This is our Friday rubric: every week a new Science Page from the Bob Morrison’s Swine Health Monitoring Project. The previous editions of the science page are available on our website.
Mira Willkan, Lorraine Chapot, Angus Cameron, Søren Kjærgaard Boldsen, and Mette Fertner share the results of their study assessing how sensitive the current PRRS regional surveillance system in Denmark is and explore alternative surveillance systems. Could they detect a new PRRS introduction in less than a month?
Key Points
- A national PRRS control programme was launched in Denmark in May 2022.
- The PRRS surveillance consists of serological antibody testing and notification requirements in case of clinical symptoms.
- With the current surveillance, approximately 22% of new incursions of PRRS into a PRRS-free region are expected to be detected within the first month.
- If the serological surveillance were performed monthly on all farms, the probability of detection could be increased to 88%.
Introduction
In Denmark, a national PRRS control programme was launched in May 2022 [1]. Subsequently, the country has been divided into regions from which the PRRS reduction strategy is run by local administrators and veterinarians. Now, the first region has obtained PRRS-free status, and more regions are expected to obtain PRRS-free status in the near future. The objective of the present study was to quantify the early detection sensitivity (EDSSe) of the current Danish PRRS surveillance system and explore alternative surveillance systems.
Methods
The current PRRS surveillance system consists of two components: a notifiable surveillance component, whereby the farmer and veterinarian are obliged to react in the event of clinical symptoms, and an active antibody surveillance component, whereby 20 pigs are sampled per farm (monthly on multiplier/breeding farms or yearly on production farms) [2]. The Early Detection Surveillance Sensitivity (EDSSe) was calculated as a product of the average herd sensitivity, the population coverage and the temporal coverage, for each of the surveillance components accounting for overlap [3]. Information on farm demographics was extracted from national register data.
Results
The dataset included 4,368 farms enrolled in the PRRS control programme, including 4% (193/4,368) breeding/multiplier farms and 96% (4,175/4,368) production farms. The farms were distributed across 43 regions. One region, the island of Bornholm, was declared PRRS-free at the end of 2024. The remaining regions had a PRRS-farm prevalence ranging from 6.6% to 54.7% at the time of data extraction (April 2024). Under the assumption of the regions having obtained PRRS-free status, the results from the present study estimated a mean probability of detecting an incursion of PRRS into a PRRS-free region within the first month of 22% for the 43 regions of interest. Hence, it is more probable that an outbreak will remain undetected using the current surveillance system. The relatively low probability of detecting an incursion of PRRS into a PRRS-free region is mainly due to the testing frequency, since the majority of farms (96%) are tested yearly. If the active surveillance were increased in frequency to monthly testing on all farms (both breeding/multiplier and production farms), the average probability of detection could be increased to 88%.
Discussion
In Denmark, more regions are expected to obtain PRRS-free status in the near future. Hence, the question has arisen as to how the surveillance should be performed to identify new incursions of the PRRS virus into a PRRS-free region as early as possible. The current surveillance system is expected to detect approximately one fifth of newly infected regions within the first month of incursion of PRRS. This sensitivity could be increased by conducting more frequent testing, potentially in a risk-based manner.
This study was published in Prev Vet Med in February 2025, available: https://www.sciencedirect.com/science/article/abs/pii/S0167587725000467
References
[1] Anonymous, 2022. Strategy for the reduction of porcine reproductive and respiratory syndrome (PRRS) in pigs in Denmark. https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwiHifm4vPWLAxXN1gIHHR_DGZQQFnoECB4QAQ&url=https%3A%2F%2Fsvineproduktion.dk%2Faktuelt%2Ftemaer%2F-%2Fmedia%2F2098DD01D3854B71A3FB7FCF7F665F1E.ashx&usg=AOvVaw0-o7rhkkWvdGI9McZGwWc8&opi=89978449
[2] Anonymous, 2023. Order no. 997 of 29/06/2023 on Porcine Reproductive and Respiratory Syndrome (PRRS) [in Danish]. Ministry for Food, Agriculture and Fisheries. https://www.retsinformation.dk/eli/lta/2023/997 [3] Cameron, A.R., Meyer, A., Faverjon, C., Mackenzie, C., 2020. Quantification of the sensitivity of early detection surveillance. Transbound. Emerg. Dis. 67, 2532-2543. https://onlinelibrary.wiley.com/doi/10.1111/tbed.13598