Bluetongue (BT) is a non-contagious notifiable viral disease of sheep and other ruminants such as cattle and goats. The main route of infection is the bite of flies (midges) infected with the bluetongue virus (BTV). Wageningen Bioveterinary Research (WBVR) conducts research into this disease.
Notifiable means that suspicions of a bluetongue infection must be reported to the NVWA (Netherlands Food and Consumer Product Safety Authority). Bluetongue is a “Categorie A” disease (Animal Health Law).
The disease is caused by many different serotypes of the bluetongue virus (Family of Rotaviridae, genus Orbivirus). Only serotypes 1-24 are notifiable. The pathogenicity of virus variants of the same serotype can vary greatly. Serotype 25 and above are less/non-virulent.
Bluetongue was reported for the first time in the Netherlands in 2006. This introduction of bluetongue virus serotype 8 led to the largest documented outbreak of bluetongue.
The clinical signs are summarized with FFF (Fever, Face, Feet). These phenomena are mainly observed in sheep; high fever, salivation, swelling in the head, including tongue and lips, and pain and inflammation of the coronary margin near the hooves. Animals no longer eat, lie a lot, walk lame or stand with a rounded back to relieve the pain in the legs. In very severe cases, the tongue turns blue. Most sheep recover after 2-3 weeks. The susceptibility of sheep breeds to bluetongue virus infection is very different.
Bluetongue virus infections in cattle and goats often progress without apparent clinical signs. Strikingly, African ruminant species appear to have natural immunity. Infected cattle produce longer and more virus in the blood and therefore play an important role in virus spread.
Bluetongue occurs permanently in tropical and subtropical areas (endemic), but outbreaks also occur occasionally in areas with a temperate climate (epidemic). Multiple serotypes occur simultaneously in many areas across all continents, except Antarctica. Affected areas are mainly limited by the distribution of the midge, the biological vector for the spread of bluetongue virus.
Only specific midge species are biological vectors and spread the virus by ingesting infected blood and releasing the virus with the next blood meal. BTV multiplies in both ruminants and these midge species. Once infected, midges continue to shed virus for life. Direct contact between ruminants does not spread the virus, but serotypes 25-27 are an exception.
Bluetongue has been reported from the end of the last century in countries around the Mediterranean, including the Balkan countries. Serotypes 1, 2, 4, 9, and 16 have since been detected in various parts of this area. Serotype 9 is no longer found in Europe. In 2014, a new variant of serotype 4 was found in Greece. This virus quickly spread across the Balkan countries and the European countries bordering the Mediterranean Sea. In 2017, serotype 3 originating from Tunisia was found in Sicily (Italy).
It was initially assumed that midge species in North-West Europe would not be a biological vector for bluetongue virus. The 2006-2008 outbreak in North-West Europe caused by serotype 8 has changed this view. The midge species Culicoides imicola is the main biological vector for the spread of the virus in Africa and Asia. Other midge species, such as C. obsoletus (Europe), C. sonorensis (America) and C. brevitarsis (Australia) can spread bluetongue virus on the respective continents.
The general expectation is that global warming will lead to expansion of these areas. It may also mean that other midge species may become suitable biological vectors for bluetongue virus due to climatic changes. Either way, global warming will increase the likelihood of bluetongue outbreaks in current temperate zones.
Atypical bluetongue viruses
From 2007, new serotypes were discovered through improved and more intensive monitoring programs. Serotype 25 was discovered in Switzerland, serotype 26 in Kuwait and serotype 27 in Corsica (France). Additional new serotypes were discovered in South Africa, Mongolia and Israel/Jordan. The serotype of these new viruses is generally determined on the basis of genetic relatedness and not, as was done for serotypes 1-24, on the basis of VNTs with standard sera. More than 35 bluetongue virus serotypes are known based on these genetic relatedness.
Serotypes 25 and higher only infect small ruminants. The infection is subclinical and this is probably why these non-virulent serotypes have not been discovered earlier by laboratory diagnosis studies. It is striking that BTV serotypes 25, 26 and 27 are transmitted without midges, thus through direct contact between sheep or goats. This is consistent with the laboratory finding that these serotypes cannot be cultured in midge-derived cell lines. Because of these specific properties, these new serotypes are therefore named atypical bluetongue viruses.
Bluetongue caused by serotypes 1-24 is a notifiable animal disease. Wageningen Bioveterinary Research (WBVR) is the national reference laboratory for Bluetongue. WBVR and has ISO-certified tests to diagnose suspicions of Bluetongue.
Antibodies against BTV after infection or vaccination can be detected with the ELISA and serum neutralization test (SNT). The ELISA detects antibodies against all serotypes, whereas the SNT detects antibodies specific for one serotype. Clotted blood (serum blood) is used in these serological tests.
All serotypes can be detected with the BTV PCR test. The serotype can (subsequently) be determined with serotype-specific PCR tests or by determining the genetic code of the virus ('sequencing').
The unknown new BTV is preferably cultured in the laboratory first. Then, the serotype can be also determined by the virus neutralization assay (VNT) using a set of standardized sera against each of the many BTV serotypes to neutralize the unknown BTV. However, this set of standardized sera is incomplete. Whole blood (EDTA blood) or organs (spleen, kidney lymph nodes) is used in these virological tests. Samples of blood or organs must be transported on ice (not frozen).
Live attenuated viruses are available as bluetongue vaccines for many serotypes in Africa, but are not allowed outside of Africa. Vaccination for one serotype does not protect against other serotypes. Live-attenuated vaccines are used in Africa, but are unsafe. In addition, these are not DIVA vaccines, which means that infections in vaccinated ruminant populations cannot be detected. They are used as emergency vaccines in other parts of the world, e.g. USA. In Europe, inactivated bluetongue vaccine is available in Europe for a limited number of serotypes (1, 2, 4, 8). These more expensive vaccines are safe and effective, but are not DIVA compatible. Preventive vaccination with inactivated vaccine is allowed, but it is scarcely used in European countries. Until now, no DIVA vaccines for bluetongue have been registered.
In 2008, a massive vaccination campaign was started with inactivated bluetongue vaccine against serotype 8. The number of infections declines sharply because of this vaccination and due to high natural immunity from infections in 2006 and 2007. After 2009 there was hardly any vaccination taken place. After a number of years, many Nort-West European countries were again declared free for serotype 8 in 2012. Unfortunately, spread of bluetongue virus serotype 8 was reported again in France in 2015 and subsequently in neighbouring countries. The risk of new outbreaks therefore remains. The situation also remains volatile in southern Europe with repeated incursions from the south, for example serotype 3 from Tunisia to Sicily in 2017.
Prevention is better than cure. It is therefore important to quarantine and test all ruminants from infected countries, which are imported to a country free of AHS, to prevent the introduction of the disease. The quarantine period usually consists of two parts, namely before transport and after transport. The individual quarantine periods are usually 21-30 days. The quarantine facility must be closed to insects and animals are often also treated with an insect repellent and/or with insecticides. In addition to clinical monitoring, blood samples are regularly tested for the presence of BTV and/or BTV antibodies before the animals are transported or admitted into a country. Special rules have been put in place for pregnant cattle, as they can carry an infected foetus that can become a source of virus spread after birth.
Containing or removing infected ruminants reduces virus uptake and thus also virus spread by midges. Midges can also be controlled to reduce the spread of the disease. After a BTV infection has been detected, various zones are set up around the source of infection, in which measures apply, such as intensive monitoring and a ban on animal movements. This will help contain the outbreak.
In addition to laboratory diagnostics, Wageningen Bioveterinary Research (WBVR) does a lot of research into all kinds of facets of bluetongue. Such as the pathology, development and validation of diagnostic tests, the spread by midges and into new BT vaccines.
This research is conducted in collaboration with national and international partners, such as the Animal Health Service (GD, Deventer) and the USDA, Manhattan, KA, USA, but also in European consortia, such as ORBINET, ORBIVAC and PALE-Blu. A new concept, the DISA principle (DISA=Disabled Infectious Single Animal), was developed for bluetongue vaccines. Knockout of one protein results in attenuation, DIVA and the vaccine cannot be spread by midges. The bluetongue DISA/DIVA platform is applicable for multiple bluetongue virus serotypes and is extensively tested in sheep and cattle.
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