Bordetella bronchiseptica

Bordetella bronchiseptica (Bb) is a Gram-negative, aerobic small bacillus that widely colonizes the ciliated epithelial cells of the respiratory tract in livestock, wild animals, and laboratory animals. It can cause respiratory diseases in a variety of mammals, including rabbits, pigs, horses, dogs, and rodents, with infected animals typically exhibiting symptoms such as rhinitis and bronchopneumonia. In recent years, there have also been reports of human infections caused by this bacterium, particularly among young children and elderly individuals with weakened immune systems. Bb is one of the pathogens responsible for atrophic rhinitis in pigs, and when it infects dogs, it can lead to kennel cough. So, how exactly does Bordetella bronchiseptica spread, and what are its implications for animal health and scientific research? This article will delve into these questions.

Bordetella bronchiseptica, also known as Bordetella bronchialis, is an aerobic, motile, Gram-negative coccobacillus commonly found on the ciliated respiratory epithelium of animals such as dogs, rabbits, and pigs. While strains isolated from pigs appear to exhibit host specificity, other hosts show relatively little host specificity. Notably, Bb readily undergoes variation—whether in the nasal cavities of animals or during artificial culture—and exists in three distinct bacterial phases. Among these, the highly virulent phase is the encapsulated Type I strain, which possesses a K antigen and produces a potent necrotic toxin. This toxin shares strong homology with the dermonecrotic toxin produced by toxigenic Pasteurella multocida (T+Pm). In contrast, the Type II and Type III strains are much less virulent. Under the influence of antibodies or inappropriately favorable conditions, the highly pathogenic Type I strain can even revert to the less virulent Type III form. Once infected, newborn piglets harboring Type I Bb bacteria can experience prolonged colonization in their nasal passages, with the bacteria persisting for up to a year. Importantly, both T+Pm and Bb display limited resistance to common disinfectants, making them effectively susceptible to inactivation under standard sanitization protocols.

Bordetella bronchiseptica is present in experimental animals such as dogs, pigs, rabbits, and guinea pigs, but it is rarely found in modern rodent and mouse facilities. However, it commonly occurs in rodents or rabbits, especially those that have been exposed to dogs or cats. This disease can infect dogs, pigs of various breeds, sexes, and ages, though young dogs and pigs are particularly susceptible and experience more severe health impacts. In China, the bacterium has a relatively high infection rate among rabbit populations—infected animals include neonatal rabbits, young adults, and breeding stock alike—resulting in significant economic losses for the rabbit farming industry. Transmission occurs via aerosols, contaminated surfaces, and direct contact, while various stress factors can further elevate the incidence of the disease.

Most cases of Bordetella bronchiseptica primarily present as subclinical infections. Bordetella bronchiseptica is considered a major pathogen in dogs and is one of the most common causes of canine infectious tracheobronchitis, commonly known as "kennel cough." Clinical signs include mucoid to purulent nasal discharge, as well as wet or dry coughs; respiratory distress may occasionally be observed in pneumonia cases, though fatalities are rare. The disease is more prevalent in young puppies. Concurrent infections with pathogens such as canine mycoplasma, canine parainfluenza virus type 2, canine adenovirus, and canine herpesvirus can further complicate the condition. Without secondary infections, however, the illness typically resolves within 1 to 2 weeks without intervention—but the bacterium may continue to be shed for up to 3 months. Clinically, dogs usually exhibit lesions characterized by rhinitis, tracheitis, and bronchitis. These lesions involve mucosal congestion, bleeding, and edema, along with thickened mucosa and blood-tinged, frothy secretions in the trachea. Meanwhile, lung surfaces appear smooth and edematous, with dark red areas of consolidation. Upon incision, a small amount of fluid oozes out, and some lungs reveal tiny, milky-white pustules that yield creamy white pus when cut open. In addition, the liver may become enlarged and brittle, with dark red fluid leaking from its cut surface. The spleen shows signs of congestion, appearing dark brown, while the stomach contains digested food mixed with reddish-brown fluid, and its lining has partially sloughed off.

For rabbits, when this pathogen is combined with Pasteurella multocida infection, clinical symptoms may appear, including ocular and nasal discharge ("sniffling"), lethargy, anorexia, and difficulty breathing—occasionally leading to death. Additionally, abscesses may develop as a result of the infection. The nasal mucosa shows congestion accompanied by copious serous or mucoid secretions, while the bronchial mucosa becomes highly congested and filled with mucus, some cases even containing foamy mucus, and in others, thin purulent fluid. Meanwhile, the lungs, liver, kidneys, and abdominal cavity may harbor pus-filled lesions of varying sizes.

In swine, the toxin-producing strain of Bordetella bronchiseptica can cause a disease known as Non-Progressive Atrophic Rhinitis (NPAR). Additionally, co-infection with toxin-producing Pasteurella multocida may lead to Progressive Atrophic Rhinitis (PAR). Clinical signs following infection include mild nasal turbinate hypoplasia, resulting in varying degrees of distortion of the muzzle and face. In severe cases, this can even interfere with breathing. Affected pigs may sneeze, snort, or cough excessively, accompanied by copious nasal discharge. The age at which infection occurs correlates with the severity of clinical symptoms: newborn piglets are most susceptible and prone to developing lesions, though susceptibility markedly declines by 6 weeks of age. Beyond NPAR, Bordetella bronchiseptica can also suppress appetite and weight gain, occasionally triggering pneumonia in neonatal piglets. Typical lower respiratory tract pathology in diseased pigs includes bronchitis, suppurative bronchopneumonia, and interstitial pneumonia. Microscopically, prominent peribronchial lymphocytic cuffs are readily observed. Further examination under the microscope reveals vasculitis, followed by endothelial proliferation, alveolar hemorrhage, necrosis, interlobular edema, perivascular and peribronchiolar fibrosis, and neutrophil infiltration.

Bordetella bronchiseptica is primarily a respiratory pathogen. Its effects on host physiology include inhibiting the respiratory clearance mechanism through ciliary paralysis, as well as altering the functions of alveolar macrophages and dendritic cells. Clearly, clinical bordetellosis can influence the role of experimental animals in respiratory system research and may also impact studies involving the lymphoreticular system.

Bordetella bronchiseptica readily develops resistance to antimicrobial drugs, so during treatment, the most sensitive antibiotic should be selected based on the results of susceptibility testing. Infected animals should receive antibiotic therapy, such as tetracycline, chloramphenicol, amoxicillin-clavulanate, potentiated sulfonamides, or any fluoroquinolone-based medication.

For individual animals infected with Bordetella bronchiseptica pneumonia, their experimental value should be assessed before deciding whether euthanasia is necessary. High fever and common disinfectants, such as diluted bleach, can effectively eliminate Bordetella bronchiseptica. Currently, numerous commercial vaccines are available on the market, including non-toxic recombinant toxin vaccines prepared via genetic engineering methods, which have demonstrated significant protective efficacy and hold great promise for practical applications. Promptly isolate affected rabbits and thoroughly disinfect the rabbit housing facilities. For treatment, administer sulfadiazine via nasal drops or intramuscular injection, combined with oral administration of synergistic sulfonamide tablets. Rabbits with chronic, unresolved cases of rhinitis complicated by bronchopneumonia should be culled promptly. Meanwhile, asymptomatic "healthy" rabbits can be proactively protected by mixing sulfonamide drugs with antimicrobial synergists into their feed for 5 to 7 consecutive days. Immediately implement emergency vaccination against Bordetella bronchiseptica for rabbit populations that remain disease-free. Colostrum-based immunity can shield piglets from developing neonatal paratyphoid (NPAR), though it does not prevent infection altogether; vaccinating sows, however, can delay infection in their offspring. Under strict isolation and disinfection protocols, closely monitor, care for, and treat affected animals to contain the spread and transmission of the disease. All cages and equipment previously used by sick animals must be disinfected with a 3–4% sodium hypochlorite solution, and these items should remain isolated for at least one week before reuse. Alternatively, solutions like Newjel or Hibitane can also be employed for effective disinfection. Disease control can be further enhanced by thoroughly cleaning and disinfecting premises, isolating or removing animals identified as shedding the pathogen through nasal swab cultures, and implementing preventive measures such as prophylactic antibiotic use in feed and timely vaccination with appropriate bacterial strains.

References

1. David G. Baker, Natural Pathogens of Laboratory Animals, 2003

2. GB/T 14926.6-2001 Laboratory Animals — Method for Detecting Bordetella bronchiseptica

3. Chen Puyan et al. Veterinary Infectious Diseases, 5th Edition