Physical properties of Infectious Bronchitis Virus (IBV)

Infectious bronchitis virus (IBV) infects exclusively birds, with different clinical presentations depending on factors such as the virus variant and the age of the birds (see ProAgri Zambia issues 69, 77, and 79). IB virus (IBV) is known for being highly sensitive to different environmental factors; for example, it is easily destroyed by common disinfectants, UV radiation, high temperatures, and extreme pH.

Such sensitivity to external factors is not only verified in field viruses, but also in vaccine strains. This means that, by nature, live IBV vaccines could be easily destroyed if certain preventative measures are not put in place during their transport, storage, preparation, and administration.

Vaccination of broilers

IBV enters the birds by colonising the upper respiratory tract (nasal cavity and upper trachea) and the mucous membrane of the eye (conjunctiva). Such structures constitute the portals of entry from which the virus reaches the circulation and spreads to different organs, such as the kidneys.

In young broilers, in which their immune systems are still developing, the application of IB live vaccines aims, mainly, to locally protect of the portals of entry. Immediately after vaccination, the IB vaccine virus colonises the surface of the upper trachea, nostrils, and conjunctiva, avoiding the attachment of the field strain (competitive exclusion). This is followed by the generation of local immunity at the portal of entry, increasing the number of immune cells in the area and the secretion of immunoglobulin A.

Figure 1. Good practices for coarse spray vaccination in broilers.

Optimising immunisation of broilers against IB

Considering the objectives of early immunisation with live vaccines, as well as the sensitivity of the IBV to environmental factors, different strategies can be practiced increasing the chances of delivering viable IB vaccine virus to the portals of entry.

Below we discuss some procedures that, when performed correctly, can help to improve the immunisation of young birds against IBV field strain.

Figure 2. Blue stain around the eyes and nostrils, showing successful vaccination (Udi Ashash, 2022).

  1. The highest level of immunisation against infectious bronchitis is reached when live vaccines are applied in a spray form. As mentioned above, IB vaccination in early life aims to increase the immunity at the portal of entry (upper respiratory tract and conjunctiva). This can only be achieved if live vaccine virus reaches the portal of entry in spray form. Vaccination in drinking water against IB has erratic and suboptimal results, so it should be avoided.
  2. In our region, it is recommended to use coarse spray (droplet size between 100 and 200 microns) (Figure. 1). Firstly, in our frequently hot and/ or dry environmental conditions, coarse spray reduces the chances of vaccine being evaporated before reaching the chickens. Secondly, very fine spray would reach deep down the respiratory tract. In our region, where mycoplasma positive flocks are common, this may result in the reactivation of the mycoplasma infection.
  3. Use 25 to 30 ml of water for every 100 birds when preparing the vaccine for spray application.
  4. Apply a coloured water stabiliser to the vaccine water before vaccine preparation. Before vaccine preparation, commercial water stabilisers should be added to the water, with the objective of dechlorinating it and improving its pH (the ideal pH range is between 5.5 and 7.5). Some of those products are also able to reduce the negative effect of water hardness and bind heavy metals. The addition of a stabiliser is key to a successful immunisation. Furthermore, the blue dye contained in these products helps to monitor the vaccine distribution. When appropriate vaccinating technique is performed, the areas around the eyes and the nostrils of the broilers are blue (Figure. 2).
  5. The ideal temperature of the water for preparing and applying IB vaccine is between 6 and 10° C (Figure. 1). Studies have demonstrated that, beyond 10 °C, there is a 3.2% reduction on vaccination effectiveness per degree Celsius of temperature increase. For example, in cases in which the water is at 25° C, there will be a drop of almost 50% in effectiveness. In Zambian field conditions it may be quite difficult to reach the ideal thermal range, but effort should focus on bringing the water temperature as close to the ideal range as possible. Chilling the water with ice packs before vaccine preparation and maintaining the low temperature with ice packs inside the sprayer is of great help. Do not add ice cubes on their own since they will dilute the vaccine as they melt. Finally, control periodically the integrity of the ice packs before refreezing them since the leakage of their content into the vaccine solution will be detrimental.
  6. Minimise the interval between the preparation of the vaccine and its application. This will increase the chances of the IB vaccine virus to remain alive by the time it reaches the portal of entry.
  7. Vaccinate the birds during the cooler parts of the day.
  8. Before spray vaccination, switch off the fans or close the curtains (Figure. 1). This will reduce the chances of the vaccine being dispersed away from the birds.
  9. Vaccinate with the lights ON (Figure. 1). This would result on the chickens looking at the moving sprayer, improving the chances of the vaccine reaching the eyes, nostrils, and trachea.
  10. Crowd the birds on one side of the house, to reduce their movements and improve vaccine distribution (Figure. 1).
  11. Walk up and down the house, spraying 50 to 100 cm above the chickens (Figure. 1)
  12. Wear protective gear, especially goggles and mask (Figure. 1)
  13. Open the curtains and restart ventilation 15 to 20 min after the end of the vaccination.
  14. Make sure the vaccines are properly scheduled. There should be a minimum interval of, at least, 12 to 14 days between 2 doses of live infectious bronchitis vaccine, to avoid interferences between them. Shorter periods may compromise the effectiveness of the second vaccine, rendering sub-optimal local protection.
  15. In order to reach the highest immunisation against IB variant 2, use homologous vaccines. Homologous vaccines are those that have, at least, 95% similarity with the field strain. The utilisation of the only homologous vaccine against variant 2 (TAbic IB VAR206) has showed the highest protection in laboratory and field trials.

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