By Dr Ann Wachira
Newcastle disease (ND) affects birds globally, and chicken are particularly vulnerable. It is caused by a virus called avian paramyxovirus serotype type 1 (APMV-1). There are five types of APMV-1 strains, which are categorized based on their strength in chicken: a) strong and affecting organs (viscerotropic velogenic), b) strong and affecting nerves (neurotropic velogenic), c) moderately strong (mesogenic), d) weak or affecting respiratory system (lentogenic), and e) showing no symptoms (asymptomatic). Lentogenic strains are the mildest, mesogenic strains are moderately severe, and velogenic strains are the most severe. Velogenic and Lentogenic strains are present in poultry worldwide.
Economic Impact
Newcastle disease outbreaks are widespread, resulting in significant economic losses in chicken production systems globally. In developing countries such as Kenya, where indigenous chicken production systems constitute over 70% of the poultry population and play a crucial role in food, nutrition, and income generation, controlling Newcastle disease is of paramount economic importance. Unvaccinated chicken are the most severely affected, with up to 100% experiencing infections or mortality. This not only reduces chicken numbers but also disrupts the entire supply chain, affecting livelihoods dependent on chicken farming. The economic strain also affects industries like feed suppliers, vets, processors and consumers. Overall, preventing and controlling Newcastle disease is crucial for the poultry sector to avoid these economic losses.
Epidemiology
Newcastle Disease virus can infect the chicken through the nose and mouth, and sometimes even through their eyes, although it typically requires a high amount of virus to cause an infection, depending on the strength of the virus. The virus is also shed through the nose and mouth, and also in the droppings or faeces by an infected chicken. Most types of Newcastle Disease virus cannot last long in the environment because they cannot handle high temperatures. However a few can survive heat, and these are mostly the weaker types that spread through droppings into the mouth.
In commercial poultry farms, Newcastle disease can infect flocks through various breaches in biosecurity, such as contaminated food, individuals, eggs, or vehicles. It can also enter through infected birds introduced into multi-age farms or via aerosols from neighboring properties or wild birds. Once a few birds get infected, the virus tends to spread within the flock mainly through aerosols. In large flocks, significant amounts of aerosol virus can be generated, which may disseminate to other flocks through air movements.
Wild birds and rodents can potentially spread Newcastle Disease. They can carry the virus in their feces, nasal secretions, or feathers and transmit it to chicken through direct contact or by contaminating feed, water, or the environment. While wild birds are considered natural reservoirs for Newcastle Disease virus, rodents may also play a role in its transmission, especially in situations where they have access to chicken facilities and can contaminate feed or water sources. Therefore, controlling access of wild birds and rodents into chicken houses and maintaining good biosecurity practices are important measures to prevent the spread of Newcastle Disease.
Outbreaks of Newcastle Disease known as epizootic ND, typically stem from infected chicken, with the spread often linked to the movement of chicken in live bird markets and among traders. A chicken carrying ND can transmit the virus to a separate flock that is entirely susceptible, leading to mortality rates of up to 100%.
In indigenous chicken populations, there’s a recognized form of ND that occurs regularly but only causes occasional deaths. This form has not gained official attention because the number of deaths is relatively low. The affected flocks typically arise from breeding birds that have survived a previous outbreak. Although many birds become immune, the virus can still pass from one susceptible bird to another. This endemic form often results in mortalities among young birds. Over time, as more birds remain susceptible, the virus can spread rapidly, leading to numerous deaths. Computer models suggest that a population of 1,000 birds is large enough to sustain this endemic virus. Such a population could be flocks of chicken in a large village or several nearby small villages.
The occurrence of Newcastle Disease outbreaks exhibits a seasonal pattern influenced by human activities. For instance, during festive seasons the increased turnover of chicken in live bird markets has historically coincided with outbreaks of ND, which were previously attributed to seasonal weather conditions. However, it’s likely not directly linked to weather but rather to the practice of farmers visiting relatives and gifting chicken during these times. In many regions, villagers are aware of the season when ND tends to occur or recognize the early cases. As a response, they often sell their chicken, which can inadvertently contribute to the initiation or continuation of outbreaks.
Vaccination can change the pattern of ND occurrence to a degree by preventing disease, but they don’t stop infection entirely. When vaccinated birds come into contact with the virulent virus, they will not show any signs of illness. However, some of the virus they encounter will replicate, and the birds will still shed ND virus. Although vaccinated birds may not shed as much virus as susceptible birds, there will still be enough to infect other chicken.
Clinical Signs
The incubation period for ND in chicken varies from two to 15 days. Lentogenic strains typically result in subclinical infections or mild respiratory disease. Mesogenic strains can lead to acute respiratory disease and neurologic signs in some chicken, although the mortality rate is low. Velogenic strains prompt a range of clinical signs, from sudden death to lethargy, conjunctival reddening, swelling of the head, watery greenish diarrhea, and respiratory signs. Additionally, they induce neurologic signs such as tremors, paralysis, and torticollis (twisted neck).
Control
Vaccination stands as the primary method for controlling Newcastle Disease. However, the currently available vaccines predominantly cater for commercial chicken producers who manage large flocks of chicken confined to specific areas and of the same age. These vaccines, which are sensitive to heat, are manufactured in multi-dose vials containing either 500, 1,000, or 2,000 doses and above. Ensuring a consistent cold temperature throughout the entire supply chain, known as the “cold chain,” is essential from manufacturing to the point of use.
On the other hand, local chicken are typically raised in small, free-range flocks of varying ages, making the use of large multi-dose vaccine vials impractical. Furthermore, sustaining the cold chain in rural settings poses significant challenges, and the cost of purchasing large multi-dose vaccines is prohibitive.
AVIVAX I-2 Newcastle Disease vaccine, locally produced by KEVEVAPI, is a live, non-pathogenic, and thermotolerant vaccine addressing limitations associated with large multi-dose vaccines and access to cold chain infrastructure in rural areas, within the indigenous chicken sector. AVIVAX I-2 is packaged in vials with minimum 50 doses.
AVIVAX I-2 Newcastle disease vaccine is stored at +2 to +8 degrees Celsius or under refrigeration, away from direct sunlight. The vaccine can maintain its protective capabilities for 8 weeks at 28 degrees Celsius when in freeze-dried form and stored in the dark. Once reconstituted, the vaccine will remain effective for two days under field conditions. AVIVAX-1-2 vaccine is best administered using a dropper to place one drop into the eye of the chicken.
To control outbreaks of Newcastle disease in chicken, measures such as quarantine, removal of all infected and exposed birds, and thorough cleaning and disinfection of chicken facilities should be employed. In regions where outbreaks typically happen annually, the vaccine may be strategically given before the usual time when seasonal outbreaks are expected to begin. Commercially available vaccines for Newcastle disease come in live, inactivated, and recombinant forms. However, it’s important to note that these vaccines do not offer complete immunity, and chicken need to be revaccinated from time to time. Talk to your animal health service provider for more information.
Dr Ann Wachira is a scientist in the livestock department at KALRO Kakamega
Email: ann.wachira@kalro.org