Newcastle Disease (ND)

Viral disease of domestic poultry and wild birds, which is characterized by gastrointestinal, respiratory and nervous signs.

Disease Details

Introduction: Newcastle disease (ND) is a highly contagious viral disease affecting poultry of all ages. Affected species include chickens, turkeys, pigeons and ducks. It is listed in OIE (Office International des Epizooties) list of diseases as per its severe nature. The virus involved is Paramyxovirus PMV-1, which is of variable pathogenicity. Signs are typically of disease of the nervous, respiratory or reproductive systems. Morbidity is usually high and mortality varies 0-100%. Higher mortality is seen in velogenic disease in unvaccinated stock. 

Epizootiology

ND virus is highly contagious, horizontally transmitting virus through infected aerosol droplets, nasal discharges and droppings either by ingestion or inhalation affecting over 250 species of birds. Spread of the disease occurs by movement of live birds, by means of contaminated poultry products, people, equipment, feed and water and by aerosol route. Though it is controversial, vertical transmission can occur rarely in newly hatched chicks.

Clinical signs and lesions

ND virus has been divided into five groups or pathotypes on the basis of signs, lesions and tissue tropism viz: 1) viscerotropic velogenic; highly virulent disease with high mortality and typical haemorrhagic and necrotic gastro-intestinal lesions; 2) neurotropic velogenic; respiratory and nervous signs with very high mortality; 3) pneumotropic mesogenic; respiratory and in some cases nervous signs with considerable mortality in young birds but low in adult birds; 4) pneumotropic lentogenic; mild or unapparent infection of the respiratory tract, with no mortality and used for vaccine production; 5) naturally apathogenic; mostly thermo-resistant and use more recently as non-stressing vaccines.

Common signs include respiratory signs as labored breathing, rales and sneezing; greenish diarrhea and twisted neck or torticollis, leg weakness including paralysis. These lesions can all be present at the same time however are not strictly pathogonomonic. There is rapid drop in egg production sometimes may reach upto zero. Also, shell less eggs are common and sometimes thin and discolored shelled eggs are seen. Degenerated ovary is frequently seen with egg peritonitis. Morbidity is 100% and mortality varies according to virulence of virus and immune status of birds and reach upto 80%.

Diagnosis

Tentative diagnosis is made on the basis of typical lesions and signs. Inoculation of suspected samples from trachea and intestinal swabs or tissues in 9-11 days old embryonated eggs, HA and HI tests with allantoic fluid is done for isolation and identification of virus. Recently RT-PCR and nucleotide sequencing which are molecular diagnostic approach has been developed. The mean death time (MDT) in embryonated eggs is used for assessment of pathogenicity of virus, Intracerebral pathogenic index (ICPI) for one-day old chicks and Intravenous pathogenic index (IVPI) for six-week-old SPF chickens are also used for assessment of pathogenicity. These measures and some other molecular properties allow distinct viral profiles and helps in distinguishing virulent and avirulent isolates. Haemagglutination inhibition (HI) and ELISA tests are used for demonstration of antibodies against NDV.

Control of Disease

Vaccination helps to prevent susceptible birds from infection and at the same time it reduces the number of susceptible birds. For the control of the disease various international, national and farm level policies are applied consisting the application of all biosecurity measures directed at preventing the introduction and spread of the virus within the farm and countries. Restrictive measures on movement of birds and their products, quarantine procedures for importation any kind of domestic and wild birds need to be applied. Stamping out of outbreaks of disease, surveys at national and international level is organized and controlled by OIE. Vaccination has been adopted throughout the world except Scandinavia, New Zealand and Switzerland. Most recently in intensive areas of poultry rearing vaccination has been started. Vaccination usually protects birds from consequences of the disease, interfering with the spread of virulent virus, although virulent virus replication and shedding may still occur. Vaccination if well applied acts as barrier to the spread of disease. Live and inactivated vaccines are used.

Live Vaccines

These vaccines are prepared from the infected allantoic fluid of embryonated eggs kept freeze-dried. The virus strains used for vaccines production are fully apathogenic, lentogenic and mesogenic.

Mesogenic strains are used only in countries or areas where ND is endemic with widespread presence of backyard birds and not very intensive rearing. It cause severe post-vaccinal reactions, with some signs of disease thus, it is only used where it is permitted for revaccination intramuscularly after priming with apathogenic or lentogenic vaccine.

Lentogenic strains as Hitchner B1 and La Sota (ICPI 0.2-0.4) which replicate particularly in the mucosa of the respiratory tract inducing post vaccinal reactions and some respiratory signs, particularly in young chicks and in primary vaccination, depending on health status of birds, concurrent bacterial infection like Mycoplasma, E. coli and environmental stress like ammonia, dust, smokes etc. La Sota strain is the most stressing and not recommended  for use by spray as first vaccination.

Apathogenic strains multiplies well in enteric and respiratory tissues like NDV 6/10, Ulster 2C, Queensland V4 isolated from waterfowl and chickens are fully asymptomatic with and ICPI < 0.15. These strains induce no post-vaccination reactions even after fine spray vaccination (droplets < 70-80 microns) and at any age. They can be used with fewer problems in storage even in hot seasons and in tropical regions. Apathogenic and lentogenic strains can be applied by individual treatment like intra nasal or eye drop administration or beak dipping. Vaccination can also be done drinking water application at concentrations carefully calculated to give each bird a sufficient dose of virus with addition of skimmed milk powder (SMP) to fresh drinking water (1-2%). SMP is used to prevent viral inactivation by physical and chemical impurities with no concurrent use of disinfectants. Birds should be withheld for water for 1-2 hours before vaccination depending on the season. Live vaccines are used for mass application by spray or aerosol route as a large number of birds can be treated in very short time. Spray vaccination can be performed in hatchery or on the large scale farms with special consideration to achieve the correct droplet size. Coarse spray of large particles (> 100 microns) do not penetrate deeply in respiratory tract of birds and produces less reactions. The major problem with spray method at one day old chicks is it may result in establishment of infection with vaccinal virus inspite of the maternally derived immunity.

Inactivated Vaccines

These vaccines are produced with allantoic fluids from embryonated eggs, infected with different lentogenic strains, such as La Sota or Ulster 2C, rarely with mesogenic strains (Roakin strain), generally no longer with velogenic strains, inactivated with β-propiolactone or formalin and mixed with mineral oil to form a stable emulsion. One or more additional viral or bacterial antigens may be incorporated into the same emulsion as polyvalent vaccines. This vaccine is administered by intramuscular or subcutaneous injection. These vaccines are usually used in pullets, once or twice before going to egg production. It gives long-lasting immunity. Sometimes it has been used in broilers, at one day old together with live vaccine. This type of practice has been done in endemic areas of disease or in case of very severe epidemics.

 Source:https://www.farm.com.np