Bluetongue disease is a
noncontagious,
insect-borne,
viral disease of
ruminant
Ruminants (suborder Ruminantia) are ungulate, hoofed herbivorous grazing or browsing mammals that are able to acquire nutrients from plant-based food by Enteric fermentation, fermenting it in a specialized stomach prior to digestion, principally ...
s, mainly
sheep
Sheep or domestic sheep (''Ovis aries'') are domesticated, ruminant mammals typically kept as livestock. Although the term ''sheep'' can apply to other species in the genus ''Ovis'', in everyday usage it almost always refers to domesticated s ...
and less frequently
cattle
Cattle (''Bos taurus'') are large, domesticated, cloven-hooved, herbivores. They are a prominent modern member of the subfamily Bovinae and the most widespread species of the genus ''Bos''. Adult females are referred to as cows and adult mal ...
,
yak
The domestic yak (''Bos grunniens''), also known as the Tartary ox, grunting ox or hairy cattle, is a species of long-haired domesticated cattle found throughout the Himalayan region of the Indian subcontinent, the Tibetan Plateau, Kachin Sta ...
s,
goat
The goat or domestic goat (''Capra hircus'') is a domesticated species of goat-antelope typically kept as livestock. It was domesticated from the wild goat (''C. aegagrus'') of Southwest Asia and Eastern Europe. The goat is a member of the a ...
s,
buffalo,
deer
Deer or true deer are hoofed ruminant mammals forming the family Cervidae. The two main groups of deer are the Cervinae, including the muntjac, the elk (wapiti), the red deer, and the fallow deer; and the Capreolinae, including the reindeer ...
,
dromedaries
The dromedary (''Camelus dromedarius'' or ;), also known as the dromedary camel, Arabian camel, or one-humped camel, is a large even-toed ungulate, of the genus ''Camelus'', with one hump on its back.
It is the tallest of the three species of ...
, and
antelope
The term antelope is used to refer to many species of even-toed ruminant that are indigenous to various regions in Africa and Eurasia.
Antelope comprise a wastebasket taxon defined as any of numerous Old World grazing and browsing hoofed mammals ...
. It is caused by ''Bluetongue virus'' (''BTV''). The virus is transmitted by the
midge
A midge is any small fly, including species in several families of non-mosquito Nematoceran Diptera. Midges are found (seasonally or otherwise) on practically every land area outside permanently arid deserts and the frigid zones. Some mid ...
s ''
Culicoides imicola
''Culicoides imicola'' (''Culicoides imicola'' Kieffer, former name ''C. pallidipennis'' Carter) is a species of Ceratopogonidae that transmits the bluetongue virus (BTV) and the African horse sickness virus.E. J. Wittmann, P. S. Mellor, and M. ...
'', ''
Culicoides variipennis
''Culicoides variipennis'' is a long biting midge. It is a part of the subgenus ''Monoculicoides'' and has many subspecies. Found in North America, ''C. variipennis'' transmits Bluetongue virus, African horse sickness virus, akabane virus, and ep ...
'', and other
culicoids.
Signs
![Bluetongue in Captive Yak](https://upload.wikimedia.org/wikipedia/commons/d/df/Bluetongue_in_Captive_Yak.png)
In sheep, BTV causes an acute disease with high morbidity and mortality. BTV also infects goats, cattle, and other domestic animals, as well as wild
ruminants
Ruminants (suborder Ruminantia) are hoofed herbivorous grazing or browsing mammals that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions. The ...
(for example,
blesbuck
The blesbok or blesbuck (''Damaliscus pygargus phillipsi'') is a subspecies of the bontebok antelope endemic to South Africa, Eswatini and Namibia. It has a distinctive white face and forehead which inspired the name, because ''bles'' is the Afri ...
,
white-tailed deer
The white-tailed deer (''Odocoileus virginianus''), also known as the whitetail or Virginia deer, is a medium-sized deer native to North America, Central America, and South America as far south as Peru and Bolivia. It has also been introduced t ...
,
elk
The elk (''Cervus canadensis''), also known as the wapiti, is one of the largest species within the deer family, Cervidae, and one of the largest terrestrial mammals in its native range of North America and Central and East Asia. The common ...
, and
pronghorn antelope
The pronghorn (, ) (''Antilocapra americana'') is a species of artiodactyl (even-toed, hoofed) mammal indigenous to interior western and central North America. Though not an antelope, it is known colloquially in North America as the American ante ...
).
[
Major signs are high fever, excessive ]saliva
Saliva (commonly referred to as spit) is an extracellular fluid produced and secreted by salivary glands in the mouth. In humans, saliva is around 99% water, plus electrolytes, mucus, white blood cells, epithelial cells (from which DNA can be ...
tion, swelling of the face and tongue, and cyanosis
Cyanosis is the change of body tissue color to a bluish-purple hue as a result of having decreased amounts of oxygen bound to the hemoglobin in the red blood cells of the capillary bed. Body tissues that show cyanosis are usually in locations ...
of the tongue. Swelling of the lips and tongue gives the tongue its typical blue appearance, though this sign is confined to a minority of the animals. Nasal signs may be prominent, with nasal discharge and stertorous respiration.
Some animals also develop foot lesions, beginning with coronitis, with consequent lameness. In sheep, this can lead to knee-walking. In cattle, constant changing of position of the feet gives bluetongue the nickname the dancing disease. Torsion of the neck (opisthotonos
Opisthotonus or opisthotonos (from grc, ὄπισθεν, translit=opisthen, lit=behind and grc, τόνος, translit=tonos, lit=tension, label=none) is a state of severe hyperextension and spasticity in which an individual's head, neck and spinal ...
or torticollis
Torticollis, also known as wry neck, is a dystonic condition defined by an abnormal, asymmetrical head or neck position, which may be due to a variety of causes. The term ''torticollis'' is derived from the Latin words ''tortus, meaning "twisted ...
) is observed in severely affected animals.
Not all animals develop signs, but all those that do lose condition rapidly, and the sickest die within a week. For affected animals that do not die, recovery is very slow, lasting several months.
The incubation period
Incubation period (also known as the latent period or latency period) is the time elapsed between exposure to a pathogenic organism, a chemical, or radiation, and when symptoms and signs are first apparent. In a typical infectious disease, the i ...
is 5–20 days, and all signs usually develop within a month. The mortality rate
Mortality rate, or death rate, is a measure of the number of deaths (in general, or due to a specific cause) in a particular population, scaled to the size of that population, per unit of time. Mortality rate is typically expressed in units of de ...
is normally low, but it is high in susceptible breed
A breed is a specific group of domestic animals having homogeneous appearance (phenotype), homogeneous behavior, and/or other characteristics that distinguish it from other organisms of the same species. In literature, there exist several slig ...
s of sheep. In Africa, local breeds of sheep may have no mortality, but in imported breeds, it may be up to 90%.
In cattle, goats, and wild ruminants, infection
An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dise ...
is usually asymptomatic
In medicine, any disease is classified asymptomatic if a patient tests as carrier for a disease or infection but experiences no symptoms. Whenever a medical condition fails to show noticeable symptoms after a diagnosis it might be considered asy ...
despite high virus levels in blood. Red deer
The red deer (''Cervus elaphus'') is one of the largest deer species. A male red deer is called a stag or hart, and a female is called a hind. The red deer inhabits most of Europe, the Caucasus Mountains region, Anatolia, Iran, and parts of wes ...
are an exception, and in them the disease may be as acute as in sheep.
Microbiology
Bluetongue is caused by the pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
ic virus
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsky's 1 ...
, ''Bluetongue virus'' (''BTV''),[ of the genus '']Orbivirus
''Orbivirus'' is a genus of double-stranded RNA viruses in the family ''Reoviridae'' and subfamily '' Sedoreovirinae''. Unlike other reoviruses, orbiviruses are arboviruses. They can infect and replicate within a wide range of arthropod and ver ...
'', of the Reoviridae
''Reoviridae'' is a family of double-stranded RNA viruses. Member viruses have a wide host range, including vertebrates, invertebrates, plants, protists and fungi. They lack lipid envelopes and package their segmented genome within multi-layered ...
family. Twenty-seven serotypes are now recognised for this virus. The twenty-seven serotypes are the result of the high variability of a single outer capsid protein, VP2.
The virus particle consists of 10 strands of double-stranded RNA surrounded by two protein shells. Unlike other arboviruses
Arbovirus is an informal name for any virus that is transmitted by arthropod vectors. The term ''arbovirus'' is a portmanteau word (''ar''thropod-''bo''rne ''virus''). ''Tibovirus'' (''ti''ck-''bo''rne ''virus'') is sometimes used to more sp ...
, BTV lacks a lipid
Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include ...
envelope. The particle has a diameter of 86 nm. The structure of the 70 nm core was determined in 1998 and was at the time the largest atomic structure to be solved.
The two outer capsid proteins, VP2 and VP5, mediate attachment and penetration of BTV into the target cell. VP2 and VP5 are the primary antigenic targets for antibody targeting by the host immune system. The virus makes initial contact with the cell with VP2, triggering receptor-mediated endocytosis
Receptor-mediated endocytosis (RME), also called clathrin-mediated endocytosis, is a process by which cells absorb metabolites, hormones, proteins – and in some cases viruses – by the inward budding of the plasma membrane (invagination). Thi ...
of the virus. The low pH within the endosome
Endosomes are a collection of intracellular sorting organelles in eukaryotic cells. They are parts of endocytic membrane transport pathway originating from the trans Golgi network. Molecules or ligands internalized from the plasma membrane can ...
then triggers BTV's membrane penetration protein VP5 to undergo a conformational change that disrupts the endosomal membrane. Uncoating yields a transcriptionally active 470S core particle which is composed of two major proteins VP7 and VP3, and the three minor proteins VP1, VP4 and VP6 in addition to the dsRNA genome. There is no evidence that any trace of the outer capsid remains associated with these cores, as has been described for reovirus. The cores may be further uncoated to form 390S subcore particles that lack VP7, also in contrast to reovirus. Subviral particles are probably akin to cores derived ''in vitro'' from virions by physical or proteolytic treatments that remove the outer capsid and causes activation of the BTV transcriptase. In addition to the seven structural proteins, three non-structural (NS) proteins, NS1, NS2, NS3 (and a related NS3A) are synthesised in BTV-infected cells. Of these, NS3/NS3A is involved in the egress of the progeny virus. The two remaining non-structural proteins, NS1 and NS2, are produced at high levels in the cytoplasm and are believed to be involved in virus replication, assembly and morphogenesis.
Evolution
The viral genome is replicated via structural protein VP1, an RNA-dependent RNA polymerase. The lack of proof-reading abilities results in high levels of transcription errors, resulting in single nucleotide mutations. Despite this, the BTV genome is quite stable, exhibiting a low rate of variants arising in populations. Evidence suggests this is due to purifying selection across the genome as the virus is transmitted alternately through its insect and animal hosts. However, individual gene segments undergo different selective pressures and some, particularly segments 4 and 5, are subject to positive selection.
Genetic diversification of BTV occurs primarily through reassortment of the gene segments during co-infection of the host cell. Reassortment can lead to a rapid shift in phenotypes independent of the slow rate of mutation. During this process, gene segments are not randomly reassorted. Rather, there appears to be a mechanism for selecting for or against certain segments from the parental serotypes present. However, this selective mechanism is still poorly understood.
Epidemiology
Bluetongue has been observed in Australia, the US, Africa, the Middle East, Asia, and Europe. An outline of the transmission cycle of BTV is illustrated in article Parasitic flies of domestic animals
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...
.
Its occurrence is seasonal in the affected Mediterranean
The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the e ...
countries, subsiding when temperatures drop and hard frosts kill the adult midge vectors.
Viral survival and vector longevity is seen during milder winters.
A significant contribution to the northward spread of bluetongue disease has been the ability of ''C. obsoletus'' and ''C.pulicaris'' to acquire and transmit the pathogen, both of which are spread widely throughout Europe. This is in contrast to the original ''C.imicola'' vector, which is limited to North Africa and the Mediterranean. The relatively recent novel vector has facilitated a far more rapid spread than the simple expansion of habitats north through global warming.
In August 2006, cases of bluetongue were found in the Netherlands, then Belgium, Germany, and Luxembourg.
In 2007, the first case of bluetongue in the Czech Republic was detected in one bull near Cheb
Cheb (; german: Eger) is a town in the Karlovy Vary Region of the Czech Republic. It has about 30,000 inhabitants. It lies on the river Ohře.
Before the 1945 Expulsion of Germans from Czechoslovakia, expulsion of the German-speaking population ...
at the Czech-German border.
In September 2007, the UK reported its first ever suspected case of the disease, in a Highland cow on a rare-breeds farm near Ipswich, Suffolk
Ipswich () is a port town and borough in Suffolk, England, of which it is the county town. The town is located in East Anglia about away from the mouth of the River Orwell and the North Sea. Ipswich is both on the Great Eastern Main Line railw ...
.
Since then, the virus has spread from cattle to sheep in Britain.
By October 2007, bluetongue had become a serious threat in Scandinavia and Switzerland
and the first outbreak in Denmark was reported. In autumn 2008, several cases were reported in the southern Swedish provinces of Småland, Halland, and Skåne,
as well as in areas of the Netherlands bordering Germany, prompting veterinary authorities in Germany to intensify controls.
Norway had its first finding in February 2009, when cows at two farms in Vest-Agder
Vest-Agder (; "West Agder") was one of 18 counties (''fylker'') in Norway up until 1 January 2020, when it was merged with Aust-Agder to form Agder county. In 2016, there were 182,701 inhabitants, around 3.5% of the total population of Norway. I ...
in the south of Norway showed an immune response to bluetongue. Norway have since been declared free of the disease in 2011.
Although the disease is not a threat to humans, the most vulnerable common domestic ruminants in the UK are cattle, goats, and especially, sheep.
Overwintering
A puzzling aspect of BTV is its survival between midge seasons in temperate regions. Adults of ''Culicoides'' are killed by cold winter temperatures, and BTV infections typically do not last for more than 60 days, which is not long enough for BTV t until the next spring. It is believed that the virus somehow survives in overwintering midges or animals. Multiple mechanisms have been proposed. A few adult ''Culicoides'' midges infected with BTV may survive the mild winters of the temperate zone. Some midges may even move indoors to avoid the cold temperature of the winter. Additionally, BTV could cause a chronic or latent infection in some animals, providing another means for BTV to survive the winter. BTV can also be transmitted from mother to fetus. The outcome is abortion or stillbirth if fetal infection occurs early in gestation and survival if infection occurs late. However infection at an intermediate stage, before the fetal immune system is fully developed, may result in a chronic infection that lingers until the first months after birth of the lamb. Midges then spread the pathogen from the calves to other animals, starting a new season of infection.
Treatment and prevention
Prevention is effected via quarantine
A quarantine is a restriction on the movement of people, animals and goods which is intended to prevent the spread of disease or pests. It is often used in connection to disease and illness, preventing the movement of those who may have been ...
, inoculation
Inoculation is the act of implanting a pathogen or other microorganism. It may refer to methods of artificially inducing immunity against various infectious diseases, or it may be used to describe the spreading of disease, as in "self-inoculati ...
with live modified virus vaccine
A vaccine is a biological Dosage form, preparation that provides active acquired immunity to a particular infectious disease, infectious or cancer, malignant disease. The safety and effectiveness of vaccines has been widely studied and verifie ...
, and control of the midge vector
Vector most often refers to:
*Euclidean vector, a quantity with a magnitude and a direction
*Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism
Vector may also refer to:
Mathematic ...
, including inspection of aircraft.
Livestock management and insect control
Vaccines
Protection by live attenuated vaccines (LAVs) are serotype specific. Multiserotype LAV cocktails can induce neutralizing antibodies against unincluded serotypes, and subsequent vaccinations with three different pentavalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with other atoms when it forms chemical compounds or molecules.
Description
The combining capacity, or affinity of a ...
LAV cocktails induce broad protection. These pentavalent cocktails contain 15 different serotypes in total: serotypes 1 through 14, as well as 19.
Immunization with any of the available vaccines, though, precludes later serological monitoring of affected cattle populations, a problem that could be resolved using next-generation subunit vaccines.
In January 2015, Indian researchers launched a vaccine named Raksha Blu that is designed to protect livestock against five strains of the bluetongue virus prevalent in the country.
History
Although bluetongue disease was already recognized in South Africa
South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...
in the early 19th century, a comprehensive description of the disease was not published until the first decade of the 20th century. In 1906, Arnold Theiler
Sir Arnold Theiler KCMG (26 March 1867 – 24 July 1936) Pour le Mérite is considered to be the father of veterinary science in South Africa. He was born in Frick, Canton Aargau, Switzerland. He received his higher education, and later qu ...
showed that bluetongue was caused by a filterable agent. He also created the first bluetongue vaccine, which was developed from an attenuated BT V strain. For many decades, bluetongue was thought to be confined to Africa. The first confirmed outbreak outside of Africa occurred in Cyprus
Cyprus ; tr, Kıbrıs (), officially the Republic of Cyprus,, , lit: Republic of Cyprus is an island country located south of the Anatolian Peninsula in the eastern Mediterranean Sea. Its continental position is disputed; while it is geo ...
in 1943.[
Recently, a vessel owned by Khalifeh Livestock Trading and managed by Talia Shipping Line, both based in Lebanon, has been denied right to dock in Spain, as it has about 895 male calves suspected to be infected by bluetongue disease.]
Related diseases
African horse sickness
African horse sickness (AHS) is a highly infectious and deadly disease caused by ''African horse sickness virus''. It commonly affects horses, mules, and donkeys. It is caused by a virus of the genus ''Orbivirus'' belonging to the family ''Reov ...
is related to bluetongue and is spread by the same midges (''Culicoides'' species). It can kill the horses it infects and mortality may go as high as 90% of the infected horses during an epidemic.
''Epizootic hemorrhagic disease virus
''Epizootic hemorrhagic disease virus'', often abbreviated to EHDV, is a species of the genus ''Orbivirus'', a member of the family ''Reoviridae''. It is the causative agent of epizootic hemorrhagic disease, an acute, infectious, and often fatal ...
'' is closely related and crossreacts with ''Bluetongue virus'' on many blood tests.
References
Further reading
*
External links
*
*
*
*
*
* Canadian Food Inspection Agency (CFIA) Animal Disease Information
*
Bluetongue disease fact sheet
*
Biosecurity training video
*
Farm-level biosecurity practices
News and announcements on the Bluetongue outbreak in the UK
''Farmers Guardian
''Farmers Guardian'' is a weekly newspaper aimed at the British farming industry. It provides comprehensive and topical news with Livestock, Arable and Machinery sections; as well as business information and latest market prices. It is sold n ...
''
{{DEFAULTSORT:Bluetongue Disease
Insect-borne diseases
Ruminant diseases
Animal viral diseases