Co-infections with Respiratory Viruses in Dogs with Bacterial Pneumonia

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

Dogs with viral co-infections were significantly heavier than those without virus infection. This finding might be influenced by the structure of the virus-negative group: All 4 dogs with another predisposing factor for the development of BP were <20 kg (West Highland White Terrier, Dachshund, Spanish Water Dog, and Schnauzer). Dogs with viral co-infections also were younger than those without viral co-infection, although this did not reach statistical significance. This finding is not unexpected, because young animals might have insufficient acquired immunity against CIRD viruses. 2 Clinical findings, arterial blood gas analysis, and hematology, as well as respiratory sample cytology in both groups were in accordance with previously reported findings in BP and did not differ between virus-negative and virus-positive groups. 42, 43, 48 On thoracic radiographs, an alveolar pattern in the cranial and the middle lobes was predominant in both groups without group predisposition. Radiographic findings in dogs with BP have been thoroughly reported previously for cases of aspiration etiology. 55, 56 In our study, radiographic findings in dogs with BP caused by other etiologies were similar to those reported for aspiration pneumonia. Aspiration etiology was considered unlikely, because none of the dogs with BP had a history of vomiting, regurgitation, recent anesthesia or signs compatible with laryngeal paralysis. Our findings could indicate that an alveolar pattern in cranial and middle lung lobes may be typical for pneumonia, regardless of etiology. On the other hand, aspiration pneumonia might have played a role in some dogs but could not be confirmed or denied based on available history, examination findings, or imaging. We were unable to identify clinical variables to reliably distinguish dogs with BP and viral co-infection, and PCR testing therefore appears to be required to identify viral respiratory infections in dogs with BP. A similar finding was reported in humans. 57 There were no significant differences in the duration of hospitalization (P = .427) or partial pressures of arterial oxygen at presentation (P = .343) between BP dogs with and without viral co-infection, indicating that viral co-infections do not appear to cause a more severe course of BP. In dogs, limited information is available on the severity of BP of different etiological origins, and in humans the reports are contradictory. Some studies have shown that mixed infections with viruses and bacteria induce a more severe clinical disease, whereas others have been unable to demonstrate significant differences in disease severity. 49, [58] [59] [60] [61] Previous studies reporting microbiological findings in dogs with pneumonia have found growth of a single species of bacteria in 40-74% of cases. 42, 43 All of these studies used TTW as a sampling method. Factors that might have influenced the finding of primarily a single species of bacteria in our study may be the use of BAL as a sampling method in majority of cases, compared to previous studies where TTW was used and the widespread use of prior antimicrobial treatments in these dogs.