Investigation, management and control of a maedi outbreak in Norway in 2019-2020

Abstract

Background: Visna-maedi is a notifiable disease in Norway, and eliminating the disease is a national goal. The import of sheep into Norway is very limited, and strict regulations apply to the movement of small ruminants between flocks and within defined geographical regions. Several outbreaks have occurred in the last 50 years, and the most recent before 2019 occurred in Trøndelag county in Central Norway in 2002. A national surveillance programme for small ruminant lentivirus infection exists since 2003.

Results: In 2019, the national surveillance programme detected seropositive animals for small ruminant lentivirus in a sheep flock in Trøndelag. Based on the result of polymerase chain reaction analysis and histopathological findings, the Norwegian Food Safety Authority concluded the diagnosis of maedi. Further investigations detected maedi in eight additional sheep flocks in the same county. The flocks were placed under restrictions, and the authorities also imposed restrictions on 82 contact flocks. Sequencing of partial gag genes indicated that the virus in the current outbreak was related to the small ruminant lentivirus detected in the same area between 2002 and 2005.

Conclusions: The outbreak investigation shows the need for sensitive and specific diagnostic methods, and an improved and more targeted surveillance strategy. It also demonstrates the risk of disease spreading between flocks through animal movements, and highlights the importance of biosecurity and structured livestock trade. In addition to allowing livestock trade only from flocks documented free from maedi, it may be necessary to monitor sheep flocks over many years, when aiming to eliminate maedi from the Norwegian sheep population.

Keywords: Disease elimination; Outbreak investigation; Ovine progressive pneumonia; Sheep; Small ruminant lentivirus; Surveillance; Visna-maedi virus infection.

Fig. 1

The number of detected maedi infected flocks from 1972 to 2022. The figure is modified after Krogsrud et al. [13], and thereafter updated with NVI journal system data and previous outbreak descriptions [10, 12]. The colours indicate the geographical location of affected flocks

Fig. 2

Sheep flocks involved in the maedi outbreak in Trøndelag in 2019 to 2020. Red triangles mark infected flocks. Green triangles mark negative contact flocks. Green dots mark monitored flocks within the zone of regulations. Grey dots represent other, not tested sheep flocks