Evaluation of the Pathogenicity and Transmissibility of a Chilean Isolate of Porcine Reproductive and Respiratory Syndrome Virus

The objective of this study was to determine clinical features, shedding and transmission of a Chilean Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) strain upon experimental inoculation of 4‐week‐old pigs. Six groups of five animals each were used. The G1 (donor) group was inoculated with PRRSV, maintained in an isolation unit for 35 days, and sampled daily to determine shedding in mucosal secretions and faeces, viraemia and seroconversion. An uninfected control group (G6) was equally maintained and sampled under strict isolation. Four other groups (G2 to G5) were exposed to PRRSV via direct contact with G1 for 5‐day periods in a staggered manner, throughout the 35‐day period, and were later placed in an independent isolation unit to monitor infection status for 7 days. All the animals in G1 and G6 were killed at 35 days post‐inoculation (dpi) and the contact groups at 12 days post‐contact (dpc). Samples were obtained from diverse organs for histopathological, immunohistochemical (IHC) and virological analysis. No clinical symptoms were evident in any group, except for a transient fever observed in G1. Histopathologically, all the animals of G1 had interstitial pneumonia, although scarce PRRSV‐positive cells were detected in the lung using IHC. PRRSV‐positive cells (IHC) were detected in the lymphoid tissue of all animals in infected groups, but especially in G3 and G4. Viraemia was detected in G1 (3–35 dpi) and in the all contact groups (5–12 dpc). Likewise, ranging from 3 to 19 dpi, PRRSV was detected in at least one animal from the tonsils and lungs in all infected groups, in nasal and ocular secretions, saliva or faeces. These results indicate that the donor group excreted infectious PRRSV and was able to transmit the infection to susceptible pigs. The critical shedding period was 7–19 dpi, during which, most likely, transmission took place.     

Porcine Reproductive and Respiratory Syndrome Virus Vaccines: Current Status and Strategies to a Universal Vaccine

Porcine reproductive and respiratory syndrome virus (PRRSV) is the causative agent of PRRS, the most significant infectious disease currently affecting swine industry worldwide. In the United States alone, the economic losses caused by PRRS amount to more than 560 million US dollars every year. Due to immune evasion strategies and the antigenic heterogeneity of the virus, current commercial PRRSV vaccines (killed‐virus and modified‐live vaccines) are of unsatisfactory efficacy, especially against heterologous infection. Continuous efforts have been devoted to develop better PRRSV vaccines. Experimental PRRSV vaccines, including live attenuated vaccines, recombinant vectors expressing PRRSV viral proteins, DNA vaccines and plant‐made subunit vaccines, have been developed. However, the genetic and antigenic heterogeneity of the virus limits the value of almost all of the PRRSV vaccines tested. Developing a universal vaccine that can provide broad protection against circulating PRRSV strains has become a major challenge for current vaccine development. This paper reviews current status of PRRSV vaccine development and discusses strategies to develop a universal PRRSV vaccine.     

Clustering of and Risk Factors for the Porcine High Fever Disease in a Region of Vietnam

Porcine high fever disease (PHFD) emerged in 2006 in China and spread to Vietnam. Little work has been carried out to investigate PHFD risk factors and space–time dynamics. To fill this gap, we investigated probable cases of PHFD at household level as the outcome. A study area, approximately 100 sq. km, was selected from a province of southern Vietnam that had reported the outbreak of PHFD in 2008. A survey was conducted in the study area to collect information about swine health problems during 2008. The questionnaire included three sections: general information, clinical signs of disease in pigs and production factors believed to be risk factors. Cases were defined at the household level and included interpretation of clinical signs in series. Logistic regression with a random intercept at the hamlet level was used to assess risk factors for PHFD at the household level. Spatial clustering was investigated using the D‐function and a Cuzick–Edward’s test. Spatial clusters were evaluated using a spatial relative risk surface and the spatial scan statistic using a Bernoulli model. Space–time clustering was explored using a space–time K‐function and Knox’s test. Space–time clusters were evaluated using a space–time permutation model in SaTScan. Of 955 households with questionnaire data, 33.4% were classified as cases. The statistical significance of space and space–time clustering differed between methods employed. The risk factors associated with occurrence of cases were higher numbers of sows and finishing pigs (log 2 transformed), receiving pigs from an external source and the interaction between using ‘water green crop’ (WGC) as pig feed and owning ducks with or without direct contact with pigs. The interaction between the presence of ducks and feeding WGC to pigs suggested the involvement of pathogens that might be present in water (environment) and could further replicate in or on ducks.     

Naturally Co‐Infected Boars with both Porcine Reproductive and Respiratory Syndrome Virus and Porcine Circovirus Type 2

In this study, the humoral response against porcine reproductive and respiratory syndrome virus (PRRSV) and porcine circovirus type 2 (PCV2), the presence of the virus in semen and serum and the genetic characteristics of the virus detected in 15 boars from a commercial farm were analysed. The results showed that 53% of the boars presented anti‐PRRSV antibodies and 100% presented anti‐PCV2 antibodies. Porcine reproductive and respiratory syndrome virus was detected in 43% of the boars and 73% were positive to PCV2. The complete ORF5 gene of PRRSV of 14 samples and a fragment of the ORF2 gene of PCV2 of 22 samples were sequenced. Porcine reproductive and respiratory syndrome virus analysis revealed <92% identity in viruses from semen and serum of two boars, whereas in the rest of the boars the identity was >97.5%. As for PCV2, two boars presented an identity <95% in serum and semen and the rest had an identity >96%. The results showed that PRRSV‐ and PCV2‐naturally infected boars can be found, and at least two different strains of viruses from semen and serum can be detected.     

Comparative Analysis of Immune Responses in Pigs to High and Low Pathogenic Porcine Reproductive and Respiratory Syndrome Viruses Isolated in China

The CH‐1a and HuN4 strains of porcine reproductive and respiratory syndrome virus (PRRSV) show different pathogenicities in pigs. To understand host immune responses against these viruses, we investigated the dynamic changes in cytokine levels produced in peripheral blood of piglets infected with the highly pathogenic PRRSV HuN4 strain or the CH‐1a strain. Clinical signs, virus loads and serum cytokine levels [interferon(IFN)‐α, Interleukin (IL)‐1, TNF‐α, IL‐6, IL‐12, IFN‐γ, IL‐10 and TGF‐β] were tested. The results showed that while piglets developed effective cellular immune responses against CH‐1a infection, those infected with HuN4 displayed ineffective cellular immunity, organ lesions and persistent elevated levels of immunoregulatory cytokines (IL‐10 and TGF‐β), which delayed the development of PRRSV‐specific immune responses. These results demonstrated that HuN4 infection induced higher cytokine levels than that of CH‐1a infection induced. The changes in inflammatory cytokines intensified the inflammatory reaction and damaged the tissues and organs.     

A recombination between two Type 1 Porcine Reproductive and Respiratory Syndrome Virus (PRRSV‐1) vaccine strains has caused severe outbreaks in Danish pigs

Porcine reproductive and respiratory syndrome virus (PRRSV) is prevalent in Danish swine herds. In July 2019, PRRSV‐1 was detected in a PRRSV‐negative boar station and subsequently spread to more than 38 herds that had received semen from the boar station. Full genome sequencing revealed a sequence of 15.098 nucleotides. Phylogenetic analyses showed that the strain was a recombination between the Amervac strain (Unistrain PRRS vaccine; Hipra) and the 96V198 strain (Suvaxyn PRRS; Zoetis AH). The major parent was the 96V198 strain that spanned ORFs 1–2 and part of ORF 3 and the minor parent was the Amervac strain, which constituted the remaining part of the genome. The virus seems to be highly transmissible and has caused severe disease in infected herds despite a high level of genetic identity to the attenuated parent strains. The source of infection was presumable a neighbouring farm situated 5.8 km from the boar station.     

Emergence of Porcine Reproductive and Respiratory Syndrome in Sweden: Detection,Response and Eradication

Porcine reproductive and respiratory syndrome (PRRS) is characterized by reproductive failure in sows and respiratory problems in growing pigs. The disease is present in most countries throughout the world but was not diagnosed in Sweden until the summer of 2007 when it was first detected through the national PRRS surveillance program. The immediate mobilization of veterinary authorities, field veterinarians and the pig industry was a prerequisite for preventing the spread of the disease. Within 10 days seven herds were verified as infected and the measures taken included stamping out, cleaning, disinfection and a vacancy period of 3 weeks before the herds were repopulated. To evaluate the effectiveness of these measures, a national sero‐surveillance was carried out during the autumn of 2007. Approximately 90% of the pig production was covered by this screening and all samples tested were negative with regard to antibodies to PRRS virus.     

Differential Expression of Proinflammatory Cytokines in the Lymphoid Organs of Porcine Reproductive and Respiratory Syndrome Virus‐Infected Pigs

Porcine reproductive and respiratory syndrome (PRRS) is considered one of the most important diseases in the swine industry. Although several studies have been carried out to elucidate the host immune response evoked against PRRS virus (PRRSV), there are several aspects that still remain unclear. The aim of this study was to determine the expression of IL‐1α, IL‐6 and TNF‐α in the lymphoid organs (mediastinal and retropharyngeal lymph nodes and tonsil) of PRRSV‐infected pigs and to determine their correlation with the expression of PRRSV antigen. Proinflammatory cytokine expression was different depending on the body compartment examined. Thus, whereas IL‐1α and TNF‐α were the main cytokines expressed in the mediastinal lymph node, IL‐6 was the most highly expressed cytokine in the retropharyngeal lymph node, and no expression of proinflammatory cytokines was observed in the tonsil. These findings may be related to the impairment of the host immune response evoked after PRRSV infection. Therefore, lymphoid organs and proinflammatory cytokines represent an important target of study for clarifying the immunopathogenesis of PRRS.     

Two Commercial Type 1 Porcine Reproductive and Respiratory Syndrome Virus (PRRSV)‐Modified Live Vaccines Reduce Seminal Shedding of Type 1 PRRSV but not Type 2 PRRSV in Infected Boars

The objective of this study was to compare the effects of two commercial type 1 porcine reproductive and respiratory syndrome virus (PRRSV)‐modified live vaccines on type 1 and type 2 PRRSV shedding in the semen of experimentally infected boars. Upon challenge with PRRSV, unvaccinated boars exhibited an increase in daily rectal temperature (39.4–39.7°C). Vaccination of boars with type 1 PRRSV significantly reduced the amount of type 1 PRRSV load in blood and semen after challenge with type 1 PRRSV, but barely reduced the amount of type 2 PRRSV load in blood and semen after the type 2 PRRSV challenge. There were no significant differences in the reduction of viremia and seminal shedding of type 1 and type 2 PRRSV between the two commercial vaccines. The seminal shedding of PRRSV is independent of viremia. The reduction of type 1 PRRSV seminal shedding coincided with the appearance of type 1 PRRSV‐specific interferon‐γ secreting cells (IFN‐γ‐SC) in vaccinated type 1 PRRSV‐challenged boars. The frequencies of type 1 PRRSV‐specific IFN‐γ‐SC induced by type 1 PRRSV vaccine are relatively high compared to type 2 PRRSV‐specific IFN‐γ‐SC induced by the same vaccine which may explain why type 1 PRRSV vaccine is more effective in reducing seminal shedding of type 1 PRRSV when compared to type 2 PRRSV in vaccinated challenged boars. These results provide clinical information on how to reduce seminal shedding of type 1 PRRSV in boars using type 1 PRRSV‐modified live vaccine.     

Downregulation of Antigen‐Presenting Cells in Tonsil and Lymph Nodes of Porcine Reproductive and Respiratory Syndrome Virus‐Infected Pigs

Porcine reproductive and respiratory syndrome virus (PRRSV) can persist in different organs of infected pigs, which suggests a failure in the immune response. Antigen‐presenting cells (APCs) play a pivotal role in the induction of effective T‐ and B‐cell responses. In this study, we investigated the changes in the different APC subpopulations and T‐ and B‐cell counts in the tonsil, retropharyngeal and mediastinal lymph nodes of pigs experimentally infected with a European PRRSV field isolate. Our results demonstrated that the expression of S100, SWC3, HLA‐DR molecule and CD3 was diminished in the studied organs throughout the study, observing a significant negative correlation between viral antigen and HLA‐DR expression in both retropharyngeal and mediastinal lymph nodes. In contrast, λ‐light chains showed an increase during the study. Taking all into account, after PRRSV infection, no enhancement in the number of APCs and T cells was observed, suggesting an impairment of the immune function which may allow the persistence of PRRSV into the organism.     

Investigation of the Occurrence of Porcine Reproductive and Respiratory Virus in Swine Herds Participating in an Area Regional Control and Elimination Project in Ontario,Canada

The main goal of this study was to investigate the occurrence of porcine reproductive and respiratory syndrome virus (PRRSV)‐specific genotypes in swine sites in Ontario (Canada) using molecular, spatial and network data from a porcine reproductive and respiratory syndrome (PRRS) regional control project. For each site, location, animal movement service provider (truck companies), PRRSV status and sequencing data of the open reading frame 5 (ORF5) were obtained. Three‐kilometre buffers were created to evaluate neighbourhood characteristics for each site. Social network analysis was conducted on swine sites and trucking companies to assemble the network and define network components. Three different PRRSV genotypes were used as outcomes for statistical analysis based on the region's phylogenetic tree of the ORF5. Multivariable exact logistic regression was conducted to investigate the association between being positive for a specific genotype and two main exposures of interest: (i) having at least one neighbour within three km also positive for the same genotype outside the production system and (ii) having at least one positive site for the same genotype in the same truck network component outside the production system. Results showed that the importance of area spread and truck network on PRRSV occurrence differed according to genotype. Additionally, the Ontario PRRS database appears suitable for conducting regional disease investigations. Finally, the use of relatively new tools available for network, spatial and molecular analysis could be useful in investigation, control and prevention of endemic infectious diseases in animal populations.     

Sequence and Phylogenetic Analyses of the Nsp2 and ORF5 Genes of Porcine Reproductive and Respiratory Syndrome Virus in Boars from South China in 2015

Porcine reproductive and respiratory syndrome virus (PRRSV) is highly genetically diverse; however, little is known about the molecular epidemiology of PRRSV in the boar farms of South China. In this study, 367 samples were collected from boar farms in South China in 2015. The Nsp2 hypervariable region and ORF5 gene were PCR amplified from 66 PRRSV‐positive samples, followed by sequencing and analysis. The percentage of PRRSV antigen‐positive samples was 17.98%; 8.72% were positive for highly pathogenic PRRSV (HP‐PRRSV), and 9.26% were positive for low pathogenic PRRSV (LP‐PRRSV). Sequence alignment and phylogenetic tree analyses revealed three novel patterns of deletion in the hypervariable region of Nsp2, which had not been identified previously. Furthermore, numerous amino acid substitutions were identified in the putative signal peptide and extravirion regions of GP5. These results demonstrate for the first time that the existence of multiple different strains on the same boar farm, and extensive genetic mutation and high infection rate of PRRSV in boars from South China. Our research contributes to the understanding of the epidemiology and genetic characteristics of PRRSV on boar farms.     

Risk Assessment of the Introduction of Porcine Reproductive and Respiratory Syndrome Virus via Boar Semen into Switzerland as an Example of a PRRSV‐Free Country

Switzerland is currently porcine reproductive and respiratory syndrome virus (PRRSV) free, but semen imports from PRRSV‐infected European countries are increasing. As the virus can be transmitted via semen, for example, when a free boar stud becomes infected, and the risk of its import in terms of PRRSV introduction is unknown, the annual probability to accidentally import the virus into Switzerland was estimated in a risk assessment. A quantitative stochastic model was set up with data comprised by import figures of 2010, interviews with boar stud owners and expert opinion. It resulted in an annual median number of 0.18 imported ejaculates (= imported semen doses from one collection from one donor) from PRRSV‐infected boars. Hence, one infected ejaculate would be imported every 6 years and infect a mean of 10 sows. These results suggest that under current circumstances, there is a substantial risk of PRRSV introduction into Switzerland via imported boar semen and that measures to enhance safety of imports should be taken. The time from infection of a previously negative boar stud to its detection had the highest impact on the number of imported ‘positive’ ejaculates. Therefore, emphasis should be placed on PRRSV monitoring protocols in boar studs. Results indicated that a substantial increase in safety could only be achieved with much tighter sampling protocols than currently performed. Generally, the model could easily be customized for other applications like other countries or regions or even sow farms that want to estimate their risk when purchasing semen from a particular boar stud.     

Pathology and Virus Distribution in the Lung and Lymphoid Tissues of Pigs Experimentally Inoculated with Three Distinct Type 1 PRRS Virus Isolates of Varying Pathogenicity

Porcine reproductive and respiratory syndrome (PRRS) continues to be the most economically important disease of swine worldwide. The appearance of highly pathogenic PRRS virus (PRRSV) strains in Europe and Asia has raised concerns about this disease and initiated increased efforts to understand the pathogenesis. In this study, we have compared the pathology and the virus distribution in tissues of pigs experimentally inoculated with three different genotype 1 PRRSV isolates. Sixty 5‐week‐old pigs were inoculated intranasally with a) the Lelystad virus (LV), b) a field strain from the UK causing respiratory clinical signs (UK) or c) a highly pathogenic strain from Belarus (BE). Sixteen animals were mock‐infected and used as controls. The animals were euthanized at 3, 7 and 35 days post‐infection (dpi), and lung and lymphoid tissues collected for histopathological examination and PRRSV detection by immunohistochemistry (IHC). Histopathological lesions consisted of interstitial pneumonia with mononuclear cell infiltrates in the lungs, lymphoid depletion, apoptosis and follicular hyperplasia in the spleen, lymph nodes and tonsil and lymphoid depletion in the thymus. Porcine reproductive and respiratory syndrome virus was detected mainly in monocytes–macrophages. BE‐infected animals showed the highest pathological scores and the highest presence of virus at 3 and 7 dpi, followed by the UK field strain and then LV. Moderate lesions were observed at 35 dpi with lesser detection of PRRSV by IHC in each infected group. The highly pathogenic BE strain induced more severe pathology in both lungs and lymphoid organs of pigs compared with the classic field isolate and the prototype LV. The increased severity of pathology was in correlation with the presence of a higher number of PRRSV‐infected cells in the tissues.     

An Outbreak of Porcine Reproductive and Respiratory Syndrome Virus in Switzerland Following Import of Boar Semen

An outbreak of porcine reproductive and respiratory syndrome virus (PRRSV) occurred in November 2012 in Switzerland (CH), traditionally PRRSV‐free. It was detected after a German boar stud informed a semen importer about the detection of PRRSV during routine monitoring. Tracing of semen deliveries revealed 26 Swiss sow herds that had used semen from this stud after its last negative routine monitoring and 62 further contact herds. All herds were put under movement restrictions and examined serologically and virologically. As a first measure, 59 sows from five herds that had previously been inseminated with suspicious semen were slaughtered and tested immediately. Investigations in the stud resulted in 8 positive boars with recent semen deliveries to CH (Seven with antibodies and virus, one with antibodies only). In one boar out of six tested, virus was detected in semen. Of the 59 slaughtered sows, five from three herds were virus‐positive. In one herd, the virus had spread, and all pigs were slaughtered or non‐marketable animals euthanized. In the remaining herds, no further infections were detected. After confirmatory testings in all herds 3 weeks after the first examination gave negative results, restrictions were lifted in January 2013, and Switzerland regained its PRRSV‐free status. The events demonstrate that import of semen from non‐PRRS‐free countries – even from negative studs – poses a risk, because monitoring protocols in boar studs are often insufficient to timely detect an infection, and infections of sows/herds occur even with low numbers of semen doses. The outbreak was eradicated successfully mainly due to the high disease awareness of the importer and because immediate actions were taken before clinical or laboratory diagnosis of a single case in the country was made. To minimize the risk of an introduction of PRRSV in the future, stricter import guidelines for boar semen have been implemented.     

Two novel recombinant porcine reproductive and respiratory syndrome viruses belong to sublineage 3.5 originating from sublineage 3.2

Porcine reproductive and respiratory syndrome virus (PRRSV) is an agent of porcine reproductive and respiratory syndrome (PRRS), which causes substantial economic losses to the swine industry. PRRSV displays rapid variation, and five lineages coexist in mainland China. Lineage 3 PRRSVs emerged in mainland China in 2005 and prevailed in southern China after 2010. In the present study, two lineage 3 PRRSV strains, which are named SD110‐1608 and SDWH27‐1710, were isolated from northern China in 2017. To explore the characteristics and origins of the two strains, we divided lineage 3 into five sublineages (3.1–3.5) based on 146 open reading frame (ORF) 5 sequences. Both strains and the strains isolated from mainland China were classified into sublineage 3.5. Lineage 3 PRRSVs isolated from Taiwan and Hong Kong were classified into sublineages 3.1–3.3 and sublineage 3.4, respectively. Recombination analysis revealed that SD110‐1608 and SDWH27‐1710 were derived from recombination of QYYZ (major parent strain) and JXA1 (minor parent strain). Sequence alignment showed that SD110‐1608 and SDWH27‐1710 shared a 36‐aa insertion in Nsp2 with QYYZ isolated from Guangdong Province in 2010. Based on the evolutionary relationship among GP2a, GP3, GP4, GP5 and N proteins between sublineages 3.2 (FJ‐1) and 3.5 (FJFS), we speculated that sublineage 3.5 (mainland China) originated from sublineage 3.2 (Taiwan, China). This study provides important information regarding the classification and transmission of lineage 3 PRRSVs.     

Review of Biological Factors Relevant to Import Risk Assessments for Epizootic Ulcerative Syndrome (Aphanomyces invadans)

Epizootic ulcerative syndrome (EUS) is a disease affecting both wild and farmed fish in freshwater and estuarine environments. After it was first described in Japan in 1971, the disease has spread widely across Asia and to some regions of Australia, North America and Africa. In Asia and Africa, the spread of the disease has substantially affected livelihoods of fish farmers and fishermen. No reports are yet published showing the presence of the disease in Europe or South America. Given its epizootic nature and its broad susceptible fish species range, it would appear that the disease has the potential for further spread. This study provides a review of the scientific literature on several biological factors of the pathogen, Aphanomyces invadans, associated with the disease EUS and aspects of the disease that are relevant to undertaking import risk assessments (IRA) covering (i) Life cycle and routes of transmission; (ii) Minimum infectious dose; (iii) Tissue localization and pathogen load; (iv) Predisposing factors for infection and factors influencing expression of disease; (v) Carrier state in fish; (vi) Diagnostic methods; (vii) Survival in the environment; (viii) Permissive temperature range; (ix) Stability of the agent in aquatic animal products; (x) Prevalence of infection; and (xi) Affected life stages. Much of the biological information presented is relevant to a broad range of risk questions. Areas where data are lacking were identified, and the information provided is put into context with other aspects that need to be addressed in an IRA.     

Assessment of area spread of porcine reproductive and respiratory syndrome (PRRS) virus in three clusters of swine farms

Despite decades of porcine reproductive and respiratory syndrome (PRRS ) research, outbreaks with emerging and re‐emerging PRRS virus (PRRSV ) strains are not uncommon in North America. The role of area spread, commonly referred but not limited to airborne transmission, in originating such outbreaks is currently unknown. The main objective of this study was to explore the role of area spread on the occurrence of new PRRSV cases by combining information on genetic similarity among recovered PRRSV isolate's open‐reading frame (ORF ) 5 sequences and publicly available weather data. Three small regions were enrolled in the study for which high farm‐level participation rate was achieved, and swine sites within those regions were readily sampled after reporting of an outbreak in a sow farm. Oral fluid PCR testing was used to determine PRRSV status of farms, and wind roses were generated for assessment of prevailing wind directions during 2–14 days preceding the outbreak. Under the conditions of this study, the data did not support the area spread theory as the main cause for these outbreaks. We suggest that for future studies, analysis of animal movement and other links between farms such as personnel, equipment and sharing of service providers should be incorporated for better insights on source of the virus. Furthermore, the development of rapid and easy diagnostic methods for ruling out resident PRRSV is urgently needed.     

White Spot Syndrome Virus (WSSV) Concentrations in Crustacean Tissues – A Review of Data Relevant to Assess the Risk Associated with Commodity Trade

We have reviewed the available peer reviewed literature on pathogen load for white spot syndrome virus (WSSV) in species susceptible to infection. Data on pathogen load in traded commodities are relevant for undertaking import risk assessments for a specific pathogen. Data were available for several of the major penaeid shrimp species farmed for aquaculture and for one crab and crayfish species. Most data are based on experimental infection, but some data were available for farmed or wild shrimp. Owing to the unavailability of immortal cell lines to determine viral load of viable virus, quantitative PCR was the main method used for quantification. The viral loads measured in shrimp at the onset of mortality events were extremely high (in the order of 10⁹–10¹⁰ copy numbers gram⁻¹ of tissue). In a farm setting, the onset of increased mortalities will often trigger emergency harvests. Therefore, shrimp obtained from emergency harvests are likely to carry substantial concentrations of viral particles. Viral load did not vary greatly with tissue type. The WSSV load in wild crustaceans, farmed crustaceans not undergoing a mortality event or survivors of a mortality event was significantly lower (usually by multiple logs). Studies have also been undertaken in ‘vaccinated’ shrimp. One of the ‘vaccines’ led to a significant reduction of viral load in WSSV‐exposed animals. The data obtained from the literature review are put into context with published information on minimal infectious dose and WSSV survival in frozen commodity shrimp.