Description
Bluetongue (BT) is a vector-borne viral disease of domestic and wild ruminants caused by an orbivirus. At least 26 different serotypes of Bluetongue have been identified. A closely related orbivirus causes Epizootic Hemorrhagic Disease (EHD) in deer and other cervids.
Most ruminants are susceptible to Bluetongue virus infection. However, clinically important disease is largely restricted to sheep, where it can cause serious illness and death. Cattle are the main reservoir and amplifying hosts for the virus.
The Bluetongue virus is spread by insects, particularly through a vector species of Culicoides biting midge, or gnat. These small flies take blood from an infected animal and spread the disease to other ruminants when they take additional blood meals. The virus does not survive in the environment outside the midge (vector) or its animal host. There is no evidence that the virus is able to survive winter in Canada.
While Bluetongue is a seasonal disease, cattle can carry the virus over the winter months. Typically, the disease is present from midsummer until shortly after the first hard frost. The midge that spreads the disease is active during this period. In areas like the southern United States with milder climates, the midges can be active year-round.
How to report
If you suspect Bluetongue in your herd, call your veterinarian within 24 hours.
Bluetongue is a provincially notifiable disease under Alberta's Animal Health Act and must be monitored. It is also a reportable disease under the Health of Animal Act and all cases must be reported to the Canadian Food Inspection Agency (CFIA). This excludes BT virus serotypes 2, 10, 11, 13 and 17, which are listed as immediately notifiable.
All suspected or confirmed cases must be reported to the Office of the Chief Provincial Veterinarian (OCPV) within 24 hours:
Hours: 8:15 am to 4:30 pm (open Monday to Friday, closed statutory holidays)
Phone: 780-427-3448
Toll free: 310-0000 before the phone number (in Alberta)
Fax: 780-415-0810
Clinical signs
Signs of clinical disease vary across species and infection is often unobservable. For example, cattle, goats and elk are affected by a very mild, self-limiting infection.
However, Bluetongue can cause serious illness and death in sheep, as well as deer and potentially other wildlife.
Cattle that are persistently infected with one strain of Bluetongue will not show signs until infected with another strain. At this point, they can display ulcers of the mouth, nose and udder, lameness with inflammation on the top of the hoof, hoof crack, hair loss and a swollen protruding tongue.
Clinical signs of Bluetongue include:
- in cattle, usually clinically inapparent but some serotypes may affect cattle severely
- in sheep, usually an acute disease with high fever; death may occur in up to 70% of cases
- in sheep, clinically inapparent infection is also possible
- excessive salivation and nasal discharge; can be bloody
- swelling of the lips, tongue and tissues in the jaw area
- hemorrhages on oral and nasal mucosa
- ulceration of the buccal cavity, tongue and dental pad
- cyanosis of the tongue (‘bluetongue’) can be seen, but absence does not rule out the disease
- coronitis with lameness
- anorexia due to a sore mouth and reduced feed intake leads to emaciation
- fetal resorption, abortions, birth of ‘dummy’ lambs
- recovered sheep may show ‘wool break’ 3 to 4 weeks after the fever
Clinical signs that appear in an infected animal can be common to many other diseases or nutritional problems. Some of these signs include:
- abortion, infertility and stillbirths
- congenital defects such as weak or dummy calves, deformed legs and feet, blind, white-eye or overshot lower jaw
See also: Differential diagnosis of Bluetongue in cattle and sheep (fact sheet)
Diagnosis
It is critical to obtain confirmation from a blood sample submitted for laboratory analysis.
Samples for diagnosing Bluetongue
- Whole blood EDTA or heparin – RT-PCR tests are used to detect and identify viral RNA
- Serum – used to identify animals that have been infected or exposed to BT virus
- Fresh tissues: collect those rich in blood or hematopoietic tissues including lung, liver, spleen, lymph nodes, and bone marrow. Sample size of 3 – 4 cm3 for each tissue is sufficient. Ship chilled at 4oC
- Formalin fixed tissues: collect same ones as for fresh. Sample size of 1 – 2 cm2 x 0.5 cm thick. Ensure formalin:tissue ratio = 10:1
- Oral and mucocutaneous lesions: collect fresh and fixed samples
- Diagnostic tests for bluetongue are done in private regional laboratories
See also: Bluetongue Information Sheet – The Centre for Food Security & Public Health
Pathological lesions
- facial and head edema, exudate on skin, crusty nasal discharge
- coronary band inflammation as severe as to lead to sloughing of the hoof
- oral petechia progressing to grey necrotic debris over erosions or ulcers
- upper respiratory mucosal cyanosis, edema and hemorrhage
- hemorrhage at the base of pulmonary artery is close to pathognomonic; absence does not rule out bluetongue
- endocardial petecchiation, focal myocardial necrosis
- enlarged spleen and lymph nodes
- newborn lambs with hydranencephaly
- lesions in cattle, if seen, localize to skin with dry, crusty exudate over cervical and thoracic skin, oral vesicles to ulcers, nasal erosions with crusty exudate and coronitis
- deer with EHD show very widespread petechial to ecchymotic hemorrhage throughout the body
Differential diagnoses
- Grazing on orofacial irritants, for example, stinging nettles
- Pneumonia
- Actinobacillosis
- Malignant Catarrhal Fever
- Infestation by Oestrus ovis
- Photosensitization
- Vesiculoulcerative diseases: Foot and Mouth Disease, Vesicular Stomatitis, Sheep Pox, Contagious Ecthyma (orf)
- Peste de petits ruminants
Where it’s found
According to the Canadian Food Inspection Agency, Bluetongue can be found anywhere the Culicoides midges that transmit the virus are present. The virus is commonly found in the U.S., Mexico, Australia, Africa, the Middle East, the Indian subcontinent and China. Bluetongue is also spreading northwards in Europe.
Most of Canada is currently free of the disease. The current potential for Bluetongue to spread in Canada is limited both seasonally and geographically. However, climatic change and associated shifts in vector habitat have the potential to change Bluetongue distribution patterns.
Over the past 30 years, there have been occurrences of Bluetongue in the Okanagan Valley, British Columbia. These are believed to be the result of wind-borne introduction of infected midges from the U.S. In September 2015, Bluetongue was confirmed in southwestern Ontario.
How it spreads
The primary way that the Bluetongue virus spreads is through vector-borne transmission by the Culicoides midge.
A cow can pass on the infection to the calf in utero. The fetus is most susceptible at 60 to 140 days of pregnancy. Infected semen can infect a cow resulting in the disease being transferred to the cow and then to the calf.
Following an infection in cattle, the virus incubates for 3 to 10 days and may remain active in the blood for 60 to 80 days. During this active phase, red blood cells of the infected animal are destroyed, and the disease can be transmitted to other animals through midge bites. Red blood cells in the blood of sheep and wild ruminants have a shorter lifespan than in cattle, thus decreasing the time that the blood is infected.
Risk to humans
Bluetongue poses no risk to human health.
Prevention and control
There is no effective treatment for animals with Bluetongue apart from rest, provision of soft food, and good husbandry.
Vaccines are available for sheep in countries where Bluetongue is endemic. Currently there is no vaccine available for Bluetongue in Canada.
Midge populations can be controlled by:
- draining stagnant pools of water, debris filled water areas
- applying insecticides in areas that cannot be drained
In warmer climates and certain situations, housing animals during the day can help prevent bites as the insects tend to feed primarily in the early morning and the evening.