28In recent years nidoviruses have emerged as an important respiratory pathogen of reptiles, 29 affecting especially captive python populations. In pythons, nidovirus infection induces an 30 inflammation of the upper respiratory and alimentary tract which can develop into a severe and 31 often fatal proliferative pneumonia. We observed pyogranulomatous and fibrinonecrotic lesions 32 in organ systems other than the respiratory tract during full post mortem examinations on 30 33 nidovirus RT-PCR positive pythons of varying species originating from Switzerland and Spain.
34The observations prompted us to study whether the atypical tissue tropism associates with 35 previously unknown nidoviruses or changes in the nidovirus genome. RT-PCR and inoculation 36 of Morelia viridis cell cultures served to recruit the cases and to obtain virus isolates. 37 Immunohistochemistry and immunofluorescence staining against nidovirus nucleoprotein 38 demonstrated that the virus not only infects a broad spectrum of epithelial (respiratory and 39 alimentary epithelium, hepatocytes, renal tubules, pancreatic ducts etc.), but also intravascular 40 monocytes, intralesional macrophages and endothelial cells. By next-generation sequencing we 41 obtained full length genome for a novel nidovirus species circulating in Switzerland. Analysis of 42 viral genomes recovered from pythons showing nidovirus infection-associated respiratory or 43 systemic disease did not explain the observed phenotypes. The results indicate that python 44 nidoviruses have a broad cell and tissue tropism, further suggesting that the course of infection 45 could vary and involve lesions in a broad spectrum of tissues and organ systems as a 46 consequence of monocyte-mediated systemic spread of the virus. 47 IMPORTANCE 49During the last years, python nidoviruses have become a primary cause of fatal disease in 50 pythons. Nidoviruses represent a threat to captive snake collections, as they spread rapidly and 51 can be associated with high morbidity and mortality. Our study indicates that, different from 52 previously evidence, the viruses do not only affect the respiratory tract, but can spread in the 53 entire body with blood monocytes, have a broad spectrum of target cells, and can induce a 54 variety of lesions. Nidovirales is an order of animal and human viruses that compromise 55 important zoonotic pathogens such as MERS-CoV and SARS-CoV, as well as the recently 56 emerged SARS-CoV-2. Python nidoviruses belong to the same subfamily as the mentioned 57 human viruses and show similar characteristics (rapid spread, respiratory and gastrointestinal 58 tropism, etc.). The present study confirms the relevance of natural animal diseases to better 59 understand the complexity of viruses of the order nidovirales.