Clinical Signs, Epidemiology, Pathology and Diagnosis of an Emerging Teratogenic
Vector Borne Viral Disease of Ruminants – Schmallenberg Virus as an Example
Peter D. Kirkland
Virology Laboratory, Elizabeth Macarthur Agriculture Institute,
Woodbridge Rd, Menangle NSW Australia
peter.kirkland@dpi.nsw.gov.au
1. Introduction
Vector borne viruses that have been associated with teratogenic defects in ruminants have
mostly belong to the Orthobunyavirus genus. The most well known is Akabane virus as a
result of the large epidemics of congenital arthrogryposis and hyrdanencephaly (AG/HE) that
have occurred, mostly in Australia and Japan but to a lesser extent in some Middle Eastern
countries1 . Before the aetiological agent was recognised, a vector borne virus was suspected
due to the geographical distribution of disease outbreaks and an association with favourable
environmental conditions. However, there was some confusion about the linkage between a
vector borne viral infection and the occurrence of disease due to the lack of disease in
breeding females, the occurrence of disease in offspring during winter and spring months and
the variety of defects that were observed. The recent emergence of Schmallenberg virus
(SBV) presents another example of the issues that are faced when investigating disease
outbreaks due to a similar novel agent. This presentation will focus on some of the important
features of an incursion of a novel virus and the approaches to undertaking an effective
investigation. Key elements involved in the investigation of a new agent such as SBV include:
 Appearance of an unusual clinical presentation/syndrome
 Rapid spread of disease over large geographical area
 Use of high throughput sequencing technologies for the identification of a novel agent
 Availability of archival sera to help identify time of the incursion
 Timely development of suitable assays for detection of virus and antibodies
 Application of new assays to detection of virus in animal tissues and insect vectors.
2. History and Clinical Signs
The emergence of SBV has displayed, as time has progressed, most of the features of a typical
orthobunyavirus. The most notable difference, that was fundamental to the discovery of this
virus, was the occurrence of a transient febrile illness with a drop in milk production in
infected cows. Within a short time frame, affected animals were observed on other farms,
raising the question of a highly contagious or vector-borne disease. A very wide range of high
consequence and exotic diseases were excluded2, including bovine ephemeral fever1. The
application of high throughput sequencing and metagenomic investigations incriminated a
virus from the Simbu serogroup2. As a consequence of the identification of a Simbu virus, the
prospect of congenital defects in the progeny of infected pregnant animals was forecast and
farmers and veterinarians advised of what might be seen. Some abortions were observed and
several months later, commencing in early December, abnormal lambs were born, with a
range of defects consistent with those described for other Simbu viruses. The incidence was
not high in most flocks but in some instances, with synchronised ‘out of season’ breeding, up
to 50% of lambs were affected. Towards the end of winter, nearing the end of the occurrence
of cases in sheep, calves were born, firstly with arthrogyposis. These calves show varying
locomotor difficulties and the limb defects are often the cause of dystocia. Later, animals
affected with hydranencephaly and born. These calves are often stillborn or show severe CNS
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deficits including blindness, deafness and extreme proprioceptive deficits. In cattle the
proportion of affected calves was generally low.
3. Epidemiology
While the initial cases of acute disease due to SBV on farms in The Netherlands and Germany
near the Dutch/German border may not have raised suspicions of a vector borne virus, once
the identification of a Simbu virus became known, vector transmission became the likely
means of virus spread. Further, it was also likely that this novel virus would be spread by
various Culicoides species. This is also supported by the speed of spread and the long
distances over which affected farms were found in the first few months. In contrast, the
radiating pattern of spread would, in the early stages, be potentially consistent with the
introduction of a contagious agent into a fully susceptible population. It was soon recognised
that SBV had emerged in a region close to where BTV serotype had emerged in Europe. This
provided some indication of the potential vectors and the extent to which the virus might
spread. At the start of the first winter, when a cessation to virus transmission was expected,
both the occurrence of disease in adult cattle and, in most cases, serological evidence
indicated that the virus had spread widely through The Netherlands, Germany, Belgium,
Northern France and to England. In the subsequent years, spread has continued with infection
also detected in Switzerland, Poland and several other Eastern European countries.
Serological studies undertaken at the end of the first transmission season identified a very
high seroprevalence in cattle – with evidence of infection on most farms and in many
instances 80-90% of cattle. Surprisingly, there was also a relatively high prevalence in sheep,
with up to 60% infected. Concern was expressed that the prevalence seemed too high for
spread in a single season, based on transmission rates for BTV8 in similar locations. An
alternative explanation proposed was that perhaps this virus was being spread by means other
than just the vector – eg perhaps direct contact. The availability of archival serum collections
facilitated the confirmation that SBV had recently arrived, with no evidence of infection
before August 2011. The high transmission rates documented are consistent with knowledge
of the annual incidence of infection with other Culicoides-borne Simbu viruses, even though
the exact vector species may be different. The availability of rapid molecular diagnostic
assays for the detection of SBV has also allowed stored insect collections to be studied and
identify relatively high infection rates with SBV. These infection rates are markedly higher
than encountered with BTV and provide some explanation for the apparently efficient
transmission of virus to livestock. The available knowledge of local Culicoides species that
have the capacity to act as vectors of other arboviruses (eg BTV) has also been valuable to
explore the potential for virus transmission indoors and the likelihood of overwintering and a
resumption of transmission early in the next season.
Research undertaken in bulls has shown that SBV may be present in semen for a short period
after the acute infection. Calf inoculation studies have confirmed that some samples contain
infectious virus but it is not known whether this will establish infection in a female if used for
insemination.
4. Pathology
Both the gross and histopathological changes observed following SBV infection largely
mirror those for virus such as Akabane3. The nature, severity of lesions and incidence of
disease is determined by the host species, the stage of gestation at which foetal infection
occurs and by the virus strain involved. The clinical presentation in cattle occurs in a
sequential manner with a succession of different clinical signs and pathology at different
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times after infection while in sheep and other small ruminants, a range of lesions and defects
may occur in the same animal. With some strains of virus, modest numbers (15-20%) of
animals may be affected while other strains may cause fewer defects, or in extreme situations,
up to 80% of progeny have been affected.
In most species, the impact of the virus is greatest in mid-gestation. There is no documented
evidence of damage to the conceptus following infection in the very early stages of pregnancy
(the first 3 weeks in sheep and goats; the first 2 months in cattle). The most susceptible stages
of gestation in small ruminants range from 28-56 days (especially 28-36 days) and in cattle
from 3 to 6 months. In the latter stages of gestation (after 60 days in small ruminants and the
last 2 months in cattle), the incidence of abnormalities declines to a very low level. Naturally,
during the course of an outbreak, the first abnormalities to be seen follow infection in the
latter stages of gestation. Calves infected close to term may sometimes show a flaccid
paralysis as a result of a non-suppurative encephalomyelitis. As infection occurs at earlier
stages of gestation there may be arthrogryposis, perhaps only involving a single joint on a
limb but after infection at earlier periods of gestation, limbs are more severely affected, with
abnormalities involving multiple joints on several or even all four limbs. Arthrogryposis in
cattle usually occurs as a result of infection between 100-170 days of gestation
Infection of the foetus between approximately 80-105 days of gestation results almost
exclusively in the development of hydranencephaly and porencephaly. There may be a small
number of calves born with both mild hydranencephaly and arthogryposis due to infection
around 100-120 days. The small grossly apparent cystic defects in the cerebral cortex
(porencephaly) are seen in some calves with arthrogryposis but soon give way to calves that
have severe CNS defects with almost complete absence of the cerebral hemispheres
(hydranencephaly).
In sheep and goats, due to the shorter gestation and shorter period of susceptibility, there is
usually a combination of severe defects of the limbs concurrently with gross CNS lesions.
There can be severe defects affecting the spinal column (torticollis, scoliosis, kyphosis,
laudosis). Development of the lungs and thymus may also be retarded.
The predominant lesions detected by histological examination are degenerative changes in the
motor neurones of the spinal cord. These changes usually produce a neurogenic muscular
atrophy but a primary viral myositis sometimes occurs. Histological examination of brains
affected by hydranencephaly at term is usually unrewarding and reveals a thin shell of
relatively normal tissue surrounding the large voids caused by viral replication.
5. Diagnosis
Because it is not often observed, the occurrence of acute disease in adult animals over a large
area cannot be relied upon to indicate the emergence of a novel teratogenic agent. However,
the incursion of a virus into a susceptible population can be explosive and such signs may be
noticed. Some strains of Akabane virus in Japan have caused clinical encephalitis in adult
cattle. These may be key signs that later lead to severe outbreaks of congenital defects at a
later time. If an epidemic of unusual disease is observed in an adult livestock population, to
assist with an investigation, anticoagulant treated blood samples and serum should be
collected from animals in the earliest detectable stages of the disease. These samples may be
used for virus isolation or molecular based genome detection technologies.
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Later in the season from midwinter extending to spring a teratogenic arbovirus should be
considered when there is an outbreak of congenital defects in cattle, sheep or goats. An
important factor to consider is the potential for contact with midges (Cucicoides spp) during
the late summer and autumn. The gross pathology should provide a strong index of suspicion.
Aetiological diagnosis and confirmation of an arbovirus infection depends on either the
detection of the agent in tissues of the affected foetus or neonate or the detection of specific
antibody in blood or fluids of affected progeny that have been deprived of colostrum. Most
stillborn or aborted bovine foetuses and calves that are born at term mount a specific antibody
response to the virus. The most conclusive approach to these investigations is to initially
determine the IgG levels in foetal fluids (pericardial or pleural) or pre-colostral serum. An
elevated IgG level will incriminate an infectious agent. Virus-specific serology can then be
undertaken for known teratogenic viruses. However, low or ‘background’ IgG levels do not
completely exclude the possibility of an infectious aetiology but serological methods will not
be appropriate. Further investigations to exclude infectious agents will require cultural or
nucleic acid detection methods.
Virus detection by virus isolation or PCR may be considered from a foetus that has been
aborted in the early stages of pregnancy. However, infectious virus is rarely present in full
term neonates. Real time PCR has become a valuable diagnostic procedures and was
employed to great effect during the investigation of the SBV outbreak. Viral RNA has been
detected in tissues from a very high proportion of lambs at term. SBV virus was also detected
in swabs of placenta and in meconium. In calves, residual viral RNA is occasionally detected
at term, sometimes in conjunction with specific antibodies.
Maternal serology can only be undertaken to exclude known agents and is of value only in
regions where the virus is not endemic. In these situations, positive maternal serology will
raise the index of suspicion while a negative result will convincingly exclude the test virus as
the aetiological agent.
During the investigation of a new or emerging vector-borne virus, once an isolate is available,
or some of the genome sequence is known, it becomes possible to develop tests and further
incriminate the agent. The development of qRT-PCR assays should be a priority to confirm
involvement of the agent in the disease outbreak. Availability of the virus in culture is a key
development but then allows the rapid development of serological tests. Basic assays such as
the VNT or IFAT may used to detect specific antibody while other tests are being developed.
6. References
There have been a considerable number of publications describing the Schmallenberg
investigations. As these investigations are extensive, cover many countries and are
complimentary, there are too many to accurately cite here, There is a recent review 4 that
should be consulted to provide a detailed overview and a comprehensive list of relevant
references. Several key papers are cited below.
1. Kirkland, P.D. 2002 Akabane and bovine ephemeral fever virus infections Vet Clin Food
Anim, 18: 501-514.
2. Hoffmann, B., Scheuch, M., Hoper, D., Jungblut, R., Holsteg, M., Schirrmeier, H.,
Eschbaumer, M., Goller, K.V., Wernike, K., Fischer, M., Breithaupt, A., Mettenleiter,
T.C., Beer, M., 2012. Novel orthobunyavirus in cattle, Europe, 2011. Emerging Infect.
Dis. 18: 469-472
3. Kirkland, P.D., Barry, R.D., Harper, P.A.W. and Zelski, R.Z., 1988. The development of
Akabane virus induced congenital abnormalities in cattle. Vet Rec, 122:582 586.
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4. Tarlinton, R., Daly, J., Dunham, S. and Kydd, J. 2012. The challenge of Schmallenberg
virus emergence in Europe. Vet. Journal. 194: 10-18
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