Evaluation of the efficacy provided by a Recombinant Canarypox-Vectored Equine West Nile Virus vaccine against an experimental West Nile Virus intrathecal challenge in horses

Efficacy of the Recombitek Equine West Nile Virus (WNV) vaccine was evaluated against a WNV intrathecal challenge model that results in WNV-induced clinical disease. Ten vaccinated (twice at days 0 and 35) and 10 control horses were challenged 2 weeks after administration of the second vaccine with a virulent WNV by intrathecal administration. After the challenge, eight of 10 controls developed clinical signs of encephalomyelitis whereas one vaccinate exhibited muscle fasciculation only once. Nine controls and one vaccinate developed a fever. Histopathology revealed mild to moderate nonsuppurative encephalitis in eight controls and one vaccinate. None of the vaccinates and all of the controls developed WNV viremia after challenge. All vaccinated horses developed antibodies to WNV after vaccination. These and results of previous studies demonstrate efficacy of the Recombitek WNV vaccine against WNV-induced clinical disease and natural challenge with WNV-infected mosquitoes.     

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.     

A West Nile virus (WNV) recombinant canarypox virus vaccine elicits WNV-specific neutralizing antibodies and cell-mediated immune responses in the horse

Successful vaccination against West Nile virus (WNV) requires induction of both neutralizing antibodies and cell-mediated immune responses. In this study, we have assessed the ability of a recombinant ALVAC-WNV vaccine (RECOMBITEK WNV) to elicit neutralizing antibodies and virus-specific cell-mediated immune responses in horses. In addition, we examined whether prior exposure to ALVAC-WNV vaccine would inhibit B and cell-mediated immune responses against the transgene product upon subsequent booster immunizations with the same vaccine. The results demonstrated that the recombinant ALVAC-WNV vaccine induced neutralizing antibodies and prM/E insert-specific IFN-gamma(+) producing cells against WNV in vaccinated horses. Prior exposure to ALVAC-WNV vaccine did not impair the ability of horses to respond to two subsequent booster injections with the same vaccine, although anti-vector-specific antibody and cell-mediated immune responses were induced in vaccinated horses. This report describes, for the first time, the induction of antigen-specific cell-mediated responses following vaccination with an ALVAC virus recombinant vaccine encoding WNV antigens. Moreover, we showed that both WNV-specific IFN-gamma producing cells and anti-WNV neutralizing antibody responses, are not inhibited by subsequent vaccinations with the same vector vaccine.     

Humoral response to West Nile virus vaccination in alpacas and llamas

OBJECTIVE: To determine humoral responses to an equine West Nile virus (WNV) vaccine in healthy alpacas and llamas and compare responses in alpacas and llamas with responses in horses. DESIGN: Clinical trial. ANIMALS: 28 alpacas, 56 llamas, and 16 horses. PROCEDURE: Horses received 2 vaccinations at 4-week intervals, and alpacas and llamas received 3 vaccinations at 3-week intervals. Fifty-five llamas received a fourth vaccination 3 weeks after the third. Blood samples were collected immediately prior to each vaccination, 3 weeks after the last vaccination for alpacas and llamas, and 4 weeks after the last vaccination for horses and tested for virus-neutralizing antibodies. Samples from 29 randomly selected vaccinated llamas were used. RESULTS: None of the animals developed any local or systemic adverse reactions. Four of 28 (14%) alpacas, 4 of 29 (14%) llamas, and 7 of 16 (44%) horses were seropositive 3 (llamas and alpacas) or 4 (horses) weeks after administration of the first vaccination; 27 of 28 (96%) alpacas, 26 of 29 (90%) llamas, and 15 of 16 (94%) horses were seropositive after administration of the second vaccination; and all 28 alpacas and 28 of 29 (97%) llamas were seropositive 3 weeks after administration of the third vaccination. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that vaccination with the equine WNV vaccine is safe in alpacas and llamas. Administration of 3 vaccinations generally resulted in virus-neutralizing antibody titers similar to those observed following 2 vaccinations in horses; however, because it is not known what antibody titer would be protective against clinical WNV disease in alpacas or llamas, we cannot conclude that the vaccine was efficacious.     

Immunologic responses to West Nile virus in vaccinated and clinically affected horses

OBJECTIVE: To compare neutralizing antibody response between horses vaccinated against West Nile virus (WNV) and horses that survived naturally occurring infection. DESIGN: Cross-sectional observational study. ANIMALS: 187 horses vaccinated with a killed WNV vaccine and 37 horses with confirmed clinical WNV infection. PROCEDURE: Serum was collected from vaccinated horses prior to and 4 to 6 weeks after completion of an initial vaccination series (2 doses) and 5 to 7 months later. Serum was collected from affected horses 4 to 6 weeks after laboratory diagnosis of infection and 5 to 7 months after the first sample was obtained. The IgM capture ELISA, plaque reduction neutralization test (PRNT), and microtiter virus neutralization test were used. RESULTS: All affected horses had PRNT titers > or = 1:100 at 4 to 6 weeks after onset of disease, and 90% (18/20) maintained this titer for 5 to 7 months. After the second vaccination, 67% of vaccinated horses had PRNT titers > or = 1:100 and 14% had titers < 1:10. Five to 7 months later, 33% (28/84) of vaccinated horses had PRNT titers > or = 1:100, whereas 29% (24/84) had titers < 1:10. Vaccinated and clinically affected horses' end point titers had decreased by 5 to 7 months after vaccination. CONCLUSIONS AND CLINICAL RELEVANCE: A portion of horses vaccinated against WNV may respond poorly. Vaccination every 6 months may be indicated in certain horses and in areas of high vector activity. Other preventative methods such as mosquito control are warranted to prevent WNV infection in horses.     

Detection of antibodies to West Nile virus in equine sera using microsphere immunoassay.

One hundred and ninety-one sera from horses that recently were exposed to West Nile virus (WNV) by either vaccination or natural infection or that were not vaccinated and remained free of infection were used to evaluate fluorescent microsphere immunoassays (MIAs) incorporating recombinant WNV envelope protein (rE) and recombinant nonstructural proteins (rNS1, rNS3, and rNS5) for detection of equine antibodies to WNV. The rE MIA had a diagnostic sensitivity and specificity, respectively, of 99.3% and 97.4% for detection of WNV antibodies in the serum of horses that were recently vaccinated or naturally infected with WNV, as compared to the plaque reduction neutralization test (PRNT). The positive rE MIA results were assumed to be WNV-specific because of the close agreement between this assay and the PRNT and the fact that unvaccinated control horses included in this study were confirmed to be free of exposure to the related St Louis encephalitis virus. The NS protein-based MIA were all less sensitive than either the rE MIA or PRNT (sensitivity 0-48.0), although the rNSI MIA distinguished horses vaccinated with the recombinant WNV vaccine from those that were immunized with the inactivated WNV vaccine (P < 0.0001) or naturally infected with WNV (P < 0.0001). The rE MIA would appear to provide a rapid, convenient, inexpensive, and accurate test for the screening of equine sera for the presence of antibodies to WNV.     

Efficacy, duration, and onset of immunogenicity of a West Nile virus vaccine, live Flavivirus chimera, in horses with a clinical disease challenge model

REASON FOR PERFORMING STUDY: West Nile virus (WNF) is a Flavivirus responsible for a life-threatening neurological disease in man and horses. Development of improved vaccines against Flavivirus infections is therefore important. OBJECTIVES: To establish that a single immunogenicity dose of live Flavivirus chimera (WN-FV) vaccine protects horses from the disease and it induces a protective immune response, and to determine the duration of the protective immunity. METHODS: Clinical signs were compared between vaccinated (VACC) and control (CTRL) horses after an intrathecal WNV challenge given at 10 or 28 days, or 12 months post vaccination. RESULTS: Challenge of horses in the immunogenicity study at Day 28 post vaccination resulted in severe clinical signs of WNV infection in 10/10 control (CTRL) compared to 1/20 vaccinated (VACC) horses (P<0.01). None of the VACC horses developed viraemia and minimal histopathology was noted. Duration of immunity (DPI) was established at 12 months post vaccination. Eight of 10 CTRL exhibited severe clinical signs of infection compared to 1 of 9 VACC horses (P<0.05). There was a significant reduction in the occurrence of viraemia and histopathology lesion in VACC horses relative to CTRL horses. Horses challenged at Day 10 post vaccination experienced moderate or severe clinical signs of WNV infection in 3/3 CTRL compared to 5/6 VACC horses (P<0.05). CONCLUSIONS: This novel WN-FV chimera vaccine generates a protective immune response to WNV infection in horses that is demonstrated 10 days after a single vaccination and lasts for up to one year. POTENTIAL RELEVANCE: This is the first USDA licensed equine WNV vaccine to utilise a severe challenge model that produces the same WNV disease observed under field conditions to obtain a label claim for prevention of viraemia and aid in the prevention of WNV disease and encephalitis with a duration of immunity of 12 months.     

Incidence and effects of West Nile virus infection in vaccinated and unvaccinated horses in California

A prospective cohort study was used to estimate the incidence of West Nile virus (WNV) infection in a group of unvaccinated horses (n = 37) in California and compare the effects of natural WNV infection in these unvaccinated horses to a group of co-mingled vaccinated horses (n = 155). Horses initially were vaccinated with either inactivated whole virus (n = 87) or canarypox recombinant (n = 68) WNV vaccines during 2003 or 2004, prior to emergence of WNV in the region. Unvaccinated horses were serologically tested for antibodies to WNV by microsphere immunoassay incorporating recombinant WNV E protein (rE MIA) in December 2003, December 2004, and every two months thereafter until November 2005. Clinical neurologic disease attributable to WNV infection (West Nile disease (WND)) developed in 2 (5.4%) of 37 unvaccinated horses and in 0 of 155 vaccinated horses. One affected horse died. Twenty one (67.7%) of 31 unvaccinated horses that were seronegative to WNV in December, 2004 seroconverted to WNV before the end of the study in November, 2005. Findings from the study indicate that currently-available commercial vaccines are effective in preventing WND and their use is financially justified because clinical disease only occurred in unvaccinated horses and the mean cost of each clinical case of WND was approximately 45 times the cost of a 2-dose WNV vaccination program.     

West Nile encephalitis.

WNV encephalitis in horses, previously reported in Africa, Asia, and Europe, occurred for the first time in the Western Hemisphere in 1999. The causative agent, WNV, is a flavivirus maintained in nature by a bird-mosquito cycle. The disease in horses is manifested primarily by ataxia of variable severity. Outbreaks of encephalitis may have a case fatality rate in excess of 40%, although this virus infection is inapparent in some horses. Early evidence indicates that WNV has overwintered in the northeastern United States and poses a threat for future disease occurrences in horses. No vaccine is available to protect against WNV infection in horses; disease control is predicated on mosquito abatement.     

Safety of an attenuated West Nile virus vaccine, live Flavivirus chimera in horses

REASON FOR PERFORMING STUDY: West Nile virus (WNV) infection is endemic and able to cause disease in naive hosts. It is necessary therefore to evaluate the safety of new vaccines. OBJECTIVES: To establish: 1) the safety of a modified live Flavivirus/West Nile virus (WN-FV) chimera by administration of an overdose and testing for shed of vaccine virus and spread to uninoculated sentinel horses; 2) that this vaccine did not become pathogenic once passaged in horses; and 3) vaccine safety under field conditions. METHODS: There were 3 protocols: 1) In the overdose/shed and spread study, horses were vaccinated with a 100x immunogenicity overdose of WN-FV chimera vaccine and housed with sentinel horses. 2) A reversion to virulence study, where horses were vaccinated with a 20x immunogenicity overdose of WN-FV chimera vaccine. Horses in both studies were evaluated for abnormal health conditions and samples obtained to detect virus, seroconversion and dissemination into tissues. 3) In a field safety test 919 healthy horses of various ages, breeds and sex were used. RESULTS: Vaccination did not result in site or systemic reactions in either experimental or field-injected horses. There was no shed of vaccine virus, no detection of vaccine virus into tissue and no reversion to virulence with passage. CONCLUSIONS: WN-FV chimera vaccine is safe to use in horses with no evidence of ill effects from very high doses of vaccine. There was no evidence of reversion to virulence. In addition, administration of this vaccine to several hundred horses that may have been previously exposed to WNV or WNV vaccine resulted in no untoward reactions. POTENTIAL RELEVANCE: These studies establish that this live attenuated Flavivirus chimera is safe to use for immunoprophylaxis against WNV disease in horses.     

Investigation of antigen specific lymphocyte responses in healthy horses vaccinated with an inactivated West Nile virus vaccine

West Nile virus (WNV) is a single-stranded, enveloped RNA virus capable of causing encephalitic disease in horses. Unvaccinated horses are at risk for developing WNV disease in endemic geographic regions. Effective vaccination reduces disease frequency and diminishes disease severity in vaccinated individuals that become infected with WNV. Recent data indicate CD4+ lymphocytes are required for effective protection against disease; in particular, cross talk between CD4+ and CD8+ lymphocytes must be functional. The objective of this project was to investigate immune responses in horses throughout a series of three vaccinations using a commercial inactivated vaccine under natural conditions. Immune responses to vaccination were determined by neutralizing antibody titers with plaque reduction neutralization test (PRNT), IgM titer (capture ELISA), WNV specific antibody Ig subclass responses, WNV lymphocyte proliferative responses and intracellular cytokine expression. Horses were vaccinated with a series of three vaccines at 3-week intervals using an inactivated product. An initial measure of immune activation following vaccination was determined by evaluating changes in lymphocyte cytokine expression. Interferon (IFN) gamma and interleukin (IL)-4 expressing CD4+ lymphocytes significantly increased 14 days following initial vaccination compared to unvaccinated horses (P<0.05). IFN-gamma expressing CD8+ lymphocytes also increased and remained elevated for 110 days. Antigen specific lymphocyte proliferative responses were significantly increased up to 90 days following the third vaccination (P<0.05). As expected, vaccinated horses produced increased neutralizing antibody based on PRNT data and WNV antigen-specific Ig subclass responses compared with unvaccinated horses (P<0.05). Our data indicate that WNV vaccination with an inactivated product effectively induced an antigen-specific antibody responses, as well as CD4+ and CD8+ lymphocyte activation.     

Use of DNA and recombinant canarypox viral (ALVAC) vectors for equine herpes virus vaccination

In this study, experimental canarypox virus (ALVAC) and plasmid DNA recombinant vaccines expressing the gB, gC and gD glycoproteins of EHV-1 were assessed for their ability to protect conventional ponies against a respiratory challenge with EHV-1. In addition, potential means of enhancing serological responses in horses to ALVAC and DNA vaccination were explored. These included co-administration of the antigen with conventional adjuvants, complexation with DMRIE-DOPE and co-expression of the antigen along with equine GM-CSF. Groups of EHV primed ponies were vaccinated twice intra-muscularly with one dose of the appropriate test vaccine at an interval of 5 weeks. Two to 3 weeks after the second vaccination, ponies were infected intra-nasally with the virulent Ab4 strain of EHV-1 after which they were observed clinically and sampled for virological investigations. The results demonstrated that DNA and ALVAC vaccination markedly reduced virus excretion after challenge in terms of duration and magnitude, but failed to protect against cell-associated viremia. Noteworthy was the almost complete absence of virus excretion in the group of ponies vaccinated with ALVAC-EHV in the presence of Carbopol adjuvant or DNA plasmid formulated with aluminium phosphate. The administration of the DNA vaccine in the presence of GM-CSF and formulated in DMRIE-DOPE and of the ALVAC vaccine in the presence of Carbopol adjuvant significantly improved virus neutralising antibody responses to EHV-1. These findings indicate that DNA and ALVAC vaccination is a promising approach for the immunological control of EHV-1 infection, but that more research is needed to identify the immunodominant protective antigens of EHV-1 and their interaction with the equine immune system.     

An African horse sickness virus serotype 4 recombinant canarypox virus vaccine elicits specific cell-mediated immune responses in horses

A recombinant canarypox virus vectored vaccine co-expressing synthetic genes encoding outer capsid proteins, VP2 and VP5, of African horse sickness virus (AHSV) serotype 4 (ALVAC(?)-AHSV4) has been demonstrated to fully protect horses against homologous challenge with virulent field virus. Guthrie et al. (2009) detected weak and variable titres of neutralizing antibody (ranging from <10 to 40) 8 weeks after vaccination leading us to hypothesize that there could be a participation of cell mediated immunity (CMI) in protection against AHSV4. The present study aimed at characterizing the CMI induced by the experimental ALVAC(?)-AHSV4 vaccine. Six horses received two vaccinations twenty-eight days apart and three horses remained unvaccinated. The detection of VP2/VP5 specific IFN-γ responses was assessed by enzyme linked immune spot (ELISpot) assay and clearly demonstrated that all ALVAC(?)-AHSV4 vaccinated horses developed significant IFN-γ production compared to unvaccinated horses. More detailed immune responses obtained by flow cytometry demonstrated that ALVAC(?)-AHSV4 vaccinations induced immune cells, mainly CD8(+) T cells, able to recognize multiple T-epitopes through all VP2 and only the N-terminus sequence of VP5. Neither VP2 nor VP5 specific IFN-γ responses were detected in unvaccinated horses. Overall, our data demonstrated that an experimental recombinant canarypox based vaccine induced significant CMI specific for both VP2 and VP5 proteins of AHSV4.     

The anamnestic serologic response to vaccination with a canarypox virus-vectored recombinant West Nile virus (WNV) vaccine in horses previously vaccinated with an inactivated WNV vaccine

A new recombinant West Nile virus (WNV) vaccine has been licensed for use in horses. Prior to the availability of the recombinant vaccine in 2004, the only equine WNV vaccine available on the market had been an inactivated vaccine. Since the recombinant vaccine only expresses selected viral genes, the question could be posed as to whether a single dose of the recombinant vaccine would be effective in producing an anamnestic serologic response in horses previously vaccinated with an inactivated WNV vaccine. In this study we demonstrate that vaccination of horses with a canarypox-vectored recombinant vaccine, under field conditions, results in a marked anamnestic response in horses previously vaccinated with an inactivated WNV vaccine.     

Investigation of an outbreak of encephalomyelitis caused by West Nile virus in 136 horses

OBJECTIVE: To describe an outbreak of encephalomyelitis caused by West Nile virus (WNV) in horses in northern Indiana. DESIGN: Case series. ANIMALS: 170 horses. PROCEDURES: Horses with clinical signs suggestive of encephalomyelitis caused by WNV were examined. Date, age, sex, breed, and survival status were recorded. Serum samples were tested for anti-WNV antibodies, and virus isolation was attempted from samples of brain tissue. Climate data from local weather recording stations were collected. An epidemic curve was constructed, and case fatality rate was calculated. RESULTS: The most common clinical signs were ataxia, hind limb paresis, and muscle tremors and fasciculations. Eight horses had been vaccinated against WNV from 2 to 21 days prior to the appearance of clinical signs. West Nile virus was isolated from brain tissue of 2 nonvaccinated horses, and anti-WNV IgM antibodies were detected in 132 nonvaccinated horses; in 2 other nonvaccinated horses, anti-WNV antibodies were detected and WNV was also isolated from brain tissue. Thirty-one (22.8%) horses died or were euthanatized. The peak of the outbreak occurred on September 6, 2002. Ambient temperatures were significantly lower after the peak of the outbreak, compared with prior to the peak. CONCLUSIONS AND CLINICAL RELEVANCE: The peak risk period for encephalomyelitis caused by WNV in northern Indiana was mid-August to mid-September. Reduction in cases coincided with decreasing ambient temperatures. Because of a substantial case fatality rate, owners of horses in northern Indiana should have their horses fully protected by vaccination against WNV before June. In other regions of the United States with a defined mosquito breeding season, vaccination of previously nonvaccinated horses should commence at least 4 months before the anticipated peak in seasonal mosquito numbers, and for previously vaccinated horses, vaccine should be administered no later than 2 months before this time.     

West Nile virus infection of Thoroughbred horses in South Africa (2000-2001)

REASONS FOR PERFORMING STUDY: West Nile virus (WNV) infection is endemic in southern Africa. With the recent emergence of WNV infection of horses in Europe and the USA the present study was performed to estimate the risk of seroconversion to WNV in a cohort of 488 young Thoroughbred (TB) horses. OBJECTIVES: To estimate the risk of seroconversion to WNV among a cohort of South African TB yearlings sold at the 2001 National Yearling Sales (NYS) and to determine whether the risk varied geographically. Two horses were also infected with a recent South African isolate of WNV to evaluate its virulence in horses. METHODS: Serum samples were collected from the cohort of 488 TB yearlings at the 2001 NYS. Serum samples that were collected from the same horses at the time that they were identified were sourced from our serum bank. Sera from 243 of the dams that were collected at the time that the foals were identified were also sourced from our serum bank. These sera were subjected to serum neutralisation (SN) tests for antibody to WNV. RESULTS: Approximately 11% of yearlings seroconverted to WNV on paired serum samples collected from each animal approximately 12 months apart. Studfarms with WNV-seropositive yearlings were widely distributed throughout South Africa and SN tests on sera from their dams indicated that exposure to WNV was even more prevalent (75%) in this population. Neurological disease was not described in any of the horses included in this study and 2 horses inoculated with a recent lineage 2 South African isolate of WNV showed no clinical signs of disease after infection and virus was not detected in their blood. CONCLUSIONS: Infection of horses with WNV is common in South Africa, but infection is not associated with neurological disease. POTENTIAL RELEVANCE: In contrast to recent reports from Europe, North Africa, Asia and North America, the results of our field and experimental studies indicated that exposure of horses to the endemic southern African strains of WNV was not associated with neurological disease.     

Pathogenesis of West Nile virus lineage 1 and 2 in experimentally infected large falcons

West Nile virus (WNV) is a zoonotic flavivirus that is transmitted by blood-suckling mosquitoes with birds serving as the primary vertebrate reservoir hosts (enzootic cycle). Some bird species like ravens, raptors and jays are highly susceptible and develop deadly encephalitis while others are infected subclinically only. Birds of prey are highly susceptible and show substantial mortality rates following infection. To investigate the WNV pathogenesis in falcons we inoculated twelve large falcons, 6 birds per group, subcutaneously with viruses belonging to two different lineages (lineage 1 strain NY 99 and lineage 2 strain Austria). Three different infection doses were utilized: low (approx. 500 TCID50), intermediate (approx. 4 log10 TCID50) and high (approx. 6 log10 TCID50). Clinical signs were monitored during the course of the experiments lasting 14 and 21 days. All falcons developed viremia for two weeks and shed virus for almost the same period of time. Using quantitative real-time RT-PCR WNV was detected in blood, in cloacal and oropharyngeal swabs and following euthanasia and necropsy of the animals in a variety of neuronal and extraneuronal organs. Antibodies to WNV were first time detected by ELISA and neutralization assay after 6 days post infection (dpi). Pathological findings consistently included splenomegaly, non-suppurative myocarditis, meningoencephalitis and vasculitis. By immunohistochemistry WNV-antigens were demonstrated intralesionally. These results impressively illustrate the devastating and possibly deadly effects of WNV infection in falcons, independent of the genetic lineage and dose of the challenge virus used. Due to the relatively high virus load and long duration of viremia falcons may also be considered competent WNV amplifying hosts, and thus may play a role in the transmission cycle of this zoonotic virus.     

Experimental West Nile virus infection in Eastern Screech Owls (Megascops asio)

Eastern Screech Owls (EASOs) were experimentally infected with the pathogenic New York 1999 strain of West Nile virus (WNV) by subcutaneous injection or per os. Two of nine subcutaneously inoculated birds died or were euthanatized on 8 or 9 days postinfection (DPI) after <24 hr of lethargy and recumbency. All subcutaneously inoculated birds developed levels of viremia that are likely infectious to mosquitoes, with peak viremia levels ranging from 10(5.0) to 10(9.6) plaque-forming units/ml. Despite the viremia, the remaining seven birds did not display signs of illness. All birds alive beyond 5 DPI seroconverted, although the morbid birds demonstrated significantly lower antibody titers than the clinically normal birds. Cagemates of infected birds did not become infected. One of five orally exposed EASOs became viremic and seroconverted, whereas WNV infection in the remaining four birds was not evident. All infected birds shed virus via the oral and cloacal route. Early during infection, WNV targeted skin, spleen, esophagus, and skeletal muscle. The two morbid owls had myocardial and skeletal muscle necrosis and mild encephalitis and nephritis, whereas some of the clinically healthy birds that were sacrificed on 14 DPI had myocardial arteritis and renal phlebitis. WNV is a significant pathogen of EASOs, causing pathologic lesions with varying clinical outcomes.     

Evaluation of antibody response to vaccination against West Nile virus in thick billed parrots (Rhynchopsitta pachyrhyncha)

West Nile virus (WNV) was first documented in North America in New York City in 1999. Several deaths attributable to WNV have been reported in captive thick-billed parrots (Rhynchopsitta pachyrhyncha), an endangered psittacine native to North America. The serologic responses in 12 captive adult thick-billed parrots after a series of three initial WNV vaccine injections with annual boosters over 6 yr was evaluated. In addition, the serologic responses of 11 thick-billed parrot chicks following an initial vaccination series to determine if there were seroconversions were also reported. Most adults (67%) had seroconverted after 5 yr of annual vaccination, with a median titer of 1:80 (range 1:40-1:160) for those that seroconverted. After the first year, birds were likely naturally exposed to WNV, which limited interpretation of titers. None of the chicks seroconverted during the initial three-vaccine series; only two of four chicks (50%) had seroconverted when tested at the 1-yr yearly booster, and at 2 yr, three of four chicks had seroconverted. Although some birds had detectable antibody titers, it is unclear whether this vaccine can reliably provide protection against WNV in thick-billed parrots.     

Evaluation of an outbreak of West Nile virus infection in horses: 569 cases (2002)

OBJECTIVE: To characterize an outbreak of West Nile virus (WNV) infection in horses in North Dakota in 2002, evaluate vaccine effectiveness, and determine horse characteristics and clinical signs associated with infection. DESIGN: Retrospective study. ANIMALS: 569 horses. PROCEDURE: Data were obtained from veterinary laboratory records, and a questionnaire was mailed to veterinarians of affected horses. RESULTS: Affected horses were defined as horses with typical clinical signs and seroconversion or positive results of virus isolation; affected horses were detected in 52 of the 53 counties and concentrated in the eastern and northeastern regions of the state. Among affected horses, 27% (n = 152) were vaccinated against WNV, 54% (309) were not, and 19% (108) had unknown vaccination status; 61 % (345) recovered, 22% (126) died, and 17% (98) had unknown outcome. The odds of death among nonvaccinated horses were 3 and 16 times the odds among horses that received only 1 or 2 doses of vaccine and horses that were vaccinated according to manufacturer's recommendations, respectively. Horses with recumbency, caudal paresis, and age > 5 years had higher odds of death, whereas horses with incoordination had lower odds of death, compared with affected horses without these characteristics. CONCLUSIONS AND CLINICAL RELEVANCE: Vaccination appears to have beneficial effects regarding infection and death caused by WNV.