Pathology and tissue distribution of West Nile virus in North American owls (family: Strigidae)

West Nile Virus–Induced Drop in Egg Production in Commercial Pekin Duck Breeders

A sudden drop in egg production in commercial poultry flocks can be economically devastating, and rapid identification of the cause often requires a combined effort between the producer, veterinarian, and pathologist. In September 2019, a 35-wk-old commercial Pekin breeder duck flock in Indiana suffered a drop in egg production from 1700 to 1000 eggs daily (41.2% drop). Again, in September 2021, three Pekin breeder duck flocks aged 32, 58, and 62 wk from the same company suffered a similar drop in egg production, with a mild increase in weekly mortality of 1.0% to 2.5%. In 2019 and in 2021, birds from affected flocks were submitted to the Veterinary Diagnostic Laboratory at Michigan State University for postmortem examination. Common gross examination findings included flaccid, shrunken, or atrophied ova (all hens), pododermatitis, airsacculitis, hepatomegaly, splenomegaly, ascites, and pallor of the left ventricle. Histopathologic examination of cerebrum, cerebellum, and brainstem revealed mild lymphocytic perivascular cuffing, vasculitis, and gliosis, suggesting viral encephalitis. In the heart, there was mild multifocal cardiomyocyte necrosis, mineralization, and infiltration by lymphocytes and macrophages. PCR for Newcastle disease virus, avian influenza virus, eastern equine encephalitis virus, and West Nile virus (WNV) was performed. Brain and heart samples were positive for WNV by PCR, and WNV antigen was detected in the cerebellum by immunohistochemistry. This is the first report to associate WNV infection with a drop in egg production in waterfowl, which are known to be important reservoir species for WNV and, as such, are generally asymptomatic.

Pathogenesis of Two Western Mediterranean West Nile Virus Lineage 1 Isolates in Experimentally Infected Red-Legged Partridges (Alectoris rufa)

West Nile virus (WNV) is the most widespread flavivirus in the world with a wide vertebrate host range. Its geographic expansion and activity continue to increase with important human and equine outbreaks and local bird mortality. In a previous experiment, we demonstrated the susceptibility of 7-week-old red-legged partridges (Alectoris rufa) to Mediterranean WNV isolates Morocco/2003 and Spain/2007, which varied in virulence for this gallinaceous species. Here we study the pathogenesis of the infection with these two strains to explain the different course of infection and mortality. Day six post-inoculation was critical in the course of infection, with the highest viral load in tissues, the most widespread virus antigen, and more severe lesions. The most affected organs were the heart, liver, and spleen. Comparing infections with Morocco/2003 and Spain/2007, differences were observed in the viral load, virus antigen distribution, and lesion nature and severity. A more acute and marked inflammatory reaction (characterized by participation of microglia and CD3+ T cells) as well as neuronal necrosis in the brain were observed in partridges infected with Morocco/2003 as compared to those infected with Spain/2007. This suggests a higher neurovirulence of Morocco/2003, probably related to one or more specific molecular determinants of virulence different from Spain/2007.

Emerging Diseases of Avian Wildlife

Climate change and the interaction with humans and domestic species influences disease in avian wildlife. This article provides updated information on emerging disease conditions such as the spread of an Asian tick, Haemaphysalis longicornis, and its associated diseases among migratory birds in the eastern United States; lymphoproliferative disease virus in wild turkeys in the United States; and salmonellosis, particularly among passerines, which has zoonotic potential. In addition, it includes updated information on West Nile virus, Wellfleet Bay virus, and avian influenza and is intended to serve as a complement to the current veterinary literature for veterinarians treating avian wildlife species.

The Role of Birds of Prey in West Nile Virus Epidemiology

Reported human cases of West Nile virus (WNV) in Europe increased dramatically in 2018. Lineage 1 strains had been circulating in Euro-Mediterranean countries since the early 1990s. The subsequent introduction of WNV lineage 2 has been responsible for the remarkable upsurge of European WNV outbreaks since 2004, including the dramatic increase in human cases observed since 2018. The virus exists in a natural cycle between mosquitoes and wild birds, with humans and horses acting as dead-end hosts. As the key vertebrate hosts in the transmission cycle of WNV, avian species have been the focus of surveillance across many countries. Raptors appear particularly susceptible to WNV infection, resulting in higher prevalence, and in some cases exhibiting neurological signs that lead to the death of the animal. In addition, birds of prey are known to play an important role as WNV reservoir and potentially amplifying hosts of infection. Importantly, raptor higher susceptibility/prevalence may indicate infection through predation of infected prey. Consequently, they are considered important target species when designing cost-effective surveillance for monitoring both seasonal WNV circulation in endemic countries and its emergence into new areas, where migrating raptors may play a critical role in virus introduction. This review summarizes the different aspects of the current knowledge of WNV infection in birds of prey and evaluates their role in the evolution of the epizootic that is spreading throughout Europe.

West Nile Virus: An Update on Pathobiology, Epidemiology, Diagnostics, Control and "One Health" Implications

West Nile virus (WNV) is an important zoonotic flavivirus responsible for mild fever to severe, lethal neuroinvasive disease in humans, horses, birds, and other wildlife species. Since its discovery, WNV has caused multiple human and animal disease outbreaks in all continents, except Antarctica. Infections are associated with economic losses, mainly due to the cost of treatment of infected patients, control programmes, and loss of animals and animal products. The pathogenesis of WNV has been extensively investigated in natural hosts as well as in several animal models, including rodents, lagomorphs, birds, and reptiles. However, most of the proposed pathogenesis hypotheses remain contentious, and much remains to be elucidated. At the same time, the unavailability of specific antiviral treatment or effective and safe vaccines contribute to the perpetuation of the disease and regular occurrence of outbreaks in both endemic and non-endemic areas. Moreover, globalisation and climate change are also important drivers of the emergence and re-emergence of the virus and disease. Here, we give an update of the pathobiology, epidemiology, diagnostics, control, and “One Health” implications of WNV infection and disease.

Current Progress of Avian Vaccines Against West Nile Virus

Birds are the main natural host of West Nile virus (WNV), the worldwide most distributed mosquito-borne flavivirus, but humans and equids can also be sporadic hosts. Many avian species have been reported as susceptible to WNV, particularly corvids. In the case that clinical disease develops in birds, this is due to virus invasion of different organs: liver, spleen, kidney, heart, and mainly the central nervous system, which can lead to death 24–48 h later. Nowadays, vaccines have only been licensed for use in equids; thus, the availability of avian vaccines would benefit bird populations, both domestic and wild ones. Such vaccines could be used in endangered species housed in rehabilitation and wildlife reserves, and in animals located at zoos and other recreational installations, but also in farm birds, and in those that are grown for hunting and restocking activities. Even more, controlling WNV infection in birds can also be useful to prevent its spread and limit outbreaks. So far, different commercial and experimental vaccines (inactivated, attenuated, and recombinant viruses, and subunits and DNA-based candidates) have been evaluated, with various regimens, both in domestic and wild avian species. However, there are still disadvantages that must be overcome before avian vaccination can be implemented, such as its cost-effectiveness for domestic birds since in many species the pathogenicity is low or zero, or the viability of being able to achieve collective immunity in wild birds in freedom. Here, a comprehensive review of what has been done until now in the field of avian vaccines against WNV is presented and discussed.

West Nile Virus Infection in Human and Mouse Cornea Tissue

The purpose of this study was to determine the in vitro and ex vivo susceptibility of human corneal cells to West Nile virus (WNV) infection and evaluate the ability of the virus to disseminate to the corneas of infected mice. Human corneal epithelial cells were challenged with WNV, incubated for 1-6 days, and tested for evidence of WNV infection. Viral RNA and antigen were detected at every time point, and the virus reached a peak titer of 2.5 × 10⁷ plaque-forming units (pfu)/mL at 3 days postinoculation (PI). Corneas procured from donors were incubated in culture dishes containing WNV for 1-5 days and tested for evidence of WNV. Viral RNA and antigen were detected, and the virus reached a mean peak titer of 4.9 × 10⁴ pfu/mL at 5 days PI. Mice were inoculated intraperitoneally with WNV, and their eyes were harvested at 2, 5, and 8 days PI and tested for evidence of WNV. Viral RNA was detected in corneas of four of nine systemically infected mice as early as 2 days PI. We conclude that human corneal cells support WNV replication in vitro and ex vivo, and WNV may disseminate into the corneas of experimentally infected mice. These findings indicate that corneal transmission cannot be ruled out as a novel mode of human-to-human WNV transmission and additional experiments should be conducted to assess this risk further.

OPHTHALMIC REFERENCE VALUES AND LESIONS IN TWO CAPTIVE POPULATIONS OF NORTHERN OWLS: GREAT GREY OWLS (STRIX NEBULOSA) AND SNOWY OWLS (BUBO SCANDIACUS)

This study established ophthalmic reference values and characterized ocular lesions in two captive populations of boreal owls, including 46 eyes of 23 great grey owls (Strix nebulosa) and 38 eyes from 19 snowy owls (Bubo scandiacus). A complete ophthalmologic exam was conducted, including neuro-ophthalmic reflexes, Schirmer tear test I (STT-I), intraocular pressure (IOP) using rebound tonometry, fluorescein staining, horizontal corneal measurements using Jameson calipers, direct and indirect ophthalmoscopy, and ocular ultrasound biometry. Eyes with an STT of <5 mm/min, outliers, and eyes with severe diseases were excluded from reference value analysis. No statistically significant differences were found between right or left eyes in either species or among individuals in different age groups and sexes. Mean intraocular pressures and Schirmer tear tests were also not statistically significantly different between great grey owls and snowy owls (IOP: 9.6 ± 2.6 mm Hg and 9.1 ± 1.9 mm Hg, respectively, and STT-I: 9.8 ± 2.8 mm/min and 9.8 ± 2.4 mm/min, respectively). However, snowy owls overall had a significantly larger eye than did great grey owls, reflected in corneal diameters (23.4 ± 1 vs. 20.0 ± 0.8 mm, respectively) and sonographic biometry. In both species, the most common ocular lesions included keratitis, cataracts, chorioretinal lesions, and abnormal pecten. Establishment of reference ocular parameters will help wildlife veterinarians and rehabilitators determine an appropriate treatment plan and will aid in correctly identifying the presence of ocular disease.

DNA vaccines encoding the envelope protein of West Nile virus lineages 1 or 2 administered intramuscularly, via electroporation and with recombinant virus protein induce partial protection in large falcons (Falco spp.)

As West Nile virus (WNV) can cause lethal diseases in raptors, a vaccination prophylaxis of free-living and captive populations is desirable. In the absence of vaccines approved for birds, equine vaccines have been used in falcons, but full protection against WNV infection was not achieved. Therefore, two DNA vaccines encoding the ectodomain of the envelope protein of WNV lineages 1 and 2, respectively, were evaluated in 28 large falcons. Four different vaccination protocols were used, including electroporation and booster-injections of recombinant WNV domain III protein, before challenge with the live WNV lineage 1 strain NY99. Drug safety, plasmid shedding and antibody production were monitored during the vaccination period. Serological, virological, histological, immunohistochemical and molecular biological investigations were performed during the challenge trials. Antibody response following vaccination was low overall and lasted for a maximum of three weeks. Plasmid shedding was not detected at any time. Viremia, mortality and levels, but not duration, of oral virus shedding were reduced in all of the groups during the challenge trial compared to the non-vaccinated control group. Likewise, clinical scoring, levels of cloacal virus shedding and viral load in organs were significantly reduced in three vaccination groups. Histopathological findings associated with WNV infections (meningo-encephalitis, myocarditis, and arteritis) were present in all groups, but immunohistochemical detection of the viral antigen was reduced. In conclusion, the vaccines can be used safely in falcons to reduce mortality and clinical signs and to lower the risk of virus transmission due to decreased levels of virus shedding and viremia, but full protection was not achieved in all groups.

Laboratory blood analysis in Strigiformes-Part I: hematologic reference intervals and agreement between manual blood cell counting techniques

BACKGROUND

While hematologic reference intervals (RI) are available for multiple raptorial species of the order Accipitriformes and Falconiformes, there is a lack of valuable hematologic information in Strigiformes that can be used for diagnostic and health monitoring purposes.

OBJECTIVES

The objective was to report RI in Strigiformes for hematologic variables and to assess agreement between manual cell counting techniques.

METHODS

A multi-center prospective study was designed to assess hematologic RI and blood cell morphology in owl species. Samples were collected from individuals representing 13 Strigiformes species, including Great Horned Owl, Snowy Owl, Eurasian Eagle Owl, Barred Owl, Great Gray Owl, Ural Owl, Northern Saw-Whet Owls, Northern Hawk Owl, Spectacled Owl, Barn Owl, Eastern Screech Owl, Long-Eared Owl, and Short-Eared Owl. Red blood cell count was determined manually using a hemocytometer. White blood cell count was determined using 3 manual counting techniques: (1) phloxine B technique, (2) Natt and Herrick technique, and (3) estimation from the smear. Differential counts and blood cell morphology were determined on smears. Reference intervals were determined and agreement between methods was calculated.

RESULTS

Important species-specific differences were observed in blood cell counts and granulocyte morphology. Differences in WBC count between species did not appear to be predictable based on phylogenetic relationships. Overall, most boreal owl species exhibited a lower WBC count than other species. Important disagreements were found between different manual WBC counting techniques.

CONCLUSIONS

Disagreements observed between manual counting techniques suggest that technique-specific RI should be used in Strigiformes.

Clinical, pathological, and immunohistochemical findings in bald eagles (Haliaeetus leucocephalus) and golden eagles (Aquila chrysaetos) naturally infected with West Nile virus

Fifteen bald eagles (Haliaeetus leucocephalus) and 3 golden eagles (Aquila chrysaetos) were diagnosed with West Nile disease based on 1) presence of lesions in brain, eyes, and heart, 2) viral antigen detection in brain, eyes, heart, kidney, and/or liver by immunohistochemical staining, 3) detection of viral RNA in tissue samples and/or cerebrospinal fluid (CSF) by polymerase chain reaction, and/or 4) detection of West Nile virus (WNV)-specific antibodies in CSF by serum neutralization assay. West Nile virus-associated gross lesions included cerebral pan-necrosis with hydrocephalus ex vacuo (7/15 bald eagles), fibrin exudation into the fundus in 1 golden eagle, retinal scarring in 1 bald eagle, and myocardial pallor and rounded heart apex in 4 bald eagles. Histologic lesions included lymphoplasmacytic encephalitis, most prominently in the cerebrum (17 eagles), lymphoplasmacytic pectenitis and choroiditis (15 and 8 eagles, respectively), and myocarditis (12 eagles). West Nile virus antigen was detected in the majority of the eagles in neurons of the brain (cerebrum and cerebellum), and less commonly present in neurons of the retina, tubular epithelial cells of the kidney, and cardiomyocytes. West Nile disease was diagnosed in 2 bald eagles based on the presence of cerebral pan-necrosis and WNV-specific antibodies in the CSF despite lacking viral antigen and RNA. In conclusion, WNV infection causes a fatal disease in bald and golden eagles. A variety of gross and histologic lesions are highly suggestive of WN disease in most eagles. A combination of detection of viral antigen and/or RNA or virus-specific antibodies proved useful in confirming the diagnosis.

Limited efficacy of West Nile virus vaccines in large falcons (Falco spp.)

West Nile virus (WNV) can lead to fatal diseases in raptor species. Unfortunately, there is no vaccine which has been designed specifically for use in breeding stocks of falcons. Therefore the immunogenicity and protective capacity of two commercially available WNV vaccines, both approved for use in horses, were evaluated in large falcons. One vaccine contained adjuvanted inactivated WNV lineage 1 immunogens, while the second represented a canarypox recombinant live virus vector vaccine. The efficacy of different vaccination regimes for these two vaccines was assessed serologically and by challenging the falcons with a WNV strain of homologous lineage 1. Our studies show that the recombinant vaccine conveys a slightly better protection than the inactivated vaccine, but moderate (recombinant vaccine) or weak (inactivated vaccine) side effects were observed at the injection sites. Using the recommended 2-dose regimen, both vaccines elicited only sub-optimal antibody responses and gave only partial protection following WNV challenge. Better results were obtained for both vaccines after a third dose, i.e. alleviation of clinical signs, absence of fatalities and reduction of virus shedding and viraemia. Therefore the consequences of WNV infections in falcons can be clearly alleviated by vaccination, especially if the amended triple administration scheme is used, although side effects at the vaccination site must be accepted.

Emerging and reemerging diseases of avian wildlife

Of the many important avian wildlife diseases, aspergillosis, West Nile virus, avipoxvirus, Wellfleet Bay virus, avian influenza, and inclusion body disease of cranes are covered in this article. Wellfleet Bay virus, first identified in 2010, is considered an emerging disease. Avian influenza and West Nile virus have recently been in the public eye because of their zoonotic potential and links to wildlife. Several diseases labeled as reemerging are included because of recent outbreaks or, more importantly, recent research in areas such as genomics, which shed light on the mechanisms whereby these adaptable, persistent pathogens continue to spread and thrive.

Pathology and tissue tropism of natural West Nile virus infection in birds: a review

West Nile virus (WNV) is a globally distributed arthropod-borne flavivirus capable of infecting a wide variety of vertebrates, with birds as its natural reservoir. Although it had been considered a pathogen of little importance for birds, from the 1990's, and especially after its introduction in the North American continent in 1999, thousands of birds have succumbed to West Nile infection. This review summarizes the pathogenesis and pathology of WNV infection in birds highlighting differences in lesion and antigen distribution and severity among bird orders and families. Despite significant species differences in susceptibility to infection, WNV associated lesions and viral antigen are present in the majority of organs of infected birds. The non-progressive, acute or more prolonged course of the disease accounts for part of the differences in lesion and viral antigen distribution and lesion severity. Most likely a combination of host variables and environmental factors in addition to the intrinsic virulence and pathogenicity of the infecting WNV strain influence the pathogenesis of the infection.

Pathology and immunohistochemical findings of west nile virus infection in psittaciformes

West Nile virus (WNV) infection was diagnosed in 38 psittacine birds based on histology, immunohistochemistry, and reverse transcriptase polymerase chain reaction (RT-PCR). Rosellas (Platycercus spp, n = 13), conures (Enicognathus, Aratinga, and Nandayus spp, n = 6), and lorikeets (Trichoglossus spp, n = 6) represented the most commonly affected species. Clinical signs ranged from lethargy, ruffled feathers, anorexia, and weight loss in most birds to sudden death in others. Except for mild to moderate enlargement of liver and spleen, there were no significant gross lesions at necropsy. Histopathologic findings included lymphoplasmacytic and histiocytic hepatitis, interstitial nephritis, myocarditis, splenitis, enteritis, pancreatitis, and occasionally, encephalitis. Viral antigen was detected by immunohistochemistry in 34 of 35 hearts (97.1%), 29 of 32 pancreata (90.6%), 33 of 37 kidneys (89.2%), 31 of 35 intestines (88.6%), 27 of 33 gizzards (81.8%), 8 of 10 ovaries (80%), 27 of 34 spleens (79.4%), 30 of 38 livers (78.9%), 23 of 32 lungs (71.9%), 21 of 31 proventriculi (67.7%), 14 of 21 adrenals (66.7%), 10 of 16 testes (62.5%), 17 of 30 brains (56.7%), 15 of 27 skins (55.5%), 3 of 6 oviducts (50%), 15 of 34 skeletal muscles (44.1%), 11 of 27 crop or esophagus (40.7%), and 1 of 6 thymuses (16.7%). Kidney was positive for WNV by RT-PCR in all the cases tested. In conclusion, Psittaciformes are susceptible to West Nile virus infection, and WNV infections are often associated with nonspecific clinical signs and widespread viral distribution in this order of birds.

Clinical evaluation and outcomes of naturally acquired West Nile virus infection in raptors

West Nile virus (WNV) infection and associated disease and mortality have been documented in numerous North American raptor species. Information regarding clinical presentations and long-term outcomes of WNV-infected raptors is important in the clinic for the diagnosis, treatment, and assessment of prognosis, as well as for understanding potential population level effects on raptor species. Raptors of 22 species admitted to a rehabilitation clinic were tested, from 2002 to 2005, for previous and acute WNV infection, while comparing clinical syndromes, trauma, and rehabilitation outcomes. Forty-two percent of admitted raptors (132/314) had been infected with WNV, and these presented with a WNV-attributed clinical disease rate of 67.4% (89/132). West Nile virus-infected raptors were less likely to be released (79/132 [59.8%]) than negative raptors (138/182 [75.8%]) and more likely to die or be euthanized (47/132 [35.6%] for WNV-infected vs. 32/182 [17.6%] for WNV-negative). However, WNV-infected raptors with neurologic disease were no less likely to be released (29/53 [54.7%]) than those without neurologic disease (50/79 [63.3%]). Clinical WNV-associated syndromes varied among species. Great horned owls (Bubo virginianus) were more likely to have neurologic signs, whereas American kestrels (Falco sparverius) and Swainson's hawks (Buteo swainsonii) were less likely to have neurologic signs. These results suggest that free-ranging raptors are frequently infected with WNV and that clinical syndromes differ among species. WNV has potentially devastating effects on raptors; however, rehabilitation of WNV-infected raptors can lead to positive outcomes, even for those having had severe neurologic disease.

Clinical findings, lesions, and viral antigen distribution in great gray owls (Strix nebulosa) and barred owls (Strix varia) with spontaneous West Nile virus infection

West Nile Virus (WNV) infection manifests itself clinically a nd pathologically differently in various species of birds. The clinicopathologic findings and WNV antigen tissue distribution of six great gray owls (Strix nebulosa) and two barred owls (Strix varia) with WNV infection are described in this report. Great gray owls usually live in northern Canada, whereas the phylogenetically related barred owls are native to the midwestern and eastern United States and southern Canada. Naturally acquired WNV infection caused death essentially without previous signs of disease in the six great gray owls during a mortality event. Lesions of WNV infection we re dominated by hepatic and splenic necrosis, with evidence o f disseminatedintravascular coagulation in the great gray owls. WNV antigen was widely distributed in th e organs of the great gray owls and appeared totarget endothelial cells, macrophages, and hepatocytes. The barred owls represented two sporadic cases. They had neurologic disease with mental dullness that led to euthanasia. These birds had mild to moderate lymphoplasmacytic encephalitis with glial nodules and lymphoplasmacytic pectenitis. WNV antigen was sparse in barred owls and only present in a few brain neurons and renaltubular epithelial cells. The cause of the different manifestations of WNV disease in these fairly closely related owl species is uncertain.