Dengue fever of cattle

| An emerging viral calamity for commercial dairy

Dr Muhammad Khalil  - This disease has been prevailing in the country since the creation of Pakistan but it never caused so much consternation amongst the farmers. It could be due to the reason that it never caused considerable problem in local cattle breeds. The disease is locally known as Vill or three days sickness. In technical language it is known as bovine ephemeral fever/ dengue fever of cattle/ three days sickness. Imported breeds like Friesian, Jersey or cross-bred are probably highly susceptible, so show high morbidity and mortality at dairy farms.
Pertinent literature indicates that ephemeral fever in cattle is prevailing since mid-nineteenth century in East and South Africa, India, Middle East, Australia and Japan. The disease appears in the form of outbreaks in countries having hot and humid environment which favours the survival or growth of arthropods. The biting arthropods act as biological vectors which provide favourable environment for multiplication of virus and transmission from infected to susceptible animals. The disease is prevailing in local cattle breeds of Pakistan for last many years. Severity of the disease is moderate without any mortality in these animals. It was noticed during 2014 with great concern that disease was also prevailing in local buffalo breeds like Nili, Ravi, Nili-Ravi and Kundi breeds. In previous years low incidence of the disease in Pakistan could be due to low density of animals but recent development of intensive commercial cattle farming has escalated its spread to all over Punjab province.
Bovine ephemeral virus is a single stranded negative sense RNA, enveloped, bullet or conical shaped and belongs to Rhabdoviridae. It is sensitive to fat-solvents like diethylether, chloroform, acetone and sodium deoxycholate. The virus in citrated whole blood remains infective when exposed to even 48oC. It loses its infectivity within 10 minutes at acidic (2.5) or alkaline (12.0) pH or 12 minutes at 56oC or 18 hours at 37oC.
Hot and humid environment near coastal or plain areas after rainy season supports the propagation of flying insect vectors that act as biological vectors for the virus. The virus can be isolated from the febrile animals as well as the vectors such as culicoides (C. coarctatus, C. imocola and C. brevitarsis) and mosquito species (Anopheles bancrofti and Culicine species). It is not a contagious and even the virus is not detectable from other mechanical vectors. It could be due to poor survival of the virus in the mechanical vectors. Moreover, the disease can be transmitted by blood transfusion from infected to other susceptible animals.
The virus enters the circulation and disseminates to all parts of the body. The BEF multiplies in fibroblasts, skeletal muscles, epithelial cells of synovial membrane of joints, alveolar cells and endothelial lining of blood vessels; hence it causes serofibrinous polysinovitis, polyarthritis, polytendovaginitis, cellulitis and focal necrosis of skeletal muscles. The lungs may show fatty oedema and lymph nodes are edematous. The lesions in the upper cervical region of spinal cord can be observed.
In acute form, affected cattle show fever, recumbency for 24-72 hours, off-feed/water, reluctant to move and spontaneous recovery within few days. But in some of the cases, there is development of lung emphysema and emphysematous oedema under skin in withers area. This sign is related with selenium and vitamin E deficiency. In less than 10 percent cases, there is blood in urine, feces or nostrils. On postmortem examination of a dead animal, there was clotted blood in the whole trachea and alveolar sacs. This could be a cause of death of the animal. Similar type of disease was seldom recorded in buffaloes but the disease was not severe.
The disease is diagnosed on the basis of 1) history, 2) signs and symptoms, 3) isolation and identification of the virus, 4) molecular characterisation (RT-PCR), 5) antigen detection (indirect sandwich ELISA) and 6)  virus specific antibody detection (complement fixation test-CFT, virus neutralisation test-VNT and indirect ELISA).
BEF infected animals are treated symptomatically. The level of calcium, vitamin C, E and selenium were increased two times in ration of farm animals. The febrile animals are given bath with fresh running preferably cold water 3-4 times a day (1-2 hours each time). The febrile animals were also treated with NSAIDs (Ketoject) and antibiotics (oxytetracycline/exenel). The animals showing labored breathing should not be given any fluid therapy.
It is known fact that the disease is not contagious but is transferred through biological vectors/ blood sucking insects. The population of such insects is increased tremendously in hot and humid environment particularly after rainy season. Control of such vectors is the best way to control the disease. The preferred chemotherapy to control biting insects is cypermethrine admixed with liquid paraffin that may be applied on animals via pour-on method. It is further added acaricidal drugs may be used to kill the biting flies/ insects in and around animal sheds.
Outbreaks of BEF result in lifelong immunity in the immunocompetant recovered animals. Moreover, live attenuated and adjuvated vaccines against BEF virus are available. In Pakistan, dairy cattle (Friesian, Jersey or cross-bred) are vaccinated using live attenuated vaccines. In the current year, first time antibody response of the vaccinated animals was recorded and found that vaccinated animals have more than 8 units of anti-BEF-CFT antibodies. Pertinent literature indicates that the buffy layer from BEF-infected blood having anti-coagulant when passed through unweaned mice through intracerebral inoculation in unweaned mice results in a live attenuated virus. The virus loses its virulence as well as immunogenicity. Addition of oil adjuvant or aluminium hydroxide gel potentiates its immunogenicity in the animals primed with killed or live attenuated BEF vaccines. In practice, animals are primed at age of 4-8 months and given first boost on 4-6 months post-priming and later on vaccinated annually. It was recorded that vaccinated animals usually had titer more than 8 units of anti-BEF-CFT antibodies.
(The writer is PhD Scholar of Microbiology)

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