What is Bluetongue?
Bluetongue (BT) is an infectious disease, transmitted mainly by culicoides vectors affecting different species of ruminants. The agent is the Bluetongue virus (BTV), an orbivirus from which 26 possible serotypes have been described.
BTV affects ruminants, both domestic and wild. Sheep are considered to be the main hosts, since it is the species that develops the disease, presenting high rates of mortality and morbidity. Moreover, other non-ruminants may be infected by BTV, such as shrews or dogs.
The main route of transmission is by mosquitoes of genus Culicoides, noting that not all species of this genus are capable of transmitting the BTV. Other sporadic transmission ways are by semen, embryos, transplacentally and iatrogenic.
The presence of the disease is directly related to the distribution of competent vectors. This distribution is closely linked to ecological and climatic conditions, which determine the survival of the vector. Currently, the distribution of competent vectors is located in 65°N latitude in Europe. The maximum activity period of the vector in this continent occurs in late summer or early fall. Climate change is one of the determining factors in the increased expansion of competent vectors and thus of the disease.
Historical development and status in Europe
Bluetongue virus (BTV) was described in South Africa in the early twentieth century. Until 1943, when the disease was described in Cyprus, it was believed that the virus was present only in Africa, where the infection was endemic in wild ruminants. Since that time, the virus has been spreading across the Mediterranean Basin and other regions such as USA, Canada or Australia, always determined by the presence of competent vectors.
Currently, the geographic distribution of the disease encompasses a worldwide distribution. However, until 2006 it was restricted to latitudes between parallels 35º N and 40º S (Figure 2).
In Europe, the disease had been located in the southern countries until 2006, particularly in the Mediterranean Basin, where serotypes 1, 2, 4, 9, 10 and 16 had been registered. In August of this year, the first outbreak of serotype 8 was declared in the Netherlands. The disease spread rapidly to Germany, Belgium, France and Luxembourg. This serotype had never been reported in Europe and these countries had never had BTV outbreaks. In subsequent years (2007 and 2008) the disease spread to Switzerland, Denmark, Czech Republic and the United Kingdom, reaching Spain in January 2008.
Furthermore, in 2007 the serotype 1 was introduced in Spain from where it spread to Portugal and Italy. The following year, serotype 6 was detected in the Netherlands and Germany, and serotype 25 in Switzerland. The last serotype detected, serotype 11, was found in 2009 in Belgium. The outbreaks of both serotypes 6 and 11 occurred by vaccine strains.
Currently, the serotypes 1, 2, 4, 8, 9 and 16 have been circulating in the European Union.
Fig. 3: Áreas de restricción para los diferentes serotipos de la Lengua Azul en Europa a octubre de 2014
Fuente: Comisión Europea.
Evolution of the disease in Spain in the XXI century
Since 2000 until now, Spain has reported four Bluetongue Virus (BT) serotypes (serotypes 2, 4, 1 and 8). Serotypes 2, 4 and 1 seem to have been introduced into Spain by infected culicoides carried by air drafts from northern Africa. However, the route of introduction of serotype 8 may be due to trade of infected animals from the central and northern Europe. Control measures have been based on vaccination, achieving the eradication on several occasions (Balearic Islands in 2002 and 2005; Peninsula in 2009).
The restriction zones of Spain as of November 2012 are: Canary and Balearic Island are considered free of disease; Spain mainland would be considered restriction zone for serotype 1, four provinces in southern Spain also have this category for serotype 4 and the region of Campo de Gibraltar also includes serotype 8.
Fig. 6: Zonas de restriccion Lengua Azul en España a fecha de enero 2013
Fuente: Ministerio de Agricultura, Alimentacion y Medio Ambiente (MAGRAMA), 2013
Diagnosis of Bluetongue
With the emergence of an outbreak, early suspicions are based on characteristic clinical signs and on the presence of vectors in the outbreak area. However, laboratory testing is required for confirmation of the diagnosis. The laboratory techniques are prescribed by the World Organisation for Animal Health (OIE) in Chapter 2.1.3 of the Manual of Diagnostic Tests and Vaccines for Terrestrial Animals 2011). In Spain, the diagnosis of this disease must be made in the diagnosis laboratory of the Autonomous Communities or by the National Reference Laboratory for Bluetongue virus (BTV), Laboratorio Central de Veterinaria de Algete. The diagnosis is based on virus isolation and identification from blood and tissue samples and in the detection of antibodies in unvaccinated animals:
- Competitive ELISAs to specifically detect anti-BTV antibodies. This methodology is available in National Reference Laboratory.
- The serotype-specificity of antibodies in sera is determined by neutralization tests.
Identification of agent:
- Virus isolation in embryonated hens’ eggs or in cultured cells.
- Detection, identification and characterization of viral genome by RT-PCR: This technique is available in Laboratories Bluetongue diagnosis of autonomous communities. In addition, the National Reference Laboratory has RT-PCR techniques specific to different serotypes (serotypes 1, 4 and 8).
Prevention and control of Bluetongue Virus
World Organisation for Animal Health (OIE) established in Terrestrial Animal Health Code (Chapter 8.3, Bluetongue) what kind of measures should be carried out against Bluetongue virus (BTV). These measures are included in the European Union (EU) (Directive 2000/75/EC) and national legislation (Manual Práctico de Operaciones en la Lucha Contra la Lengua Azul - Spanish). The recommendations contained in these legislations are based on:
- Notification to the competent authorities of all suspected cases.
- The total sacrifice on the farm is not justified as eradication measure.
- Restriction of animal movement from the farm or farms concerned.
- Establishment of a protection and surveillance zones around the farm or farms concerned, 100 and 50 kilometers, respectively. These distances are higher than those established for other diseases EU listed, because is transmitted by mosquitoes. The size of these areas can be modified by geographical, climatic and entomological conditions.
- Confinement of animals during the maximum activities of vectors and vector control measures using insecticides and repellents on the environment, in the accommodation of animals and the animals themselves.
- Implementation of clinical, serological, epidemiological and entomological in the areas of protection and surveillance zones established around the outbreaks.
- Vaccination programs of all host of BTV locate in the protection zone.
- Programs entomological surveillance through trapping, allow us to know the species of Culicoides which can transmit the disease.
- Serological surveillance programs allow early detection of the presence of the disease.
Vaccination is the most effective measure to minimize losses associated with the disease, eventually interrupt the cycle of the animal to the vector and allow the eradication of the disease. Due to the existence of many serotypes should be used vaccines against one or several serotypes isolated in the outbreak. There are several types of vaccines against Bluetongue:
- Live attenuated vaccine polyvalent or monovalent.
- Inactivated vaccine polyvalent or monovalent.
Vaccination in Spain
Bluetongue vaccination is recommended by the World Organisation for Animal Health (OIE) and the European Union (Directive 2000/75/EC) since an effective method for the Bluetongue virus (BTV) eradication. This technique stimulates the immune system preventing viral replication in animals, therefore decreases the viral circulation. Furthermore, it facilitates the movement of animals of susceptible species from restricted zone to free area with adequate sanitary guarantees. Finally, this goal is the final eradication of the disease.
In Spain, we have used two types of vaccines for the disease control: live vaccines monovalent used from 2000 until 2006; and inactivated vaccines monovalent since 2006, or polivalent (serotypes 1 and 4; serotypes 1 and 8), since 2009. Currently, the inactivated vaccines are the only ones in use. Vaccinations of the different provinces are established according to the restriction zones established by the Spanish Ministry of Agriculture, Food and Environment (MAGRAMA).
Since 2000, through the use of prophylactic measures, especially vaccination campaigns, has been eradicated serotype 2 in 2002 (Balearic Islands), serotype 4 in 2005 (Balearic Islands) and serotype 4 in 2009 (Peninsula).
Nowadays, Spanish government continues with the same control measures to eradicate the disease. However, vaccination performed against serotypes 1, 4 and 8 has caused a reduction in the number of outbreaks, a high level of vaccine coverage and favorable epidemiological situation. These facts have led to modify the vaccine strategy in June 2011, which is expected to continue in 2012. Nowadays, the vaccination is voluntary according to the discretion of the farmer which is also the responsible of the economic cost. However, the vaccine will be prescribed by a veterinarian, who apply or supervise their implementation, as required Order ARM/3373/2010 of December 27.