Newcastle disease (N D) is regarded as the most important disease to be controlled in poultry. The importance is not only due to the rapid spread and devastating effects on the infected birds, with flock mortality rare up to 100%, but also to the economic impact that might ensue, due to trading restrictions and embargoes placed on countries, where outbreaks have occurred. Because of the severe nature of the disease ND is included in the list of diseases reported by office International des Epizooties (OIE) Which contains transmissible diseases with a potential for very serious and rapid spread Most member countries enforce statutory control measures in the event of outbreaks of disease. ND is caused by a paramyxovirus serogroup 1 (APMV-1) belonging to the family Paramyxoviridae genus Avulavirus only one serotype is known, but different pathotypes exit: very virulent (velogenic), medium virulent (mesogenic), mildly virulent (lentogenic), or apathogenic strains. These latter two types of strains are commonly used in the preparation of live vaccines.
Over 250 species of birds have been reported to be susceptible. N D virus is highly contagious, horizontally transmitted through infected respiratory discharge and droppings, by either inhalation or ingestion. The spread among farms, even at a long distance, occurs by movement of live birds (game and exotic birds, feral pigeons, commercial market poultry), movement of contaminated poultry products, people, equipment, feed and water and by airborne spread. Vertical transmission is very controversial, probably occurring only very rarely in newly hatched chicks.
Clinical signs and lesions
Depending on the signs, lesions and tissue tropism, ND virus strains have been divided into five groups or pathotypes :
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 : respiratoty and,in some cases, nervous signs with considerable mortality in young birds but low in adults;
4) pneumotropic lentogenic: mild or unapparent infection of the respiratory tract, with no mortality (used as vaccines)
5) naturally apathogenic, mostly thermoresistant, used more recently as non-stressing vaccines. In the field, respiratory (laboured breathing, rales and sneezing), enteric (greenish diarrhoea) and nervous signs (twisted neck or torticollis, leg weakness or paralysis, etc.) and respective lesions can all be present at the same time, but are not strictly pathognomonic.
The drop in egg production is very rapid, sometimes to zero eggs may have thin and discoloured shells or no shell; the ovary deoenerate and egg yolk is present in the abdominal cavity, with consequent peritonitis. Morbidity may reach 100%; mortality varie , depending on virus virulence and the residual immune state of bird , sometimes reaching over 80%.
A preumptive diagnosis is based on the characteristic signs and lesions and is confirmed by virus isolation and identification (inoculation of specimens, taken from tracheal and intestinal swabs or tissues, in 9-11 day-old embryonated eggs, HA and HI tests with allantoic fluids). More recently molecular diagnostic techniques have been developed, such as RT-PCR and nucleotide sequencing. The laboratory assessment of virus pathogenicity is determined by the mean death time (MDT) in embryonated eggs, the intracerebral pathogenic index (ICPI) for one-day-old chicks and the intravenous pathogenic index (IVPI) for six-week-old SPF chickens (see table). These and other molecular properties allow distinct viral profiles to be developed, distinguishing between avirulent and virulent isolates. Antibodies are demonstrated using the haemagglutination inhibition (HI) or ELISA tests.
Control of disease
The objectives are both to prevent susceptible birds from becoming infected and to reduce the number of susceptible birds by vaccination. Control policies are applied at international, national and farm level and consist of the application of all biosecurity measures directed at preventing the introduction and spread of virus within countries and areas. The establishment of restrictive measures on the movement of birds and their products, quarantine procedures for importation of all kind of domestic or wild birds, the stamping out of outbreaks of disease, national and international surveys and reports of disease outbreaks - everything is organized and controlled by the Health Authorities under OIE. Except for a few countries
(Scandinavia, New Zealand and Switzerland), vaccination is adopted universally, most recently even in Australia, particularly in intensive rearing areas. Vaccination usually protects birds from the consequences of the disease, interfering with the spread of virulent virus, although virulent virus replication and shedding may still occur, albeit at reduced levels. In any event, if well applied, vaccination represents an effective barrier to the spread of infection. Two types of vaccines are used, live and inactivated.
Live vaccines: These are prepared with the infected allantoic fluid of embryonated eggs kept freeze-dried. The strains of virus used for vaccine production are divided into 3 groups, fully apathogenic, lentogenic and mesogenic (see table).
Mesogenic strains are often used only in countries or areas where ND is endemic, with widespread presence of backyard birds and not very intensive rearing. They cause very severe post-vaccinal reaction, sometime with some signs of disease; therefore they are used, where it is permitted, mostly for revaccination and intramucularly after a priming with a pathogenic or lentogenic vaccine. Lentogenic strains genotype II, such as Hitchner Bl and La Sota (ICPI 0.2-0.4), which replicate particularly in the mucosa of the respiratory tract and can induce post-vaccinal reactions and respiratory signs, particularly in young chicks and in primary vaccination, depending on the health status, intercurrent bacterial infections ( Myco plasma, E. coli) of the birds and environmental conditions (ammonia, dust, etc.) the La Sota strain is the most stressing and not recommended for use by spray as a first vaccination. Apathogenic strains genotype I (virus multiplies well in enteric and respiratory tracts), such as NDV 6/ 10, Ulster 2C, Queensland V4, isolated from waterfowl and chickens, are fully asymptomatic with an ICPI < 0.15. Even with fine spray vaccination (droplets < 70-80 microns), and at any age, they induce no post-vaccinal reactions; being thermoresistant, they can be used with fewer problems of storage even in hot seasons and in tropical areas. The application of live apathogenic, lentogenic strains as vaccines can be done by individual treatment, such as nasal- or eye-drop administration or beak-dipping. The mass application of vaccines is done via the drinking water, at concentrations carefully calculated to give each bird a sufficient dose of virus; addition of dried skim med milk powder to fresh drinking water (1-2%) is suggested to prevent viral inactivation by physical and chemical impurities, with no concurrent use of disinfectants. Before vaccination, the birds have to be deprived
Of water for 1-2 hours, depending on the season (see later).
The mass application of live vaccines by spray or aerosol is also very widespread, because a large number of birds can be treated in a very short time. Spray vaccination can be performed in the hatchery or on the farm. To achieve the correct droplet size is very important.
A coarse spray of relatively large particles (> 100 microns) does not penetrate deeply into the respiratory tract of birds and produces less of a reaction; this is less conditioning with apathogenic strains having an lCPI < 0.15. The spray method at one-day-old may result in the establishment of infection with vaccinal virus, despite maternally-derived immunity.
lnadivated vaccines: These are prod uced with allantoic fluids from embryonated eggs, infected with d ifferent lentogenic strains,such as La Sota or Ulster 2C, rarely with mesogenic strains (Roakin strain), generally no longer with velogenic strains, inactivated with B-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.
The vaccine is ad ministered by intramuscular or subcutaneous injection. These vaccines are usually used in pullets, once or twice, before going into lay, giving long-lasting immunity. They have sometimes been used in broilers, at one-day-old, together with live vaccine, particularly in endemic areas of disease or in case of very severe epidemics.