Studies on the carriage and transmission of porcine reproductive and respiratory syndrome virus by individual houseflies (Musca domestica )

Survival of Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) in the Environment

Porcine reproductive and respiratory syndrome (PRRS) is one of the most economically important diseases of swine, with losses due to poor reproductive performance and high piglet and growing pig mortality. Transmission of porcine reproductive and respiratory syndrome virus (PRRSV) may occur by both direct and indirect routes; the latter includes exposure to PRRSV-contaminated fomites, aerosols, and arthropod vectors. This review has collected available data on the ex-vivo environmental stability and persistence of PRRSV in an effort to highlight important sources of the virus and to determine the role of environmental conditions on the stability of the virus, especially temperature. The ex-vivo settings include fomites (solid, porous, and liquid fomites), insects, people, and pork meat, as well as the role of environmental conditions on the stability of the virus, especially temperature.

Farm biosecurity measures to prevent hepatitis E virus infection in finishing pigs on endemically infected pig farms

Hepatitis E virus (HEV) can be transmitted from pigs to humans and cause liver inflammation. Pigs are a major reservoir of HEV and most slaughter pigs show evidence of infection by presence of antibodies (ELISA) or viral RNA (PCR). Reducing the number of HEV infected pigs at slaughter would likely reduce human exposure, yet how this can be achieved, is unknown. We aimed to identify and quantify the effect of biosecurity measures to deliver HEV negative batches of pigs to slaughter. A case-control study was performed with Dutch pig farms selected based on results of multiple slaughter batches. Case farms delivered at least one PCR and ELISA negative batch to slaughter (PCR^-ELISA^-), indicating absence of HEV infection, and control farms had the highest proportion of PCR and/or ELISA positive batches (PCR⁺ELISA⁺), indicating high within-farm transmission. Data about biosecurity and housing were collected via a questionnaire and an audit. Variables were selected by regularization (LASSO regression) and ranked, based the frequency of variable selection. The odds ratios (OR) for the relation between case-control status and the highest ranked variables were determined via grouped logistic regression. Thirty-five case farms, with 10 to 60% PCR^-ELISA^- batches, and 38 control farms with on average 40% PCR⁺ELISA⁺ batches, were included. Rubber and steel floor material in fattening pens had the highest ranking and increased the odds of a PCR^-ELISA^- batch by 5.87 (95%CI 3.03-11.6) and 7.13 (95%CI 3.05-16.9) respectively. Cleaning pig driving boards weekly (OR 1.99 (95%CI 1.07-3.80)), and fly control with predatory flies (OR 4.52 (95%CI 1.59-13.5)) were protective, whereas a long fattening period was a risk. This study shows that cleaning and cleanability of floors and fomites and adequate fly control are measures to consider for HEV control in infected farms. Yet, intervention studies are needed to confirm the robustness of these outcomes.

Synanthropic Flies-A Review Including How They Obtain Nutrients, along with Pathogens, Store Them in the Crop and Mechanisms of Transmission

An attempt has been made to provide a broad review of synanthropic flies and, not just a survey of their involvement in human pathogen transmission. It also emphasizes that the crop organ of calliphorids, sarcophagids, and muscids was an evolutionary development and has served and assisted non-blood feeding flies in obtaining food, as well as pathogens, prior to the origin of humans. Insects are believed to be present on earth about 400 million years ago (MYA). Thus, prior to the origin of primates, there was adequate time for these flies to become associated with various animals and to serve as important transmitters of pathogens associated with them prior to the advent of early hominids and modern humans. Through the process of fly crop regurgitation, numerous pathogens are still readily being made available to primates and other animals. Several studies using invertebrate-derived DNA = iDNA meta-techniques have been able to identify, not only the source the fly had fed on, but also if it had fed on their feces or the animal's body fluids. Since these flies are known to feed on both vertebrate fluids (i.e., from wounds, saliva, mucus, or tears), as well as those of other animals, and their feces, identification of the reservoir host, amplification hosts, and associated pathogens is essential in identifying emerging infectious diseases. New molecular tools, along with a focus on the crop, and what is in it, should provide a better understanding and development of whether these flies are involved in emerging infectious diseases. If so, epidemiological models in the future might be better at predicting future epidemics or pandemics.

Landscape distribution and abundance of animal-associated adult filth flies on commercial swine facilities in North Carolina, US

Filth flies associated with animal production transmit pathogens to humans and animals, propagate antimicrobial resistance in microbial communities and provoke nuisance litigation. Although dispersal of flies from facilities is often responsible for these negative effects, filth fly research on swine facilities has been limited to within the barns. Filth fly adaptations in space and time, as well as influences of abiotic and biotic factors impact distribution and abundance of animal-associated filth flies on swine production facilities. In this study, fly surveillance was conducted around four swine facilities in Bladen County, North Carolina, U.S.A. from January 2019 to October 2019. Traps were replaced weekly and animal-associated filth flies were identified. Flies were grouped for comparison based on biology and differences in pest management strategies. There were distinct differences in abundance and spatial distribution of different filth fly groups on the swine facilities, which are likely linked to environmental factors like spatial relation to crop production and species phenology. The impact of the observed temporal and spatial distribution and abundance is discussed in the context of filth fly management.

A Systematic Review on Viruses in Mass-Reared Edible Insect Species

Edible insects are expected to become an important nutrient source for animals and humans in the Western world in the near future. Only a few studies on viruses in edible insects with potential for industrial rearing have been published and concern only some edible insect species. Viral pathogens that can infect insects could be non-pathogenic, or pathogenic to the insects themselves, or to humans and animals. The objective of this systematic review is to provide an overview of the viruses detected in edible insects currently considered for use in food and/or feed in the European Union or appropriate for mass rearing, and to collect information on clinical symptoms in insects and on the vector role of insects themselves. Many different virus species have been detected in edible insect species showing promise for mass production systems. These viruses could be a risk for mass insect rearing systems causing acute high mortality, a drastic decline in growth in juvenile stages and in the reproductive performance of adults. Furthermore, some viruses could pose a risk to human and animal health where insects are used for food and feed.

Interpretable machine learning applied to on-farm biosecurity and porcine reproductive and respiratory syndrome virus

Effective biosecurity practices in swine production are key in preventing the introduction and dissemination of infectious pathogens. Ideally, on-farm biosecurity practices should be chosen by their impact on bio-containment and bio-exclusion; however, quantitative supporting evidence is often unavailable. Therefore, the development of methodologies capable of quantifying and ranking biosecurity practices according to their efficacy in reducing disease risk has the potential to facilitate better-informed choices of biosecurity practices. Using survey data on biosecurity practices, farm demographics, and previous outbreaks from 139 herds, a set of machine learning algorithms were trained to classify farms by porcine reproductive and respiratory syndrome virus status, depending on their biosecurity practices and farm demographics, to produce a predicted outbreak risk. A novel interpretable machine learning toolkit, MrIML-biosecurity, was developed to benchmark farms and production systems by predicted risk and quantify the impact of biosecurity practices on disease risk at individual farms. By quantifying the variable impact on predicted risk, 50% of 42 variables were associated with fomite spread while 31% were associated with local transmission. Results from machine learning interpretations identified similar results, finding substantial contribution to predicted outbreak risk from biosecurity practices relating to the turnover and number of employees, the surrounding density of swine premises and pigs, the sharing of haul trailers, distance from the public road and farm production type. In addition, the development of individualized biosecurity assessments provides the opportunity to better guide biosecurity implementation on a case-by-case basis. Finally, the flexibility of the MrIML-biosecurity toolkit gives it the potential to be applied to wider areas of biosecurity benchmarking, to address biosecurity weaknesses in other livestock systems and industry-relevant diseases.

Identifying outbreaks of Porcine Epidemic Diarrhea virus through animal movements and spatial neighborhoods

The spread of pathogens in swine populations is in part determined by movements of animals between farms. However, understanding additional characteristics that predict disease outbreaks and uncovering landscape factors related to between-farm spread are crucial steps toward risk mitigation. This study integrates animal movements with environmental risk factors to identify the occurrence of porcine epidemic diarrhea virus (PEDV) outbreaks. Using weekly farm-level incidence data from 332 sow farms, we applied machine-learning algorithms to quantify associations between risk factors and PEDV outbreaks with the ultimate goal of training predictive models and to identify the most important factors associated with PEDV occurrence. Our best algorithm was able to correctly predict whether an outbreak occurred during one-week periods with >80% accuracy. The most important predictors included pig movements into neighboring farms. Other important neighborhood attributes included hog density, environmental and weather factors such as vegetation, wind speed, temperature, and precipitation, and topographical features such as slope. Our neighborhood-based approach allowed us to simultaneously capture disease risks associated with long-distance animal movement as well as local spatial dynamics. The model presented here forms the foundation for near real-time disease mapping and will advance disease surveillance and control for endemic swine pathogens in the United States.

Review on the transmission porcine reproductive and respiratory syndrome virus between pigs and farms and impact on vaccination

Porcine reproductive and respiratory syndrome (PRRS) is considered to be one of the most costly diseases affecting intensive pig production worldwide. Control of PRRS is a complex issue and involves a combination of measures including monitoring, diagnosis, biosecurity, herd management, and immunization. In spite of the numerous studies dealing with PRRS virus epidemiology, transmission of the infection is still not fully understood. The present article reviews the current knowledge on PRRSV transmission between and within farm, and the impact of vaccination on virus transmission.

Factors associated with herd-level PRRSV infection and age-time to seroconversion in farrow-to-finish herds

Factors associated with porcine reproductive and respiratory syndrome virus (PRRSV) infection were investigated in 109 herds. Serums from four batches of pigs (4, 10, 16 and 22 weeks, 15 pigs/batch) were tested by ELISA for PRRSV antibodies. Infection by Mycoplasma hyopneumoniae (Mhp), Actinobacillus pleuropneumoniae, H1N1 and H1N2 swine influenza A viruses (swIAV) and PCV2 were detected by specific serological or PCR tests. Data related to herd characteristics, biosecurity, management housing and climatic conditions were collected during a herd visit. Factors associated with the herd's PRRSV seropositive status were identified by logistic regression. Large herd size, the lack of disinsectisation in the gestation facilities, on-farm semen collection, a short time-period for gilt quarantine and a low temperature setpoint for the ventilation controller in the fattening room significantly increased the odds of a herd being seropositive for PRRSV. Infection by Mhp and H1N2 swIAV were associated with a PRRSV seropositive status. A Cox proportional hazards model was used to identify the factors associated with the age-time to seroconversion in infected herds. Joint housing for the gilts and sows when lactating, a large nursery pen, a small number of pens per fattening room and lack of all-in all-out management in the fattening section significantly reduced the age-time to seroconversion. A small range of temperatures controlling ventilation rate in the nursery room was also associated with time to PRRSV seroconversion. Infection by Mhp and a high PCV2 infection pressure were associated with a shorter time to seroconversion. Biosecurity measures minimising the risk of introducing PRRSV into the herd, management practices reducing contacts between animals from different batches and within batches and favourable climatic conditions should be implemented to better control PRRSV infection.

Risk factors for porcine reproductive and respiratory syndrome outbreaks in Vietnamese small stock farms

AIM

To examine risk factors that could have played a role in the 2010 porcine reproductive and respiratory syndrome (PRRS) outbreak in Yenhung district, Quangninh province, North-Vietnam, with the purpose of establishing why existing control measures implemented after previous outbreaks had failed to prevent further outbreaks.

METHODS

A case-control study was carried out in Yenhung district. Data were obtained by an interview-based questionnaire survey. The sampling unit was households, which equated to small-scale pig farms. A total of 150 case and 150 control households were selected at communes affected by the 2010 PRRS epidemic during April to June. Risk factors were analysed using binary logistic regression and unconditional multiple logistic regression.

RESULTS

Households infected with PRRS were significantly associated with multiple variables belonging to three main groups: (1) location of the farms: i.e. farms positioned <1,000 m from a pig abattoir or within 500 m of local markets or 100 m of main roads; (2) farm management: i.e. where there was non-application of weekly farm disinfection, feeding uncooked swill, new introduction of purchased pigs without isolation, or usage of water from irrigation systems for raising pigs; (3) people and animal contact: i.e. where households kept animals with either no confinement or partial confinement, had visits by family members to other affected farms or had frequent visits by neighbours. The use of water from irrigation systems was found to be the risk factor most strongly associated with infected households in the 2010 outbreak (OR=22; 95% CI=12-42).

CONCLUSIONS

The results show that the epidemiology of PRRS in Quangninh province was linked to sociological and cultural practices, and that effective PRRS control needs an integrated approach coupled with behavioural changes in the pig raising practices of the general public. Failure to recognise this could explain why further outbreaks have occurred.

Survival of porcine reproductive and respiratory syndrome virus in fresh pork

Porcine reproductive and respiratory syndrome virus (PRRSV) remains one of the most economically important diseases of pigs. Transmission of PRRS virus has been reported through many routes, with aerosol route being the most predominant. There may also be a potential risk of transmission through contami-nated pork, but this has never been investigated. The purpose of this study was to experimentally contaminate fresh pork with three different concentrations of PRRSV and to study virus survival at ambient (25 °C), refrigerated (4 °C), and frozen (-20 °C) temperatures. Concentrations of virus representing natural infectivity level and 'worst case scenario' were studied. The virus was detected in fresh pork at all three virus concentrations for up to 48 h at ambient temperature. At 4 °C, the virus survived for 6 days in pork inoculated with the higher virus concentration and for 3 days in pork inoculated at the lower concentration. At frozen temperature, PRRSV was detected for up to 60 days in pork inoculated at the higher concentration and for 7 days in pork inoculated at the lower concentration. These results suggest that fresh pork has the potential to be a vehicle for virus dissemination depending upon temperature and time of storage.

Survival of Porcine Reproductive and Respiratory Syndrome Virus in Pork Products

There is a risk of virus transmission through contaminated pork, and many viruses are considered potential hazards for both humans and livestock. The risk of transmission may be elevated with importation/exportation of meat between countries globally. Survival of porcine reproductive and respiratory syndrome virus (PRRSV) in different pork products has not been studied. The present study evaluated PRRSV survival in four different products: fresh sausage, ham, bacon, and acidified sausage prepared with experimentally contaminated pork. These products were prepared according to standard methods used by the manufacturers of pork products, and then stored at room temperature, 4 °C and -20 °C. PRRSV was detected only in fresh sausage for up to 15 days at 4 °C and for 30 days at -20 °C. No PRRSV was detected at any temperature in any of the other three products. These preliminary data provide valuable information for the pork processing industry, as well as in planning for import/export of these products among different countries.

Potential role of noncommercial swine populations in the epidemiology and control of porcine reproductive and respiratory syndrome virus

OBJECTIVE

To assess the role of noncommercial pigs in the epidemiology of porcine reproductive and respiratory syndrome (PRRS) virus.

DESIGN

Seroepidemiologic study and survey study.

ANIMALS

661 pigs from which blood samples were collected at slaughter and 32 pigs from which blood samples were collected longitudinally.

PROCEDURES

Spatial databases of commercial farms and 4-H participation were evaluated by use of commercial geographic information systems software. Information on disease knowledge and management methods of 4-H participants was obtained by mail survey and personal interview. Serum samples for antibody testing by PRRS ELISA were obtained from pigs at slaughter or at county fairs and on farms.

RESULTS

Participation in 4-H swine programs was geographically associated with commercial swine production in Minnesota, and 39% of 4-H participants reared pigs at locations with commercial pigs. High seroprevalence at fairs (49%; range, 29% to 76%) and seroconversion after fairs indicated that PRRS virus exposure was common in pigs shown by 4-H participants and that transmission could occur at fairs.

CONCLUSIONS AND CLINICAL RELEVANCE

The small swine population shown by 4-H members (estimated 12,000 pigs) relative to the population of commercial swine in Minnesota (estimated 6.5 million pigs) suggested the former overall was likely of minor importance to PRRS virus epidemiology at present. However, the relative frailty of knowledge of biosecurity practices, evidence that PRRS virus exposure was frequent, common intentions to show pigs at multiple events, and often close interactions with commercial herds suggested that the 4-H community should be involved in regional efforts to control PRRS.

Correlation among genetic, Euclidean, temporal, and herd ownership distances of porcine reproductive and respiratory syndrome virus strains in Quebec, Canada

BACKGROUND

Porcine reproductive and respiratory syndrome (PRRS) is a viral disease that has a major economic impact for the swine industry. Its control is mostly directed towards preventing its spread which requires a better understanding of the mechanisms of transmission of the virus between herds. The objectives of this study were to describe the genetic diversity and to assess the correlation among genetic, Euclidean and temporal distances and ownership to better understand pathways of transmission.

RESULTS

A cross-sectional study was conducted on sites located in a high density area of swine production in Quebec. Geographical coordinates (longitude/latitude), date of submission and ownership were obtained for each site. ORF5 sequencing was attempted on PRRSV positive sites. Proportion of pairwise combinations of strains having ≥98% genetic homology were analysed according to Euclidean distances and ownership. Correlations between genetic, Euclidean and temporal distances and ownership were assessed using Mantel tests on continuous and binary matrices. Sensitivity of the correlations between genetic and Euclidean as well as temporal distances was evaluated for different Euclidean and temporal distance thresholds. An ORF5 sequence was identified for 132 of the 176 (75%) PRRSV positive sites; 122 were wild-type strains. The mean (min-max) genetic, Euclidean and temporal pairwise distances were 11.6% (0-18.7), 15.0 km (0.04-45.7) and 218 days (0-852), respectively. Significant positive correlations were observed between genetic and ownership, genetic and Euclidean and between genetic and temporal binary distances. The relationship between genetic and ownership suggests either common sources of animals or semen, employees, technical services or vehicles, whereas that between genetic and Euclidean binary distances is compatible with area spread of the virus. The latter correlation was observed only up to 5 km.

CONCLUSIONS

This study suggests that transmission of PRRSV is likely to occur between sites belonging to the same owner or through area spread within a 5 km distance. Both should be considered in the perspective of prevention.

Assessment of Stomoxys calcitrans (Diptera: Muscidae) as a vector of porcine reproductive and respiratory syndrome virus

Porcine Reproductive and respiratory syndrome (PRRS) is a globally significant swine disease caused by an arterivirus. The virus replicates in alveolar macrophages of infected pigs, resulting in pneumonia in growing pigs and late-term abortions in sows. Outbreaks occur on disparate farms within an area despite biosecurity measures, suggesting mechanical transport by arthropods. We investigated the vector potential of stable flies, Stomoxys calcitrans (L.) (Diptera: Muscidae), in the transmission of porcine reproductive and respiratory syndrome virus (family Arteriviridae, genus Arterivirus, PRRSV) under laboratory conditions. Stable flies were collected around PRRS-negative boar stud barns in North Carolina and tested for presence of the virus. Stable flies were collected on alsynite traps placed near the exhaust fan of the close-sided tunnel-ventilated buildings, suggesting blood seeking flies are attracted by olfactory cues. No flies were positive for PRRSV. We assessed transmission of the virus through an infective bite by feeding laboratory reared stable flies on blood containing virus and transferring them to naive pigs for subsequent bloodmeals. Transmission of the virus to naive pigs by infective bites failed in all attempts. The volume of blood contained within the closed mouthparts of the stable fly seems to be insufficient to deliver an infective dose of the virus. Stable flies are unlikely to transmit PRRSV from one pig to another while blood feeding. The fate of the virus after a bloodmeal remains to be determined.

One year's study of dynamic and evolution of types I and II PRRSV in a swine farm

This study was to investigate dynamic and evolution of PRRSV in a seed-stock farm by monitoring PRRSV status from 11 June 2009 to 4 August 2010. For laboratory test, around 18-24 umbilical cords from farrowed sows and 5-95 sera from nursery and grow/finish pigs were submitted around every 2 weeks interval during the study. The submitted samples were tested for PRRSV using IDEXX PRRS 2XR ELISA kit, RT-nested PCR. The PRRSV-positive samples were further sequences based on ORF5 and analyzed using MEGA 3.1 program and Beast 1.5.4 package. The surveyed farm was first infected with type II PRRSV but it was infected newly with type I PRRSV of unknown origin, showing rapid substitution to type I PRRSV as a dominant strain in 2 weeks. The type I PRRSV was first detected from umbilical cord of a farrowed sow in 12 January 2010, and secondly from nursery pigs in 26 January 2010. Although sudden increase of mean S/P ratio was found in grow/finish pigs around 2 months earlier than first type I PRRSV detection, no type I PRRSV viremia was found. Thirty three ORF5 full sequences from 14 type II to 19 type I PRRSVs were obtained chronologically in this farm and the genetic characteristics and evolution rates of those sequences were analyzed. The substitution rates (/site/day) of two types were 4.03×10(-5) (type I), 3.09×10(-5) (type II), respectively, which was more frequent than previous reports. The calculated divergence time of type I PRRSV was consistent with the time when the sudden elevation of serum IgG in grow/finish barn was first observed. This study provided fundamental data for type I PRRSV dynamic in a previously type II PRRSV-infected farm and suggested grow/finisher barn could be a primary site for PRRSV introduction.

A stochastic mathematical model of the within-herd transmission dynamics of Porcine Reproductive and Respiratory Syndrome Virus (PRRSV): fade-out and persistence

A stochastic, mathematical model of a farrow-finish pig herd was developed and used to investigate the within-herd transmission dynamics of PRRSV, and to examine patterns of on-farm persistence and fade-out. The model was structured to represent the management of a typical European pig herd. Three parameters determining the natural history of infection were derived from the literature. Transmission parameters were chosen using PRRSV antibody data from a cross-sectional study of 103 pig herds (Evans et al., 2008). The seroprevalence by age was generated from the model at 21-day intervals and was compared to the cross-sectional field data using log-likelihood, accounting for the accuracy of the ELISA test used. The model was run for various isolation practices of purchased gilts, contact structure, herd size and the frequency of re-introduction of infectious gilts. The time-dependent log-likelihood patterns varied between herds in a similar way to patterns observed from serological values from the 103 farms. Essentially they indicated two patterns of seroprevalence: herds in which PRRSV was stably persistent, and herds in which PRRSV was unstable, either recently introduced or recently faded-out. With a herd size of 327 sows with identical management, fade-out of virus occurred within 4 weeks in 21.9% of simulations. Without isolation of gilts from sows, fade-out within 250 days decreased from 81.6% to 14.3% and for herd sizes of 75, 150, 300 and 600, the probability of persistence of virus for >1200 days was 4%, 13.4%, 20.4% and 18.2%, respectively. Introduction of virus at a rate of approximately 0.37 times per year resulted in virus persisting for >1200 days in 32.4% of simulations, compared with 17.6% for no re-introduction. Fade-out of virus was most likely to occur within breeding females before virus reached young stock. Persistence was more likely once PRRSV was present in piglets which in turn infected rearing-pigs. The probability of persistence was higher with increased herd size, increased contact between different age groups and increased re-introduction of infectious gilts. The ability of the model to capture the variability in cross-sectional, age-related serological patterns suggests that the processes of re-introduction, persistence and fade-out of PRRSV play critical roles in PRRSV epidemiology. The potential importance to pig production and transmission of virus between herds is discussed.

Porcine reproductive and respiratory syndrome virus (PRRSV) in GB pig herds: farm characteristics associated with heterogeneity in seroprevalence

Background

The between- and within-herd variability of porcine reproductive and respiratory syndrome virus (PRRSV) antibodies were investigated in a cross-sectional study of 103 British pig herds conducted 2003–2004. Fifty pigs from each farm were tested for anti-PRRSV antibodies using ELISA. A binomial logistic model was used to investigate management risks for farms with and without pigs with PRRSV antibodies and multilevel statistical models were used to investigate variability in pigs' log ELISA IRPC (relative index × 100) in positive herds.

Results

Thirty-five herds (34.0%) were seronegative, 41 (39.8%) were seropositive and 27 (26.2%) were vaccinated. Herds were more likely to be seronegative if they had < 250 sows (OR 3.86 (95% CI 1.46, 10.19)) and if the nearest pig herd was ≥ 2 miles away (OR 3.42 (95% CI 1.29, 9.12)). The mean log IRPC in seropositive herds was 3.02 (range, 0.83 – 5.58). Sixteen seropositive herds had only seropositive adult pigs. In these herds, pigs had -0.06 (95% CI -0.10, -0.01) lower log IRPC for every mile increase in distance to the nearest pig unit, and -0.56 (95% CI -1.02, -0.10) lower log IRPC when quarantine facilities were present. For 25 herds with seropositive young stock and adults, lower log IRPC were associated with isolating purchased stock for ≥ 6 days (coefficient -0.46, 95% CI -0.81, -0.11), requesting ≥ 48 hours 'pig-free time' from humans (coefficient -0.44, 95% CI -0.79, -0.10) and purchasing gilts (coefficient -0.61, 95% CI -0.92, -0.29).

Conclusion

These patterns are consistent with PRRSV failing to persist indefinitely on some infected farms, with fadeout more likely in smaller herds with little/no reintroduction of infectious stock. Persistence of infection may be associated with large herds in pig-dense regions with repeated reintroduction.

Porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory disease (PRRS) is an economically important disease around the globe; it has been estimated to cost the swine industry in USA approximately 560 million US dollars annually. It is well established that PRRS is caused by an enveloped, single-stranded positive-sense RNA virus known as porcine reproductive and respiratory syndrome virus (PRRSV). The inability to successfully control PRRS across farms via traditional methods (e.g. vaccine and animal flow) has led to a growing interest in area-based eradication. Important to such an initiative is information on PRRSV transmission within and between herds and intervention strategies to prevent its spread. This paper will review the current literature on selected areas of PRRS known to be important to the topic of pathogen elimination, including etiology, clinical manifestations, direct and indirect routes of transmission, as well as discuss measures for disease control, prevention and eradication.