Minchinia nelsoni (Haplosporida) Disease Syndrome in the American Oyster Crassostrea virginica

SYNOPSIS. Minchinia nelsoni disease was studied in 3 populations of oysters from Marumsco Bar, Pocomoke Sound, Maryland. The native population was sampled regularly beginning in 1961 shortly after the disease invaded this area. Two populations of healthy year-old oysters were introduced (one in 1965 and one in 1966) into Pocomoke Sound and studied concurrently with the native population to determine the course of development of the disease and epizootic differences in populations. Gross and microscopic examination of oysters showed progressive stages of infection. Initial infection occurred in epithelia of the filtering organs (gill, palp, water tubes) and spread into connective tissue where hyaline hemocytes infiltrated. Intermediate infection was characterized by local infection and infiltration of connective tissue in and adjacent to epithelia of gill, palp, esophagus, stomach, gut, diverticula, and gonadal alveoli. Advanced infection was recognized by the general invasion and infiltration of connective tissue and the circulatory system by hyaline hemocytes. Terminal infections showed histologically massive pyknosis of nuclei and necrosis of tissues before outward signs of death were apparent. Remission of the disease was recognized by diminution of infection intensity and infiltration; localization of parasites near external epithelia; increased pigment cell formation; and diapedesis and deposition of necrotic parasites and tissue against the shell, followed by external conchiolinous encapsulation. Death from M. nelsoni disease may be attributed in part to seasonal environmental or physiologic stresses, which infected oysters, weakened and in poor condition due to the pathologic manifestations of the disease, are unable to tolerate. The gross pathologic signs of disease were: pale digestive gland, poor condition, mantle recession, weakness, and conchiolinous shell depositions. The microscopic signs were: diapedesis, relative decrease in numbers of phagocytes; relative increase in numbers of hyaline hemocytes; phagocytosis, fibrosis, cellular infiltration, abscess, ulceration, excessive pigment cell formation, mechanical disruption, pyknosis, and necrosis.     

History and epizootiology of Haplosporidium nelsoni (MSX), an oyster pathogen in Delaware Bay, 1957–1980

Between 1957 and 1959, a previously unknown sporozoan parasite, now designated as Haplosporidium nelsoni (formerly Minchinia nelsoni), or MSX, killed 90–95% of the oysters in lower Delaware Bay. Native oysters have been studied for more than 20 years since then to determine long-term disease and mortality patterns resulting from this host-parasite association. Development of resistance to MSX-kill in native oysters has reduced disease mortality to about half the original level, even though the pathogen continues to be very active in the bay. Since the initial epizootic, MSX levels have fluctuated in a cyclic pattern with peaks every 6 to 8 years. Periods of low disease pressure follow very cold winters, while average or above average winter temperatures correlate with high MSX activity. During peak years, every oyster in the lower bay may become infected. Although the parasite is salinity limited, salinities in the lower bay, the area from which oysters are marketed, are nearly always favorable for MSX, and fluctuations in river flow have almost no effect on MSX in this region. Infection periods recur each summer. Some oysters die soon after becoming infected; others survive through winter, but die in spring as the pathogen compounds normal overwinter stresses. Many survivors are able to suppress or rid themselves of infections when temperatures approach 20°C in late spring. Resistance to MSX-kill in native oysters is not correlated with an ability to prevent infection, but with restriction of parasites to localized, nonlethal lesions. The persistence of “hot spots” for infection in areas where oysters are sparse, the lack of spores in infected oysters, and failure to transmit the disease experimentally lead to the hypothesis that an alternate or reservoir host produces infective stages of MSX.     

Effects of natural and field-simulated blooms of the dinoflagellate Prorocentrum minimum upon hemocytes of eastern oysters, Crassostrea virginica, from two different populations

Oysters, Crassostrea virginica, from two populations, one from a coastal pond experiencing repeated dinoflagellate blooms (native), and the other from another site where blooms have not been observed (non-native), were analyzed for cellular immune system profiles before and during natural and simulated (by adding cultured algae to natural plankton) blooms of the dinoflagellate Prorocentrum minimum. Significant differences in hemocytes between the two oyster populations, before and after the blooms, were found with ANOVA, principal components analysis (PCA) and ANOVA applied to PCA components. Stress associated with blooms of P. minimum included an increase in hemocyte number, especially granulocytes and small granulocytes, and an increase in phagocytosis associated with a decrease in aggregation and mortality of the hemocytes, as compared with oysters in pre-bloom analyses. Non-native oysters constitutively had a hemocyte profile more similar to that induced by P. minimum than that of native oysters, but this profile did not impart increased resistance. The effect of P. minimum on respiratory burst was different according to the origin of the oysters, with the dinoflagellate causing a 35% increase in the respiratory burst of the native oysters but having no effect on that of the non-native oysters. Increased respiratory burst in hemocytes of native oysters exposed to P. minimum in both simulated and natural blooms may represent an adaptation to annual blooms whereby surviving native oysters protect themselves against tissue damage from ingested P. minimum.     

Epizootiology of diseases of oysters(Crassostrea virginica), and parasites of associated organisms in eastern North America

Haplosporidan parasites of oysters have been reported from four continents. Those of the generalMinchinia, Haplosporidium, andMarteilia, which cause serious diseases of oysters, have been intensively studied. Epizootiology of these highly pathogenic species is well known. Life cycles are obscure for all haplosporidans because artificial infections have not been achieved. The high pathogenicity of newly-discovered haplosporidan diseases to native oysters in eastern North America and western Europe may indicate that these are exotic pathogens parasitizing susceptible oysters not previously exposed to these disease agents. Epizootiology of two haplosporidan pathogens along the middle Atlantic Coast of North America during 25 years of disease activity is discussed.Haplosporidium nelsoni sporulates only rarely and its life cycle remains unconfirmed. Resistant oysters were developed in nature and from laboratory breeding.Haplosporidium costale which causes Seaside Disease in high-salinity waters appears to be a more acclimated disease with regular patterns of infection and mortality. Several minor parasites whose life cycles and host species need more study are mentioned.Contribution No. 1147. Virginia Institute of Marine Science; Gloucester Point, Virginia 23062     

Avian malaria Plasmodium relictum in native Hawaiian forest birds: epizootiology and demographic impacts on ‵apapane Himatione sanguinea

The role of introduced avian malaria Plasmodium relictum in the decline and extinction of native Hawaiian forest birds has become a classic example of the potential effect of invasive diseases on biological diversity of naïve populations. However, empirical evidence describing the impact of avian malaria on fitness of Hawai‵i's endemic forest birds is limited, making it difficult to determine the importance of disease among the suite of potential limiting factors affecting the distribution and abundance of this threatened avifauna. We combined epidemiological force‐of‐infection with multistate capture––recapture models to evaluate a 7‐year longitudinal study of avian malaria in ‵apapane, a relatively common native honeycreeper within mid‐elevation Hawaiian forests. We found that malaria transmission was seasonal in this mid‐elevation forest; transmission peaked during fall and during some years produced epizootic mortality events. Estimated annual mortality of hatch‐year birds typically exceeded 50% and mortality of adults exceeded 25% during epizootics. The substantial impact of avian malaria on this relatively common native species demonstrates the key role this disease has played in the decline and extinction of Hawaiian forest birds.     

Epizootiology of Minchinia nelsoni in susceptible wild oysters in Virginia, 1959 to 1971

Twelve years after the haplosporidian parasite Minchinia nelsoni erupted causing severe epizootics of oysters in lower Chesapeake Bay, regular patterns of mortalities and disease prevalence persisted. Distribution changed with salinity and weather patterns, but in mesohaline areas (about 15–25 ‰) infective pressure remained high and relatively stable despite scarcity of oysters.Susceptible disease-free oytsers from low-salinity areas of the James River, the major seed area in Virginia, were transplanted annually to disease-prevalent areas for monitoring in trays. Mortalities were usually over 50% the first year and almost as high in the second year. Prevalences of the pathogen, called MSX, ranged from 35 to 50% in live oysters. Seasonal patterns of disease activity are depicted from 1960 through 1971, and they exhibit exceptional regularity for open-water conditions. Source and history of oysters, and timing of exposure are important variables that affect disease activity as well as size and age. The disease caused by MSX appears to be not contagious in trays.The patterns of disease and mortality obtained from susceptible wild oysters without previous exposure provided a basis for evaluating other stocks including genetic strains selected for disease resistance.     

Population Ecology of Hybrid Mesquite (Prosopis Species) in Western Australia: How Does it Differ from Native Range Invasions and What are the Implications for Impacts and Management?

Beneficial exotic trees and shrubs have been widely spread throughout semiarid and arid regions of the world. These trees and shrubs can however cause severe negative impacts. Mesquite (Prosopis species), native to the New World, is one example which continues to be promoted despite causing serious impacts both in its native and introduced ranges. We describe the population structure of the largest population of fire-tolerant hybrid mesquite (P. velutina × P. glandulosa var. glandulosa × P. pallida) in Australia, which was intentionally established in the 1930s. We compare it with invasive populations within its native range, and consider the implications for managing exotic mesquite invasions. We found relatively high juvenile densities at all levels of canopy cover (<30% to 90–100%), and low mortality rates for both juveniles and adults (<2%/y), which suggests that populations are still in an early phase of invasion. Exotic populations differed from native range populations in being more dense (average 4,859 adults/ha), having a sizable sapling (seedling and juvenile) bank that can remain quiescent under canopy cover (average 10,914 seedlings and juveniles/ha), failing to act as nurse plants for native shrubs (<8 native shrubs/ha), and almost totally excluding the herbaceous (grass) layer (average 0.3% cover). Our results suggest that ecosystem impacts in the introduced range are likely to be even worse, and management even more difficult, than has already been reported for invasions within its native range. The lack of feasible means for managing highly invasive, broad-scale mesquite populations need to be addressed, and needs to be considered explicitly when promoting mesquite as a beneficial plant.     

Epizootiology of Minchinia costalis in susceptible oysters in Seaside Bays of Virginia's Eastern Shore, 1959–1976

“Seaside Disease” of oysters caused by Minchinia costalis (Haplosporida, Sporozoa) produced annual mortalities on the Seaside of the Delmarva Peninsula along the middle Atlantic Coast from Chesapeake Bay to Delaware Bay, U.S.A. The May–June mortalities occurred from 1959 to 1976 without exception; deaths began in late May, peaked in June, and were usually over by July 1. The pathogen developed rapidly from March to May, and sporulation occurred in connective tissues of all organs in May and June. Exposure to a May–June enzootic was required to obtain infections. The pathogen remained subclinical until late winter of the following year. A sympatric pathogen, Minchinia nelsoni, which kills oysters extensively in lower Chesapeake Bay, was present but caused only minor mortalities. Salinities > 30 parts per thousand seem to favor M. costalis and inhibit M. nelsoni. Prevalences of both diseases in live oysters or gapers are given for 11 of the 18 years monitored.     

Population Dynamics and Behavior of Bighorn Sheep with Infectious Keratoconjunctivitis

ABSTRACT Introduced disease is a major mortality factor in some populations of bighorn sheep (Ovis canadensis). Epizootics of infectious keratoconjunctivitis (IKC) and contagious ecthyma occurred in bighorn sheep in the Silver Bell Mountains of south-central Arizona, USA, from 1 December 2003 to 31 March 2004. Our objectives were to 1) investigate the influence of the epizootic on abundance and demographics and 2) examine how IKC affected the mortality, behavior, and movements of clinically affected animals. Morbidity was 39%, and all sex and age classes were affected. The population declined 23%, with most mortality in the adult female (1 M, 11 F) segment of the population. Of the diseased animals that were marked (n = 27), 44% recovered and 44% died. Predation (50%) and starvation (33%) were the primary causes of mortality of diseased bighorn sheep. Bighorn sheep that were infected spent less time feeding and moved less than noninfected animals during the epizootic. Managers might be able to minimize losses of infected animals through predator control. To minimize losses to starvation, managers should refrain from any activity that disturbs infected animals (including treatment) because disturbances increase energy expenditures and expose infected animals to injury.     

Gyrodactylus unicopula Glukhova, 1955, from young plaice Pleuronectes platessa L. with notes on the ecology of the parasite

A population of Gyrodactylus unicopula Glukhova, 1955, is described from the gill filaments of young plaice from a nursery ground on the north-west coast of Scotland. The seasonal incidence of this parasite was followed by sampling the 1965 year class of plaice at intervals of from one to three months from April 1965 to April 1967. The level of intensity is usually low and shows no progressive increase, although the fish are shown to be capable of supporting much higher populations of the parasite, which suggests the existence of some mechanism controlling the level of intensity in the natural population. The distribution of G. unicopula on the filamentous surface of the gills is shown in detail. χ² analyses indicated no significant association or antagonism between this parasite and two ciliates commonly found on the gills of the same population of plaice. Gyrodactyliasis, the disease caused by parasites of the genus Gyrodactylus, is discussed with special reference to marine fish farming.     

Population‐level influence of a recurring disease on a long‐lived wildlife host

Despite a heightened interest regarding the role of infectious diseases in wildlife conservation, few studies have explicitly addressed the impacts of chronic, persistent diseases on long‐term host population dynamics. Using mycoplasmal upper respiratory tract disease (URTD) within natural gopher tortoise Gopherus polyphemus populations as a model system, we investigated the influence of chronic recurring disease epizootics on host population dynamics and persistence using matrix population models and Markov chain models for temporally autocorrelated environments. By treating epizootics as a form of environmental stochasticity, we evaluated host population dynamics across varying levels of outbreak duration (ρ), outbreak recurrence (f), and disease‐induced mortality (μ). Baseline results indicated a declining growth rate (λ) for populations under unexposed or enzootic conditions (λ_Enzootic= 0.903, 95% CI: 0.765–1.04), and a median time to quasi‐extinction of 29 years (range: 28–30 years). Under recurring epizootics, stochastic growth rates overlapped with baseline growth rates, and ranged between 0.838–0.902. Median quasi‐extinction times under recurring epizootics also overlapped for most scenarios with those of baseline conditions, and ranged between 18–29 years, with both metrics decreasing as a function of f and μ. Overall, baseline (enzootic) conditions had a greater impact on λ than epizootic conditions, and demographic vital rates were proportionately more influential on λ than disease‐ or outbreak‐associated parameters. Lower‐level elasticities revealed that, among disease‐ and outbreak‐associated parameters, increases in μ, force of infection (φ), and f negatively influenced λ. The impact of disease on host population dynamics depended primarily on how often a population underwent an epizootic state, rather than how long the epizootic persisted within the exposed population. The modeling framework presented in this paper could be widely applied to a range of wildlife disease systems in which hosts suffer from persistent recurring diseases.     

Application of a Demographic Model for Evaluating Proposed Oyster-Restoration Actions in Chesapeake Bay

A demographic model was developed for oyster populations in the Chesapeake Bay, USA, to explore population responses to proposed management actions in support of an Environmental Impact Statement and Environmental Risk Assessment for oyster restoration. The model indicated that high natural mortality due to disease strongly controlled the population of native Eastern oysters. Continuing restoration effort at recent levels was predicted not to increase oyster populations. An enhanced restoration program that included habitat improvement and stocking would likely increase populations, particularly in areas with lower salinity where disease prevalence was lower. However, population numbers would likely reach a plateau much less than the restoration goal a few years after enhanced restoration efforts ended. A harvest moratorium was predicted to have a smaller positive effect than enhanced restoration. A moratorium likely would take much longer than the 10-year restoration period to meet restoration goals given the present high natural mortality rates. The proposed introduction of non-native Suminoe oysters was not modeled because insufficient data existed with which to parameterize the model. These results were used semi-quantitatively in the Ecological Risk Assessment to evaluate population trajectories and speculate about population changes more than 10 years after implementation of a management action.     

SNPs across time and space: population genomic signatures of founder events and epizootics in the House Finch (Haemorhous mexicanus)

Identifying genomic signatures of natural selection can be challenging against a background of demographic changes such as bottlenecks and population expansions. Here, we disentangle the effects of demography from selection in the House Finch (Haemorhous mexicanus) using samples collected before and after a pathogen‐induced selection event. Using ddRADseq, we genotyped over 18,000 SNPs across the genome in native pre‐epizootic western US birds, introduced birds from Hawaii and the eastern United States, post‐epizootic eastern birds, and western birds sampled across a similar time span. We found 14% and 7% reductions in nucleotide diversity, respectively, in Hawaiian and pre‐epizootic eastern birds relative to pre‐epizootic western birds, as well as elevated levels of linkage disequilibrium and other signatures of founder events. Despite finding numerous significant frequency shifts (outlier loci) between pre‐epizootic native and introduced populations, we found no signal of reduced genetic diversity, elevated linkage disequilibrium, or outlier loci as a result of the epizootic. Simulations demonstrate that the proportion of outliers associated with founder events could be explained by genetic drift. This rare view of genetic evolution across time in an invasive species provides direct evidence that demographic shifts like founder events have genetic consequences more widespread across the genome than natural selection.     

Demography and dynamics of three wild horse populations in the Australian Alps

Wild horses (Equus caballus) are a non‐native species occupying over 2800 km² of the nationally significant Australian Alps National Parks. We estimated key demographic parameters (fecundity, adult and juvenile survival and annual finite population growth rate) over 3 years and related these to horse body condition and available food for three populations under natural conditions, and found a trend consistent with food limitation. The populations were independent, with different site characteristics and occupied areas, identified by land managers, as areas of concern about possible conservation impacts. Annual fecundity and juvenile survival varied across sites averaging between 0.21 and 0.31 female young per adult female, and 0.83 and 0.90 per annum, respectively, and annual adult survival was consistent across sites averaging 0.91 per annum. One population was increasing (λ = 1.09 year^?1; 95% CI 1.04–1.14) and two populations were stable (λ ～ 1.0 year^?1). Mean body condition of horses was positively correlated with mean pasture biomass rank. Across the three populations, fecundity, recruitment, body condition and annual finite population growth rate were lowest when mean pasture biomass rank was lowest and conversely highest when pasture rank was highest. We conclude that food limitation appears to be operating across these three sites. We used our results to assess the sensitivity of annual finite rate of increase (λ) to changes in key demographic parameters and found that λ was most sensitive to a change in adult survival, with the second most sensitive parameter being fecundity. Thus, if the aim of management is to reduce the size of the wild horse population then targeting adult survival is most important, followed by fecundity. Finally, we estimated the linear, negative, numerical response for wild horses between annual λ and horses per unit pasture biomass.     

Possible effects of ingested lead gunshot on populations of ducks wintering in the UK

Although the use of lead ammunition for shooting wildfowl and/or over listed wetlands in the UK has been banned, c. 70% of ducks shot in England (the only UK country with compliance monitoring) are still shot with lead and the proportion of ducks found dead with signs of lead poisoning from ingested gunshot has not declined significantly since the ban. However, there is little quantitative evidence of the impacts of additional mortality from lead poisoning on duck populations. For the eight duck species that winter in freshwater habitats in the UK, we found that inter‐specific variation in mean population growth rate during the period 1990/1991 to 2013/2014 was significantly negatively correlated with two independent measures of the prevalence of ingested lead gunshot in the UK and Europe. This relationship was found for a wide range of different periods over which population growth was estimated, and also for annual growth rates in the period 1966/1967 to 2013/2014, derived from smoothed population trajectories. These findings support the hypothesis that ingested lead gunshot might affect population trend. An alternative hypothesis, that migratory short‐stopping driven by climate change affected trends in numbers of ducks wintering in the UK, was not supported by simple or partial correlation results. The possible impact of ingested lead gunshot on the Common Pochard Aythya ferina, a species listed as globally threatened, is of special concern.     

Absence of a genetic bottleneck in a wild rabbit (Oryctolagus cuniculus) population exposed to a severe viral epizootic

Infectious diseases and their demographic consequences are thought to influence the genetic diversity of populations. In Europe, during the last 50 years, the European rabbit (Oryctolagus cuniculus) has suffered two important viral epizootics: myxomatosis and rabbit viral haemorraghic disease (RVHD). Although mortality rates were very high, the impact of these diseases on genetic diversity has never been assessed directly. The subject of this paper is a wild rabbit population in France, which has been studied since the beginning of the 1980s. The first outbreak of RVHD occurred in 1995 and provoked a demographic crash. The population, sampled for the first time in 1982 and 1994, was sampled again at the end of 1996 to examine the impact of the epizootic on genetic diversity. In spite of the observed high mortality rate ( approximately 90%), analysis of 14 polymorphic loci (allozymes and microsatellites) showed no loss in genetic diversity after the epizootic. Determination of temporal changes in allele frequencies indicated that the population evolved under genetic drift. The temporal method of Waples demonstrated a significant decrease in the effective population size (Ne) correlated with the demographic crash due to the epizootic. However, the population had only been studied for two generations after the epizootic and the remnant population size probably stayed high enough ( approximately 50 individuals) to keep its genetic diversity at the precrash level. These results suggest that, contrary to what is usually thought and in spite of the subsequent high mortality rates, past epizootics (especially myxomatosis) may have had little effect on the genetic diversity of wild rabbit populations in Europe.     

A genetic test for recruitment enhancement in Chesapeake Bay oysters, Crassostrea virginica, after population supplementation with a disease tolerant strain

Many of the methods currently employed to restore Chesapeake Bay populations of the eastern oyster, Crassostrea virginica, assume closed recruitment in certain sub-estuaries despite planktonic larval durations of 2–3 weeks. In addition, to combat parasitic disease, artificially selected disease tolerant oyster strains are being used for population supplementation. It has been impossible to fully evaluate these unconventional tactics because offspring from wild and selected broodstock are phenotypically indistinguishable. This study provides the first direct measurement of oyster recruitment enhancement by using genetic assignment tests to discriminate locally produced progeny of a selected oyster strain from progeny of wild parents. Artificially selected oysters (DEBY strain) were planted on a single reef in each of two Chesapeake Bay tributaries in 2002, but only in the Great Wicomico River (GWR) were they large enough to potentially reproduce the same year. Assignment tests based on eight microsatellite loci and mitochondrial DNA markers were applied to 1579 juvenile oysters collected throughout the GWR during the summer of 2002. Only one juvenile oyster was positively identified as an offspring of the 0.75 million DEBY oysters that were planted in the GWR, but 153 individuals (9.7%) had DEBY ×wild F1 multilocus genotypes. Because oyster recruitment was high across the region in 2002, the proportionately low enhancement measured in the GWR would not otherwise have been recognized. Possible causes for low enhancement success are discussed, each bearing on untested assumptions underlying the restoration methods, and all arguing for more intensive evaluation of each component of the restoration strategy.     

Importance of ambient saturation deficits in an epizootic of the fungus Neozygites floridana in cassava green mites (Mononychellus tanajoa)

The mite-pathogenic fungus Neozygites floridana Fisher (Entomophthorales: Neozygitaceae) is considered to have potential for the biological control of the cassava green mite, Mononychellus tanajoa (Bondar). However, its activity is sporadic and laboratory data suggest a strong dependence on night-time saturation deficits for transmission. We report on an epizootic of this fungus in a mite population in northeastern Brazil. During the epizootic, host populations appeared to be limited by a combination of the pathogen and a predatory mite Neoseiulus idaeus (Acari: Phytoseiidae). When temperatures increased, the epizootic finished and the host population began to grow. Abiotic conditions could not explain the variation in host mortality following pickup of infective propagules in this epizootic. However, night-time saturation did help to explain the variation in transmission from infective cadavers to newly killed hosts. This supports laboratory observations that horizontal transmission between hosts is determined mainly by saturation deficits, while the process of infection is little affected by abiotic conditions. A further field observation was the near-absence of resting spores in dead mites (ca. 0.1% of cadavers), suggesting that the pathogen population was unsuccessful in producing inoculum to infect future M. tanajoa populations. The implications are that this pathogen will only be effective as a biological control agent in periods of high relative humidity, and establishment in new areas may be limited by resting spore formation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Natural history of a bighorn sheep pneumonia epizootic: Source of infection,course of disease,and pathogen clearance

A respiratory disease epizootic at the National Bison Range (NBR) in Montana in 2016–2017 caused an 85% decline in the bighorn sheep population, documented by observations of its unmarked but individually identifiable members, the subjects of an ongoing long‐term study. The index case was likely one of a small group of young bighorn sheep on a short‐term exploratory foray in early summer of 2016. Disease subsequently spread through the population, with peak mortality in September and October and continuing signs of respiratory disease and sporadic mortality of all age classes through early July 2017. Body condition scores and clinical signs suggested that the disease affected ewe groups before rams, although by the end of the epizootic, ram mortality (90% of 71) exceeded ewe mortality (79% of 84). Microbiological sampling 10 years to 3 months prior to the epizootic had documented no evidence of infection or exposure to Mycoplasma ovipneumoniae at NBR, but during the epizootic, a single genetic strain of M. ovipneumoniae was detected in affected animals. Retrospective screening of domestic sheep flocks near the NBR identified the same genetic strain in one flock, presumptively the source of the epizootic infection. Evidence of fatal lamb pneumonia was observed during the first two lambing seasons following the epizootic but was absent during the third season following the death of the last identified M. ovipneumoniae carrier ewe. Monitoring of life‐history traits prior to the epizootic provided no evidence that environmentally and/or demographically induced nutritional or other stress contributed to the epizootic. Furthermore, the epizootic occurred despite proactive management actions undertaken to reduce risk of disease and increase resilience in this population. This closely observed bighorn sheep epizootic uniquely illustrates the natural history of the disease including the (presumptive) source of spillover, course, severity, and eventual pathogen clearance.     

ECOLOGY AND POPULATIONS

Bagge , P., Lehtovouri , M. & Lindqvist , O. 1963. [Quantitative investigations of the bird fauna of Inari and Enontekiö in northern Lappland.] Bailey , R. S. 1967. An index of bird population changes on farmland. Brenner , F. J. 1966. The influence of drought on reproduction in a breeding population of Redwinged Blackbirds Agelaius phoeniceus. Davis , B. N. K. 1967. Bird feeding preferences among different crops in an area near Huntingdon. Dick , J. A. & Rising , J. D. 1965. A comparison of foods eaten by Eastern Kingbirds and Western Kingbirds in Kansas. Evans , F. C. 1964. The food of Vesper, Field, and Chipping Sparrows nesting in an abandoned field in Southeastern Michigan. Haapanen , A. 1965. Bird fauna of the Finnish forests in relation to forest succession. Hildén , O. 1964. Ecology of duck populations in the island group of Valassaaret, Gulf of Bothnia. Jenkins , D., Watson , A. & Miller , G. R. 1964. Current research on Red Grouse in Scotland. Kear , J. 1962. Feeding habits of the Grey Lag Goose Anser anser on the Island of Bute. Kear , J. 1965. Recent changes in Scottish barley acreages and the possible effect on wild geese. KEAR, J. 1967. Feeding habits of the Greylag Goose Anser unser in Iceland, with reference to its interaction with agriculture. Kikkawa , J. 1966. Population distribution of land birds in temperate rainforest of southern New Zealand. Kulczycki , A. 1966. [The birds of the Lańcut Park.] Murton , R. K. 1965. Natural and artificial population control in the Wood-Pigeon. Murton , R. K. c. 1966. A statistical evaluation of the effect of Wood-Pigeon shooting as evidenced by the recoveries of ringed birds. Nelson , J. B. 1966. Population dynamics of the Gannet (Sula bassana) at the Bass Rock, with comparative information for other Sulidae. Paynter , R. A. 1966. A new attempt to construct life tables for Kent Island Herring Gulls. Pulliainen , E. 1963. On the history, ecology, and ethology of the Mallards (Anas platyrhynchos L.) overwintering in Finland. Ratcliffe , D. A. 1967. The Peregrine situation in Great Britain 1965–66. Schemnitz , S. D. 1964. Comparative ecology of Bobwhite and Scaled Quail in the Oklahoma Panhandle. Skead , C. J. 1967. Ecology of birds if the Eastern Cape Province. Sladen , W. J. L., Wood , R. C. & Emison , W. B. 1966. Antarctic avian population studies, 1965–66. Snow , D. W. & Snow , B. K. 1961. Breeding seasons and annual cycles of Trinidad land-birds. Snow , D. W. 1966. Population dynamics of the Blackbird. Soikkeli , M. 1965. On the structure of the bird fauna of some coastal meadows in western Finland. Watson , A. 1965. Research on Scottish Ptarmigan. Watson , A. 1966. Hill birds of the Cairngorms. Westerskov , K. 1965. Winter ecology of the Partridge Perdix perdix in the Canadian Prairie. Williamson , K. 1964. Bird Census work in Woodland. Williamsonj , K. 1967. The bird community of farmland. Willson , M. F. 1966. Breeding ecology of the Yellow-headed Blackbird.