The bluetongue prevalent in the Netherlands, caused by the virus serotype 3, appears to occur almost exclusively clinically in sheep. As a result, the sheep have to go through the disease to produce antibodies.
This is evident from a study by the Dutch Sheep and Goat Breeders Organization (NSFO) to see to what extent subclinical bluetongue infections occur on sheep farms. For this purpose, 156 blood samples were taken between October 19 and 23 from a total of nine herds in the municipalities of Wijdemeren, Zaanstad, Schagen, Stichtse Vecht, Nijkerk and Oldebroek.
The trial was conducted with different breeds, in different regions and with different herd sizes. The first infections were reported in all these herds in the first weeks of the bluetongue outbreak in September. This means that there are four to six weeks between the first clinical symptoms and the taking of blood samples.
Antibodies against the bluetongue virus have been detected in three of the 156 animals examined where no complaints associated with bluetongue were previously seen. No antibodies were found in the samples of seven of the nine couples.
This is inconsistent with previous studies with other serotypes. For example, in an infection with serotype 4 in Greece, all sheep had antibodies six to ten days after infection. Some of these also became ill. A similar picture also emerges from studies in the United States and Italy.
According to veterinarian Reinard Everts, director of the NSFO, subclinical bluetongue occurred in certain breeds during the test in Italy. ‘We also found subclinical infections in the last outbreak with type 8 in Belgium, a few years ago. This means that the animals do not become visibly ill and are still protected.’
Going through infection
There appear to be hardly any subclinical infections among the current infections in the Netherlands. ‘This also means that sheep that have not shown any symptoms of disease do not yet have antibodies and have therefore not built up immunity. Actually, only animals that have been sick have antibodies. Of all sheep that are stung by the midge, about 70 percent die,’ Everts continues.
The veterinarian expects that a repetition of a similar examination will give the same result. ‘It also corresponds to our gut feeling. You still see new infections in the weeks after the first infection at a company. You wouldn’t expect that when subclinical infections occur substantially. We also hear similar sounds from sheep farmers who have submitted blood samples themselves.’
Everts therefore points out the importance of a vaccine. ‘Without a vaccine, 70 percent of infected sheep will die. This vaccine is therefore not only a must for the lambs, but also for the sheep that are still around and have not been sick.’