The Schmallenberg virus was first diagnosed in a small town in Germany of the same name in August 2011. It is related to viruses affecting ruminant animals in Africa and Oceania. Subsequently, the virus was found in Belgium, Denmark France and in the UK in early 2012. It was first notified in Ireland, North and South, in Autumn 2012. Schmallenberg virus has now been diagnosed in cattle, sheep, goats and Alpaca throughout the North Western European Area. Deer and wild boar are considered potential wildlife hosts. There is no known risk to humans from this family of viruses.
Infection with the virus can have different clinical manifestations depending on the herd’s immune and reproductive status. Disease is generally considered to be more severe in sheep than cattle.
The causative agent of Schmallenberg virus (SBV) infection is a virus belonging to a family called Bunyaviridae, Genus Orthobunyavirus. Within this genus is a serogroup called the Simbu viruses which contains this SBV as well as others including the closely related Shamonda virus and Akabane virus - agents which cause disease in other countries. The Oropouche virus, which causes diseases in humans also belongs to this group though it is currently believed that SBV is not zoonotic (incapable of causing illness in humans). The virus is thought to be transmitted mainly by midges of the Culicoides spp.
This disease is vector borne i.e. it is spread from animal to midge to animal. However, venereal spread has not been ruled out (See below). The main species of midge involved are Culicoides spp. Mosquitos are thought not to be involved significantly in spread. However, midges can carry massive quantities of virus which they innoculate into sheep and cattle when feeding. Midges may even allow multiplication of the virus within their own bodies before innoculation. After an animal becomes infected by a virus-vector midge, a viraemia (virus circulation in blood) develops which lasts 2-5 days. After an incubation period (time from initial infection to development of clinical signs) of 1 to 6 days, the animals may or may not develop clinical signs with recovery over a 2-7 day period. Since initial introduction into Ireland, the disease has continued to spread rapidly throughout Ireland, carried by Culicoides midges at an alarming rate of spread of up to 20 km/day. The midge season in Ireland is from April to November and ability for the disease to spread is strongly influenced a number of factors including the following:
- Variety of midge (some Culicoides spp. are better at transmitting than others)
- Environmental temperature influences the speed at which midges develop, their lifespan (varying from 10 days to 3 months) and the success of viral multiplication within the midge
- Wind – influences geographical spread of midges on a daily basis
- Availability of host (cattle, sheep, other spp?) – Density of animals in a geographical area and the species mix
The exact mechanism of over-wintering of disease has not been fully established. Various theories exist that the virus persists over winter :
- In adult midges
- In animals – the virus is known to persist over long periods in certain tissues in infected animals including the lymph nodes, spleen and testes (The impact of virus shedding in semen from infected testes is currently unknown)
- In an infected foetus
- In midge larvae (vertical transmission from the adult midge to larvae has not been definitively ruled out)
The signs of the acute infection in cattle are often seen as fever, milk drop, inappetence, weight loss and occasionally diarrhoea, with a herd level outbreak lasting a few weeks. Signs of acute infection are often absent in sheep though fevers have been reported with associated milk drop described in milking sheep. The signs of acute infection usually disappear within one to two week. However, abortions occur in pregnant ewes (or cows) from early pregnancy onwards but if ewes are infected between 25 and 50 days into the pregnancy (cows from 62 to 173 days) then a range of severe developmental abnormalities can occur in the fœtus as follows :
• Bent limbs
• Fixed joints
• Twisted neck/spine
• Short jaw
• Domed skull
• Brain & spinal cord deformities
• Dummy calf – won’t suck
• HAS – hydranencephaly arthrogryposis syndrome
Most clinical disease occurs during the midge season though occasional new infections have been described during the winter period. The source of these winter-infections remains unexplained and may reflect alternative mechanisms of spread including venereal or mosquito mediated.
Diagnosis can be based on:
- Clinical signs - Deformities are fairly distinctive for this disease though they do not provide a definitive diagnosis
- Viral antigen can be detected by PCR – This test can identify virus in acutely infected animal or in an aborted foetus
- Serology - An ELISA test for antibodies is available. This identifies development of immunity in recovered animals or in dummy calves/lambs. Sampling may also be conducted on bulk milk.
Control of Schmallenberg virus (SBV) infection is based on three possible aspects:
Midge control - There are no products specifically licensed for the control of Culicoides spp. Furthermore, any products that could be used for flies are thought to be of limited value in preventing spread of SBV as the products may not kill the midges on treated animals prior to inoculation of the virus into the animal. Thus spread of infection is not prevented by the use of these products.
- Vaccination - The most important control option is to ensure animals are immune prior to becoming pregnant to prevent the formation of deformed foetuses as this is the main source of loss. This is best achieved by the appropriate use of a SBV vaccine to prevent foetal infection during the danger period when deformities are caused. A Schmallenberg virus vaccine has recently been given a provisional authorisation in Ireland. The vaccine against Schmallenberg virus infection has been licensed for use in both cattle and sheep. For further information on vaccination please consult your local veterinary practitioner.
Monitoring - This varies depending on the nature and risk status of your herd. Appropriate screening programmes can be discussed with your local veterinary practitioner.
The major objective of Schmallenberg virus (SBV) vaccination is the prevention of foetal infection. Therefore, vaccination usually focuses on breeding animals.
Breeding animals should receive their basic vaccination prior to service and, thereafter, should be re-vaccinated with Schmallenberg virus vaccine at regular intervals to assure solid immunity during the critical first phase of subsequent gestations. It is essential to continue vaccination until virus has ceased to circulate in a region or country.
Vaccination with Schmallenberg virus vaccine is therefore an important tool in ensuring reduction of losses associated with this infection.
The development of Bovilis SBV, a vaccine against Schmallenberg virus (SBV) infection that can be used in control programmes, has been a major step forward for Schmallenberg virus control.
For information on aetiology (Cause of disease) click here.
For information on epidemiology (Factors influencing occurrence of disease) click here.
For information on clinical signs click here.
For information on diagnosis click here.
For information on control click here.