Co-infections with Respiratory Viruses in Dogs with Bacterial Pneumonia

Viitanen, S.J.; Lappalainen, A.; Rajamäki, M.M.

Canine infectious respiratory disease, also known as infectious tracheobronchitis or kennel cough, is considered 1 of the most common infectious diseases in dogs worldwide. 4 It is highly contagious and affects the larynx, trachea, bronchi, and nasal mucosa, causing acute respiratory signs, mainly cough, which usually are selflimiting. 4 Because of the contagious nature of CIRD, it is most prevalent in dense dog populations such as in kennels and rehoming centers. 4 Canine infectious respiratory disease has a multifactorial etiology, and several respiratory viruses as well as bacterial pathogens have been shown to contribute to the disease complex. [4] [5] [6] Canine parainfluenza virus (CPIV) and canine adenovirus type 2 (CAV-2) were first detected in dogs with contagious tracheobronchitis in the 1960s, and since then these viruses have been considered as the principal etiologic agents in CIRD. [7] [8] [9] [10] Canine herpes virus (CHV) also was detected in dogs with respiratory signs decades ago, but its role in CIRD remains controversial. [11] [12] [13] [14] In addition to these pathogens, novel respiratory viruses recently have been reported. Canine respiratory coronavirus (CRCoV) was first identified in 2003 in dogs at a rehoming center in the United Kingdom. 15 Subsequently, evidence of contagious tracheobronchitis caused by CRCoV has been reported worldwide. [16] [17] [18] [19] [20] Canine influenza virus (CIV) was first described in a racing Greyhound population in [27] [28] [29] [30] [31] [32] Although not considered part of the CIRD complex, canine distemper virus (CDV) is an important respiratory pathogen causing severe systemic disease characterized by a variety of clinical signs, including respiratory signs. 33 Viral-bacterial co-infections in the respiratory tract are well documented in humans with community-acquired pneumonia (CAP), and epidemiologic data as well as laboratory studies support the conclusion that respiratory viruses predispose to the development of secondary bacterial infections. 34, 35 Several mechanisms have been shown to contribute to this situation: Viruses destroy the respiratory epithelium and facilitate bacterial adhesion, viral infection up-regulates expression of molecules that bacteria utilize as receptors, and virus-induced immunosuppression promotes secondary bacterial infection. 35, 36 Recent advances in molecular diagnostic techniques have greatly increased understanding of the etiology of CAP in humans. Viruses affecting the lower respiratory tract currently are recognized as causative and complicating factors in up to 40% of humans with CAP. [37] [38] [39] Preceding or concurrent infections with CIRD viruses also have been considered as a possible etiological factor in dogs with BP, although there currently are only isolated reports suggesting this connection. 1, 40, 41 In dogs, BP most often is caused by opportunistic bacteria that belong to the normal oral flora. 42, 43 However, published studies on viral-bacterial co-infections in dogs almost exclusively describe dogs housed in dense populations, such as kennels or rehoming centers, that are infected with respiratory viruses and bacteria belonging to the CIRD complex. 6, 7, 44 Nevertheless, it is likely that respiratory viruses also are an important etiologic factor in dogs with BP caused by opportunistic bacteria in a similar manner as has been reported in humans.