The recent bluetongue epidemic in northern Europe began in the summer
(August) of 2006 and cases have been detected up until January 2007. The
outbreak has been caused by serotype 8 of the virus and of the 2124, 695 cases
were reported from Belgium, 7 from France, 952 from Germany, 460 from the
Netherlands, and 8 from Luxembourg. 1111 of the cases were in cattle, 951
in small ruminants, and 47 in wild animals.
This epidemic is unusual for a few reasons. It is very unusual for an outbreak
to occur so far north. There has been concern for a number of years that the
virus could spread from North Africa to Europe, and cases have been detected in
southern Europe, mainly in Italy and Spain. As the disease is spread by Culicoides
midges it is unusual that the outbreaks would continue throughout the winter
months of December and January. Concern about the spread of the disease
have been heightened by the prospect of climate change and the possibility of
the vector species moving northwards. Another unusual feature of this epidemic
is the fact that so many cattle have been affected clinically, as the disease
normally only clinically affects sheep.
Bluetongue is caused by a virus within the Orbivirus genus of the family
Reorvirades. At present 24 distinct serotypes have been identified as a result
of serum neutralisation tests. The virus is transmitted by a small number of
species of biting midges of the genus Culicoides. These vectors – insects
that carry the disease- prefer to feed on large animals. The main transmission
cycle is between the Culicoides midge and cattle, with sheep being
infected when cattle are not present or the midge population is high. Thus,
cattle can be used to detect the presence of the virus. Peak populations of
vector Culicoides occur in the late summer and autumn and therefore this
is the time when Bluetongue is most commonly seen.
Only about 20 of the more than 1,400 Culicoides
species worldwide are actual or possible vectors of bluetongue virus. Continued
cycling of the virus among competent Culicoides
vectors and susceptible ruminants is critical to viral ecology. In the USA, the
principal biological vector is C. variipennis
sonorensis , which limits distribution of the
virus to southern and western regions. In Australia the principal vector is C.
brevitarsis , while in Africa, Europe, and the
Middle East it is C. imicola
.
Infection with the bluetongue virus causes inflammation, swelling, and
haemorrhage of the mucous membranes of the mouth, nose, and tongue. It can also
cause inflammation and soreness of the feet. In sheep, the tongue and mucous
membranes of the mouth become swollen, haemorrhagic, and may look red or dirty
blue in colour, which gives the disease the name bluetongue. Affected sheep may
die after acute or chronic disease, or may recover with weight loss and/or wool
breaks.
Bluetongue can cause spectacular disease outbreaks and is classified by the
Office International des Epizooties (OIE) as having the potential for rapid
spread and has important implications for international trade.
The disease was first described in South Africa after Merino sheep from Europe
were introduced in the late eighteenth century. It was thought to be confined to
South Africa and for many years the research into the virus and the disease were
exclusively undertaken in that country, mostly at the Onderstepoort Veterinary
Institute. The viral nature of the disease was established, as was its insect-borne
spread and multiple virus serotypes.
Although all ruminant species can be infected by the bluetongue virus, clinical
signs of the disease are usually restricted to domesticated breeds of sheep.
Other animals such as goats and cattle rarely show any symptoms. The infection
is therefore mostly ‘silent’.
The Northern European epidemic is another example of a disease or disease agent
behaving in new and un-typical ways. As viruses, vectors and other disease
agents can change their range and behaviour it means that it is very important
for scientists and veterinarians to keep an eye on the international situation
and watch out for any new developments. An understanding of the reasons for
these differences will be important for controlling outbreaks that may
occur in summer and autumn 2007.
[To see more information see CAB Abstracts Database which has more than 2000
references on this important disease]