Pathological features of West Nile and Usutu virus natural infections in wild and domestic animals and in humans: A comparative review.

Mosquito-borne flaviviruses are emerging pathogens with zoonotic potential. Due to the recent climate and environmental changes, they are spreading across Europe, becoming a major threat for public and veterinary health. West Nile virus (WNV) and Usutu virus (USUV) are arboviruses that are responsible for multiple disease outbreaks in different species of birds, reptiles, and mammals, including humans. This review reports and compares the clinical signs as well as the gross and microscopic pathological features during natural infection with WNV and USUV in wild and domestic animals, as well as in humans. The main objective of this comparative review is to delineate the common features and the specific differences that characterize WNV- and USUV-induced diseases in each group of species and to highlight the main gaps in knowledge that could provide insight for further investigation on the pathogenesis and neurovirulence of these viruses.

The pathology of co-infection with Usutu virus and Plasmodium spp. in naturally infected Eurasian blackbirds (Turdus merula).

Usutu virus (USUV) is a mosquito-borne zoonotic flavivirus causing mortality in Eurasian blackbirds (Turdus merula) in Europe. In dead blackbirds, avian malaria co-infection due to mosquito-borne hemosporidians (e.g., Plasmodium spp.) has been reported. In humans, a similar co-infection of a flavivirus, Dengue virus, and Plasmodium spp. is causing increased severity of clinical disease. Currently, the effects of co-infection of arboviruses and hemosporidians in blackbirds remain unclear. This study investigates the rate of USUV and Plasmodium spp. co-infection in found-dead blackbirds (n = 203) from 2016 to 2020 in the Netherlands. Presence of Plasmodium spp. was evaluated by cytology (43/203; 21,2%), histopathology (94/186; 50,5%) and qPCR (179/203; 88,1%). The severity of histological lesions in USUV and Plasmodium spp. co-infected dead blackbirds (121/203; 59,6%) were compared with those in Plasmodium spp. single-infected cases. Additionally, since no knowledge is present on the infection rate on live birds and mosquitoes in the Netherlands, a small group of live blackbirds (n = 12) and selected in the field-collected mosquito pools (n = 96) in 2020 were tested for the presence of Plasmodium spp. The latter was detected in the tested live blackbirds by qPCR (8/10; 80%), and cytology (3/11; 27,3%) and in the mosquito pools by qPCR (18/96; 18,7%). For this study, co-infection between USUV and Plasmodium spp. was observed only in the dead blackbirds. The high Plasmodium spp. presence, associated with lower lesions score, in single infected found dead birds suggest a predominantly smaller pathogenic role as single agent. On the other hand, the higher histological lesion scores observed in USUV and Plasmodium spp. co-infected birds suggests a major pathogenic role for the virus or an increased severity of the lesions due to a possible interplay of the two agents.

SARS-CoV-2 Infection in Captive Hippos (Hippopotamus amphibius), Belgium.

Two adult female hippos in Zoo Antwerp who were naturally infected with SARS-CoV-2 showed nasal discharge for a few days. Virus was detected by immunocytochemistry and PCR in nasal swab samples and by PCR in faeces and pool water. Serology was also positive. No treatment was necessary.

West Nile Virus and Usutu Virus: A Post-Mortem Monitoring Study in Wild Birds from Rescue Centers, Central Italy.

West Nile virus (WNV) and Usutu virus (USUV) are mosquito-borne flaviviruses that have been associated with neurological diseases in humans and wild birds. Wild bird rescue centers are potential significant hot spots for avian infection surveillance, as recognized in the Italian Integrate National Surveillance Plan for Arboviruses. Here we report the results of a post-mortem active monitoring study conducted from November 2017 to October 2020 on animals hosted in five wild bird rescue centers of Central Italy. Five hundred seventy-six (n = 576) wild birds were tested by real-time polymerase chain reaction (RT-PCR) for the presence of WNV or USUV RNA fragments. No birds tested positive for USUV RNA (n = 0; 0.00%). Evidence of WNV RNA (Ct value = 34.36) was found in one bird (n = 1; 0.17%), an adult little grebe (Tachybaptus ruficollis subsp. ruficollis), that tested WNV positive in December 2019. This study highlights the strategic role of wildlife rescue centers in monitoring both the introduction and circulation of avian emerging zoonotic diseases. In addition, the presence of WNV during the cold season evidences the possible role of birds in overwintering mechanisms in the Italian territory and requires further investigations.

Neuronal satellitosis is a common finding in the avian brain.

Perineuronal or neuronal satellitosis is the term describing the presence of glial cells in the satellite space surrounding the neuronal perikaryon. Confusingly, this finding has been described both as a physiologic and pathologic condition in humans and animals. In animals, neuronal satellitosis has been described in mammals, as well as in avian species. For the latter, the authors wondered whether neuronal satellitosis is expressed in the normal telencephalon of different avian orders and families and whether this pattern in different species shows a specific brain-region association. For these aims, this study explored the presence of neuronal satellitosis in the major areas of the healthy telencephalon in wild and domestic avian species of different orders and families, evaluating its grade in different brain regions. Neuronal satellitosis was seen in the hyperpallium and mesopallium as areas with the highest grade. Passeriformes showed the highest grade of neuronal satellitosis compared to diurnal or nocturnal raptors, and Charadriiformes. To clarify the exact role of neuronal satellitosis in animals without neurological disease, further studies are needed.RESEARCH HIGHLIGHTSNeuronal satellitosis is a common finding in the healthy avian telencephalon.Neuronal satellitosis is a species- and brain-region-associated finding in birds.Passeriformes have the highest grade of neuronal satellitosis.

Pathology and Pathogenesis of Eurasian Blackbirds (Turdus merula) Naturally Infected with Usutu Virus.

The Usutu virus (USUV) is a mosquito-borne zoonotic flavivirus. Despite its continuous circulation in Europe, knowledge on the pathology, cellular and tissue tropism and pathogenetic potential of different circulating viral lineages is still fragmentary. Here, macroscopic and microscopic evaluations are performed in association with the study of cell and tissue tropism and comparison of lesion severity of two circulating virus lineages (Europe 3; Africa 3) in 160 Eurasian blackbirds (Turdus merula) in the Netherlands. Results confirm hepatosplenomegaly, coagulative necrosis and lymphoplasmacytic inflammation as major patterns of lesions and, for the first time, vasculitis as a novel virus-associated lesion. A USUV and Plasmodium spp. co-infection was commonly identified. The virus was associated with lesions by immunohistochemistry and was reported most commonly in endothelial cells and blood circulating and tissue mononucleated cells, suggesting them as a major route of entry and spread. A tropism for mononuclear phagocytes cells was further supported by viral labeling in multinucleated giant cells. The involvement of ganglionic neurons and epithelial cells of the gastrointestinal tract suggests a possible role of oral transmission, while the involvement of feather follicle shafts and bulbs suggests their use as a diagnostic sample for live bird testing. Finally, results suggest similar pathogenicity for the two circulating lineages.