Ranikhet disease, also known in the west as Newcastle disease, is a contagious and highly fatal disease of fowls caused by a virus of the genus Rubulavirus in the family Paramyxoviridae. It affects turkeys, pigeons, crows, ducks, geese, koel, pheasants, guineafowl, partridges and doves. Hedgehogs are suspected as reservoirs of the disease. The disease is also suspected to cause conjunctivitis among laboratory workers and persons handing infected birds. The virus is readily cultivated in developing chick embryos. It has ability to adsorb into the surface of red cells of chicken and a few other species of animals and to induce their aggregation.
Epidemilology: The virus is shed in all secretions and excretions for at least 4 weeks. Transmission occurs by direct contact between birds by airborne route via aerosols and dust particles and also by contaminated feed and water. Mechanical spread between flocks also takes place. With lentogenic strains, transovarian transmission is important.
Clinical signs: The first symptoms usually observed in young birds are sneezing,gasping and often droopiness. Among growing birds and in adults, sudden deaths occur in a few instances, followed by a number of birds showing respiratory symptoms. The affected birds are dull and depressed with cyanosis of comb and wattles and clonic muscular spasms and ruffled feathers. These symptoms are accompanied by diarrhoea, characterized by the passing of a watery stool with an offensive smell. There is profuse salivation. The saliva often accumulates in the mouth and obstructs respiration, which results in the production of a gurgling sound. There is a rapid decline in egg production. In Turkeys, the disease runs a very mild course. In adults, in particular, it may pass off unnoticed except for some dullness, loss of appetite and other minor symptoms. Such birds, however, may act as source of infection for in- contact chicks. Birds with nervous symptoms show paralysis of wings or legs, torticollis, ataxia or circular movements of head and clonic spasm.
Diagnosis: Respiratory distress in fowls, accompanied or followed by nervous symptoms with a marked drop in egg production in laying hens are suggestive of the disease. Isolation of the virus from spleen, brain or lungs of affected fowls by cultivation in chick embryos is the most certain method of diagnosis. Positive serum-neutralization tests in embryos can be obtained in 10 to 21 days after the onset. The haemagglutination inhibition (HI) test is a simple in vitro test which is positive 5 days after the appearance of respiratory symptoms. Small pieces of spleen, lungs, brain and liver in at least 10 times the volume of 50% glycerol saline, are required for dispatch to the laboratory for diagnostic examination. In dead birds, typical pinpoint haemorrhagic spots covered with bran-like deposits are seen in the proventriculus. Various serological tests, viz. fl uo r e s c e nt a n ti b o d y, c o mp le me n t fi xa t io n, vi r u s n e u tr a li z a tio n, a g a r-g e l immunodiffusion, ELISA and electron microscopy are also used to diagnose the disease.
Treatment, prevention and control: Some important measure for its prevention include slaughtering of all apparently ailing birds, segregation of in-contacts in groups of 10 to 15 each; removal of all infective materials such as droppings, residues of poultry feed and litter to a distance and their incineration; observance of general cleanliness and provision of separate attendants for each group of birds. All purchased birds or those introduced from outside should be kept in segregation for at least 10 days before taking them into the poultry farm which should be at a distance from places of traffic. The poultry runs should be ploughed from time to time and lime should be applied there as a general disinfectant. As far as possible, the pens and runs should be made inaccessible to free-flying birds by providing a barrier or wire-netting.The maintenance of flock in deep-litter system and stopping all unauthorized entries, even of human being into the battery brooders, disposal of fowl carcasses by burning or deep burial to reduce the scope of carrion-eating birds like crows, kites and vultures perching near fowl pens or poultry farms helps to reduce the hazard of this infection.
Two types of vaccines
(i) a mesogenic strain for the adult birds
(ii) a lentogenic strain for younger birds or baby chicks are useful. The 'Mukteswar' or 'R2B' mesogenic vaccine strain developed at the Indian Veterinary Research Institute, Mukteswar is used for adult birds (age over 6 weeks). The vaccine consists of freeze-dried virus grown in chick embryos. Vaccination of birds 6- week- old and above confers lasting immunity. Some side reactions include incoordination of limbs and sometimes paralysis in 1 to 3% of the vaccinated birds. The reactions may become more acute, if the birds are affected with roundworms, coccidiosis or are weak on account of malnutrition.
A second vaccine, Ranikhet disease vaccine 'F' strain for young birds (week-old chicks) is also produced in India. This virus strain was first experimented upon in England. The vaccine consists of freeze-dried virus grown in chick embryos. The vaccine is instilled into the eyes and nostrils of week-old baby chicks. It confers immunity for a short period. When the chicks become 6- week- old they should be revaccinated with the Ranikhet disease 'Mukteswar' strain vaccine for a more lasting protection.Another virus strain, 'La Sota' of American origin is also in use in India. The vaccine is prepared in the same manner as described for 'F' strain vaccine. It has been used successfully for the protection of baby chicks till they are 6- weeks old. The vaccines for mass-scale in young birds/baby chicks are used as 'nasal drops'. Although in developed and well-controlled poultry establishments, effective vaccination can be carried out by giving the vaccine in drinking water or as eye-drops / aerosols.