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Bluetongue
Disease
Randall
J Berrier, DVM
Staff Veterinarian
Technical Service
Bluetongue is an insect-transmitted,
non-contagious viral disease that affects domestic and wild
ruminants. The
worst affected domestic species is sheep.
Goats and cattle usually have mild, self-limiting
cases. White-tail
deer and pronghorn are among the wild species that can be
affected by bluetongue virus.
Bluetongue virus is an Orbiviris.
It is primarily spread by biting insects but can also
rarely be transmitted venereally by infected semen and
transplacentally from mothers to offspring.
In the
United States
the primary insect vector is the biting gnat or midge (Culicoides
variipennis). Other
Culicoides species
can also potentially serve as vectors.
Large outbreaks of the disease are seen when these gnat
populations reach pea
k
numbers. The
disease is considered seasonal in areas where insect activity
is limited by inclement weather (freezing) and is more
prevalent in temperate to tropical areas; therefore the
southern and southwest states are the most common areas of
prevalence in the
U.S.
Currently there are at least 25 serotypes
of bluetongue virus worldwide, of which 5 have been identified
in the
United States
: 2, 10, 11, 13 and 17.
The clinical signs in animals include high
fever, depression, labored breathing, sores or vesicles on the
tongue, mouth or nostrils; lameness associated with laminitis
and coronitis of the hooves; facial and tongue edema
(swelling), where the disease gets its name; loss of wool,
weight loss, abortion and even death.
As mentioned earlier, sheep are the hardest hit
domestic species. The
degree of susceptibility in sheep varies depending on age,
breed and disease serotype. In
floc
k
s infected for the first time, the morbidity (% of animals
affected) can reach 50-75% and mortality can reach 20-50%.
Again, cattle are not usually affected with clinical
signs as badly as sheep but the Culicoides
spp. are much more attracted to cattle and cattle serve as
the primary reservoir and amplifying host for the virus as
they develop a high level of viremia (high levels of virus in
the blood stream).
One of the biggest problems with the
bluetongue disease is the restrictions imposed in
international trade. The
OIE (Office International Des Epizootics) has placed
bluetongue on the List
A of veterinary diseases.
The result is restrictions being placed on
international movement of cattle and sheep and their
by-products from countries that have this infection to
countries that do not. These
restrictions do not recognize regional and seasonal freedom
from virus activity. This
has a huge negative economic impact on the
United States
.
Prevention and control of the disease
involves minimizing or eliminating exposure of livestoc
k
to the Culicoides-gnat
during insect season. Eliminate
breeding areas for the biting gnats.
Move animals away from areas where biting gnats are
numerous and during pea
k
hours for insect activity (dawn and night time).
Vaccination for bluetongue is of limited effectiveness
since the only USDA licensed commercial attenuated vaccine in
the
U.S.
is against serotype 10 only (
Colorado Serum's Bluetongue
Vaccine). The
vaccine is specific for type 10 immunity and cross protection
to other bluetongue serotypes has been reported to be limited
and variable.1
In spite of the lac
k
of specific immunity to other bluetongue serotypes many
ranchers choose to use the serotype 10 vaccine early in an
outbrea
k
because the identity of the infecting serotype can ta
k
e wee
k
s to determine after an outbrea
k
occurs.1 In
addition, bluetongue virus has been reported to be a potent
interferon inducer and as a result, some believe the available
vaccine may provide antiviral protection.1 Pregnant
animals should not be vaccinated.
Epizootic hemorrhagic disease (EHD) virus of deer
mimics bluetongue disease in deer.
The commercial type 10 vaccine does not provide cross
protection immunity for EHD.
Treatment involves supportive care based
on symptoms and antibiotics for secondary bacterial
infections.
References;
1.
Current Veterinary Therapy 3, Food Animal Practice,
Howard.
2.
APHIS-USDA-GOV, News and Info, June 2003.
3.
Veterinary Medicine, Radostits, Gay, Blood and
Hinchcliff, 9th edition. |
