Infection with Schmallenberg virus (SBV) can lead to (febrile) diarrhoea in domestic and wild ruminants, with severely reduced general condition and milk loss. If the infection occurs in the womb, serious malformations of the young animals can occur. Transmission occurs through the bite of gnats (biting-sucking insects) and from the infected mother to the unborn offspring. The first evidence of the disease in animals from Austria dates back to 2012. Humans are not susceptible to the infection.
Infection of fetuses can lead to abortions or various malformations of the head and extremities and to the birth of weak calves, lambs or kids, depending on the time of infection. In born animals, the infection remains unnoticed or leads to a mild, transient disease with diarrhoea, fever and a decrease in milk yield.
Situation in Austria
In the course of the first occurrence of SBV in Austria, a very rapid infection of a large part (almost 100 %) of the Austrian cattle population occurred in 2012. With a time lag, many malformations were observed in calves, lambs and kids in the spring of the following year 2013. The detection of antibodies in wild ruminants provided evidence that these animals were also affected by SBV infection. Since the first occurrence, the frequency of new annual infections has been much lower due to existing baseline immunity in the ruminant population (presence of protective antibodies), but isolated detections occur annually mostly during export surveys.
In 2020, 1,981 blood samples from cattle, sheep, goats and deer were tested for antibodies to SBV, of which 704 (36%) were positive. In contrast, the virus itself was detected in only 69 of 29,679 virologically tested samples (0.2 %).
SBV is an enveloped RNA virus with a single-stranded, segmented genome from the family Bunyaviridae, genus Orthobunyavirus. Related viruses (Shamonda, Aino and Akabane viruses; "Simbu serogroup") are widespread in Australia, Asia and Africa, where they usually initially cause only a very mild clinic. However, if pregnant animals are infected, considerable congenital damage, premature births and disturbances in fertility can occur after a time delay.
According to the current state of knowledge, SBV does not pose a risk to humans.
If malformations or stillbirths occur in calves in a herd, the responsible official veterinarian must be contacted, who will take the necessary steps to send samples or the aborted foetus to suitable laboratories. Since infection with SBV is not notifiable, the detection has no consequences under epidemic law.
Transmission occurs through gnats(Culicoides spp.). Due to the seasonal occurrence of these vectors, acute infections as well as accidental detections of SBV during routine examinations (e.g. export examinations) are limited to the summer and autumn months. Naive dams may transmit the infection to the fetuses transplacentally.
Acute infection: In most cases there are no or only mild and short-lived clinical symptoms characterised by fever, diarrhoea or a drop in milk yield. In the course of the acute infection, abortions may also occur in pregnant animals.
intransplacental infection: if pregnant, naive dams are infected during a susceptible period (in sheep probably between the 30th and 50th day, in cattle approximately during the 75th to 175th day of gestation), the infection may lead to severe malformations of the extremities (arthrogryposis) and head (hydrocephaly, torticollis, hydranencenphaly). The birth of malformed and/or weak young animals occurs in winter and spring, depending on the time of infection and the gestation period of the different affected species.
The examination for antibodies from blood (serum) is carried out by ELISA or (rarely) serum neutralisation test. The result allows a statement as to whether the animal had contact with the virus. A serum neutralisation test can be carried out on paired serum samples to differentiate between a recent and a more recent infection. A passed infection usually leads to immunity.
The examination by means of PCR is carried out on blood, semen and organs (brain, spinal cord, spleen) as well as abortive material: PCR directly detects the virus or virus components. In blood, SBV is only detectable for a very short period of time (a few days). In semen, too, SBV is usually detectable for a short period in the course of an acute infection; in rare cases, SBV may also be detectable in semen for a longer period.
In all cases, samples should ideally be shipped to the laboratory with coolants and in compliance with the relevant transport regulations (UN3373) by an authorised logistics company.