Avian bordetellosis is a highly infectious, acute upper respiratory tract disease of turkeys characterized by high morbidity and usually low mortality. Other synonyms previously used for the disease include Alcaligenes rhinotracheitis, adenovirus-associated respiratory disease, acute respiratory disease syndrome, and turkey rhinotracheitis.
Although the disease primarily affects turkeys, it has also been observed in quail and ostrich chicks. In cockatiels, B avium is known to cause temporomandibular rigidity (lockjaw syndrome). It is an opportunistic pathogen in chickens. Damage to the upper respiratory tract resulting from prior exposure to other respiratory pathogens or related, live vaccine strains, such as infectious bronchitis virus or Newcastle disease virus, or from an environmental irritant such as ammonia, is necessary to induce signs in chickens. B avium has also been isolated from healthy individuals of many other wild or domesticated avian species, including ducks, geese, owls, partridges, parrot finches, and others, but there is currently no evidence to suggest it causes disease in these hosts.
Bordetellosis has been identified in almost every area of the world where turkeys are intensively reared. It appears to be rare or inapparent in some locations, whereas severe outbreaks occur in other geographic regions. The reasons for these epidemiologic differences are not known.
The major etiologic agent causing bordetellosis is Bordetella avium but, in 2009, it was reported that B hinzii also causes clinical signs of bordetellosis in experimentally infected turkey poults. Since that time, B hinzii has also been identified from turkeys diagnosed with the disease. Reanalysis of bacterial isolates from historical cases of bordetellosis suggests that some B hinzii infections may have been mistakenly ascribed to B avium or other closely related bacteria for a considerable length of time.
The mechanism of pathogenesis for B avium and B hinzii depends on their ability to proficiently attach to the cilia of the pseudostratified columnar epithelium. From this location, toxins and other effectors produced by the bacteria are optimally positioned to damage the underlying tracheal cartilage. Bacteria initially adhere to ciliated cells of the nasal mucosa, subsequently progressing to the trachea and primary bronchi. Potential attachment factors of B avium, which may collectively participate in adherence, include pili, a hemagglutinin, the protein autotransporter Baa1, and components associated with lipopolysaccharide. Attachment factors of B hinzii have yet to be identified, but genome sequence comparisons and other data suggest that at least some may be unique from those used by B avium.
Damage to the tracheal cartilage caused by B avium is likely due to the actions of an osteotoxin and a tracheal cytotoxin. It is not currently known whether B hinzii also produces these toxins or whether tracheal damage is mediated by other bacterial products. A dermonecrotic toxin produced by B avium may further contribute to virulence, but its specific role is unclear. B hinzii is not known to produce the toxin, and no corresponding gene has been identified in any of the 13 isolates for which genome sequences are publicly available.
As for many other bacterial pathogens, iron acquisition is necessary for colonization and spread of B avium in the host. Mechanisms of iron uptake identified in B avium include heme receptors, siderophore receptors, and a transferrin-binding protein. Genome sequence annotations for B hinzii identify several genes potentially involved in iron acquisition, but whether they are expressed and how they might function has not been investigated.
In several other Bordetella spp, the timing and level of expression of most virulence factors is controlled by proteins encoded by the bvg (Bordetella virulence genes) locus. This process, known as phenotypic modulation, leads to reversible up- or down-regulation of virulence gene expression in response to local environmental conditions. A bvg locus has been identified in B avium and is required for virulence. B hinzii also has a recognizable bvg locus, but little is known about its possible role in virulence. Information currently available suggests the precise manner in which virulence genes are regulated in both B avium and B hinzii may be unique as compared with other Bordetella spp in which phenotypic modulation has been more intensively studied.
Damage to the upper respiratory tract resulting from bordetellosis can lead to secondary infections with Escherichia coli or other agents, which can significantly increase the severity of the disease. In many cases, turkeys infected solely with B avium recover within 4–6 weeks without serious consequences.
Morbidity in young turkeys with bordetellosis is usually 80%–100%. Mortality ranges from 0% in birds with uncomplicated disease to >40% when secondary invaders such as E coli or Newcastle disease virus are present. When environmental conditions in turkey barns are less than optimal or when disease is complicated by secondary agents, mortality often increases and clinical signs are more severe. Turkeys appear to become relatively resistant to bordetellosis after 5–6 weeks of age, although disease in breeders and older flocks has occasionally been reported. Nonetheless, mature birds exposed to B avium may become clinically inapparent carriers capable of transmitting bordetellosis to susceptible turkeys. Prior infection with B avium can predispose turkeys to colibacillosis and increase the severity of related airsacculitis.
B avium is highly contagious and easily transmitted from infected turkeys to susceptible birds by direct contact. It can also be spread through contaminated drinking water, feed, and litter, which can remain infectious for as long as 6 months. Other domesticated and wild birds from which B avium has been isolated should be considered possible reservoirs of infection. No studies have yet directly addressed transmission of B hinzii, but considering its close relationship to B avium, it seems likely these two species have similar patterns of transmission.
Signs of bordetellosis usually occur 7–10 days after infection and include sinusitis, with a clear nasal discharge that can be observed when pressure is applied to the nares. Foamy-watery eyes, a snick or cough, mouth breathing, dyspnea, tracheal rales, and altered vocalization are also characteristic. Older turkeys may develop a dry cough. During the first 2 weeks of disease, the nares and feathers of the head and wings often appear crusted with wet, sticky exudate. By 1 week after disease onset, tracheal softening can sometimes be palpated through the skin of the neck. Copious production of mucus in the trachea and tracheal collapse can result in mortality due to suffocation. Complicated disease often triggers more exaggerated signs, including airsacculitis.
Lesions are primarily found in the upper respiratory tract and consist of nasal and tracheal exudates, collapse of cartilaginous rings, and progressive loss of ciliated epithelium. In uncomplicated disease, the tracheal epithelium can return to normal 4–6 weeks after the onset of signs.
At necropsy, turkeys with characteristic bordetellosis have watery eyes and extensive mucus in the sinuses and trachea, which rarely extends below the tracheal bifurcation. Mild hemorrhage in the lining of the trachea may be apparent in some cases, and softening of the tracheal rings is usually evident, sometimes accompanied by a dorsal/ventral flattening of the trachea. Pneumonia and airsacculitis are observed only when the disease is complicated by another disease agent.
Diagnosis of bordetellosis is based on clinical signs and lesions and isolation of B avium or B hinzii. The bacteria are best isolated from the anterior trachea because samples taken from the sinuses frequently produce cultures that are overgrown with other, faster-replicating bacteria, such as Proteus spp.
Both B avium and B hinzii are gram-negative, nonfermentative, motile, aerobic bacilli that grow on a variety of media, including MacConkey agar, Bordet-Gengou agar, blood agar, veal infusion broth, trypticase soy broth, and brain-heart infusion broth. B hinzii, but not B avium, grows on minimal essential medium. When B avium is grown in broth media high in nutrients, filamentous forms may arise. MacConkey agar is recommended for primary culture because it will differentiate nonfermentative Bordetella from fermentative opportunists, including E coli.
After incubation at 37°C for 24–36 hours, B avium typically produces translucent, glistening, pearl-like colonies with smooth edges, ~0.2–1 mm in diameter. After serial passage in the laboratory, a rough colony type with a dry appearance and serrated, irregular edges can be observed for some isolates. Such colonies, which are nonpathogenic, are the visible manifestation of phase variation, a process in which spontaneous mutations in the bvg locus irreversibly abolish production of virulence factors. Colonies of B hinzii are typically described as round, convex, glistening, and grayish and are generally larger than those formed by B avium, up to 2 mm in diameter.
Standard biochemical tests can be used to distinguish B avium and B hinzii from other nonfermentative bacteria, but discriminating between the two species using this approach is challenging (see table titled Properties of B Avium and B hinzii). Although isolation of either bacterium from birds with appropriate clinical signs is sufficient for a diagnosis of bordetellosis, identification to the species level is desirable because it provides information about species-specific prevalence.
Properties of B avium and B hinzii
Highly sensitive and specific PCRs for B avium (targeting a putative membrane protein gene, as annotated in the genome sequence of reference isolate 197N) and B hinzii (targeting the ompA gene) have been reported that can establish the status of suspect colonies, but neither assay is currently recommended for direct testing of clinical samples. It should be noted that a novel species of Bordetella first recognized in 2016, B pseudohinzii, may test positively with the B hinziiompA PCR. Because the host range of B pseudohinzii appears to be restricted to mice, it seems unlikely that false positives might arise from poultry isolates.
Hemagglutination of guinea pig erythrocytes can also be used to differentiate between B avium and B hinzii. A monoclonal antibody produced from mice immunized with B avium has been used to develop both a latex bead-based agglutination test and an indirect immunofluorescence test, but whether the monoclonal antibody also reacts with B hinzii is unknown.
Serology can also help to establish a diagnosis of bordetellosis due to B avium. A microagglutination test has been developed that detects B avium-specific IgM ~1 week after infection. An ELISA that detects IgG >2 weeks after infection is also available and has the added benefit of detecting maternal antibody. There are no data to indicate whether birds infected with B hinzii test positively by either of these methods. There is currently no serologic test suitable for identification of birds infected with B hinzii.
Although antimicrobial therapy may be helpful for secondary colibacillosis, treatment with antimicrobial agents by aerosol, injection, or in the water has not been effective for the control of B avium, even when isolates appear to be highly sensitive based on testing in vitro. This likely reflects the difficulty of achieving therapeutic levels at the site of bacterial colonization, the tracheal epithelium, even when blood levels are deemed adequate.
For Bavium, resistance to ampicillin, aztreonam, erythromycin, streptomycin, ceftiofur, lincomycin, sulfonamides, and tetracycline has been reported. Resistance profiles appear to vary from by region, which is perhaps related to local or regional antibiotic usage. In a few instances, plasmid-mediated transfer of resistance between isolates has been demonstrated.
No information is available regarding the efficacy of antibiotic treatment when bordetellosis is due to B hinzii. Human isolates are often resistant to many antibiotics, including β-lactams, quinolones, macrolides, and cephalosporins. Resistance to aztreonam has been reported for poultry isolates.
Addition of niacin or an oxy-halogen formulation to drinking water has been reported to lessen the severity of clinical signs.
B avium vaccines comprised of bacterins or modified, live mutants generally provide limited efficacy. Results for a given vaccine vary depending on:
Vaccines may reduce the severity of bordetellosis, but none are known to prevent infection. Vaccination is not widely practiced by turkey breeders, and the immunity passed to progeny generally comes from naturally occurring infections.
No B hinzii-derived vaccines have yet been developed. B hinzii and B avium are antigenically related, but whether B avium vaccines affect the course of disease after infection with B hinzii has not been examined.
B avium is easily carried between farms, and this may also be the case for B hinzii. Thus, prevention should include a good biosecurity program. Rigorous cleanup and disinfection after field outbreaks is essential. Most of the commonly used disinfectants are effective against B avium when applied as directed. There are no studies addressing the susceptibility of B hinzii to chemical disinfectants. Maintaining optimal air quality and temperature, minimizing environmental stressors, and other sound husbandry practices will lessen the severity of an outbreak.
Both B avium and B hinzii are opportunistic pathogens in people. The prevalence of human infection with B hinzii is greater than with B avium, but both are rare in people and largely occur in immunocompromised individuals. B avium has been isolated only from cases of respiratory disease, whereas B hinzii has been reported as a cause of respiratory disease, endocarditis, septicemia, and infections of the digestive tract. A few cases provide evidence for possible transmission from an avian source through occupational exposure.
Avian bordetellosis is a highly infectious, acute upper respiratory tract disease of turkeys caused by either Bordetellaavium or Bhinzii.
It is characterized by high morbidity and low mortality, but secondary infections can significantly increase the severity of disease.
Antibiotic treatment of outbreaks is rarely effective. Maintaining high air quality, eliminating environmental stressors, and other sound husbandry practices will reduce the impact of an outbreak.
Vaccines currently available may reduce the severity of disease but do not prevent infection. Rigorous biosecurity measures are required to prevent infection of clean flocks.