Causes and Prevention and Control Methods of Newcastle Disease in White-Feathered Broiler Chickens

Newcastle disease is also called Asian fowl plague. Newcastle disease is a highly contagious, acute septicemic infectious disease caused by a virus. The main characteristics of Newcastle disease are difficulty breathing, diarrhea, nervous dysfunction, and hemorrhage of mucous membranes and serous membranes. It mainly attacks chickens and turkeys, and other poultry can also be infected.

1. Pathogen The pathogen of this disease is Newcastle disease virus, a paramyxovirus. It has relatively strong resistance to heat; usually at 100°C for 1 minute, 50°C for 5—6 minutes, and 37°C for several hours to several days, it still retains infectivity, immunogenicity and the ability to agglutinate red blood · cells. It has a relatively broad adaptability to acid and alkali, and is infectious within the pH range of 2—10. It is also tolerant to low temperatures; the lower the temperature, the longer it survives. Its resistance to disinfectants is relatively weak; the virus in infected allantoic fluid can lose infectivity within a few minutes when exposed to 2% formalin or 3% Lysol.

2. Epidemiological characteristics Chickens, turkeys, quail, pheasants and other poultry are all susceptible to this disease, among which chickens are the most sensitive, followed by pheasants; Leghorn chickens are more sensitive than local chickens, and older chickens have lower sensitivity. The main sources of infection of this disease are sick chickens, dead chickens and virus-carrying chickens. Some birds can spread this disease. Sick chickens shed the virus from oral and nasal secretions and feces. Virus-carrying chickens after an epidemic has passed show a chronic course, which is one of the causes of Newcastle disease outbreaks,   Newcastle disease is mainly transmitted through the respiratory tract and digestive tract, and eggs can also carry the virus. Wild birds, external parasites, humans and livestock can all mechanically transmit the pathogen. The secretions and excretions, blood, meat, internal organs, chicken feathers and digestive tract contents of sick chickens are the main sources of infection.

3. Clinical symptoms The incubation period is generally 3—5 days. According to the strength of the virus virulence and the length of the disease course, it can be divided into three types: peracute, acute, and subacute or chronic.

(1) Peracute type Sick chickens often die suddenly without any symptoms.

(2) Acute type The body temperature of sick chickens rises to 4312, feed intake decreases or they do not eat at all. They are listless, stand apart from the flock, draw in the neck and close the eyes, and the comb and wattles become purplish red or purplish black. They have difficulty breathing, stretch the neck and open the beak, shake the head, make “gugu” or “gege” sounds, and sometimes sneeze. When a chicken is held upside down, a large amount of pale white liquid flows from the mouth. The crop is filled with liquid or gas, and there is diarrhea, sometimes with blood. Laying hens show reduced or stopped egg production. Sick chickens generally die within 1-2 days or 3—5 days.

(3) Subacute or chronic type Mostly converted from the acute type; the initial symptoms of sick chickens are the same as the acute type, showing obvious respiratory symptoms, and those with a slightly longer disease course develop neurological symptoms, lameness, and paralysis of one or both legs. The wings droop, they turn in circles, move backward, and the head tilts backward or twists to one side. The disease course is about 10 days, and a small number of sick chickens can recover on their own. When adult chickens are affected, in addition to milder general symptoms, the main manifestation is a sharp drop in egg production, with soft-shelled eggs noticeably increasing compared with usual, or diarrhea symptoms.

4. Pathological changes The crop is filled with acidic liquid containing bubbles; the mucous membrane of the proventriculus is edematous, with hemorrhage at the tips of the papillae or between the papillae, and some have hemorrhagic spots under the koilin layer of the gizzard; the cecal tonsils are swollen, hemorrhagic and necrotic. The small intestinal mucosa has date-pit-like fibrinous necrotic foci; the rectal mucosa is hemorrhagic.

5. Prevention and control measures The occurrence of Newcastle disease is prevented through comprehensive measures of sanitary management and vaccination. Sanitary management mainly means controlling the invasion of pathogens into the chicken flock. Outsiders must not enter the chicken farm, and flying birds and wild animals must be prevented from entering. Market quarantine must be strictly enforced, and the sale of sick and dead chickens is prohibited. Once Newcastle disease occurs on a chicken farm, strict comprehensive measures such as blockade, isolation, disinfection, culling of sick chickens and emergency vaccination are required to quickly stamp out the epidemic. Guiding immunization based on the results of antibody monitoring is the most effective means of preventing Newcastle disease in chickens. Where conditions permit, chicken farms should preferably determine the optimal time for flock immunization based on the results of antibody monitoring of the flock. At present, the Newcastle disease vaccines produced in China have 4 strains: I, II, III, and IV. The I strain vaccine (also called the Indian strain) is a moderately virulent vaccine, and the other three vaccines are attenuated vaccines. Among the attenuated vaccines, the IV strain (Lasota) vaccine has relatively strong virulence, followed by the II strain, and the III strain is the weakest. Attenuated vaccines are suitable for chicks and adult chickens, and generally do not cause serious reactions after inoculation. The I strain vaccine is only for use in chickens over two months old; after inoculation, some individual chickens may show obvious reactions, but antibodies are produced quickly after immunization, and immunity is strong, making it suitable for use in areas where the disease is severely epidemic.

However, for safety reasons, before using the I strain vaccine, attenuated vaccine should be used for basic immunization. Oil-emulsion inactivated vaccine produces antibodies slowly, but the antibodies produced are high and uniform and last a long time; when used in combination with attenuated vaccine, it can produce obvious effects on some chicken farms severely contaminated by Newcastle disease virus. Generally, before 10 days of age, attenuated vaccine is used for immunization by nasal drops, eye drops or large-droplet aerosol; at 10 days of age, half a dose of oil-emulsion inactivated vaccine is injected for immunization. After that, antibody monitoring is carried out every 15—20 days, and when 20% of the chickens in the flock have antibody titers below 4Lg2, attenuated vaccine is used for aerosol immunization. At 18—20 weeks of age, white-feathered broiler chickens are injected once with oil-emulsion inactivated vaccine. After that, antibody monitoring of the flock should be persisted in every month, and when 20% of the chickens in the flock have antibody titers below 8Lg2, IV strain vaccine is used for aerosol immunization, or IV strain vaccine is used for aerosol immunization while simultaneously injecting oil-emulsion inactivated vaccine, so that the antibody level of the flock is always maintained above 8Lg2. Only in this way can the egg production level of the flock remain stable. Once an epidemic occurs, Newcastle disease hyperimmune yolk antibody can be injected promptly, or 4 times the dose of IV strain vaccine can be used for emergency injection.