Identification of hyperendemic foci of horses with West Nile virus disease in Texas

OBJECTIVE: To determine whether West Nile virus (WNV) disease hyperendemic foci (hot spots) exist within the horse population in Texas and, if detected, to identify the locations. SAMPLE POPULATION: Reports of 1,907 horses with WNV disease in Texas from 2002 to 2004. Procedures: Case data with spatial information from WNV epidemics occurring in 2002 (1,377 horses), 2003 (396 horses), and 2004 (134 horses) were analyzed by use of the spatial scan statistic (Poisson model) and kriging of empirical Bayes smoothed county attack rates to determine locations of horses with WNV disease in which affected horses were consistently (in each of the 3 study years) clustered (hyperendemic foci, or hot spots). RESULTS: 2 WNV hot spots in Texas, an area in northwestern Texas and an area in eastern Texas, were identified with the scan statistic. Risk maps of the WNV epidemics were qualitatively consistent with the hot spots identified. Conclusions and CLINICAL RELEVANCE: WNV hot spots existed within the horse population in Texas (2002 to 2004). Knowledge of disease hot spots allows disease control and prevention programs to be made more efficient through targeted surveillance and education.     

The relationship between equine and human West Nile virus disease occurrence

Cases of human and equine West Nile virus (WNV) disease reported in Texas in 2002 were analyzed to assess their temporal relationship. For each human case with a known residential location, the closest equine case (within a 5 km radius) was selected. A total of 80 human–equine case pairs were identified, 51 (64%) of which were located in urban areas. Dates-of-onset of human and equine cases were positively correlated (r_SP = 0.494, P < 0.001). Although overall there was no significant (P = 0.207) difference between the dates-of-onset of human and equine cases, in urban areas of Texas equine cases were reported significantly (P = 0.011) earlier (August 7) than corresponding human cases (August 19). Monitoring equine populations that are susceptible to WNV disease within close proximity to urban human populations might be useful for predicting disease risk in human populations.     

Epidemic West Nile virus encephalomyelitis: a temperature-dependent, spatial model of disease dynamics

Since first being detected in New York in 1999, West Nile virus (WNV) has spread throughout the United States and more than 20,000 cases of equine WNV encephalomyelitis have been reported. A spatial model of disease occurrence was developed, using data from an outbreak of serologically confirmed disease in an unvaccinated population of horses at 108 locations in northern Indiana between 3 August and 17 October 2002. Daily maximum temperature data were recorded at meteorological stations surrounding the study area. The distribution of the total number of degree-days elapsing between July 4 and the date of diagnosis of each case was best described by a normal distribution (mean = 5243 °F, S.D. = 1047). The days on which the average risk was >25, >50 and >75% were predicted (versus observed) to occur on August 23 (August 9), August 31 (September 2) and September 9 (September 9). The epidemic was predicted to occur 3 days earlier, or 4 days later, than observed if temperatures in the study area were uniformly increased, or decreased, by 5 °F, respectively. Maps indicated that WNV encephalomyelitis risk always remained greater in the northwest quadrant of the study area. Since WNV might exist at a hypoendemic level of infection, and occasionally re-emerge as a cause of epidemics in equine populations, by identifying factors that contributed to this epidemic, the potential impact of future epidemics can be reduced. Such studies rely on a GIS framework, availability of meteorological and possibly remotely sensed data and information on host and landscape factors. An early-warning system for WNV transmission in equine populations could be developed.     

Characteristics of an outbreak of West Nile virus encephalomyelitis in a previously uninfected population of horses

Equine West Nile virus (WNV) encephalomyelitis cases - based on clinical signs and ELISA serology test results - reported to Texas disease control authorities during 2002 were analyzed to provide insights into the epidemiology of the disease within a previously disease-free population. The epidemic occurred between June 27 and December 17 (peaking in early October) and 1,698 cases were reported. Three distinct epidemic phases were identified, occurring mostly in southeast, northwest and then central Texas. Significant (P<0.05) disease clusters were identified in northwest and northern Texas. Most (91.1%) cases had no recent travel history, and most (68.9%) cases had not been vaccinated within the previous 12 months. One-third of cases did not survive, 71.2% of which were euthanatized. The most commonly reported presenting signs included ataxia (69%), abnormal gait (52%), muscle fasciculations (49%), depression (32%) and recumbency (28%). Vaccination status, ataxia, falling down, recumbency and lip droop best explained the risk of not surviving WNV disease. Results suggest that the peak risk period for encephalomyelitis caused by WNV may vary substantially among regions within Texas. Recumbent horses have a poor prognosis for survival. Vaccines, even if not administered sufficiently in advance of WNV infection within a district, may reduce the risk of death by at least 44%.     

Assessing human risk of illness with West Nile virus mosquito surveillance data to improve public health preparedness

Surveillance for West Nile virus (WNV) and other mosquito‐borne pathogens involves costly and time‐consuming collection and testing of mosquito samples. One difficulty faced by public health personnel is how to interpret mosquito data relative to human risk, thus leading to a failure to fully exploit the information from mosquito testing. The objective of our study was to use the information gained from historic West Nile virus mosquito testing to determine human risk relative to mosquito infection and to assess the usefulness of our mosquito infection forecasting models to give advance warning. We compared weekly mosquito infection rates from 2004 to 2013 to WNV case numbers in Illinois. We then developed a weather‐based forecasting model to estimate the WNV mosquito infection rate one to 3 weeks ahead of mosquito testing both statewide and for nine regions of Illinois. We further evaluated human illness risk relative to both the measured and the model‐estimated infection rates to provide guidelines for public health messages. We determined that across 10 years, over half of human WNV cases occurred following the 29 (of 210) weeks with the highest mosquito infection rates. The values forecasted by the models can identify those time periods, but model results and data availability varied by region with much stronger results obtained from regions with more mosquito data. The differences among the regions may be related to the amount of surveillance or may be due to diverse landscape characteristics across Illinois. We set the stage for better use of all surveillance options available for WNV and described an approach to modelling that can be expanded to other mosquito‐borne illnesses.     

Surveillance of West Nile virus in horses in Canada: A retrospective study of cases reported to the Canadian Food Inspection Agency from 2003 to 2019

The objectives of the study were to describe the regional and provincial incidence rates and the weekly distribution of 842 reported West Nile virus (WNV) cases in horses in Canada between 2003 and 2019. This study also investigated characteristics of cases reported to the Canadian Food Inspection Agency (CFIA) between 2015 and 2019. The western region (British Columbia, Alberta, Saskatchewan, and Manitoba) had higher incidence rates than the eastern region (Ontario, Quebec, and Atlantic provinces) and overall, Saskatchewan registered the highest incidence. Over the study period, an earlier weekly preliminary onset of WNV cases was observed in the western region. The vast majority of cases were unvaccinated (96%), most cases were Quarter Horses (68%) and the risk of mortality was 31.9%. The findings of this study may be useful in informing veterinary equine practitioners about measures to prevent WNV disease in horses in Canada.     

Simulation of the seasonal cycles of bird, equine and human West Nile virus cases

The West Nile virus (WNV) is an arthropod-borne virus (arbovirus) circulating in a natural transmission cycle between mosquitoes (enzootic vectors) and birds (amplifying hosts). Additionally, mainly horses and humans (dead-end hosts) may be infected by blood-feeding mosquitoes (bridge vectors). We developed an epidemic model for the simulation of the WNV dynamics of birds, horses and humans in the U.S., which we apply to the Minneapolis metropolitan area (Minnesota). The SEIR-type model comprises a total of 19 compartments, that are 4 compartments for mosquitoes and 5 compartments or health states for each of the 3 host species. It is the first WNV model that simulates the seasonal cycle by explicitly considering the environmental temperature. The latter determines model parameters responsible for the population dynamics of the mosquitoes and the extrinsic incubation period. Once initialized, our WNV model runs for the entire period 2002-2009, exclusively forced by environmental temperature. Simulated incidences are mainly determined by host and vector population dynamics, virus transmission and herd immunity, respectively. We adjusted our WNV model to fit monthly totals of reported bird, equine and human cases in the Minneapolis metropolitan area. From this process we estimated that the proportion of actually WNV-induced dead birds reported by the Centers for Disease Control and Prevention is about 0.8%, whereas 7.3% of equine and 10.7% of human cases were reported. This is consistent with referenced expert opinions whereby about 10% of equine and human cases are symptomatic (the other 90% of asymptomatic cases are usually not reported). Despite the restricted completeness of surveillance data and field observations, all major peaks in the observed time series were caught by the simulations. Correlation coefficients between observed and simulated time series were R=0.75 for dead birds, R=0.96 for symptomatic equine cases and R=0.86 for human neuroinvasive cases, respectively.     

Anti-insect defensive behaviors in equines post-West Nile virus infection

West Nile virus (WNV), a zoonotic mosquito transmitted Flavivirus, has had significant health effects on horses in the United States, with over 23,000 United States equine cases since the disease was first recognized in 1999. Previous research has focused on how this disease progresses and affects equids days to weeks post infection. The purpose of this study was to evaluate if permanent equine behavioral changes had occurred in horses that had recovered from acute West Nile fever or encephalitis. Specifically, we examined if surviving this disease caused changes in the defensive behaviors of the animal against biting and stinging insects, presumably because of neurological sequelae that can result from the infection. Results from behavioral observations and neurologic reflex testing suggest that long-term survivors of WNV do not show a change in the frequency or types of behaviors used compared to uninfected horses, supporting the concept that lasting deficits from WNV usually resolve within the following 1–3 years post-infection. However, microhabitat and grouping behavior did have a significant impact on the frequency of defensive behaviors, with indoor locales and larger groups of horses showing less insect avoidance behaviors. These principles may play a more pivotal role in protecting equines from biting insects and disease than thought previously.     

A case-control study of factors associated with development of clinical disease due to West Nile virus, Saskatchewan 2003

REASON FOR PERFORMING STUDY: West Nile virus (WNV) was first diagnosed in Saskatchewan equids in 2002. AWNV epidemic was considered highly likely for 2003, which would provide a unique opportunity to study all aspects of WNV subclinical infection and clinical disease development in a relatively naive population. HYPOTHESIS: There are individual equid attributes and management risk factors associated with development of clinical disease. Specifically, this study could address the question of vaccine efficacy for the prevention of development of clinical disease. METHODS: A case-control study was conducted in the summer of 2003 during a province-wide outbreak of WNV. Between 5 and 10 equids were sampled from each of 23 case premises with clinical disease and 23 control premises with no apparent or confirmed clinical disease. Data were analysed to identify risk factors for the development of clinical disease. RESULTS: The proportion of equids serologically positive for natural exposure to West Nile virus was 64% (193/300). Nonvaccinated equids were 23 times (95%CI limits 3.0, 168.5, P = 0.002) more likely to develop clinical disease than those vaccinated. The estimate of vaccine efficacy in this field study was 96% (95%CI limits 67%, 99%). CONCLUSIONS: The study demonstrated that vaccination was strongly associated with the prevention of clinical disease. POTENTIAL RELEVANCE: Vaccination is an effective, practical method of prevention of clinical disease.     

Seroepidemiology of Selected Alphaviruses and Flaviviruses in Bats in Trinidad

A serosurvey of antibodies against selected flaviviruses and alphaviruses in 384 bats (representing 10 genera and 14 species) was conducted in the Caribbean island of Trinidad. Sera were analysed using epitope‐blocking enzyme‐linked immunosorbent assays (ELISAs) specific for antibodies against West Nile virus (WNV), Venezuelan equine encephalitis virus (VEEV) and eastern equine encephalitis virus (EEEV), all of which are zoonotic viruses of public health significance in the region. Overall, the ELISAs resulted in the detection of VEEV‐specific antibodies in 11 (2.9%) of 384 bats. Antibodies to WNV and EEEV were not detected in any sera. Of the 384 sera, 308 were also screened using hemagglutination inhibition assay (HIA) for antibodies to the aforementioned viruses as well as St. Louis encephalitis virus (SLEV; which also causes epidemic disease in humans), Rio Bravo virus (RBV), Tamana bat virus (TABV) and western equine encephalitis virus (WEEV). Using this approach, antibodies to TABV and RBV were detected in 47 (15.3%) and 3 (1.0%) bats, respectively. HIA results also suggest the presence of antibodies to an undetermined flavivirus(es) in 8 (2.6%) bats. Seropositivity for TABV was significantly (P < 0.05; χ²) associated with bat species, location and feeding preference, and for VEEV with roost type and location. Differences in prevalence rates between urban and rural locations were statistically significant (P < 0.05; χ²) for TABV only. None of the aforementioned factors was significantly associated with RBV seropositivity rates.     

Ecological Surveillance for West Nile in Catalonia (Spain), Learning from a Five‐Year Period of Follow‐up

To enhance early detection of West Nile virus (WNV) transmission, an integrated ecological surveillance system was implemented in Catalonia (north‐eastern Spain) from 2007 to 2011. This system incorporated passive and active equine surveillance, periodical testing of chicken sentinels in wetland areas, serosurveillance wild birds and testing of adult mosquitoes. Samples from 298 equines, 100 sentinel chickens, 1086 wild birds and 39 599 mosquitoes were analysed. During these 5 years, no acute WNV infection was detected in humans or domestic animal populations in Catalonia. WNV was not detected in mosquitoes either. Nevertheless, several seroconversions in resident and migrant wild birds indicate that local WNV or other closely related flaviviruses transmission was occurring among bird populations. These data indicate that bird and mosquito surveillance can detect otherwise silent transmission of flaviviruses and give some insights regarding possible avian hosts and vectors in a European setting.     

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.     

Spatio-temporal epidemiology of Tritrichomonas foetus infection in Texas bulls based on state-wide diagnostic laboratory data

Texas is the largest cattle producing state and suffers severe economic losses due to abortions caused by the protozoan parasite Tritrichomonas foetus. The objective of this study was to use data from the state-wide diagnostic laboratory system of Texas to investigate the occurrence and spatio-temporal distribution of bovine trichomoniasis (BT) in Texas, and to identify spatial disease clusters within the state. The study population consisted of bulls tested for BT in 2010 by the Texas Veterinary Medical Diagnostic Laboratory system that performs at least 95% of all T. foetus testing in the state. Preputial samples were cultured and diagnosis was made by real-time polymerase chain reaction (PCR). Data on BT was aggregated at the county level with time aggregation of one month. The scan statistics was used to identify spatial disease clusters. The database included 31,202 test results with a proportion of positives of 3.7%. As expected, BT was present throughout Texas. Testing prevalence was highest in the summer (5.5%). The scan statistics identified a spatial cluster in southeastern Texas, which could only partially be explained by cattle herd density. The findings of this study provide baseline data to monitor the success of BT control activities in Texas and aids in generating hypotheses regarding specific risk factors for the disease. The identification of high-risk areas and periods is also essential to improve intervention efforts.     

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 in horses in Saudi Arabia (in 2013–2015)

West Nile virus (WNV) is an important emerging zoonotic arbovirus giving rise to clinical syndromes of varying severity in humans and horses. Culex mosquitoes are the main vector. Although WNV has been reported in many countries in the Middle East and Asia, little is known about its prevalence in equine populations in the Arabian Peninsula. We have carried out a serological study on 200 horses to assess WNV infection in the Eastern and Central regions of Saudi Arabia in 2013–2015. Sera were tested for the presence of WNV antibodies in parallel using a commercial enzyme‐linked immunosorbent assay (ELISA) kit and microneutralization (MN) tests. In comparison with the MN assay used as “gold standard,” we find the ELISA had a sensitivity of 94.7% and specificity of 80.1%. The prevalence of WNV neutralizing antibody ranged from 5 (17.3%) of 29 sera collected in Riyadh up to 15 (55.6%) of 27 sera collected from Al‐Qateef. These findings highlight the need to be aware of the possibility of WNV disease in humans and horses presenting with central nervous system disease in the Kingdom of Saudi Arabia.     

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.     

Equine colic: clinical epidemiology and associated risk factors in and around Debre Zeit

A prospective study was conducted to describe clinical epidemiology of equine colic in the Society for Protection of Animal Abroad and Donkey Sanctuary Project Clinic, at Debre Zeit, Ethiopia, from November 2014 to April 2015. The objectives were to describe clinical epidemiology of equine colic, to characterize the main types of equine colic, and to determine the major risk factors associated with equine colic. The method which was used in the study was attending clinical case of equine and assessing physiological parameters, fecal egg count, abdominal sounds, and rectal examination as well as questioner interviewing of the owners. The data were collected and analyzed using Statistical Package for Social Science. The incidence of colic was 10.3% in the study period. Colicky were typed as unknown, flatulent, impaction, spasmodic, and enterolithiasis. The proportion of case incidence were 63.1 (41/65), 33.8 (22/65), and 3.1% (2/65), in donkey, horse, and mule, respectively. The total mean (±SD) of temperature 37.80 ± 1.003, heart rate 57.54 ± 10.098, fecal egg count 236.922 ± 67.990, respiratory rate 30.92 ± 7.315, and packed cell volume 41.40 ± 10.221 were recorded. The case fatality rate of equine colic was 15.38% (10/65). There were statistically highly significant (p < 0.01) differences in impaction colic in relation to species. Interview with 183 equine owners revealed incidence of equine colic as the sixth major disease condition affecting equine. A long-term epidemiological study of the true representative population should be carried out to determine the incidence rate and associated risk factors of equine colic in the study area.