Analysis of the risk of introduction and spread of porcine reproductive and respiratory syndrome virus through importation of raw pigmeat into New Zealand

AIMS: To determine the frequency with which porcine reproductive and respiratory syndrome (PRRS) virus (PRRSv) would become established in a non-commercial pig herd in New Zealand due to illegal feeding of uncooked food waste containing virus-contaminated pigmeat. To determine the likelihood of a single incursion resulting in a multi-farm outbreak of the disease, and describe the spatio-temporal characteristics of such an outbreak. METHODS: A Monte Carlo simulation model was constructed to determine the expected annual frequency of PRRSv infection being initiated in a non-commercial pig herd as a result of inadvertent feeding of pigmeat imported from countries endemically infected with the disease. Once the likelihood of PRRSv becoming established in a single pig herd was determined, stochastic spatially explicit infectious disease modelling software was utilised to model the temporal and spatial characteristics of the resulting epidemic. RESULTS: Assuming the proportion of imported pigmeat remained at current levels, consumption patterns of pigmeat in households in New Zealand remained steady, and limited compliance with recently reintroduced regulations to prevent feeding of uncooked food waste, at least 4.3 pig herds per year were predicted to become infected with PRRSv. Simulation modelling of PRRSv epidemics related to initial infection of a non-commercial farm produced an estimate that 36% of these incursions would spread from the initial herd, and that these outbreaks would involve 93 herds on average in the first year. By increasing the estimated persistence of PRRSv infection in small herds, an average of 205 herds became infected in the first year. CONCLUSIONS: Given a mean of 4.3 infected premises per year and a 36% probability of infection spreading beyond the initial infected herd, there was a 95% likelihood of a multi-farm PRRS outbreak occurring within 3 years. CLINICAL RELEVANCE: Introduction of PRRSv through importation of virus-contaminated pigmeat presents a high risk for establishment of the disease in the pig industry in New Zealand.     

Analysis of the risk of introduction and spread of porcine reproductive and respiratory syndrome virus through importation of raw pigmeat into New Zealand

AIMS: To determine the frequency with which porcine reproductive and respiratory syndrome (PRRS) virus (PRRSv) would become established in a non-commercial pig herd in New Zealand due to illegal feeding of uncooked food waste containing virus-contaminated pigmeat. To determine the likelihood of a single incursion resulting in a multi-farm outbreak of the disease, and describe the spatio-temporal characteristics of such an outbreak.

METHODS: A Monte Carlo simulation model was constructed to determine the expected annual frequency of PRRSv infection being initiated in a non-commercial pig herd as a result of inadvertent feeding of pigmeat imported from countries endemically infected with the disease. Once the likelihood of PRRSv becoming established in a single pig herd was determined, stochastic spatially explicit infectious disease modelling software was utilised to model the temporal and spatial characteristics of the resulting epidemic.

RESULTS: Assuming the proportion of imported pigmeat remained at current levels, consumption patterns of pigmeat in households in New Zealand remained steady, and limited compliance with recently reintroduced regulations to prevent feeding of uncooked food waste, at least 4.3 pig herds per year were predicted to become infected with PRRSv. Simulation modelling of PRRSv epidemics related to initial infection of a non-commercial farm produced an estimate that 36% of these incursions would spread from the initial herd, and that these outbreaks would involve 93 herds on average in the first year. By increasing the estimated persistence of PRRSv infection in small herds, an average of 205 herds became infected in the first year.

CONCLUSIONS: Given a mean of 4.3 infected premises per year and a 36% probability of infection spreading beyond the initial infected herd, there was a 95% likelihood of a multi-farm PRRS outbreak occurring within 3 years.

CLINICAL RELEVANCE: Introduction of PRRSv through importation of virus-contaminated pigmeat presents a high risk for establishment of the disease in the pig industry in New Zealand.     

Lungworm outbreaks in adult dairy cows: estimating economic losses and lessons to be learned

Two lungworm outbreaks in dairy herds were investigated in order to estimate the resulting economic costs. On the two farms, with 110 and 95 cows, total costs were estimated at ?59 and ?67 per cow, respectively. Overall, milk production reduced by 15 to 20 per cent during the outbreaks. Five cows died on one farm, while on the other farm seven cows died as a result of the lungworm outbreak. On one farm, 51.7 per cent of the total costs was due to reduced milk production and 33.1 per cent was due to disposal of dead animals. On the other farm, it was 36.3 and 50.9 per cent, respectively. The remaining 13 to 15 per cent of the total costs were due to extra inseminations, laboratory diagnosis and treatments. The history and development of the outbreaks are described. One lesson from these outbreaks is that recognising that potentially lungworm-na?ve animals are to be introduced into the adult herd allows for timely measures (for example, vaccination) to prevent a lungworm outbreak.     

The economic impact of clinical transmissible gastroenteritis for swine producers participating in the Missouri mail-in-record program

Economic loss from transmissible gastroenteritis (TGE) was estimated at $0.18 per hog marketed for the average Missouri Mail-in-Record swine panel producer in 1978. This cost was $0.19 per hog marketed in 1979. These estimates were averaged for all producers in the record system, those with TGE in their herds as well as TGE-free herds. Estimates were also prepared for those herds with TGE outbreaks. In 1978, the average loss due to TGE for producers who had the disease on their farms was estimated at $1.74 per hog marketed, or 18% of the average return earned above total production costs. In 1979, this cost was $1.25 per hog marketed, or 13% of the average return earned above total production costs.     

The additional costs of segregated transport to slaughter to decrease Salmonella prevalence in pork--a simulation study

A stochastic simulation model was used to assess the additional costs related to segregated transport to slaughter. This control measure was assumed to be implemented within a producers' association to decrease Salmonella prevalence in pork. Calculations were based on the additional shipments caused by the separate transport of low- and high-prevalence herds and on the additional transport distance caused by changed routing. The results showed that there is not necessarily a considerable increase in the number of shipments due to herd status separation for transport. The percentage of shipments changed due to segregated transport varied between 43% and 69% depending on the threshold prevalence. The additional costs per slaughtered pig varied between 0.07€/pig and 0.58€/pig under the given assumptions. Costs were governed by the percentage of changed shipments and the additional distance of a changed shipment. Due to the fact that the percentage of changed shipments is related to the distribution of herd prevalence within the producers' association, there is no cost-effective threshold in general. Different producers' associations incur different costs caused by segregated transport to slaughter at the same threshold prevalence. The current study supports producers' associations in evaluating the additional costs of segregated transport for their members.     

Impact of different approaches to calculate the economics of disease in pig farming

Diagrams for analyzing the economics of sow and fattening farms are described and discussed. The diagrams are used to quantify the economic significance of a change in technical results for pig farms in the Netherlands. Sensitivity analyses show the importance of taking into account feed savings of piglets and sows due to, for instance, less litters per sow per year. To see whether there was an effect of the economic environment and the structure of the fanning system, the same calculations were carried out for Minnesota (USA). The economic effect of a change in results (e.g. days open) was smaller for Minnesota, but economic effects were similarly influenced by specific factors, such as feed intake and price. Subsequently, several scientific publications on the estimation of the costs of Aujeszky's disease in pigs were compared with respect to the economic methods used and the outcomes obtained. Because of different estimation procedures, the costs of the disease varied significantly. Differences are mainly due to the loss items that were taken into account, the way the loss items were calculated and the structure of the farming system that was included. There is a need for a standardized framework of how to conduct a sound economic evaluation, in order to make the outcome of different calculations more comparable and interpretable.     

Simulation of the economic impact of transmissible gastroenteritis on commercial pig production in Australia

A simulation model (AUSPIG) was used to predict the effect of an increase in piglet deaths, and a reduction in growth rate and an increase in feed conversion ratio of grower pigs on the profitability of two herds representative of the Australian pig industry caused by the introduction of transmissible gastroenteritis virus (TGE) into those herds. For each herd, mortality rates for piglets under 1 week of age of 50% and 95% were assumed to represent a 'moderate' and a 'severe' outbreak, respectively. A reduction in net revenue of 70% was predicted to occur in the 6 months after a 'moderate' outbreak of TGE (100% for a 'severe' outbreak). This represents a total loss of between $260 and $330 per breeding sow in the 12 months after infection with the TGE virus. The likely financial impact of an outbreak of TGE on an Australian piggery is substantial and should be considered when addressing quarantine issues.     

Acute phase response and clinical manifestation in outbreaks of interdigital phlegmon in dairy herds

Several Finnish dairy herds have suffered from outbreaks of interdigital phlegmon (IP). In these new types of outbreaks, morbidity was high and clinical signs severe, resulting in substantial economic losses for affected farms. In our study, we visited 18 free stall dairy herds experiencing an outbreak of IP and 3 control herds without a similar outbreak. From a total of 203 sampled cows, 60 suffered from acute stage IP. We demonstrated that acute phase response of bovine IP was evident and therefore an appropriate analgesic should be administered in the treatment of affected animals. The response was most apparent in herds with high morbidity in IP and with a bacterial infection comprising Fusobacterium necrophorum and Dichelobacter nodosus, indicating that combination of these two bacterial species affect the severity of the disease.     

Herd-level economic losses associated with Johne's disease on US dairy operations

Johne's disease (‘paratuberculosis') is a chronic, infectious, wasting disease that affects dairy cattle. Estimation of its impact on herd productivity and corresponding economic loss on US dairy operations was part of the USDA National Animal Health Monitoring System's (NAHMS) 1996 national dairy study. Johne's-positive herds experience an economic loss of almost US$ 100 per cow when compared to Johne's-negative herds due to reduced milk production and increased cow-replacement costs. For Johne's-positive herds that reported at least 10% of their cull cows as having clinical signs consistent with Johne's disease, economic losses were over US$ 200 per cow. These high-prevalence herds experienced reduced milk production of over 700 kg per cow, culled more cows but had lower cull-cow revenues, and had greater cow mortality than Johne's-negative herds. Averaged across all herds, Johne's disease costs the US dairy industry, in reduced productivity, US$ 22 to US$ 27 per cow or US$ 200 to US$ 250 million annually.     

Haemophilus somnus Infections I. A Ten Year (1969-1978) Retrospective Study of Losses in Cattle Herds in Western Canada

A total of 838 outbreaks of fatal Haemophilus somnus infections in herds of cattle were diagnosed at provincial veterinary laboratories in Manitoba, Saskatchewan, Alberta and British Columbia during the period 1969-1978. The index cases from these outbreaks included 759 cases of thromboembolic meningoencephalitis, 78 cases of fibrinous pneumonia with pleuritis and one case of H. somnus abortion. The epizootics were subdivided on the basis of province, class and age of cattle and seasonal occurrence. Most outbreaks occurred in a feedlot-type of operation, approximately four weeks after arrival of the cattle. There was often a history of respiratory disease prior to an outbreak and in some cases thromboembolic meningoencephalitis had occurred in the herd the preceding year. The average morbidity-mortality ratio was 2.7/1. The average economic loss per herd was $3 190 based on an average of 15 sick animals and five deaths per affected herd.     

The effect of an outbreak of respiratory disease on herd-level milk production of Norwegian dairy farms

This study was done to evaluate the effect of an outbreak of acute respiratory disease associated with bovine respiratory syncytial virus (BRSV) on the daily milk yield per cow in Norwegian dairy-cattle farms. Retrospective data from 184 dairy herds located in two neighbouring veterinary districts during the study period (December 1994–May 1995, during which an epidemic of acute respiratory disease associated with BRSV occurred in this area) were analysed. Data on the bulk-milk deliveries and the date of the outbreak were collected at herd level, whereas information on calving dates and parity was collected at cow-level. The effect of the herd outbreaks on the daily milk yield was analysed with a repeated-measurement approach. The average daily milk loss was estimated to be 0.70 kg per cow for 7 days after a herd outbreak (compared with the period >1 week prior to an outbreak), adjusted for the herd-level lactation stage, parity and their interaction term. We consider the estimated milk loss associated with a herd outbreak of epidemic respiratory disease to be of minor importance.     

Epidemiological investigations of the 2001 foot-and-mouth disease outbreak in Argentina

A total of 2126 herds, an attack rate of 0.82 per cent, were affected during an epidemic of foot-and-mouth disease in Argentina in 2001. The spatial and temporal distribution of the epidemic was investigated using nearest-neighbour and spatial scan tests and by estimating the frequency distributions of the times to intervention, and distances and times between outbreaks. The outbreaks were clustered and associated significantly (P<0.01) with herd density; 94 per cent were located in the Pampeana region, where the cattle population is concentrated, which had an attack rate of 1.4 per cent. The clustering results suggested that the virus had spread locally between outbreaks. Most of the outbreaks were separated by one day and the maximum distance between outbreaks was almost 2000 km, indicating that the infection spread rapidly over large distances. The index outbreak was detected more than 15 days after the primary outbreak, and restrictions on the movement of cattle were probably not enforced until about one month after infection occurred. As in other major epidemics, the period between the first outbreaks and the effective application of control strategies was probably crucial in determining the progress of the epidemic.     

Impact of biological and economic variables on optimal parity for replacement in swine breed-to-wean herds

Voluntary and involuntary culling practices determine the average parity when sows are replaced in a herd. Underlying these practices is the economic effect of replacing a sow at different parities. A dynamic programming model was used to find the optimal parity and net present value in breed-to-wean swine herds. The model included income and costs per parity weighted by the discount rate and sow removal rate. Three scenarios that reflect a wide range of cases were considered: low removal rates per parity with no salvage value (LRNS), high removal rates per parity with no salvage value (HRNS), and high removal rates per parity with a percentage of the sows having a salvage value (HRYS). The optimal parity of replacement for the base biological and economic conditions was 4 and 5 parities in the high and low removal scenarios, respectively. Sensitivity analyses identified the variables influencing the optimal replacement parity. Optimal parity of replacement ranged from 3 to 7 parities in the low replacement scenario, compared with 1 to 5 parities in the high replacement scenarios. Sow replacement cost and salvage value had the greatest impact on optimal parity of replacement followed by revenues per piglet weaned. The discount rate and number of parities per year generally had little influence on optimal parity. For situations with high sow costs, low salvage values, and low revenues per piglet, the optimal parity at removal was as high as 6 to 10 parities, and for situations with low sow cost, high salvage values, and high revenues per piglet, the optimal parity at removal was as low as 1 to 2 parities depending on removal rates. The modified internal rate of return suggested that, for most LRNS and HRYS scenarios considered, investment in a swine breed-to-wean enterprise was favored over other investments involving a similar risk profile. Our results indicate that in US breeding herds, sows are culled on average near the optimal parity of 4. However, the optimization process should be a dynamic one that adapts to changes in replacement rates, salvage value, replacement cost, and revenues per piglet.     

Bioeconomic evaluation of sow longevity and profitability

Sow production indicators, including litter size, litter weight, and the length of time that sows remained in the herd (sow longevity), were used to characterize sow performance and profitability. Sow longevity and production records from 148,568 sows in 32 commercial herds from Central Illinois from January 1995 to May 2001 were analyzed using survival and repeatability models, respectively. The factors studied included sow genetics (32 genetic lines), with eight major lines present in multiple herds, and the combination of herd and year of entry in the herd. The largest difference in longevity between the major genetic lines was approximately one parity. There were differences (P < 0.05) in the instantaneous sow removal rate or hazard from the major lines. These differences constitute evidence that sow longevity could be improved by using replacements from specific genetic lines. The net present value per sow (present value of future cash flows and the present value of the sow) was used to evaluate the effect of sow longevity and production traits on economic returns. Assuming a zero discount rate per parity, genetic lines with longer herd life resulted in greater profit than genetic lines with shorter herd life. This difference was reduced with increasing discount rates and was reversed with high discount rates and low net income per litter. These results suggest that the magnitude of the economic improvement attained through the use of sow genetic lines with longer longevity depends on the economic context under which the evaluation is made.     

Economic and reproductive consequences of retained placenta in dairy cattle

The financial losses due to retained placenta in Dutch dairy cattle were estimated by using two different methods of calculation. A data-set containing the birth records of 160,188 Meuse-Rhine-Yssel cows provided data on the reproductive performance of cows with and without retained placenta. The fertility of cows after retention of the placenta appeared to be affected. An economic calculation made by adding the losses due to increased calving interval, increased culling rate, loss of milk production and the costs of veterinary treatment and drugs revealed that the total loss due to retained placenta was 471 pounds per year for a 100-cow farm with an average incidence of the condition (6.6 per cent). For a 'problem' farm with a 30 per cent rate, the loss was 2139 pounds per year. A computer farm simulation model, based on a stochastic determination of events, was used to make calculations for circumstances closely resembling those on farms. A 6.6 per cent rate of retained placenta caused a small but significant decrease in net return on labour and management; however, a 30 per cent rate caused highly significant changes. The economic effects of retained placenta were similar in magnitude in herds of high or low productivity and high or low fertility. Sensitivity analysis showed that the greatest financial losses were caused by loss of milk production, followed by the number of animals suffering from complications. The financial losses in herds with an average rate of retained placenta were thus of limited economic importance and therapeutic measures alone should be adequate.(ABSTRACT TRUNCATED AT 250 WORDS)     

Between-herd prevalence of Mycoplasma bovis in bulk milk in Flanders, Belgium

Mycoplasma bovis (M. bovis) is a highly infectious pathogen of cattle causing pneumonia, polyarthritis, otitis, and less frequently, subcutaneous abscesses, abortions and meningitis. Ineffective drugs treatments, culling of infected cows and loss of milk production can lead to significant economic loss on dairy farms. The early detection of cows excreting M. bovis bacteria to prevent mastitis outbreaks is warranted. Reports suggest that the risk of M. bovis mastitis is higher in larger dairy herds. The objective of this study is to estimate the herd-level prevalence of M. bovis in Flanders, Belgium by culturing bulk tank milk samples taken from dairy farms. Three bulk tank milk samples per dairy herd were taken over four weeks, with collection intervals of two weeks. Culturing was done after pre-incubation using modified Hayflicks media to increase the chances of recovery of bacteria. For the identification of M. bovis, tDNA intergenic spacer PCR was used. In three herds (1.5%) of the 200 herds sampled, M. bovis was isolated from one of the three consecutive bulk tank milk samples. We conclude that in Flanders in 2009 at least 1.5% of the dairy herds had one or more cows excreting M. bovis in the milk. The frequent monitoring of bulk tank milk to detect the presence of M. bovis, especially in expanding herds on farms that often purchase replacement animals, should be encouraged in order to detect the presence of M. bovis and to monitor the success of control procedures following an outbreak of mycoplasmal mastitis in the herd.     

Risk factors for infection of sow herds with porcine reproductive and respiratory syndrome (PRRS) virus

In 1992, the porcine reproductive and respiratory syndrome virus (PRRSV) of European type (PRRSV-EU) was introduced in Denmark. By 1996, the virus had spread to approximately 25% of the Danish herds. In January 1996, a modified-live vaccine based on the American type of the virus (PRRSV-US) was used in replacement boars for Danish artificial insemination (AI) centres and from July 1996, the vaccine was used in PRRSV-EU infected herds for prevention of disease. Soon after vaccine introduction, PRRSV non-infected herds experienced outbreaks of disease due to infection with PRRSV-US. In this study, we investigated the risk factors (biosecurity level, animals, exposure from PRRSV-US-infected neighbour herds, semen, herd size, pig density and herd density) for infection with PRRSV-US in a cohort of 1071 sow herds; we used a nested case-control study. The retrospective observation period lasted from June 1996 (when they all were non-infected) to October 1997. Seventy-three non-vaccinated, closed sow herds became infected with the vaccine strain during this period. Each case herd was matched with two control herds from the cohort (controls had not been infected at the time of infection in the case herds). The data were analysed using a Cox-regression model. The hazard of infection increased significantly with exposure from PRRSV-US-infected neighbouring herds, purchase of animals from herds incubating PRRSV-US infection, increasing herd size and purchase of semen from boars at PRRSV-US-infected AI centres. The results are consistent with the modified-live vaccine strain spread to other herds by trade with animals and semen and by neighbour (area) transmission. We suggest that virus spread by aerosols was a frequent mode of transmission.     

The classical swine fever epidemic 1997-1998 in The Netherlands: descriptive epidemiology

The objective of this paper is to describe the severe epidemic of classical swine fever (CSF) in The Netherlands in 1997–1998 under a policy of non-vaccination, intensive surveillance, pre-emptive slaughter and stamping out in an area which has one of the highest pig and herd densities in Europe.

The primary outbreak was detected on 4 February 1997 on a mixed sow and finishing pig herd. A total of 429 outbreaks was observed during the epidemic, and approximately 700 000 pigs from these herds were slaughtered. Among these outbreaks were two artificial insemination centres, which resulted in a CSF-suspect declaration of 1680 pig herds (mainly located in the southern part of The Netherlands). The time between introduction of CSF virus (CSFV) into the country and diagnosis of CSF in the primary outbreak was estimated to be approximately 6 weeks. It is presumed that CSFV was spread from The Netherlands to Italy and Spain via shipment of infected piglets in the beginning of February 1997, before the establishment of a total stand-still of transportation. In June 1997, CSFV is presumed to be introduced into Belgium from The Netherlands.

Pre-emptive slaughter of herds that had been in contact with infected herds or were located in close vicinity of infected herds, was carried out around the first two outbreaks. However, this policy was not further exercised till mid-April 1997, when pre-emptive slaughter became a standard operational procedure for the rest of the epidemic. In total, 1286 pig herds were pre-emptively slaughtered. (approximately 1.1 million pigs). A total of 44 outbreaks (10%) was detected via pre-emptive slaughter.

When there were clinical signs, the observed symptoms in infected herds were mainly atypical: fever, apathy, ataxia or a combination of these signs. In 322 out of 429 outbreaks (75%), detection was bases on clinical signs observed: 32% was detected by the farmer, 25% by the veterinary practitioner, 10% of the outbreaks by tracing teams and 8% by screening teams of the veterinary authorities. In 76% of the outbreaks detected by clinical signs, the farmer reported to have seen clinical symptoms for less than 1 week before diagnosis, in 22% for 1–4 weeks before diagnosis, and in 4 herds (1%) the farmer reported to have seen clinical symptoms for more than 4 weeks before diagnosis.

Transportation lorries played a major role in the transmission of CSFV before the primary outbreak was diagnosed. It is estimated that approximately 39 herds were already infected before the first measures of the eradication campaign came into force.

After the first measures to stop the spread of CSFV had been implemented, the distribution of the most likely routes of transmission markedly changed. In most outbreaks, a neighbourhood infection was indicated.

Basically, there were two reasons for this catastrophe. Firstly, there was the extent of the period between introduction of the virus in the region and detection of the first outbreak. As a result, CSFV had opportunities to spread from one herd to another during this period. Secondly, the measures initially taken did not prove sufficient in the swine- and herd-dense region involved.     

Transmissible gastroenteritis in endemically infected breeding herds of pigs in East Anglia, 1981-85

Endemic infection was a common sequel to primary outbreaks of transmissible gastroenteritis in large breeding herds of pigs in East Anglia. The main clinical features of the disease were diarrhoea affecting sucking piglets aged about six days or more, diarrhoea among recently weaned pigs and brief episodes of overt clinical recrudescence in part of the herd. Post mortem and bacteriological findings were often more suggestive of colibacillosis than transmissible gastroenteritis. In some herds, endemic infection remained clinically mild or inapparent for long periods. Evidence of endemic transmissible gastroenteritis infection was found in 43 (50.6 per cent) of 85 herds of pigs studied prospectively between 1981 and 1983. There was a significant correlation with herd size; the disease recurred during the 12 months after primary outbreaks in 36 (65.5 per cent) of 55 herds with over 100 sows compared with seven (23.3 per cent) of 30 herds with less than 100 sows (P less than 0.001). In the larger herds it occurred more commonly where finishers were kept (P less than 0.05). Sow morbidity and management factors during the primary outbreak had no statistically significant effect on the incidence of recrudescence. Epidemiological aspects of the findings are discussed with emphasis on the difficulties associated with the diagnosis and control of endemic transmissible gastroenteritis infection.     

Descriptive epidemiology of the outbreak of classical swine fever in Catalonia (Spain), 2001/02

Spain suffered an outbreak of classical swine fever between June 14, 2001 and May 7, 2002, which affected 49 herds; this paper describes the epidemiological characteristics of the 39 herds that were affected in Catalonia, an area of high pig density in the north east of Spain. The outbreak took place in two waves, which affected first the province of Lleida and then Barcelona. A total of 291,058 animals were slaughtered, 59,595 belonging to infected herds; 22 of the infected herds were detected on the basis of clinical suspicion on the part of the farmer or farm veterinarian, and the other 17 were detected by surveillance methods. The transmission of the virus between herds was attributed to the movement of people in 23 per cent of the cases, to animals in 13 per cent, vehicles in 10 per cent, proximity 18 per cent, the pick-up service of the rendering plant in 8 per cent and slurry in 5 per cent; in the other nine herds (23 per cent) the route of entry of the disease could not be established. The viruses isolated in the two waves of the outbreak were 100 per cent homologous and belonged to subgroup 2.3. The origin of the outbreak remains unknown.