Yersin et al. 1998). In swine, B. bronchiseptica is extensively prevalent and plays multiple roles in respiratory disease. It is the primary etiologic agent of nonprogressive atrophic rhinitis, a mild to moderately severe, reversible condition, and it promotes colonization by toxigenic strains of Pasteurella multocida, leading to severe, progressive atrophic rhinitis (Brockmeier et al. 2019). In young pigs, it is a primary cause of bronchopneumonia and in older pigs contributes to the porcine respiratory disease complex (Brockmeier et al. 2019). Its presence also enhances colonization with Glaesserella parasuis and Streptococcus suis and increases the severity of respiratory disease associated with viral pathogens, including porcine reproductive and respiratory syndrome virus, swine influenza virus, and porcine respiratory coronavirus (Brockmeier et al. 2019). B. bronchiseptica infections can be confirmed by culture or molecular methods, although infection is often diagnosed based on clinical signs and without laboratory confirmation, confounding the ability to track the prevalence of cases. The classical and non‐classical Bordetella species share several factors that contribute to their virulence in hosts. However, the function and importance of each may vary, apparently related to divergent aspects of host adaptation. Research into most of these factors has most often been examined using B. bronchiseptica in mice or pigs, or B. avium in birds. A longer list of currently known virulence factors in various Bordetella species is shown in Table 17.1. Bordetellae produce several adhesins that are important in the ciliated epithelial mucosal colonization that is characteristic of the early stages of infection in different species (see Figure 17.1). Filamentous hemagglutinin (FHA) is an adhesion protein that can be associated with the bacterial cell surface or be a soluble molecule. It facilitates attachment to many different cell types, including ciliated cells, macrophages, and leukocytes. One of the key roles of FHA is the initial colonization of the host epithelia. This was demonstrated using a B. bronchiseptica isolate lacking FHA, which exhibited decreased colonization and disease severity in a porcine infection model (Nicholson et al. 2009). This work indicated that FHA is a vital adhesion factor for initial colonization and is required to establish disease. This role in colonization may also be linked to its contribution to biofilm formation in the murine nasal cavity and trachea; reducing FHA decreases Bordetella biofilm formation (Serra et al. 2011). An FHA‐deficient mutant of B. bronchiseptica elicited an increased T helper (Th) 17‐mediated inflammatory response in mice, characterized by interleukin (IL)‐17‐positive neutrophils, macrophages, and CD4+ T cells (Henderson et al. 2012). This indicates that FHA mediates the suppression of Th17‐mediated inflammation, which primarily activates the killing mechanisms of macrophages and promote the production of antibodies, thereby allowing the infection to persist. Figure 17.1 Colonization by Bordetella bronchiseptica (red staining) of the ciliated epithelium of the conchae (A:160X) or bronchus (B:400X) of an infected pig. Areas denuded of cilia are apparent in the conchae. A neutrophilic inflammatory infiltrate is present in the lumen of the bronchus. Fimbriae are hair‐like filaments that can extend over a cell length from the cell surface. Six fimbriae subunit genes have been identified in Bordetella spp., however fim2 and fim3 are the major subunits that compose the fimbriae produced by most characterized Bordetella isolates. The virulence function of these fimbriae is unknown, but there is evidence to suggest they play a role in attachment to respiratory tract cells. A fimbriae‐deficient strain of B. bronchiseptica was observed to be deficient in attachment to the airway epithelium (Scheller et al. 2015). Fimbriae also play a role, apparently cooperative with that of FHA, in immunosuppression. A recently identified ortholog of the fimbrial locus was identified in B. avium, which mediates attachment to the avian respiratory epithelium (Loker et al. 2011). The expression of fimbriae was also determined to be regulated by temperature in B. avium,
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Bordetella
Introduction
Characteristics of the Organisms
Species
Host species
Disease
Genome size (MB)
Intracellular survival
Classical group:
B. bronchiseptica
Variety of mammals including cats, dogs, horses, humans, mice, pigs, rabbits, seals, and sheep
Respiratory disease, varying from clinically inapparent to acute bronchopneumonia (e.g. kennel cough in dogs) and rhinitis in pigs
5.3
+
B. parapertussis
Sheep and humans; distinct host‐specific lineages
Pneumonia in sheep; whooping cough‐like disease in humans
4.7
+
B. pertussis
Humans
Whooping cough
4.1
+
Non‐classical group:
B. avium
Poultry, wild birds
Respiratory disease
3.7
−
B. hinzii
Poultry; immunocompromised humans
Respiratory disease (coryza) in poultry; septicemia in immunocompromised humans
4.9
+
B. holmesii
Humans
Whooping cough‐ like disease; bacteremia
3.7
Unknown
B. pseudohinzii
Mice
Chronic otitis media
4.5
+
B. trematum
Immunocompromised humans
Wound, skin infections
4.5
+
Environmental:
B. petrii
Immunocompromised humans
Wound, ear infection
5.3
+
Pathogenic Species
Bordetella bronchiseptica
Virulence factor
B. bronchiseptica
B. parapertussis
B. avium
Filamentous hemagglutinin
+
+
+
Fimbriae
+
+
+
Pertactin
+
+
−
Adenylate cyclase toxin
+
+
−
Dermonecrotic toxin
+
+
+
Tracheal cytotoxin
+
+
+
Lipopolysaccharide
+
+
+
Type III secretion system
+
+
−
Type VI secretion system
+
+
+
Bordetella virulence gene (bvg) regulatory locus
+
+
+
Bordetella avium
Bordetella parapertussis
Bordetella hinzii
Bordetella pseudohinzii
Other and Emerging Species
Source of Infection: Ecology and Epidemiology
Types of Disease and Pathologic Changes
Virulence Factors
Adhesins
Filamentous Hemagglutinin
Fimbriae
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