Effect of Myxoma Virus Species Jump on Iberian Hare Populations

The myxoma virus species jump from European rabbits (Oryctolagus cuniculus) to Iberian hares (Lepus granatensis) has raised concerns. We assess the decline suffered by Iberian hare populations on the Iberian Peninsula and discuss the association between the effect of myxomatosis and the average abundance index, which we estimated by using hunting bags.

Quantifying resistance to myxomatosis in wild rabbits produces novel evolutionary insights

Wild rabbits in Australia developed genetic resistance to the myxoma virus, which was introduced as a biological control agent. However, little is known about the rate at which this evolutionary change occurred. We collated data from challenge trials that estimated rabbit resistance to myxomatosis in Australia and expressed resistance on a continuous scale, enabling trends in its development to be assessed over 45 years up to 1995. Resistance initially increased rapidly, followed by a plateau lasting ten years, before a second rapid increase occurred associated with the introduction of European rabbit fleas as myxoma virus vectors. By contrast, in the United Kingdom, where rabbit flea vectors were already present when the myxoma virus initially spread, resistance developed more slowly. No estimates of rabbit resistance to myxomatosis have been made for almost 30 years, despite other highly lethal rabbit pathogens becoming established worldwide. Continued testing of wild-caught rabbits in Australia to determine current levels of resistance to myxomatosis is recommended to assess its current effectiveness for managing pest rabbits. Given the economic and environmental significance of invasive rabbits, it would be remiss to manage such biological resources and ecosystem services poorly.

Nowhere to run, rabbit: the cold-war calculus of disease ecology

During the cold war, Frank Fenner (protégé of Macfarlane Burnet and René Dubos) and Francis Ratcliffe (associate of A. J. Nicholson and student of Charles Elton) studied mathematically the coevolution of host resistance and parasite virulence when myxomatosis was unleashed on Australia's rabbit population. Later, Robert May called Fenner the "real hero" of disease ecology for his mathematical modeling of the epidemic. While Ratcliffe came from a tradition of animal ecology, Fenner developed an ecological orientation in World War II through his work on malaria control (with Ratcliffe and Ian Mackerras, among others)-that is, through studies of tropical medicine. This makes Fenner at least a partial exception to other senior disease ecologists in the region, most of whom learned their ecology from examining responses to agricultural challenges and animal husbandry problems in settler colonial society. Here I consider the local ecologies of knowledge in southeastern Australia during this period, and describe the particular cold-war intellectual niche that Fenner and Ratcliffe inhabited.

Early infections by myxoma virus of young rabbits (Oryctolagus cuniculus) protected by maternal antibodies activate their immune system and enhance herd immunity in wild populations

The role of maternal antibodies is to protect newborns against acute early infection by pathogens. This can be achieved either by preventing any infection or by allowing attenuated infections associated with activation of the immune system, the two strategies being based on different cost/benefit ratios. We carried out an epidemiological survey of myxomatosis, which is a highly lethal infectious disease, in two distant wild populations of rabbits to describe the epidemiological pattern of the disease. Detection of specific IgM and IgG enabled us to describe the pattern of immunity. We show that maternal immunity attenuates early infection of juveniles and enables activation of their immune system. This mechanism associated with steady circulation of the myxoma virus in both populations, which induces frequent reinfections of immune rabbits, leads to the maintenance of high immunity levels within populations. Thus, myxomatosis has a low impact, with most infections being asymptomatic. This work shows that infection of young rabbits protected by maternal antibodies induces attenuated disease and activates their immune system. This may play a major role in reducing the impact of a highly lethal disease when ecological conditions enable permanent circulation of the pathogen.

Myxomatosis in 1950s Britain

In 1953 myxomatosis, a viral disease of rabbits, broke out in Britain for the first time. It rapidly killed tens of millions of the animals from Kent to the Shetlands. Many farmers and foresters welcomed a disease that virtually eliminated a longstanding and serious agricultural pest. Others were horrified by the sight of thousands of dead and dying animals. With meat still rationed, consumers rued the loss of a cheap and nutritious foodstuff. Rough shooters deplored the loss of prey and hatters and furriers the unavailability of the fur on which their businesses depended. Rabbits also had champions within the 'establishment'; these included Winston Churchill who was personally influential in making deliberate transmission of the disease a criminal offence. The arrival in Britain of myxomatosis presented the authorities with difficult questions: should they try to contain it, spread it or do nothing; should they take advantage of rabbit depopulation and try to exterminate such a destructive animal? In the event the outbreak was allowed to run its course and rabbit extermination became government policy. This article considers who or what was responsible for the disease reaching the UK and how it then spread throughout the country. It examines the responses of government, other institutions and members of the public. Finally, it explores the impact of rabbit de-population on agriculture, the natural environment and public opinion.

Myxoma virus in the European rabbit: interactions between the virus and its susceptible host

Myxoma virus (MV) is a poxvirus that evolved in Sylvilagus lagomorphs, and is the causative agent of myxomatosis in European rabbits (Oryctolagus cuniculus). This virus is not a natural pathogen of O. cuniculus, yet is able to subvert the host rabbit immune system defenses and cause a highly lethal systemic infection. The interaction of MV proteins and the rabbit immune system has been an ideal model to help elucidate host/poxvirus interactions, and has led to a greater understanding of how other poxvirus pathogens are able to cause disease in their respective hosts. This review will examine how MV causes myxomatosis, by examining a selection of the identified immunomodulatory proteins that this virus expresses to subvert the immune and inflammatory pathways of infected rabbit hosts.

Occurrence of atypical myxomatosis in Central Europe: clinical and virological examinations

An outbreak of the atypical form of myxomatosis struck a rabbit farm in Hungary. The animals had previously been vaccinated with a vaccine containing Shope rabbit fibroma virus strain. The disease appeared in winter when the presence of mosquitoes and fleas is not common. The virus was isolated from an eyelid specimen of a naturally infected rabbit. The surviving animals were observed for four weeks, blood samples were collected and, after euthanasia, organ specimens were also examined by morphological methods including pathology and electron microscopy. Serum samples were examined by virus neutralisation for antibodies. Genetic analysis of the isolated virus was carried out by polymerase chain reaction (PCR) and direct sequencing. The primers were designed on the basis of the major envelope gene (Env) of the Lausanne reference strain in the GenBank. The viral proteins were examined by SDS-PAGE. The isolated virus (ref. no.: BP04/2001) was able to infect the susceptible animals directly, by contact. The disease was characterised by respiratory symptoms of the upper tracheal tract, conjunctivitis and high mortality by the 11th-14th day. Aerogenic infection with strain BP04/2001 resulted in 100% morbidity among the susceptible animals. Sequencing of the amplified 400-bp-long DNA revealed 97% homology with the Env gene of the Lausanne strain, which proves that strain BP04/2001 is a variant of the Lausanne strain having been enzootic throughout Europe. The live vaccine strain used in Hungary against myxomatosis, which is also a Lausanne-derived strain, protected the animals. According to the protein analysis a protein of 200 kDa in size is not expressed in strain BP04/2001. This is the first report on atypical myxomatosis in Central Europe. The virus spreads by airborne transmission and may cause severe losses in the rabbit population.

Atypical Myxomatosis - Virus Isolation, Experimental Infection of Rabbits and Restriction Endonuclease Analysis of the Isolate

Atypical form of myxomatosis, which caused non-lethal and clinically mild disease in domestic rabbits 1 month after immunization with a commercially available vaccine MXT, is described. The isolated myxoma virus designated as Litovel 2 (Li-2) did not induce systemic disease following subcutaneous and intradermal applications in susceptible experimental rabbits but led to the immune response demonstrated by ELISA. No severe disease was induced in those Li-2 inoculated rabbits by challenge with the virulent strains Lausanne (Lu) or Sanar (SA), while the control animals showed nodular form of myxomatosis with lethal course of the illness. Restriction fragment length polymorphism (RFLP) of genomic DNA with KpnI and BamHI endonucleases was used for genetic characterization of the Li-2 isolate, the vaccine strain MXT and both virulent strains Lu and SA, respectively. In general, RFLP analysis has shown to be informative for inferring genetic relatedness between myxoma viruses. Based on restriction endonuclease DNA fragment size distribution, it was evident that the pathogenic strain SA is genetically related to the reference strain Lu and the isolate Li-2 is more related, but not identical, to the vaccination strain MXT.

Monitoring the spread of myxoma virus in rabbit Oryctolagus cuniculus populations on the southern tablelands of New South Wales, Australia. III. Release, persistence and rate of spread of an identifiable strain of myxoma virus

An identifiable strain of myxoma virus was introduced into four local populations of wild rabbits Oryctolagus cuniculus on the southern tablelands of New South Wales (NSW) and its spread in the presence of other field strains was monitored for 6 months. The main vector in this region was considered to be the European rabbit flea Spilopsyllis cuniculi. Each population of rabbits was of a high density and living in groups of warrens covering areas from 59 to 87 hectares. Rabbits occupying centrally located warrens were inoculated with the virus in late September or early October (spring) and the subsequent appearance of myxomatosis across the sites monitored by trapping, shooting and visual observations. Samples, taken from rabbits with myxomatosis, were examined by polymerase chain reaction (PCR) that allowed identification of the introduced strain. On all four sites the introduced virus spread from the inoculated rabbits in the centrally located warrens to rabbits in surrounding warrens. On Sites 1 and 3, this spread continued across the entire site persisting for at least 118 and 174 days respectively. On Sites 2 and 4, the virus was detected for 78 and 62 days respectively and the subsequent inability to detect the introduced virus correlated with the appearance of an unrelated field strain. Using three different methods of calculation, rates of spread ranged from 3.7 to 17.8 m d(-1).

Monitoring the spread of myxoma virus in rabbit Oryctolagus cuniculus populations on the southern tablelands of New South Wales, Australia. II. Selection of a strain of virus for release

To be able to study the dynamics of myxoma virus spread following a release in the field, a strain of virus is required that is both highly transmissible and readily differentiated from other field strains. Eight strains of virus of known virulence for laboratory rabbits and with previously mapped and sequenced restriction fragment length polymorphisms, were used to infect groups of seronegative wild rabbits. Based on these trials, and on the nature of the DNA polymorphism, a virus designated Brooklands/2-93 was chosen as a strain suitable for experimental release. These trials confirmed that resistance to myxomatosis within wild rabbit populations continues to be substantial and that some rabbits are highly resistant. These rabbits probably have little role in transmission of virus. Most of the virus strains tested induced very small or invisible primary lesions at the inoculation site. Thus the secondary skin sites such as eyelids, face and ears may be critical for transmission.

Monitoring the spread of myxoma virus in rabbit Oryctolagus cuniculus populations on the southern tablelands of New South Wales, Australia. I. Natural occurrence of myxomatosis

A survey of rabbit populations in the southern tablelands of New South Wales, Australia, was carried out to establish the pattern of occurrence of myxomatosis in preparation for a deliberate release of myxoma virus. Myxomatosis was first detected in December and cases were found on most sites through to May. The serological profiles of rabbit populations suggested that their susceptibility to myxoma virus was generally low in winter and highest in spring and summer reflecting the presence of increasing numbers of susceptible young rabbits. This was consistent with the pattern of rabbit breeding, as determined from the distribution of births and reproductive activity in females and males, which occurred maximally in spring and early summer. The serology and age structure of rabbit populations on sites suggested that some rabbit populations can escape an annual myxomatosis epizootic. Although fleas were present on rabbits throughout the year and therefore not considered to be a limiting factor in the spread of myxomatosis, their numbers peaked at times coincident with peak rabbit breeding. It was concluded that mid to late spring was an optimal time for a deliberate release.

Epidemiology of viral haemorrhagic disease and myxomatosis in a free-living population of wild rabbits

From January 1993 to June 1996, the epidemiology of myxomatosis and viral haemorrhagic disease (VHD) was studied in a free-living population of wild rabbits (Oryctolagus cuniculus) in Spain by means of serological surveys and radiotracking. Myxomatosis was endemic and associated with the breeding period. Its serological pattern was characterised by a 100 per cent prevalence of antibodies in adult rabbits and a rapid increase in antibodies in young rabbits in their first year. No mortality from myxomatosis was detected in adults, and mortality in young rabbits could not be estimated because of interference by predators and scavengers and the deaths of many radiotagged rabbits inside their burrows. VHD was also an endemic disease associated with the breeding period. Adults had a higher prevalence of antibodies against VHD than young rabbits, reaching values of 80 to 90 per cent. During the study, there was an increase in rabbit numbers as a result of a decrease in mortality from predation which was associated with an increase in mortality due to VHD and in the prevalence of antibodies to VHD. Mortality from VHD was lower in rabbits with VHD antibodies than in seronegative rabbits, but some mortality from the disease was also detected in seropositive rabbits. The annual mean mortality rate due to VHD in adult rabbits was estimated to be 21.8 per cent.

Patterns of parasite aggregation in the wild European rabbit (Oryctolagus cuniculus)

Understanding the factors controlling the distribution of parasites within their host population is fundamental to the wider understanding of parasite epidemiology and ecology. To explore changes in parasite aggregation, Taylor's power law was used to examine the distributions of five gut helminths of the wild rabbit. Aggregation was found to be a dynamic process that varied with year, season, host sex, age class, and myxomatosis. Yearly and seasonal changes are thought, in the main, to be the result of variations in weather conditions acting upon infectious stages (or intermediate hosts). Evidence in support of this was the comparatively low degree of fluctuation in the aggregation of the pinworm, Passalurus ambiguus, as the infectious stage of this parasite is likely to be less susceptible to environmental variation. Host age had a marked effect on the level of aggregation of all parasites, but this effect varied between parasite species. P. ambiguus, Trichostrongylus retortaeformis and Cittotaenia denticulata aggregation were lower in adult than juvenile rabbits whilst Graphidium strigosum and Mosgovoyia pectinata aggregation tended to increase with age. Host immunity is thought to be responsible for these differences. Differences in aggregation for different parasites were also seen when the rabbit population was split into males and females. Myxomatosis had a marked effect on helminth distribution with substantially less aggregation in rabbits showing clinical signs of the disease.

Cross-sectional study of the association between pathological conditions and myxoma-virus seroprevalence in intensive rabbit farms in Europe

Myxomatosis is a major viral disease of the European rabbit (Oryctolagus cuniculus). Two forms of the disease (nodular and amyxomatous) exist. The clinical diagnosis of the nodular form is easily performed on the basis of typical skin lesions whereas that of amyxomatous forms must be based on virus isolation or detection of specific antibodies to myxoma virus (MV). The seroprevalence of MV was studied between March 1998 and February 1999 in 16 farms from three European countries considered free of myxomatosis on the basis of the absence of typical clinical signs. MV antibodies were detected by enzyme-linked immunosorbent assay (ELISA) (sensitivity 100%, specificity 90%) in all 16 farms; the seroprevalences corrected for test inaccuracy (95% confidence interval) were 55+/-7.7% and 37+/-6.1% for does and broilers, respectively. The association between herd sizes, types of rabbitries, and presence of recurrent respiratory or digestive troubles and seroprevalence of MV antibodies was tested in logistic multiple regressions. In all models, the seroprevalence of MV antibodies was significantly higher in herds (does and broilers) with recurrent respiratory or digestive troubles than in herds without these problems. The seroprevalence was also higher in herds (does and broilers) where animals were housed totally or partially in outdoors rabbitries than in totally enclosed rabbitries. The effect of herd sizes on the presence of MV antibodies was the same in does and broilers; intermediate sizes were at lower risk than the smaller and larger ones.

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.

First report of myxomatosis in Mexico

An outbreak of myxomatosis occurred between September and October 1993 on a rabbit farm in Punta Colnett (Ensenada, Baja California in northwestern Mexico, Transpeninsular Highway, km 128) and was confirmed by the Mexico-USA Commission for Prevention of Foreign Diseases of Animals (CPA). This represents the first officially confirmed case of the disease in Mexico. Like the cases in California (USA), the brush rabbit (Sylvilagus bachmani) seems to be the carrier of the virus, since serum samples from wild rabbits from different areas of the peninsula of Baja California were found to contain antibodies against the myxoma virus.

Impact of viral hemorrhagic disease on a wild population of European rabbits in France

An outbreak of rabbit viral hemorrhagic disease (RVHD) and of myxomatosis occurred in a free-living population of rabbits (Oryctolagus cuniculus) near Paris (France) in 1995. Annual mortality rates were 88% in adults and 99% in juveniles. There was no difference in mortality rates between males and females. Since most adults were protected with myxoma antibodies after May, they probably died of RVHD. Mortality lasted throughout the year despite high proportions of rabbits having developed myxomatosis and RVHD antibodies, which suggests that the combination of the two diseases and the immunosuppressive characteristics of myxoma virus could be responsible for the mortality caused by RVHD. The proportion of juveniles with RVHD antibodies increased with their weight. Seroconversion against RVHD occurred in spring and autumn.

[Myxomatosis in Tunisia: seroepidemiological study in the Monastir region (Tunisia)]

A sero-epidemiological study of myxomatosis, realized in the region of Monastir, confirmed the existence and the prevalence of the disease in its nodular form. Different strains of the myxomatosis virus were isolated and identified by gel immunodiffusion test (GID) using specific polyclonal sera. Serological analyses using complement fixation (FC) and (GID) tests allowed the detection of specific antibodies in sera from both healthy and sick animals. The results also confirmed the better sensitivity of the FC over the GID. Overall rates infection of herds and animals were 54.9% and 32.63% (p < 0.05), respectively, as revealed by serological testing. These rates increase with the size of herds. Geographic distribution of myxomatosis cases suggests that the disease first appeared in the coastal region then moved inside the Sabel area. Breeding and farming conditions, associated with deficiency in sanitary and medical measures, are at the origin of the introduction and the wide distribution of myxomatosis in this region.

Control of rabbit myxomatosis in Poland

The authors present an epizootiological analysis of myxomatosis in Poland. The biological, physical and chemical properties of virus strains used for the production and control of 'Myxovac M' vaccine are discussed. The long-term stability, safety and efficacy of the vaccine are demonstrated. Laboratory experiments were confirmed in large-scale field observations.

[Myxomatosis in the sub-antarctic islands of Kerguelen, without vectors, thirty years after its introduction]

Myxoma virus was introduced into the Kerguelen archipelago in 1955-1956. Thirty years after its introduction, the virus is present in most areas inhabited by rabbits. Rabbit fleas and mosquitoes are absent from this group of islands and the disease is transmitted by contact. The timing of the beginning of new myxomatosis outbreaks, the absence of real epizootics as well as the higher percentage of infected males over females are specific observations in favour of this mode of transmission. The majority of 34 isolates tested between 1984 and 1988 are of intermediate virulence (Grades IIIA-IIIB). In these conditions, the impact of myxomatosis virus on rabbit populations estimated on two sites is low. Myxomatosis therefore plays only a minor role in the regulation of rabbit populations.

Does myxomatosis still regulate numbers of rabbits (Oryctolagus cuniculus Linnaeus, 1758) in the United Kingdom?

Myxomatosis now kills a much smaller proportion of rabbit populations than in the past, while remaining an important regulatory factor, as shown experimentally. On two separate occasions, experimental reduction of the prevalence of the disease (by reducing infestations of the main vector, the rabbit flea) led to significant increases in numbers of rabbits surviving the winter.

[Preliminary studies of the effect of diseases and environmental pollution on the population dynamics of wild rabbits (Oryctolagus cuniculus Linnaeus, 1758) in the federal state of North Rhine-Westphalia (Germany)]

A survey was conducted of wild rabbits (Oryctolagus cuniculus) shot in all districts of the Federal State of North Rhine-Westphalia (Germany). A total of 104 rabbits were examined during 1988 and 1989. Attempts to isolate myxoma virus and other viruses were unsuccessful. Two serum samples neutralised myxoma virus. In 1990, 210 rabbits were examined; 57 animals were subjected to tests for the presence of the virus of viral haemorrhagic disease of rabbits and 42 were tested for specific antibody. No virus was isolated. Antibody titres were judged to be significant in 5% of the rabbits tested. Bacteriological, histological and virological tests showed no evidence of ill health in these animals. With the exception of abnormal cadmium content of tissues, detailed examination of ovaries and testes and lead assays failed to reveal reduced fertility as a reason for the fall in population.

Myxomatosis: population dynamics of rabbits (Oryctolagus cuniculus Linnaeus, 1758) and ecological effects in the United Kingdom

In 1953-1955, myxomatosis spread among rabbits (Oryctolagus cuniculus) in the United Kingdom, causing 99% mortality. Subsequently, there was a gradual increase in rabbit numbers. By 1955, the Ministry of Agriculture, Fisheries and Food (MAFF) had already found attenuated strains of myxoma virus. By 1970, genetic resistance had appeared. In the 1970s, mortality declined to 47-69% with only approximately 25% of rabbits infected, giving a field mortality of 12-19%. However, myxomatosis is persistent, generally showing a major prevalence peak in autumn and often a minor peak in spring. An eight-year MAFF experiment in which prevalence of the disease was artificially reduced indicates that myxomatosis remains a significant factor in population regulation. After rabbit numbers fell in the 1950s, important ecological changes took place: vegetation altered due to reduced grazing pressure, predators were affected by the reduction of a major prey species and these changes also affected many other animals. Currently, rabbit numbers have returned to approximately one-third of pre-myxomatosis levels and this is causing damage to farm and conservation habitats.

Myxomatosis in farmland rabbit populations in England and Wales

The overall pattern and consequences of myxomatosis in wild rabbit populations were studied at three farmland sites in lowland southern England and upland central Wales between 1971 and 1978. When results from all years were combined, the disease showed a clear two-peaked annual cycle, with a main autumn peak between August and January, and a subsidiary spring peak during February to April. Rabbit fleas, the main vectors of myxomatosis in Britain, were present on full-grown rabbits in sufficient numbers for transmission to occur throughout the year, but the observed seasonal pattern of the disease appeared to be influenced by seasonal mass movements of these fleas. However other factors were also important including the timing and success of the main rabbit breeding season, the proportion of rabbits which had recovered from the disease and the timing and extent of autumn rabbit mortality from other causes. Significantly more males than females, and more adults and immatures than juveniles, were observed to be infected by myxomatosis. Only 25-27% of the total populations were seen to be infected during outbreaks. Using two independent methods of calculation, it was estimated that between 47 and 69% of infected rabbits died from the disease (much lower than the expected 90-95% for fully susceptible rabbits with the partly attenuated virus strains that predominated). Thus it was estimated that 12-19% of the total rabbit populations were known to have died directly or indirectly from myxomatosis. Although the effects of myxomatosis were much less than during the 1950s and 1960s, it continued to be an important mortality factor. It may still have a regulatory effect on rabbit numbers, with autumn/winter peaks of disease reducing the numbers of rabbits present at the start of the breeding season.

Observations on the seasonal incidence of myxomatosis and its interactions with helminth parasites in the European rabbit (Oryctolagus cuniculus)

European rabbits (Oryctolagus cuniculus) were collected monthly over a 10-yr period and the incidence of myxomatosis and the size of helminth populations were recorded. Myxomatosis occurred annually, always as an epidemic in the latter half of the year and was associated with both an increase in the percentage of animals infected and the size of the infections of the nematodes Trichostrongylus retortaeformis and Passalurus ambiguus and the cestode Mosgovoyia pectinata. It is suggested that myxomatosis had the effect of reducing the rabbits' overall immune response to infection and that due to the regular annual occurrence of myxomatosis, the resulting increase in the size of helminth infections must be considered an integral part of the population dynamics of these parasites.

Possible windborne spread of myxomatosis to England in 1953

An analysis of the meterological conditions showed that the first outbreaks of myxomatosis in S.E. England in 1953 could have resulted from wind carriage of insects infected with myxoma virus from northern France. South-easterly winds on the night 11-12 August would have carried the insects 120-160 km from the Départements of Nord, Pas de Calais and Somme across the English Channel to near Edenbridge, Kent. The flight would have taken 6.5-8.5 h at wind speeds of 15-22 km h-1. On the night 11-12 August, temperatures increased with height (inversion) up to 500 m; at ground level temperature was around 19 degrees C and at 500 m was 25 degrees C. Insects would have travelled up to the top of the inversion arriving on 12 August as the inversion declined. Two or possibly three generations of infection would have taken place before the disease was seen around the middle of September 1953. The most likely insect was the mosquito Anopheles atroparvus which breeds along the coastal marshes of England and northern France and which has been shown experimentally and in the field to transmit myxoma virus mechanically.

The establishment and spread of myxomatosis and its effect on rabbit populations

The establishment of myxomatosis, the spread of the disease and its effects on rabbit populations in Australia and in Britain are briefly reviewed. Though the disease is endemic, with regular outbreaks in most rabbit populations, its effect is now much less dramatic than previously. Recent epidemiological studies have shown that the rate of spread of infection, the proportion of rabbits infected and the proportion dying from the disease are very much smaller than recorded in earlier outbreaks. The reasons for these changes are discussed, and the epidemiology of the disease in Britain is compared with that in Australia.

Myxomatosis: changes in the epidemiology of myxomatosis coincident with the establishment of the European rabbit flea Spilopsyllus cuniculi (Dale) in the Mallee region of Victoria

Outbreaks of myxomatosis during the winter or spring have coincided with the establishment of the European rabbit flea in the Mallee region. The severity of these outbreaks has varied from causing complete suppression of the normal spring increase in rabbit numbers to being completely ineffective in a year in which late spring rains allowed rabbit breeding to extend into the early summer.In 1973 and 1974 effective spring myxomatosis caused heavy mortality in kittens before they emerged from the warrens. The age of the population increased as the result of few young rabbits coming into the population and of the lessened stress on old rabbits in a low summer-autumn population. This effect was reversed in the late-breeding year, 1976, when flea numbers were apparently too low to maintain a spring outbreak and rabbit numbers increased rapidly.

Myxomatosis: the occurrence of antibody to a soluble antigen of myxoma virus in wild rabbits, Oryctolagus cuniculus (L.), in Victoria, Australia

The occurrence of antibody of myxoma virus in wild rabbits following epizootics is highest in the semi-arid north-west of Victoria and lowest in temperate southern Victoria. Occurrence ranges up to about 90% in the north-west and to about 70% in the south except on the Western Plains where epizootics are rare and antibody occurrence seldom exceeds 30%. The establishment of the European rabbit flea may be changing the pattern of occurrence of antibody in the north-west by causing spring outbreaks of myxomatosis. It is suggested that the effects of the replacement of a simple recurring system of epizootic and breeding season several months apart by the occurrence of myxomatosis twice in the same year, once coincident with the breeding season, will be complex. The occurrence of detectable antibody may be less dependent on the infection rate and may be dependent to some extent on the relative timing of spring myxomatosis and the breeding season.

Myxomatosis in the Mallee region of Victoria, Australia

Sharp reductions in the wild rabbit (Oryctolagus cuniculus (L.)) population in the Mallee are associated with annual myxomatosis epizootics. The extent to which the population reductions are the direct result of the epizootics varies with time of epizootic occurrence. All grazing animals in the Mallee are under nutritional stress each summer and autumn. When the epizootic occurs during the early summer heavy losses occur in a previously healthy population. Similar losses which occur in the late summer and autumn are the result of a nutritional stress - epizootic complex. The end result in each case is a population reduction of about 80%. This reduction occurs in a population which is the most resistant to myxomatosis known in Victoria and in association with epizootics caused by field strains of myxoma virus of moderate virulence only. The earlier summer epizootics are of considerable economic importance because they sharply reduce the pressure on the limited food available for other grazing animals.

Myxomatosis on the Western Plains of Victoria

Myxomatosis on the Western Plains is an enzootic disease in contrast with the epizootic pattern which is general in eastern Australia. The most unusual aspects are the presence of significant numbers of diseased rabbits throughout the winter and the continuously low percentage of rabbits with antibodies to myxoma virus. Climatic and topographic conditions are unsuited to the production of the high densities of mosquitoes necessary for widespread epizootics. Under these conditions the effects of less efficient methods of myxomatosis transmission are apparent. The unusual epidemiology of myxomatosis has resulted in selection for virulence of the virus similar to that which has occurred under summer epizootic conditions. All field strains are now in the mid range of virulence.

Myxomatosis in domestic rabbits in Oregon

An epizootic of myxomatosis involved 26 rabbitries in western Oregon. Major clinical signs were inflammation and edema of the eyelids, conjunctiva, and anogenital area. Mortality ranged from 20 to 50%. On histologic examination, intracytoplasmic inclusion bodies were readily apparent in the epithelial cells of the conjunctiva. Lymphoid depletion of the spleen was also a common finding.