Videotutorial on Monitoring PRRSV: the key to determining PRRS epidemiological status

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Videotutorial on Monitoring PRRSV: the key to determining PRRS epidemiological status

Different diagnostic techniques are used to carry out regular monitoring of a farm. Serology allows us to detect and quantify antibodies against the PRRSV. PCR enables us to amplify the viral genome to determine its presence and to quantify it. Sequencing aims to determine the order of nucleotides in a fragment of genetic material of PRRSV.

Serology gives us an idea of how the PRRSV circulates on a farm. When interpreting the results it is essential to bear in mind the serological technique and the commercial kit used, as the kinetics of detection can vary considerably. Currently there is no serological technique that enables us to differentiate between vaccinated and infected animals. Bleeding animals at different ages (gilts, breeders and fattening pigs) will enable us to obtain the SERUM PROFILE of the farm, which will provide information on its immunological and clinical status. Analysis of the results helps to determine the age at infection, the infection pressure of the herd as well as the percentage of susceptible animals. This information is useful for determining the epidemiological status of the farm and applying the appropriate control measures.

PCR enables us to detect the presence of PRRSV in different types of sample through the amplification of its genetic material. In the breeding herd, it is commonly used to confirm the diagnosis of acute or subclinical PRRS outbreaks. In gilts, it is widely used to determine their status at the beginning and at the end of their acclimatization period. In fattening pigs, it helps to determine their epidemiological status. These results will help us to apply suitable control measures.

Determining PRRSV sequence is a useful tool for epidemiological studies. This technique enables us to create phylogenetic trees of field strains and to describe the degree of variation between strains. It can also provide information about the number of different strains present on one single farm. In the case of a PRRS outbreak, sequencing enables us to determine whether the strain causing the outbreak is the endemic one, the HOMOLOGOUS strain, or if it is a new one, a HETEROLOGOUS strain that has entered the farm. This is an important distinction, as corrective measures are different in each case. It should be taken into account that the degree of similarity of ORF5 or even the complete genomic sequence is not a useful parameter for predicting the level of cross-protection of a given vaccine strain against a specific field strain. Results obtained from these analyses provide information on the epidemiological status of the farm. This will help us to choose the appropriate control measures and intervention strategies in each case.

It is necessary to know the most appropriate biological samples for performance of the analytical techniques, and to know the type of information that can be obtained from each of these. Blood samples allow us to obtain results that are useful at an individual level, for example in the process of gilt acclimatization. It is possible to obtain results for the whole population with a sufficient number of samples from each animal group. PRRSV has a tropism for a large number of different tissues such as lungs, foetus, tonsils, lymph nodes and umbilical cord, which are appropriate for sampling. Some of these samples allow detection of the PRRSV irrespective the phases of viraemia. Saliva is becoming widely used for performing both serology and PCR. Saliva samples are obtained through cotton rope which is chewed by the pigs. The sampling process is faster, cheaper and non-invasive, thereby reducing stress in the animals. Information obtained is for the whole population and enables better detection of a low prevalence of infection.

Once monitoring has been performed, farms can be classified according to PRRS epidemiological status. This herd classification will help us to decide on corrective measures and a vaccine program strategy that have to be implemented. Evaluating the presence of PRRSV circulation in some pig populations, in other words shedding status, and evaluating the serological and clinical status in breeders will allow us to classify the PRRS status of the herd. Herds with a reproductive clinical and subclinical PRRS outbreak have virus circulation and a high antibody level. Such herds belong to the PRRS positive unstable category. Those farms where the reproductive outbreak is being controlled but antibody levels in sows are still high due to the presence of virus circulating within the sow herd will be classified as PRRS positive stable farms. Those farms where PRRSV circulation has been eliminated from all the sections of the farm will be classified as provisional negative stable in case they still have seropositive sows, or PRRS negative stable if no seropositive animals are present on the farm.

In summary, monitoring will allow us to evaluate the epidemiological PRRS status of the farm. This herd classification together with evaluation of biosecurity will enable us to decide the best control measures and vaccine strategies to be implemented.