Bluetongue in Cattle, Sheep and Goats
(Sore muzzle, pseudo foot-and-mouth disease, muzzle disease)
Description Bluetongue is an insect-spread disease of ruminants characterised by inflammation of mucous membranes, congestion, swelling and haemorrhages. The disease is variable in severity. Sheep are generally the worst affected, with cattle having milder disease. In some parts of the world, infection without clinical disease is recognised.
Bluetongue is spread by small biting midges. It is not transmitted by direct or indirect contact between animals in the absence of the insects. Rarely virus may be excreted in the semen when males are viraemic. Contaminated semen may infect recipient cows but would be unlikely to establish in an area unless abundant vectors were present.
Bluetongue is an infectious, noncontagious arthropodborne viral disease primarily of domestic and wild ruminants. Infection with bluetongue virus is common worldwide but is usually subclinical or mild in most infected ruminants. Bluetongue is almost exclusively a disease of sheep, particularly the fine-wool and mutton breeds, although white-tailed deer ( Odocoileus virginianus ), and pronghorn ( Antilocapra americana ) and desert bighorn sheep ( Ovis canadensis ) may develop severe clinical disease in North America.Bluetongue is an insect-spread disease of ruminants characterised by inflammation of mucous membranes, congestion, swelling and haemorrhages. The disease is variable in severity. Sheep are generally the worst affected, with cattle having milder disease. In some parts of the world, infection without clinical disease is recognised. Bluetongue is spread by small biting midges. It is not transmitted by direct or indirect contact between animals in the absence of the insects.Rarely virus may be excreted in the semen when males are viraemic. Contaminated semen may infect recipient cows but would be unlikely to establish in an area unless abundant vectors were present.
Tentative diagnosis of BT can be made when clinical signs appear in populations known to be susceptible, the occurrence of disease coincides with a prevalence of insect vectors, necropsy of sheep reveals characteristic gross lesions, and a flock history of recent wasting (loss of weight) and pododermatitis (foot rot).
Specimens for Laboratory
Preferred samples for confirmatory diagnosis include sterile heparinized blood samples from animals with clinical signs or spleen or bone marrow, or both, from dead animals. Samples from aborted and congenitally infected newborn animals should include heparinized blood and, if possible, spleen, lung, brain, and serum. If possible, the heparinized whole blood (erythrocytes and white cells) should be washed in saline containing antibiotics and resuspended in saline prior to shipping. This procedure will reduce the antibody that may neutralize the virus if blood-cell lysis occurs.
Specimens should be shipped refrigerated, not frozen. Freezing makes virus isolation difficult.
Diagnosis and Lesions:
The typical clinical signs of bluetongue enable a presumptive diagnosis, especially in areas where the disease is endemic. Suspicion is confirmed by the presence of petechiae, ecchymoses, or hemorrhages in the wall of the base of the pulmonary artery and focal necrosis of the papillary muscle of the left ventricle. These highly characteristic lesions are usually obvious in severe clinical infections but may be barely visible in mild or convalescent cases. These lesions are often described as pathognomonic for bluetongue, but they have also been observed occasionally in other ovine diseases such as heartwater, pulpy kidney disease, and Rift Valley fever. Hemorrhages and necrosis are usually found where mechanical abrasion damages fragile capillaries, such as on the buccal surface of the cheek opposite the molar teeth and the mucosa of the esophageal groove and omasal folds. Other autopsy findings include subcutaneous and intermuscular edema, skeletal myonecrosis, myocardial and intestinal hemorrhages, hydrothorax, hydropericardium, pericarditis, and pneumonia. In many areas of the world, bluetongue in sheep, and especially in other ruminants, is subclinical and, therefore, laboratory confirmation based on virus isolation in embryonated chicken eggs, susceptible sheep, or cell cultures, or the identification of viral RNA by PCR is necessary. The identity of isolates may be confirmed by the group-specific antigen-capture ELISA, immunofluorescence, immunoperoxidase, serotype-specific virus neutralization tests, or hybridization with complementary gene sequences of group- or serotype-specific genes. For virus isolation, blood (10-20 mL) is collected as early as possible from febrile animals into an anticoagulant such as heparin, sodium citrate, or EDTA and transported at 4C to the laboratory. For longterm storage where refrigeration is not possible, blood is collected in oxalate-phenol-glycerin (OPG). Blood to be frozen should be collected in buffered lactose peptone and stored at or below -70C. Blood collected at later times during the viremic period should not be frozen, as lysing of the RBC or thawing releases the cell-associated virus, which may then be neutralized by early humoral antibody. The virus does not remain stable for long at -20C. In fatal cases, specimens of spleen, lymph nodes, or red bone marrow are collected and transported to the laboratory at 4C as soon as possible after death. A serologic response in ruminants can be detected 7-14 days after infection and is generally lifelong. Current recommended serologic techniques for the detection of bluetongue virus antibody include agar gel immunodiffusion and competitive ELISA. The latter is the test of choice and does not detect cross-reacting antibody to other orbiviruses, especially anti-EHDV (epizootic hemorrhagic disease virus) antibody. Various forms of virus neutralization test, including plaque reduction, plaque inhibition, and microtiter neutralization can be used to detect type-specific antibody.