1. Newcastle disease vaccines: an overview

Introduction

Chickens are susceptible to many infectious diseases. One of the most important of these is the viral disease known as Newcastle disease, which causes devastating losses in both commercial and village chickens. Reducing losses of large numbers of village chickens to virulent Newcastle disease is an essential first step to improving their productivity. Newcastle disease can be controlled by the use of vaccines. There are many Newcastle disease vaccines suitable for use in commercial chickens. These are available on the international market. The I-2 Newcastle disease vaccine has been developed for local or regional production and use in controlling Newcastle disease in village chickens.

Many Newcastle disease vaccines deteriorate after storage for one or two hours at room temperature. This makes them unsuitable for use in villages where the vaccine may need to be transported for hours or in some cases days at ambient temperature. The I-2 Newcastle disease vaccine is more robust and is known as a thermostable vaccine. Thermostable vaccines still require long-term storage in the refrigerator. However during transportation of the vaccine to the field, the vaccine will not deteriorate as quickly as the traditional vaccines. Evaporative cooling provided by wrapping the vaccine in a damp cloth will be adequate for maintaining the viability of the vaccine during transportation to remote villages. However if it is stored in direct sunlight or allowed to reach high temperatures (above 37°C) for more than a few hours it too will deteriorate and be unsuitable for use as a vaccine.

Immunity to Newcastle disease virus

Chickens that survive infection with virulent Newcastle disease virus develop a long lasting immunity to further infection with Newcastle disease virus.

The basis of this immunity is:

1. Circulating antibodies.
2. Secreted antibody producing mucosal immunity.
3. Cell mediated immunity.

Newcastle disease virus of low virulence induces similar immune responses without causing severe disease. This is the basis of vaccination.

See Appendix 2 for more information about Newcastle disease virus.

Live vaccines

These vaccines are made with virus that is alive and able to infect cells. Strains of virus of low or moderate virulence are used. They mimic natural infection and induce all three immune responses.

Killed vaccines

The ability of the virus to infect cells has been destroyed by treatment with a chemical, radiation or heat. These vaccines invoke only a circulating antibody response.

Some vaccine strains of Newcastle disease virus

Strains of Newcastle disease virus have been broadly classified into four pathotypes as follows:

Avirulent	Causes no disease
Lentogenic	Low virulence, low mortalities, loss of egg production
Mesogenic	Moderate virulence, mortalities up to 50 percent, loss of egg production
Velogenic	High virulence, severe disease with high mortalities.

(Spradbrow P.B. 1987)

Many strains of Newcastle disease virus other than velogenic strains are used in the production of live vaccines. Eight of these strains are listed in Table 1.

Table 1: Eight strains of Newcastle disease virus used in live vaccines

Strain	Description
F	Lentogenic. Usually used in young chickens but suitable for use as a vaccine in chickens of all ages.
B1	Lentogenic. Slightly more virulent than F, used as a vaccine in chickens of all ages.
La Sota	Lentogenic. Often causes post vaccination respiratory signs, used as a booster vaccine in flocks vaccinated with F or B1.
V4	Avirulent. Used in chickens of all ages.
V4-HR	Avirulent. Heat Resistant V4, thermostable, used in chickens of all ages.
I-2	Avirulent. Thermostable, used in chickens of all ages.
Mukteswar	Mesogenic. An invasive strain, used as a booster vaccine. Can cause adverse reactions (respiratory distress, loss of weight or drop in egg production and even death) if used in partially immune chickens. Usually administered by injection.
Komarov	Mesogenic. Less pathogenic than Mukteswar, used as booster vaccine. Usually administered by injection.

Thermostable Newcastle disease vaccines

Thermostable Newcastle disease vaccines exhibit a relative resistance to inactivation on exposure to elevated temperatures. Strains of Newcastle disease virus vary in thermostability.

Thermostable vaccines are prepared from a strain of Newcastle disease virus that retains its ability to infect cells after storage outside a cold chain for a short period of time.

There are two basic processes used to produce a thermostable Newcastle disease vaccine.

1. Isolation of naturally occurring thermostable variants of the virus.
2. Increasing the thermostability of this variant by artificial selection in the laboratory.
A seed lot system is used to produce the vaccine. The sequential use of a master seed and a working seed minimizes the number of passages to produce a vaccine and maintains the genetic stability of the vaccine virus. The antigenicity and the thermostability of the virus in the master seed should be retained during the two passages that produce the working seed and then the vaccine.

See Section 13 for more details.