Abstract
Morbidity and mortality events caused by avian paramyxovirus-1 (APMV-1) in Double-crested Cormorant (DCCO; Phalacrocorax auritus) nesting colonies in the US and Canada have been sporadically documented in the literature. We describe APMV-1 associated outbreaks in DCCO in the US from the first reported occurrence in 1992 through 2012. The frequency of APMV-1 outbreaks has increased in the US over the last decade, but the majority of events have continued to occur in DCCO colonies in the Midwestern states. Although morbidity and mortality in conesting species has been frequently reported during DCCO APMV-1 outbreaks, our results suggest that isolation of APMV-1 is uncommon in species other than DCCO during APMV-1 outbreaks and that the cause of mortality in other species is associated with other pathogens. Populations of DCCO do not appear to have been significantly affected by this disease; however, because at least 65% of the APMV-1 outbreaks in DCCO in the US have involved APMV-1 strains classified as virulent to poultry (virulent Newcastle disease virus), its persistence and increased occurrence in DCCO warrants continued research and surveillance.
INTRODUCTION
The avian paramyxovirus serotype 1 (APMV-1) group of viruses (family Paramyxoviridae, genus Avulavirus) was discovered in 1926 when a novel virus was found to be the cause of an acute disease in poultry in Newcastle-upon-Tyne, England, and simultaneously in Java, Indonesia (Kraneveld 1926; Doyle 1927). The disease in poultry was later named Newcastle disease (ND) after the initial outbreaks in Newcastle-upon-Tyne, and the causative agent was named Newcastle disease virus (NDV). Almost a century later, ND is still considered one of the most economically important diseases of poultry worldwide (Alexander and Senne 2008).
Beginning in the 1970s the isolation of avian paramyxoviruses that were antigenically distinct from NDV led to creation of 10 avian paramyxovirus serogroups (APMV-1–APMV-10; Alexander 1987; Miller et al. 2010a). Although NDV is still frequently used synonymously with APMV-1 (Alexander and Senne 2008), multiple genotypes within the APMV-1 group have been identified. Most of the viruses classified as class I are avirulent to poultry and cause only asymptomatic infections in waterfowl and shorebirds (Miller et al. 2010b). The majority of poultry-virulent viruses circulating worldwide are in the APMV-1 class II group that also contains genotypes virulent to wild birds such as pigeons (Columba livia and others in the Columbidae) and Double-crested Cormorants (DCCO; Phalacrocorax auritus and others in the Phalacrocoracidae; Diel et al. 2012a).
In DCCO, APMV-1 viruses primarily affect nestlings and fledglings, hereafter referred to as juveniles, causing clinical disease and mortality that can exceed 90% in a nesting colony (Glaser et al. 1999). Many of the APMV-1 strains isolated from DCCO have been determined to be virulent to poultry, including the virus responsible for the 1990 outbreaks in DCCO in several Canadian provinces (Wobeser et al. 1993) and the strain that caused outbreaks in DCCO at multiple sites in the US and Canada in 1992 (Banerjee et al. 1994; Meteyer et al. 1997). Although the habitat preferences of DCCO and rigorous biosecurity in the poultry industry make transmission from infected DCCO to poultry uncommon, DCCO were believed to be the source of virus for a 1992 ND outbreak in an open-range turkey farm in North Dakota (Heckert et al. 1996).
In the US, APMV-1 in DCCO has been reported in specific locations, seasons, and years (Glaser et al. 1999; Farley et al. 2001; Allison et al. 2005; Diel et al. 2012b). We describe APMV-1 outbreaks in DCCO in the US from the first reported occurrence in 1992 through 2012. We examined these events for temporal and spatial patterns that may help guide additional research aimed at understanding the dynamics of APMV-1 in wild birds.
MATERIALS AND METHODS
We use the term virulent NDV (vNDV) to describe isolated viruses with intracerebral pathogenicity indices (ICPI) of ≥0.7 in day-old domestic specific pathogen–free chickens (Gallus gallus domesticus) or presence of multiple basic amino acids (at least three arginine or lysine residues) at the C terminus of the fusion protein cleavage site, starting at position 113, along with phenylalanine at position 117 (World Organisation for Animal Health 2012). The term APMV-1 is used to describe viruses for which strain virulence to poultry was not established (e.g., isolated virus was not characterized beyond APMV-1 classification or virus was not isolated from examined birds). A DCCO APMV-1 outbreak refers to an event involving five DCCO mortalities attributed to APMV-1 or vNDV.
Data on DCCO APMV-1 outbreaks in the US during 1992–2012 were gathered from the US Geological Survey's (USGS's) National Wildlife Health Center (NWHC) long-term (>30 yr) database (EPIZOO; USGS 2013) of US wildlife morbidity and mortality events. This database includes the results of diagnostic tests performed on carcasses sent directly to NWHC and reports from state and federal agencies of diagnostic results performed by other laboratories. Estimates of mortality were acquired through interviews with individual field biologists, and methods of estimation may not have been consistent among sites. Events recorded in the NWHC database represent the minimum number of DCCO APMV-1 outbreaks that have occurred in the US, because wildlife outbreaks are not always observed or reported and results from tests performed at other laboratories are only voluntarily submitted to NWHC. Nevertheless, these records represent the most comprehensive set of data currently available for examining trends in DCCO AMPV-1 outbreaks in the US over the past 20 yr.
Diagnosis of an APMV-1 or vNDV outbreak in DCCO was made at a nesting colony scale based on a flock or site perspective. Encephalitis characteristic of APMV-1 in juvenile DCCO (Meteyer et al. 1997) and evidence of APMV-1 or vNDV needed to be present in at least a subset of DCCO submitted from a site for that site to be considered positive. Because virus was more difficult to isolate as the outbreaks progressed and a 5 μm histologic section is an incomplete assessment of the brain, a presumptive diagnosis of APMV-1 associated mortality was made when 1) encephalitis was present but no virus was detected or 2) virus was detected but there was no encephalitis in a juvenile DCCO from a colony where an APMV-1– or vNDV–associated diagnosis had been previously confirmed.
During 1992–2005, virus isolates were characterized as APMV-1 using hemagglutination inhibition tests with APMV-1 specific antisera. A subset of APMV-1 isolates or positive tissues from juvenile DCCO was sent by NWHC to National Veterinary Services Laboratory (NVSL; Ames, Iowa, USA) for ICPI testing. In 2006, NWHC began screening samples using the National Animal Health Laboratory Network's APMV-1 matrix gene real-time reverse transcriptase PCR (mAPMV-1 RRT-PCR) assay followed by the vNDV RRT-PCR assay that recognizes historic US poultry vNDV strains (Wise et al. 2004). Positive vNDV RRT-PCR samples were sent to NVSL for further virus characterization. Virus isolation in embryonated chicken eggs and duck-embryo fibroblast cell lines was attempted on positive mAPMV-1 RRT-PCR samples at NWHC. Recovered virus isolates were rescreened by both PCR assays and the sequence of the fusion protein cleavage site was used for virulence determination according to criteria of the World Organisation for Animal Health (2012).
To provide consistency of scale when comparing DCCO APMV-1 outbreaks, we grouped events by management area (either lake or refuge) except for events in the Great Lakes. Interactions occur between adults and mobile (swimming and flying) subadults from different nesting colonies within the same lake or refuge during foraging, loafing, or roosting. Therefore, grouping at this spatial scale is reasonable to describe general temporal and geospatial patterns of APMV-1 and vNDV in DCCO.
We assessed DCCO population trends from the North American breeding bird survey (BBS) regional trend analysis data (Sauer et al. 2012). Population trend analyses were available through 2011 and results are based on a Bayesian hierarchical model that accommodates variation in variety of parameters such as survey quality, detectability, and observer effects among regions and uses Markov chain Monte Carlo fitting procedures (Link and Sauer 2002). To overlap with the DCCO APMV-1 outbreak data, we accessed DCCO population trends during 1992–2011 in US states with four or more DCCO APMV-1 outbreaks (Minnesota, Michigan, Wisconsin, North Dakota, and California). Birds from Midwestern states, where the majority of APMV-1 outbreaks have occurred, belong to the interior-US breeding population of DCCO (Hatch and Weseloh 1999). Double-crested Cormorant population trends at the breeding population level may influence disease transmission because these birds interact along migratory routes and wintering sites. However, because DCCO trends have not been compiled by BBS for the various breeding populations, we accessed trends for US Fish and Wildlife Service (USFWS) Region 3 (Illinois, Indiana, Iowa, Michigan, Minnesota, Ohio, Wisconsin) and Region 6 (Colorado, Kansas, Montana, Nebraska, North Dakota, South Dakota, Utah, Wyoming), which together encompass the majority of the interior-US population.
RESULTS
Diagnostic findings during DCCO APMV-1 outbreaks
We examined 256 DCCO from APMV-1 outbreaks during 1992–2012, but not all tests (molecular testing, virus isolation and sequencing, and histology) were performed on each bird (Table 1). There were no consistent gross lesions, but encephalitis was present in 69% (149/215) of examined DCCO. We isolated APMV-1 or vNDV from 39% (93/240) of examined DCCO. Of the DCCO tested with mAPMV-1 or vNDV RRT-PCR, 64% (82/128) were positive, although some were positive only for the virus isolated from the tissue and not when applied directly to the sample of brain, kidney, or liver. Isolates from 36 of 55 DCCO APMV-1 outbreaks during 1992–2012 were characterized and all were vNDV according to World Organisation for Animal Health standards (Table 1).
Location, timing, estimated annual mortality, and proportion of examined Double-crested Cormorants (Phalacrocorax auritus) positive for each diagnostic test during 1992–2012 avian paramyxovirus-1 outbreaks in the US.a

During 2008, 2010, and 2012 multiple DCCO APMV-1 outbreaks occurred in the upper Midwest. There was also evidence of APMV-1 and vNDV in individual DCCO not associated with known outbreaks at a specific nesting colony. During August 2008, a single moribund juvenile DCCO was found in a rural suburb in Grand Forks County, North Dakota. Avian paramyxovirus type 1 was isolated from the brain of this DCCO and encephalitis was present histologically. Virulent NDV was isolated from the brain of a single moribund juvenile DCCO (encephalitis was not present histologically) found in August 2008 at a wildlife refuge in Pike County, Missouri, where no other affected DCCO were observed. During 2010 DCCO APMV-1 outbreaks were documented in multiple DCCO nesting colonies in Lake Michigan. In August 2010 two moribund juvenile DCCO were also found 4 days apart on the shoreline of Lake Michigan in Milwaukee County, Wisconsin. Both juveniles had encephalitis and vNDV was isolated from the brain of one of them. During the 2012 DCCO APMV-1 outbreaks, two moribund juvenile DCCO were found during August in Hubbard and Ramsey Counties, Minnesota, where no other affected DCCO were observed. Histologic lesions were present in both birds, one of which also had vNDV isolated from the brain. Virulent NDV was also isolated from a pooled tissue (kidney, lungs, intestine, spleen, heart, and pancreas) sample of a single moribund juvenile DCCO found in August 2012 at a colony of several hundred apparently healthy DCCO in Rice County, Minnesota.
During APMV-1 outbreaks morbidity and mortality was reported in species conesting with DCCO. Carcasses of 104 juvenile and adult Ring-billed Gulls (Larus delawarensis; RBGU), Herring Gulls (Larus argentatus), American White Pelicans (Pelecanus erythrorhynchos; AWPE), Great Egrets (Ardea alba), Great Blue Herons (Ardea herodias), and other waterfowl and geese were submitted with DCCO carcasses for examination (Table 2). Despite numerous tests for pathogenic bacteria, fungi, viruses, and parasites and histologic examination of tissues, a cause of death was not determined for 36% (38/106) of the examined conesting individuals. When a cause of death was determined, diagnoses in conesting birds included avian botulism type C (14/106), salmonellosis (8/106), West Nile virus (9/106), or emaciation of undetermined cause (19/106; Table 2).
Causes of morbidity and mortality in species other than Double-crested Cormorants examined during 1992–2012 avian paramyxovirus-1 outbreaks in the USA.

Positive APMV-1 or vNDV diagnostic results were documented in four non-DCCO birds during APMV-1 outbreaks. Avian paramyxovirus type 1 was isolated from a cloacal swab of a RBGU collected during the 2008 DCCO APMV-1 outbreak in Faribault County, Minnesota. There were no histologic lesions and the cause of death for this RBGU was determined to be avian botulism. An AWPE collected from Meeker County, Minnesota, during a 2008 DCCO APMV-1 outbreak was positive by mAPMV-1 RRT-PCR (intestinal sample), but immunohistochemistry on the brain was negative and cause of death was determined to be avian cholera. An AWPE from a 2010 DCCO APMV-1 outbreak in Stutsman County, North Dakota, was positive by mAPMV-1 RRT-PCR (brain tissue). No virus was isolated but APMV-1 infection was suspected as the bird had mild nonsuppurative encephalitis and tests for other pathogenic bacteria and viruses were negative. The only vNDV isolation from species other than DCCO was from an AWPE (brain tissue) collected in 2008 from Marshall County, Minnesota, during an event that involved only four adult pelicans. There was no histologic evidence of encephalitis in the examined AWPE and the cause of death was determined to be emaciation of undetermined cause.
Spatial distribution of DCCO APMV-1 outbreaks
The majority (43/55) of DCCO APMV-1 outbreaks during 1992–2012 were reported in the Midwestern states of Michigan, Minnesota, North Dakota, Nebraska, South Dakota, and Wisconsin (Table 1) and all of these were associated with summer nesting colonies. The three states with the largest numbers of reported outbreaks during this 20-yr period were Minnesota (21), Wisconsin (nine), and North Dakota (six). There were 12 locations in the US where DCCO APMV-1 outbreaks were documented to have reoccurred (Table 1). The number of years between reported outbreaks for these locations ranged from 1 to 18.
Frequency of outbreaks and estimated mortality from APMV-1
During 1992–2002 there were 19 DCCO APMV-1 outbreaks, 14 of which occurred during 1992. During 2003–12 there were 42 DCCO APMV-1 outbreaks, and at least one event was documented every year except 2004 and 2009. The highest DCCO mortality was estimated as 8,090 juveniles in 1992. The lowest mortality occurred in 2005 when an estimated 30 juvenile DCCO (out of a population of approximately 2,000) were reported dead during an outbreak in a single isolated nesting colony in Washoe County, Nevada. Although twice as many DCCO APMV-1 outbreaks occurred during 2003–2012, the estimated mortality in juvenile DCCO was 50% less (5,640/11,775) than the estimated mortality for the previous decade.
Breeding bird survey analysis results for DCCO populations trends (% change/year, 95% credible intervals) during 1992–2002 were positive and significant for the US (8, 3.97–12.39), USFWS Region 6 (12.05, 4.02–19.98), Minnesota (7.07, 0.26–14.37), North Dakota (21.03, 7.33–38.67), and California (5.95, 0.62–13.12). The trends were positive but not significant (95% credible interval included zero) during 1992–2002 for USFWS Region 3 (9.93, −0.71 to 14.55), and Wisconsin (2.28, −13.34 to 19.25). Michigan was the only state examined with a negative, but not statistically significant, population trend (−22.93, −62.76 to 23.29) during 1992–2002. During 2003–11 DCCO population trends were positive and statistically significant for the US (7.35, 0.24–17.05), USFWS Region 3 (7.83, 0.25–14.99), and California (9.59, 0.34–28.68). Population trends were also positive but not statistically significant during 2003–2011 for USFWS Region 6 (2.13, −7.66 to 11.39), Minnesota (7.14, −1.19 to 15.69), North Dakota (1.96, −9.67 to 14.47), Wisconsin (4.72, −11.80 to 19.43), and Michigan (3.76, −55.56 to 132.54).
DISCUSSION
Diagnostic findings during DCCO APMV-1 outbreaks
During 1992–2012, 55 DCCO APMV-1 outbreaks were documented in the US. The majority (69%) of DCCO examined from outbreak locations had characteristic encephalitis and were positive via RRT-PCR (mAPMV-1 or vNDV; 64%) on tissue or viral isolate. Although virus (APMV-1 or vNDV) was isolated from only 39% of DCCO examined during outbreaks, previous studies have suggested that encephalitis can still be detected but the rate of virus isolations drop precipitously as the outbreak progresses and affected DCCO either die from infection or recover (Meteyer et al. 1997). Isolation of vNDV has occasionally been reported in other species during APMV-1 outbreaks (Wobeser et al. 1993; Diel et al. 2012b); however, our results suggest that isolation of APMV-1 is uncommon in species other than DCCO during APMV-1 outbreaks and that the cause of mortality in other species is associated with other pathogens (e.g., botulinum toxin, West Nile virus, or salmonella; Table 2).
Spatial distribution of DCCO APMV-1 outbreaks
The majority of DCCO APMV-1 outbreaks in the US, since first detected in 1992, have occurred in the Midwest. There are 12 locations (defined by management area) where outbreaks have reoccurred; however, the number of years between outbreaks at these locations ranged from 1 to 18. Because APMV-1 can be inactivated by heat and direct sunlight (Food and Animal Organization of the United Nations 2001), environmental persistence on a site for multiple years is unlikely. Spatial clustering of outbreaks in the Midwest is more likely a result of adults breeding at or near colonies where they hatched and fidelity to well-established colonies (Hatch and Weseloh 1999). Avian paramyxovirus type 1 may be maintained over winter months at southeastern US wintering grounds by circulation of the virus among subclinical DCCO. When these birds return to the Midwestern breeding grounds, susceptible juveniles may come into contact with virus shed by adults. Although DCCO juveniles appear to be most susceptible to APMV-1, the virus was isolated in November 2002 from a single adult DCCO from Florida showing neurologic signs (Allison et al. 2005). In another study of DCCO on their wintering grounds in Alabama and Mississippi, no virus was isolated from >200 sampled birds. However, prevalence of antibodies to APMV-1 was significantly higher for birds sampled in late winter (February and March) compared to early winter (November and December), suggesting transmission of APMV-1 on the wintering grounds (Farley et al. 2001).
Only three of 55 documented DCCO APMV-1 outbreaks have occurred in DCCO colonies in the Northeast Atlantic breeding population (Maryland, Maine, and New York). Based on recoveries of USFWS identification bands, Dolbeer (1991) reported considerable overlap on wintering grounds in the southeastern US of populations of DCCO nesting east of the Rocky Mountains. However, the migratory divide recently described for Great Lakes populations of DCCO suggests that western Great Lakes populations migrate west of the Appalachian Mountains and winter in the lower Mississippi valley and eastern Great Lakes populations migrate east of the Appalachians and winter primarily in Florida (Guillaumet et al. 2011). This type of segregation of populations along migratory routes and wintering grounds may minimize APMV-1 outbreaks outside the interior-US DCCO breeding population.
Mortality associated with APMV-1 has also been diagnosed intermittently in DCCO breeding colonies in the western US (California, Utah, Oregon, and Nevada). Double-crested Cormorants occasionally migrate to the western US from Alberta (Dolbeer 1991), where DCCO APMV-1 has also been documented (Wobeser et al. 1993). Such infections in western populations may also be due to infected interior-US birds occasionally moving through the west (Kuiken 1999). Additional studies comparing isolated viruses among years and regions and examining migration patterns of the interior-US DCCO breeding population could increase our understanding of the drivers of spatial patterns in APMV-1 outbreaks.
Frequency of outbreaks and estimated mortality caused by APMV-1
Compared to the previous decade, almost two and a half times as many DCCO APMV-1 outbreaks occurred during 2003–12 and outbreaks were more frequent, occurring almost every year. Increased recognition of field signs and monitoring may be contributing to more frequent diagnoses of DCCO APMV-1 outbreaks. When biologists and private citizens become aware of a disease, particularly one that could affect domestic animals, they may be more inclined to report observations of sick and dead wildlife. Further, once a DCCO APMV-1 outbreak is documented, natural resource agencies often begin monitoring DCCO colonies in neighboring counties and states. However, at many of the waterbird colonies, submission of specimens to NWHC for cause-of-death determination during a morbidity or mortality event occurs regardless of whether APMV-1 is suspected.
Double-crested Cormorant populations increased dramatically from the late 1970s to 1991 following restrictions on DDT use (Weseloh and Collier 1995), protection of DCCO by the Migratory Bird Treaty Act, and increased availability of prey. The continental DCCO population as a whole may still be below biological carrying capacity (Wires and Cuthbert 2006). However, growth rates in some regions, such as the Great Lakes, have begun to slow suggesting, DCCO in these regions may be at or approaching carrying capacity and more recently may be affected by management activities (Weseloh et al. 2002; Ridgway et al. 2006; Guillaumet et al. 2014). Breeding bird survey population trend estimates for DCCO were positive during 1992–2002 and 2003–10 for USFWS Regions 3 and 6, as well as most states where DCCO APMV-1 outbreaks occurred more than twice (Minnesota, Wisconsin, North Dakota, and California). Although positive trends in both decades suggest increased numbers of DCCO during 2003–10 in areas where APMV-1 outbreaks were common, we could not assess whether DCCO population growth had occurred at the same rate (i.e., it is possible that numbers increased but rate of growth decreased) because of overlap in credible intervals for the BBS population trend analysis results.
Because of concern that large DCCO populations may negatively affect vegetation at nesting sites and sport-fish populations, control measures for DCCO have occurred over the last several decades (Taylor and Dorr 2003). In 2003, a Public Resource Depredation Order (50 CFR 21.48) aimed at interior, southeastern, and Atlantic DCCO populations, authorized the nonpermit use of management activities (culling, egg oiling, and nest removal) by federal, state, and tribal officials in 24 states. The influence of management activities on DCCO movement has been evaluated in several studies (Duerr et al. 2007; Dorr et al. 2010; Strickland et al. 2011). Some proportion of DCCO may abandon a colony following management activities and typically relocate to sites within the same watershed. If management actions have resulted in the relocation of infected birds, these activities could assist in the spread of APMV-1 and may have contributed to the increased number of outbreak sites observed during 2003–10. However, the effects of management on disease transmission are undoubtedly complex, as management activities such as egg oiling would reduce the number of susceptible juveniles in a colony.
Regardless of whether DCCO are truly overabundant or have just exceeded socially acceptable numbers (Taylor and Dorr 2003; Wires and Cuthbert 2006), DCCO and other colonial waterbirds can cause habitat changes on breeding grounds (Miller 1982; Boutin et al. 2011). The accumulation of DCCO feces and annual stripping of leaves for nesting materials can destroy trees and shrubs (Taylor and Dorr 2003). In such situations DCCO, which typically nest in trees, nest on the ground. Juveniles leave ground nests at around 4 wk of age and tree nests at 6 wk. Ground-nesting juveniles may, therefore, have higher contact rates with infected individuals at an earlier age. Increased severity of APMV-1 outbreaks at ground nesting colonies has been suggested (Kuiken et al. 1998) and preliminarily supported by increased mortality of juveniles from ground nests compared to those from tree nests during a 1975 DCCO APMV-1 outbreak in Canada (Cleary 1977). Including counts of ground-nesting versus tree-nesting DCCO during future breeding colony surveys could help address the influence of nesting habitat structure on APMV-1 transmission.
Although the number of events reported by field biologists increased during 2003–12 the reported morbidity and mortality in juvenile DCCO at APMV-1 outbreak sites was lower than the previous decade. The majority of reported DCCO mortality due to APMV-1 or vNDV occurred during the first year the disease was documented in the US and diminished in subsequent years (Table 1). This pattern is similar to other wildlife diseases that caused large-scale mortality among susceptible species during initial emergence or reemergence (Berger et al. 1998; Dhondt et al. 1998; Yaremych et al. 2004; Blehert et al. 2009) and may be the result of differences in susceptibility of new hosts, pathogen evolution, or changes in environmental factors (climate change, habitat fragmentation and destruction, etc.) that influence transmission dynamics. Additionally, increased early recognition of the disease and understanding of its etiology may play a role in the observed reduction in the scale of mortality events. For example, there may have been temporal variation in the timing and intensity of management activities such as collection and incineration of carcasses. These changes would presumably have reduced the spread of the virus by removing infectious carcasses from the landscape.
Our retrospective investigation demonstrated APMV-1 continues to primarily affect DCCO on their nesting grounds in the midwestern US and the frequency of outbreaks in DCCO appears to have increased during the decade. We found APMV-1 detection to be uncommon in conesting species during DCCO APMV-1 outbreaks. In a recent study, although nine of 10 APMV-1 isolates from 2008 outbreaks in DCCO were not detected by vNDV RRT-PCR, all four that were pathotyped by ICPI met World Organisation for Animal Health criteria as vNDV (Rue et al. 2010). In this study, isolates from 36 of 55 DCCO APMV-1 outbreaks during 1992–2012 were characterized and all were vNDV according to World Organisation for Animal Health standards. Although determining the causal factors involved in the spatial and temporal patterns of this disease and evolving pathogenicity of APMV-1 isolates from cormorants was outside the scope of this project, the information presented provides a foundation for more detailed investigations, including effects of management and alteration of movement and habitat on the occurrence of APMV-1 outbreaks in DCCO.
ACKNOWLEDGMENTS
We thank the many state, federal, and tribal biologists that provided information on avian mortality events to the USGS–National Wildlife Health Center (NWHC). We are especially grateful to the biologists, pathologists, and technicians from US Fish and Wildlife Service, Michigan Department of Natural Resources, Minnesota Department of Natural Resources, New York Department of Environmental Conservation, Oregon Department of Fish and Wildlife, Utah Division of Wildlife Resources, Wisconsin Department of Natural Resources, National Parks Service, Leech Lake Band of Ojibwe, and US Department of Agriculture–Animal and Plant Health Inspection Service–Wildlife Services. We thank the various pathologists, biologists and technicians, in particular R. Long and T. Egstand, at NWHC for their contributions to the diagnostic work. We also thank J. Struthers for assistance with compiling records from the NWHC database. Use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the US government.