Novel Dermatitis and Relative Viral Nucleic Acid Tissue Loads in a Fin Whale (Balaenoptera physalus) with Systemic Cetacean Morbillivirus Infection

Summary

Cetacean morbilliviruses (CeMVs) are significant causes of mortality in many cetacean species in epizootics and smaller outbreaks. Despite the prominence of skin lesions in seals and terrestrial animals, including humans, affected by other morbilliviruses, they have not been reported in CeMV-infected cetaceans. Here we report CeMV-associated skin lesions in a fin whale (Balaenoptera physalus) with subacute, systemic CeMV infection that live-stranded in Scotland, UK. Grossly, the skin was sloughing in large sheets, presumed due to autolysis, but histological examination showed syncytia, below the dermoepidermal junction, that were strongly immunopositive for morbillivirus antigen, as were syncytia in other organs. By polymerase chain reaction (PCR), the relative load of CeMV-specific RNA was largest in the liver and urinary bladder, even in formalin-fixed, paraffin-wax embedded samples. Levels were low in skin and only detectable in frozen samples. Genetic comparison of the CeMV revealed close alignment with isolates from fin whales from the North Atlantic Ocean and Mediterranean Sea, but that it was distinct from the porpoise CeMV clade. These findings show skin samples can be used to diagnose CeMV infection in cetaceans, highlighting the potential of ante-mortem sampling for monitoring disease in current populations and assessment of changes in host and pathogen genetics.

Introduction

Cetacean morbillivirus (CeMV) is a distinct species made up of three well-characterized strains named after the species in which they were initially found (porpoise morbillivirus, dolphin morbillivirus [DMV] and pilot whale morbillivirus) plus one recovered subsequently from a Longman's beaked whale (Indopacetus pacificus), collectively called CeMV-1. Two more strains, recovered more recently from a Guiana dolphin (Sotalia guianensis) and two Indo-Pacific bottlenose dolphins (Tursiops aduncus), are collectively called CeMV-2 (Van Bressem et al, 2014). CeMV is an unsegmented, linear, negative-sense, single-stranded RNA virus belonging to the genus Morbillivirus, family Paramyxoviridae, order Mononegavirales, and has been responsible for epizootics in odontocetes and mysticetes (Van Bressem et al, 2014) including fin whales (Balaenoptera physalus) (Mazzariol et al, 2016).

Beffagna et al (2017) suggested that the lack of typical morbillivirus pathology on post-mortem examination in a subset of stranded, CeMV-1 infected striped (Stenella coeruleoalba) and bottlenose (Tursiops truncatus) dolphins (Casalone et al, 2014) may be due to genomic variations in the virus compared with the CeMV-1 originally detected in previous epizootics in the Mediterranean Sea, which affected mainly striped dolphins and long-finned pilot whales (Globicephala melas). They also suggested the genomic variations might have facilitated fin whale infections in the Mediterranean Sea, resulting in several cases in which CeMV-1-specific RNA was found in the brain, lung and spleen of a stranded neonate and the liver, spleen, lung, lymph nodes and skeletal muscle of older animals (Mazzariol et al, 2016). Previous reports of CeMV infection in fin whales have described lesions including non-suppurative encephalitis, bronchiolo-interstitial pneumonia, hepatitis, mild catarrhal enteritis and lymphoid depletion of the spleen and pulmonary and prescapular lymph nodes (Di Guardo et al, 2013; Mazzariol et al, 2016). Definitive CeMV-related skin lesions have not been reported in cetaceans nor have the relative amounts of CeMV-specific RNA present in different organs during systemic infection.