Bluetongue is a non-contagious, viral disease of domestic and wild ruminants with a high morbidity and mortality, transmitted by the bites of midges Culicoides.5 Because the potential for extremely rapid spread and its serious consequences for the international trade of animals and animal products, it is contained in the code for the terrestrial animals of the World Organization for Animal Health (OIE) and European Commission (EC), being an official control disease.8,3,4
The Bluetongue virus - BTV belongs to the family Reoviridae, genus Orbivirus,4 with a variation in the proteins that make up the outer capsid—VP2 to VP5— and determine the serotype, of which 24 have been identified.7
This virus is endemic in a worldwide band of tropical and subtropical regions from approximately 35°S to 40°N; however, outbreaks also occur outside this area, and the virus may persist long-term if the climate and vectors are suitable. The global distribution and nature of BTV infection has changed significantly in recent years, and climate change has been implicated as a potential cause of this dramatic event, especially in Europe.2 In fact, since 2006, BTV has expanded further into Western Europe reaching as far north as 58°N, representing a new wave of expansion.6
The virus can replicate in many species of ruminants, including cattle, sheep, deer, goats and camelids, although severe clinical signs are most commonly seen in improved breeds of sheep.5 Some serotypes of BTV also induces clinical cases in cattle and other ruminants, but in this species, it is most common a subclinical disease and therefore may serve as important and covert viral reservoirs for sheep.4
The most common Culicoides associated with the disease in endemic areas are C. imicola (in Africa, Europe, the Middle East and Asia), C. sonorensis (the principal vector in North America) and C. brevitarsis (Australia).5,6 The distribution of Culicoides in Southern Europe is believed to have expanded as a result of higher temperature and lower rainfall,3 and tend to disseminate in more regions where the virus in not isolated yet.3
In addition to of higher temperature and lower rainfall, it also contributes to the spread of the disease stagnant water and non-controlled animal movements.
BTV is mainly transmitted by biting midges in the genus Culicoides, which are biological vectors. These are small (0,3 mm) and haematophagous insects, a characteristic limited to females, which require protein for the production of eggs.6 The midges can fly short distances of 1-2 km,5 but they can be blown much farther by wind with more than 300 Km,3 with climate events such as storms, typhoons and hurricanes.2
The virus can replicate in many species of ruminants, including sheep, goats, cattle, African buffalo, water buffalo, cervids, bison, and wildebeest. Clinical cases tend to occur mainly in sheep, but in Europe also in cattle. Goats, buffalos, bison, South American camelids, wild or zoo ruminants, cervids and some carnivores are occasionally affected.5
Most transmission of BTV, particularly in endemic areas, occurs ‘silently’ in disease-resistant host animals. Cattle are the main such reservoir in most areas, being subclinically infected and sometimes developing prolonged viraemias up to 100 days.3
After the bite of an infected animal, the Midge ingested the virus and infect the insect midgut cells, replicate in them, escape into the body cavity of the insect (haemocoel) and infect and replicate in the salivary glands before it can be transmitted to a new host.6
The extrinsic incubation period - EIP is estimated to be approximately a week, with a range of 2-10 days.5 The midge kept at 15ºC require several weeks to complete the EIP, while those kept at 30ºC may complete incubation in a couple of days. BTV appears unable to replicate at temperatures below 12ºC, and the activity of Culicoides vectors also reduces or ceases at low temperatures.9
Bluetongue virus can also have transmitted via transplacental, only with the serotype 8. According to European Commission, the precautionary measure as regards the movement of pregnant animals now applies only for zones which are restricted only for this serotype.1
Clinical cases of BTV occur mainly in sheep, while subclinical infection seem to predominate in most other species. Clinical signs include fever, depression, hemorrhages and ulcerations of the oral and nasal tissue, swelling and oedema of lips, tongue and jaw; blue tongue as a result of cyanosis (rare), coronary inflammation and lameness, conjunctivitis, nasal discharge, salivation, hyperaemia and pain at muco-cutaneous junctions such as the gums and vulva, and death.8
Cases of subclinical disease (without symptoms) are also very important, because it makes it difficult to diagnosis of the disease and contributes to the easy replication of the virus.
There are also implications in reproduction and production like abortions, temporary infertility, reductions in milk yield, weight loss and severe wool break.
Bluetongue is a disease of greater economic importance, it can affect 100% of the flock, and mortality averages can be between 20% to 70% of animals with clinical signs.8 Direct loses associated to the disease are weight loss, reduction in milk yield, abortions up to 80% of pregnant sheep.
There are also indirect costs such as animal movement restrictions and slaughter of animals. This may be due not only to the serious health implications but also to the difficult control of the disease, because some animals can remain as asymptomatic carriers and reservoirs that spread the disease in a collective or area.
No specific treatment is available, other than supportive care. The measures to control and eradicate the disease is based on identiﬁcation, surveillance and tracing of susceptible and potentially infected animals, vaccination programs for sheep and cattle, restriction of animals and vectors control.8,1
Reverse transcriptase-polymerase chain reaction (RT-PCR) tests are widely used to identify viral RNA in clinical samples, and can also identify the serotype. Immunofluorescence and immunoperoxidase staining, or group-specific antigen-capture ELISAs may also be used.5
Vaccination is used as the most effective and practical measure to minimize losses related to the disease and to potentially interrupt the cycle from infected animal to vector. It is essential to use a vaccine designed to provide protection against the speciﬁc strain (or strains) of virus of concern in a particular area.8
1 European Commission Animal Diseases - Bluetongue. Jan 2018. https://ec.europa.eu/food/animals/animal-diseases/control-measures/bluetongue_en
2 MacLachlan, N.J and Guthrie, A. Re-emergence of Bluetongue, African horse sickness and other Orbivirus diseases. Vet. Res. (2010) 41:35
3 Purse, B; Mellor, P, Rogers, D, Samuel, R; Mertens, P; Baylis, M. Climate change and the recent emergence of bluetongue in Europe. Nature Reviews, Microbiology. Vol 3, Feb 2005
4 Saegerman, C; Berkvens,D; Mellor P. Bluetongue Epidemiology in the European Union. Emerging Infectious Diseases. www.cdc.gov/eid. Vol. 14, No. 4, April 2008
5 The center for Food Security & Public Health – CFSPH and Institute for International Cooperation in Animal Biologics - Bluetongue. Iowa State University. 2015. http://www.cfsph.iastate.edu/Factsheets/pdfs/bluetongue.pdf
6 Wilson, A and Mellor, P. Bluetongue in Europe: past, present and future. Phil. Trans. R. Soc. B (2009) 364, 2669–2681
7 World Organization for Animal Health – OIE. Technical Disease Card - Bluetongue. 2013. http://www.oie.int/fileadmin/Home/eng/Animal_Health_in_the_World/docs/pdf/Disease_cards/BLUETONGUE.pdf
8 World Organization for Animal Health – OIE. General Disease Information Sheets – Bluetongue (BT). Jan. 2018. http://www.oie.int/doc/ged/D13959.PDF
9 Wittmann, E. J., Mellor, P. S., Baylis, M. Effect of temperature on the transmission of orbiviruses by the biting midge, Culicoides sonorensis. Med. Vet. Entomol. 2002. 16, 147–156.