4. Incubation of fertile eggs

Introduction

All strains of Newcastle disease virus grow in embryonated eggs. Embryonated eggs are used for isolating the virus, producing vaccine and producing antigen for serological tests.

Source of eggs

There is a theoretical risk of contaminating vaccines with pathogens that are transmitted through embryonated eggs. The use of embryos from a Specific Pathogen Free (SPF) flock minimizes this risk. However, in many countries, embryonated eggs from an SPF flock are unavailable. In this case it is practical to use embryonated eggs from a healthy local flock.

When deciding which flock to use to supply the eggs, consider the following points. Seek veterinary advice about the health status of the flock.

Does the flock appear healthy and free from infectious disease?

Is there any serological testing performed on the flock?

What is the vaccination regime used in the flock?

Is the flock free of Salmonella pullorum?

Are the eggs clean?

What is the percentage of fertile eggs?

Is the hatching rate acceptable?

Are the newly hatched chicks healthy?

Do not purchase eggs during an outbreak of any disease in the flock supplying the eggs.

If eggs are purchased from a commercial hatchery at 8 or 9 days old, candle at the hatchery to select eggs with viable embryos. Keep the eggs warm during transport to the vaccine production centre.

Eggs with white shells are preferable as they are easier to candle.

Recording details of egg purchases

Once a suitable source of embryonated eggs has been located, an order can be placed for the delivery of the eggs. It is useful if the person responsible for placing the orders and receiving the eggs keeps records.

The following information should be recorded in a notebook set aside for this purpose.

Date when the eggs are ordered and the name of the person who received the order.

Number and age of the eggs ordered.

Date and number of the eggs received.

Colour and appearance of the eggs received.

Number of eggs damaged during transport.

Date and number of eggs placed in incubator.

Number of viable eggs after candling prior to inoculation.

Storage and cleaning of eggs

Do not buy dirty eggs.

Eggs that are stained can be disinfected by washing in a warm (37°C) solution of 0.1 percent Chloramin B (benzine sulfonamide sodium salt) or wiped with a 70 percent alcohol solution.

Fertile eggs that have not been incubated can be purchased. They can then be placed in an incubator when they are delivered. Alternatively, they can be stored for several days in cool conditions (16°C to 18°C) prior to incubation. This may reduce the number of viable embryos, as some embryos may not develop after storage.

Incubation of eggs before inoculation

Many vaccine production centres will already have large commercial incubators installed. Smaller incubators are available and are suitable for the small-scale production of vaccine.

Incubation temperature = 38°C to 39°C.

Humidity should be maintained at 60 to 65 percent. A tray filled with water and placed in the bottom of the incubator is usually sufficient to maintain this level of humidity.

Place the eggs in the incubator with the air sac on top.

Eggs should be turned three times a day.

Incubation of eggs after inoculation

Inoculated eggs contain virus and should be placed in a different incubator. Eggs inoculated with virulent strains of Newcastle disease virus should not be incubated in the same incubator as used for eggs inoculated with the avirulent I-2 strain of Newcastle disease virus.

Inoculated eggs are incubated under the same conditions as uninoculated eggs but do NOT turn the eggs.

Cleaning and decontamination of incubators

Keep surfaces clean by wiping out with a wet cloth and disinfecting with 70 percent alcohol solution or a non-corrosive disinfectant.

See Section 16 for more information about cleaning and decontamination.