Blue Tongue in Animals

Blue Tongue

It is an infectious, non-contagious, arthropod-born viral disease, which affect cattle and sheep but primarily to the sheep. It is worldwide in distribution and reported mostly in sub-clinical forms.

Etioligical Agent:

Orbivirus belongs to the family “Reoviridae” – 24 serotypes exist, distributed on different geographical regions.

Transmission:

via biting midges [Culicoids] – natural transmittors of virus.

There are about 1400 spp. of biting midges but <20 spp. are known to harber BT virus.

Blue tongue virus has special affinity for blood cells due to which prolonged viremia seen in blue tongue cases (>9 weeks) – year round cases can be seen.

Mechanical transmission by other blood sucking insects also reported but virus shedding is low in secretion and excretion of body (very low concentration).

Oral / aerosol transmission not common but transmission of virus via semen (during natural service and A.I.) reported.

Mortality rate: 2-30%

In Sheep; there are peracute to chronic type of cases.

Peracute :

-          Animal die off with 7-9 days.

-          Severe pulmonary edema

-          Dyspnae and Froathing from nostrils

-          Death due to asphyxia

Chronic:

-          Animal die off within 3-5 weeks

-          Mortality due to secondary bacterial infection i.e. pasteurolosis

-          In mild cases – recovery is very fast.

Production losses are due to mortality and prolonged convalescent period, due to which animal shows unthriftiness and possible reproduction losses.

In sheep: virus also causes vascular endothelial damage which ultimately results in change in capillaries permeability  leads to intravascular coagulation. All these conditions lead to edema, congestion, hemorrhages and inflammation.

Clinical Findings

High grade fever (105 – 108 F) Animal listless

Reluctant to move

More severe signs in young lambs Mortality rate is very much high in young lambs.

After 2 days of fever, there is edema of tongue and lips, nose, face, submandible, eyelids, ears, muzzles, congestion of mouth, conjunctiva and coronary band; which is responsible for lameness in affected animals.

There is serous nasal discharge in early stages while it becomes mucupurulent in later stages. Congestion of nose and nasal cavity called sore muzzle. Due to all these symptoms, animal is not capable to eat properly.

Physiological Exam

There are small hemorrhages in the nose and mouth.

Another characteristic sign --- swelling of tongue compromise the circulation due to which it appears as cyanotic and gives bluish appearance – that’s why disease also called Blue Tongue Disease. There is a purple to red colored band (where hoof and skin meets) and torticulus (diversion of neck).

In Cattle:

Blue tongue is a mild disease. In response to virus, IgE come into circulation and symptoms like that of sheep appears in rare cases. Then, there may be fever, decreased body weight, lacrimation, hyperesthesia (come out of abnormal exudates in response to a stimuli). There may be oral vesicles (may be vesicular and ulcerative).

Malform calves born with hydroencephaly and proencephaly (cyst in hemispheres) shows ataxia and blindness.

Diagnosis

Based on the characteristic clinical picture especially in the endemic areas.

PM Findings

Patachial, echymotic hemorrhages along the base of pulmonary artery wall. Papillary muscle necrosis in left ventricle of the heart.

Differential Diagnosis

Pulpy kidney disease, Western rift valley fever

Lab Confirmation

Virus isolation on embryonated chicken eggs.

PCR developed for viral RNA

In field conditions: two tests can be performed.

(i)      Agar gel immunodiffusion

(ii)    Competitive ELISA – test of choice because:

Cross reaction against other antibodies not occur.

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