Experimental infection of pigs with West Nile virus

Evidence of West Nile Virus Circulation in Horses and Dogs in Libya

West Nile virus (WNV) is a globally significant mosquito-borne Flavivirus that causes West Nile disease (WND). In Libya, evidence of WNV circulation has been reported in humans but never in animals. The aim of this study was to determine the seroprevalence of WNV infection in horses and dogs in Libya. In total, 574 and 63 serum samples were collected from apparently healthy, unvaccinated horses and dogs, respectively, between 2016 and 2019. A commercially available competitive enzyme-linked immunosorbent assay (c-ELISA) kit was initially used to test the collected samples for the presence of WNV Ig-G antibodies. Positive and doubtful sera were also tested using a more specific virus neutralisation assay to confirm whether the ELISA-positive results were due to WNV or other Flavivirus antibodies. The seroprevalence of WNV IgG antibodies according to ELISA was 13.2% out of 574 of total horses' samples and 30.2% out of 63 of total dogs' samples. The virus neutralisation test (VNT) confirmed that 10.8% (62/574) and 27% (17/63) were positive for WNV-neutralising titres ranging from 1:10 to 1:640. Univariable analysis using chi-square tests was conducted to measure the statistical significance of the association between the hypothesized risk factors including city, sex, breed, and age group and were then analyzed using the subsequent multivariable logistic regression model for horse samples. Age group was found to be the only significant risk factor in this study. The results of the present study provide new evidence about WNV circulation in Libya.

Serological evidence of West Nile viral infection in archived swine serum samples from Peninsular Malaysia

West Nile virus (WNV), a neurotropic arbovirus, has been detected in mosquitos, birds, wildlife, horses, and humans in Malaysia, but limited information is available on WNV infection in Malaysian pigs. We tested 80 archived swine serum samples for the presence of WNV antibody and West Nile (WN) viral RNA using ID Screen West Nile Competition Multi-species enzyme-linked immunosorbent assay kits and WNV-specific primers in reverse transcription polymerase chain reaction assays, respectively. A WNV seroprevalence of 62.5% (50/80) at 95% confidence interval (51.6%-72.3%) was recorded, with a significantly higher seroprevalence among young pigs (weaner and grower) and pigs from south Malaysia. One sample was positive for Japanese encephalitis virus antibodies; WN viral RNA was not detected in any of the serum samples.

West Nile Virus and Related Flavivirus in European Wild Boar (Sus scrofa), Latium Region, Italy: A Retrospective Study

BACKGROUND

A retrospective sero-survey for evidence of West Nile virus (WNV) infection in European wild boar (Sus scorfa) was conducted in the Latium region, Italy, on stored serum samples of the period November 2011 to January 2012.

METHODS

Sera were collected from 168 European wild boars and screened for antibodies to WNV and other Flaviviruses by competitive enzyme linked immunosorbent assay (cELISA). All sera positive for Flavivirus antibodies by cELISA were further examined by virus neutralization test (VNT). To test the presence of Flavivirus RNA in samples, an RT-PCR was performed using a pan-Flavivirus primers pair.

RESULTS

Thirteen wild boars (7.73%) were seropositive for Flaviviruses. The hemolysis of serum samples limited the interpretation of the VNT for 7 samples, confirming the presence of specific antibody against WNV in a single European wild boar serum sample. The presence of ELISA positive/VNT negative samples suggests the occurrence of non-neutralizing antibodies against WNV or other antigen-related Flaviviruses. No samples resulted positive for Flavivirus by RT-PCR assay.

CONCLUSION

Although a moderately high percentage of animals with specific antibody for WNV has been detected in wild boar in other surveillance studies in Europe, this has not been reported previously in Italy. Together, these data indicate that European wild boar are exposed to WNV and/or other related-Flavivirus in central Italy and confirm the usefulness of wild ungulates, as suitable Flavivirus sentinels.

Comparative Pathology of West Nile Virus in Humans and Non-Human Animals

West Nile virus (WNV) continues to be a major cause of human arboviral neuroinvasive disease. Susceptible non-human vertebrates are particularly diverse, ranging from commonly affected birds and horses to less commonly affected species such as alligators. This review summarizes the pathology caused by West Nile virus during natural infections of humans and non-human animals. While the most well-known findings in human infection involve the central nervous system, WNV can also cause significant lesions in the heart, kidneys and eyes. Time has also revealed chronic neurologic sequelae related to prior human WNV infection. Similarly, neurologic disease is a prominent manifestation of WNV infection in most non-human non-host animals. However, in some avian species, which serve as the vertebrate host for WNV maintenance in nature, severe systemic disease can occur, with neurologic, cardiac, intestinal and renal injury leading to death. The pathology seen in experimental animal models of West Nile virus infection and knowledge gains on viral pathogenesis derived from these animal models are also briefly discussed. A gap in the current literature exists regarding the relationship between the neurotropic nature of WNV in vertebrates, virus propagation and transmission in nature. This and other knowledge gaps, and future directions for research into WNV pathology, are addressed.

West Nile Virus: Veterinary Health and Vaccine Development

West Nile virus (WNV) (Flaviviridae: Flavivirus) was discovered in Africa more than 80 yr ago and became recognized as an avian pathogen and a cause of neurologic disease in horses largely during periodic incursions into Europe. Introduction of WNV into North America stimulated great anxiety, particularly in the equine industry, but also for pet owners and livestock producers concerned about the effect of WNV on other domestic animals. Numerous subsequent studies of naturally occurring and experimentally induced disease greatly expanded our understanding of the host range and clinical consequences of WNV infection in diverse species and led to rapid development and deployment of efficacious vaccines for horses. In addition to humans, horses are clearly the animals most frequently affected by serious, sometimes lethal disease following infection with WNV, but are dead-end hosts due to the low-magnitude viremia they develop. Dogs, cats, and livestock species including chickens are readily infected with WNV, but only occasionally develop clinical disease and are considered dead-end hosts for the virus.

Monocyte-Derived Dendritic Cells as Model to Evaluate Species Tropism of Mosquito-Borne Flaviviruses

Several mosquito-borne Flaviviruses such as Japanese encephalitis virus (JEV), West Nile virus (WNV), Dengue Virus (DENV), and Zika virus (ZIKV) can cause severe clinical disease. Being zoonotic, Flaviviruses infect a wide variety of terrestrial vertebrates, which dependent of the virus-host interactions, can enhance ongoing epidemics and maintain the virus in the environment for prolonged periods. Targeted species can vary from amphibians, birds to various mammals, dependent on the virus. For many mosquito-borne flaviviruses the spectrum of targeted species is incompletely understood, in particular with respect to their contribution to the maintenance of virus in certain geographical regions. Furthermore, little is known about virus and host factors contributing to species tropism. The present study utilized human and porcine monocyte-derived dendritic cells (MoDC) as a cell culture model to better understand Flavivirus species tropism and innate immune responses. MoDC were selected based on their presence in the skin and their role as an early target cell for several Flaviviruses and their role as immune sentinels. While differences in viral infectivity and replication were minor when comparing porcine with human MoDC for some of the tested Flaviviruses, a particularly strong replication in human MoDC was found with USUV, while JEV appeared to have a stronger tropism for porcine MoDC. With respect to innate immune responses we found high induction of TNF and IFN-β in both human and porcine MoDC after infection with JEV, WNV, and USUV, but not with DENV, ZIKV, and Wesselsbron virus. Spondweni virus induced these cytokine responses only in porcine MoDC. Overall, innate immune responses correlated with early infectivity and cytokine production. In conclusion, we demonstrate Flavivirus-dependent differences in the interaction with MoDC. These may play a role in pathogenesis but appear to only partially reflect the expected species tropism.

Serological Evidence of Mosquito-Borne Flaviviruses Circulation in Hunting Dogs in Campania Region, Italy

A Flavivirus survey on 183 hunting dogs was conducted in Campania region, Southern Italy. The seroprevalence value of 40.43% (74/183, 95% confidence intervals [CIs] 33.37-47.49) detected in our study using a competitive enzyme-linked immunosorbent serologic assay (cELISA) proves a considerable level of Flavivirus exposition of these animals. Among the 74 cELISA-positive sera, seroneutralization (SN) test showed that 24 sera resulted positive for Usutu virus with an overall prevalence of 13.11% (24/183) (95% CI 8.27-17.95), but none of cELISA-positive samples resulted positive for West Nile virus. Data analysis showed a significant difference of cELISA seropositivity risk factors in case of presence of farm animals in contact with hunting dogs and for dogs living in a rural environment but not for gender, age, management, hunting season, and hunting abroad. A RT-PCR assay was performed to detect the Flavivirus RNA, but none of the blood samples tested positive. This study documents the first report regarding the circulation of Flavivirus in hunting dog in Southern Italy and suggests the dog as an interesting target to monitor Flavivirus circulation.

Assessment of listing and categorisation of animal diseases within the framework of the Animal Health Law (Regulation (EU) No 2016/429): West Nile fever

West Nile fever (WNF) has been assessed according to the criteria of the Animal Health Law (AHL), in particular criteria of Article 7 on disease profile and impacts, Article 5 on the eligibility of WNF to be listed, Article 9 for the categorisation of WNF according to disease prevention and control rules as in Annex IV and Article 8 on the list of animal species related to WNF. The assessment has been performed following a methodology composed of information collection and compilation, expert judgement on each criterion at individual and, if no consensus was reached before, also at collective level. The output is composed of the categorical answer, and for the questions where no consensus was reached, the different supporting views are reported. Details on the methodology used for this assessment are explained in a separate opinion. According to the assessment performed, WNF can be considered eligible to be listed for Union intervention as laid down in Article 5(3) of the AHL. The disease would comply with the criteria as in Sections 2 and 5 of Annex IV of the AHL, for the application of the disease prevention and control rules referred to in points (b) and (e) of Article 9(1). The animal species to be listed for WNF according to Article 8(3) criteria are several orders of birds and mammals as susceptible species and several families of birds as reservoir. Different mosquito species can serve as vectors.

Animal Models of Human Viral Diseases

As the threat of exposure to emerging and reemerging viruses within a naive population increases, it is vital that the basic mechanisms of pathogenesis and immune response be thoroughly investigated. By using animal models in this endeavor, the response to viruses can be studied in a more natural context to identify novel drug targets, and assess the efficacy and safety of new products. This is especially true in the advent of the Food and Drug Administration's animal rule. Although no one animal model is able to recapitulate all the aspects of human disease, understanding the current limitations allows for a more targeted experimental design. Important facets to be considered before an animal study are the route of challenge, species of animals, biomarkers of disease, and a humane endpoint. This chapter covers the current animal models for medically important human viruses, and demonstrates where the gaps in knowledge exist.

Animal viral diseases and global change: bluetongue and West Nile fever as paradigms

Environmental changes have an undoubted influence on the appearance, distribution, and evolution of infectious diseases, and notably on those transmitted by vectors. Global change refers to environmental changes arising from human activities affecting the fundamental mechanisms operating in the biosphere. This paper discusses the changes observed in recent times with regard to some important arboviral (arthropod-borne viral) diseases of animals, and the role global change could have played in these variations. Two of the most important arboviral diseases of animals, bluetongue (BT) and West Nile fever/encephalitis (WNF), have been selected as models. In both cases, in the last 15 years an important leap forward has been observed, which has lead to considering them emerging diseases in different parts of the world. BT, affecting domestic ruminants, has recently afflicted livestock in Europe in an unprecedented epizootic, causing enormous economic losses. WNF affects wildlife (birds), domestic animals (equines), and humans, thus, beyond the economic consequences of its occurrence, as a zoonotic disease, it poses an important public health threat. West Nile virus (WNV) has expanded in the last 12 years worldwide, and particularly in the Americas, where it first occurred in 1999, extending throughout the Americas relentlessly since then, causing a severe epidemic of disastrous consequences for public health, wildlife, and livestock. In Europe, WNV is known long time ago, but it is since the last years of the twentieth century that its incidence has risen substantially. Circumstances such as global warming, changes in land use and water management, increase in travel, trade of animals, and others, can have an important influence in the observed changes in both diseases. The following question is raised: What is the contribution of global changes to the current increase of these diseases in the world?

Do wild ungulates allow improved monitoring of flavivirus circulation in Spain?

As a response to the need for improved and cost-efficient West Nile virus (WNV) and other flavivirus surveillance tools, we tested 887 juvenile free-living red deer, 742 free-living juvenile wild boar, and 327 farmed deer, to detect temporal variability in exposure to these viruses. Thirty of 742 wild boar samples (4%; 95% CI 2.8,5.7) yielded a positive ELISA result. Antibody-positive individuals had been sampled between 2003 and 2011 in localities from central and southern Spain. No wild boar from the northern half of Spain (n=120) tested positive. Regarding juvenile wild red deer, only two out of 887 samples yielded a positive ELISA result (0.2%; 95% CI 0.1,0.8). These two samples came from the same site and sampling year. The likelihood of detecting contact with WNV or cross-reacting flaviviruses was 18 times higher among juvenile wild boar than among juvenile red deer. ELISA positivity among farmed deer increased 10-fold after local flavivirus outbreaks recorded in the summer and autumn of 2010. This survey demonstrated the potential usefulness of juvenile wild ungulates, particularly wild boar, as suitable flavivirus sentinels in southwestern Europe, and that systematic serum banking of samples from hunter-harvested wildlife or from individual farmed ungulates provides valuable material for retrospective epidemiological surveys and future disease monitoring.

Porcine teschovirus polioencephalomyelitis in western Canada

Beginning in 2002, a small number of pig farms in western Canada began reporting 4-7-week-old pigs with bilateral hind-end paresis or paralysis. Low numbers of pigs were affected, some died, most had to be euthanized, and those that survived had reduced weight gains and neurological deficits. Necropsies revealed no gross lesions, but microscopic lesions consisted of a nonsuppurative polioencephalomyelitis, most severe in the brain stem and spinal cord. The lesions were most consistent with a viral infection. Tests for circovirus, Porcine reproductive and respiratory syndrome virus, coronavirus, Rabies virus, and Pseudorabies virus were negative. Using immunohistochemistry, virus neutralization, fluorescent antibody test, and nested reverse transcription polymerase chain reaction, Porcine teschovirus was identified in tissues from affected individuals. To the authors' knowledge, this is the first report of teschovirus encephalitis in western Canada and the first reported case of polioencephalomyelitis in pigs in Canada, where teschovirus was confirmed as the cause.

Flavivirus encephalitis: pathological aspects of mouse and other animal models

Encephalitic flaviviruses are important arthropod-borne pathogens of humans and other animals. In particular, the recent emergence of the West Nile virus (WNV) and Japanese encephalitis virus (JEV) in new geographic areas has caused a considerable public health alert and international concern. Among the experimental in vivo models of WNV and JEV infection, mice and other laboratory rodents are the most thoroughly studied and well-characterized systems, having provided data that are important for understanding the infectious process in humans. Macaca monkeys have also been used as a model for WNV and JEV infection, mainly for the evaluation of vaccine efficacy, although a limited number of published studies have addressed pathomorphology. These animal models demonstrate the development of encephalitis with many similarities to the human disease; however, the histological events that occur during infection, especially in peripheral tissues, have not been fully characterized.

Fox squirrels (Sciurus niger) develop West Nile virus viremias sufficient for infecting select mosquito species

The West Nile virus (WNV) viremia and shedding profiles of 11 adult fox squirrels (Sciurus niger) infected by needle inoculation or mosquito bite were characterized. Daily mean titers (95% confidence intervals) for all squirrels on days 1 through 6 postexposure (p.e.) were: 10(1.7 (1.32.1)), 10(4.4 (4.04.8)), 10(5.3 (5.05.6)), 10(4.4 (3.94.9)), 10(2.7 (2.03.4)), and 10(1.1 (0.52.1)) plaque-forming units (PFU)/mL. The highest WNV serum titers of individual squirrels infected by needle inoculation or mosquito bite ranged from 10(4.5) to 10(6.1) and from 10(5.1) to 10(5.3) PFU/mL, respectively. Nine (82%) squirrels, including all 4 squirrels infected by mosquito bite, had WNV serum titers > or =10(5.1) PFU/mL that persisted on average for 1.6 +/- 0.3 days. Infection and dissemination rates of Culex pipiens (L.) that fed on squirrels with serum titers of 10(4.4 +/- 0.1) PFU/mL were 56% and 13%, respectively. Both of these rates increased to over 80% when fed on squirrels with a mean WNV titer of 10(5.5 +/- 0.1) PFU/mL. Infection and dissemination also occurred in Aedes triseriatus (Say) but at a much lower rate. WNV was isolated from the oral and rectal cavities of all squirrels and from urine that was opportunistically collected from 5 squirrels. The largest quantity of WNV recovered from swabs of the oral cavity and urine was 10(3.1) PFU. The longest periods after exposure that WNV was isolated from the oral cavity and urine from a squirrel were 22 and 17 days p.e., respectively. WNV RNA was also detected in kidney tissue in 1 squirrel 29 days p.e., suggesting that fox squirrels can be persistently infected. Collectively these observations provide further evidence that squirrels can contribute to the natural history and epidemiology of WNV, especially in peridomestic environments.

Transmission dynamics and changing epidemiology of West Nile virus

West Nile virus (WNV) is a flavivirus that is maintained in a bird-mosquito transmission cycle. Humans, horses and other non-avian vertebrates are usually incidental hosts, but evidence is accumulating that this might not always be the case. Historically, WNV has been associated with asymptomatic infections and sporadic disease outbreaks in humans and horses in Africa, Europe, Asia and Australia. However, since 1994, the virus has caused frequent outbreaks of severe neuroinvasive disease in humans and horses in Europe and the Mediterranean Basin. In 1999, WNV underwent a dramatic expansion of its geographic range, and was reported for the first time in the Western Hemisphere during an outbreak of human and equine encephalitis in New York City. The outbreak was accompanied by extensive and unprecedented avian mortality. Since then, WNV has dispersed across the Western Hemisphere and is now found throughout the USA, Canada, Mexico and the Caribbean, and parts of Central and South America. WNV has been responsible for >27,000 human cases, >25,000 equine cases and hundreds of thousands of avian deaths in the USA but, surprisingly, there have been only sparse reports of WNV disease in vertebrates in the Caribbean and Latin America. This review summarizes our current understanding of WNV with particular emphasis on its transmission dynamics and changing epidemiology.

Experimental infections with West Nile virus

PURPOSE OF REVIEW

West Nile virus emerged recently in North America as a serious human and animal pathogen. This review summarizes the use of experimental infections with West Nile virus in diverse vertebrate species that have been used to answer fundamental questions about the host response, pathogenesis of West Nile virus infection and virus evolution.

RECENT FINDINGS

West Nile virus has an extremely broad vertebrate host range. Infection of common species of birds has defined those with high vs. low potential to serve as amplifying hosts for the virus. In general, mammals (primates, horses, companion animals) are dead-end hosts for West Nile virus, although some circumstances (i.e. immunosuppression) may allow individuals to become capable of transmitting the virus to mosquitoes. Some mammals (rodents, rabbits, squirrels) and reptiles (alligators) have been found to develop a viremia of sufficient magnitude to predict at least low competence for infecting feeding mosquitoes. Finally, experimental infection of rodents, horses and primates with West Nile virus has been integral to developing and evaluating the efficacy of West Nile virus vaccines.

SUMMARY

Experimental infection with West Nile virus has assisted in delineating those hosts important and not important to the transmission cycle, in understanding how the virus induces disease in susceptible hosts, and in validating the efficacy of vaccines used for control of disease.

Antibodies to West Nile Virus in Feral Swine from Florida, Georgia, and Texas, USA

West Nile virus (WNV) exposure has not yet been reported in feral swine (Sus scrofa) despite the broad geographic range and population density of this species. The objectives of this study were to determine the prevalence of antibodies to WNV in feral pigs, and to evaluate serologic diagnostics as applied to this species. Feral pig serum from three states was evaluated for antibodies to WNV. The overall WNV seroprevalence rate for 222 samples collected in 2001-2004 was 22.5%. Seroprevalence rates in Florida, Georgia, and Texas were 17.2%, 26.3%, and 20.5%, respectively. The results of this study demonstrate that feral pigs could represent useful mammalian sentinels of WNV.

Blood meal identification from mosquitoes collected at a commercial alligator farm

Outbreaks of West Nile virus on a Florida alligator farm prompted an investigation of which species of mosquitoes were feeding on the animals at the farm. Mosquitoes were collected on 4 separate overnight trips in September and October 2003 by using CO2-baited Centers for Disease Control light traps and wooden resting boxes that were placed inside or near the alligator housing pens. Mosquitoes were identified to species, bloodfed individuals were separated, and their abdomens were removed for DNA extraction. The DNA was tested to determine the vertebrate origin of the blood meal by polymerase chain reaction (PCR) amplification by using 4 primer sets specific to crocodilians, alligators, mammals, and birds. PCR products were sequenced to identify hosts. Of the 37 mosquito blood meals tested, 13 blood meals were positively identified to species, and 7 blood meals of those 13 were from Alligator mississippiensis, the American alligator. Alligator blood was found in Culex erraticus, Mansonia dyari, and Ma. titillans, and to our knowledge, this represents the first report of these mosquito species feeding on American alligators.