By Jon L. Schaeffer, DVM, PhD
JLS Research
Raleigh, North Carolina
It’s been over 30 years since in ovo vaccination was widely adopted by US broiler hatcheries. Although it may not have been entirely obvious at the outset, the move toward egg injection resulted in a seismic shift in how production systems would be structured over the ensuing decades.
In particular, removing the need for manual injection of Marek’s disease vaccine (MDV) allowed hatcheries to triple or even quadruple in size and capacity compared to what was previously possible. With this expansion, efficiencies have been realized at the complex level that remain to this day.
Vaccinating the old way
As is often the case when transitioning to new technologies, the benefits are profound, but positive features of the “old way of doing things” can get left behind. Such may be the case with post-hatch vaccination of live vaccines. Before in ovo vaccination, MDV at broiler hatcheries was administered manually, requiring individual handling of day-of-hatch chicks.
Depending on the model of the automatic injector, the operator could administer a subcutaneous MDV, as well as respiratory vaccines sprayed into the eyes or external nares. In some cases, even coccidiosis vaccine could be administered into the mouth, all in a single handling.
Clearly, this approach was more precise compared to spray cabinets that perform most of these functions today. But it was slow and impractical by today’s standards.
Automatic post-hatch injectors today advertise speeds of up to 5,500 chicks per hour, depending on how many vaccinations are performed. However, with hatcheries operating at line speeds of 30,000 to 100,000 chicks per hour, the impracticality of continuing to administer these live vaccines manually becomes obvious. Further complicating the problem is the reality that live respiratory vaccines (e.g., Newcastle disease (ND), infectious bronchitis (IB)) and live bacterial vaccines (e.g., Salmonella, Escherichia coli) are embryo-lethal and cannot be administered in ovo without some additional breakthroughs in technology.
Thus, absent an alternate approach with an input cost that makes it worthwhile, mass administration of these post-hatch vaccines via spray cabinets will be with us for the foreseeable future.
Live respiratory-vaccine concerns
A common concern in broiler production is that live respiratory vaccines result in too many “vaccine reactions” during early grow out. These respiratory reactions typically manifest as severe air sacculitis, sinusitis and excess mortality. Yes, these birds are young and economic losses are therefore less impactful, but these early infections can lead to unthrifty flocks that lack uniformity and generally perform poorly.
Oftentimes, the vaccines themselves are identified as the problem, with the more immunogenic vaccines being considered too strong. However, the USDA requires rigorous safety testing on these vaccines before licensing, and in most cases, a 10X dose must be demonstrated to be safe. So, the virulence of the vaccines is unlikely to be the root cause of the vaccine reactions. Also, it is highly unlikely that vaccines, with one or two passages through birds, would undergo meaningful changes in virulence.
Sometimes, the problem can be traced to issues with how the vaccine was handled and/or administered, resulting in too many chicks leaving the hatchery unvaccinated. Once an unvaccinated, immunologically naïve chick is placed into a house, a myriad of outcomes is possible.
For example, these birds may become immunized without major effects on flock performance if they receive a light exposure to either the vaccine virus shed by vaccinated flock mates or the field virus. Conversely, a heavier challenge from these same viruses, especially during the winter months when ventilation and litter conditions are compromised, can result in clinical disease that we often refer to as “vaccine reactions.”
Sometimes, reactions are addressed by switching to a milder tissue-culture-derived vaccine, and these vaccines may be the only recourse where the problem is intractable. However, these milder vaccines may not provide protection as broad or robust as that of stronger vaccines. Therefore, before switching, it may be worth investigating vaccine administration at the hatchery.
Vaccination considerations
There are two broad areas to consider at the hatchery: vaccine handling/mixing and spray-cabinet operations.
Vaccine handling/mixing
- Check that the vaccine is not expired.
- Establish that the cold chain has been maintained with no temperature excursions.
- Ensure that the thawing and mixing procedures are being executed correctly.
- Generally, the larger the volume that is delivered to each basket of 100 chicks, the better the coverage. For example, 14 ml is better than 7 ml, and 21 ml is even better. Concerns about wet chicks usually surface when the volume delivered approaches or exceeds 30 ml.
- When multiple spray vaccinations are administered, be mindful of the combined volume being delivered when calculating dilutions for each vaccine.
Spray-cabinet operations
Ensure that the dispensed volume is accurate and the spray pattern is correct. The entire volume should be delivered onto the chicks, not on the sides or outside the chick baskets. Adjustments can be made to the positioning of nozzles, the timing of the spray and the droplet size. The latter is generally a function of nozzle type and pressure.
With droplets that are too fine, the vaccine often settles too slowly and can get wafted away by the room’s air-handling system before ever reaching the chicks. Conversely, droplets that are too large don’t dispense widely enough across the baskets.
Consulting vaccine manufacturers about nozzle types, droplet sizes and other fine-tuning aspects of their spray cabinets may be instructive.
Molecular diagnostics
With advances in molecular diagnostics that have made PCR readily available and cost-effective, changes implemented at the hatchery can be monitored for effectiveness by measuring vaccine takes.
For IB and ND vaccines, we generally consider 5 to 7 days post-vaccination as the ideal time to sample tracheas. At the front end of this window, there is the highest likelihood that the takes are truly reflecting the vaccine administered at the hatchery. As samples are taken closer to 7 days and beyond, chances increase that the takes reflect a combination of hatchery-administered virus and the virus picked up post-placement.
Thus, day 7 takes typically produce more positives than at day 5. However, experienced scientists, based on the strength of the PCR signal and variation among sampled birds, can usually discern whether the takes represent hatchery-applied vaccine versus secondary exposure and whether it represents a problem.
For ND and IB, the profile on how the vaccines cycle through flocks and which tissues to sample and when is quite well understood. For other viruses (e.g., laryngotracheitis, avian metapneumovirus), sampling time is less well understood, and they should probably not be used as a gauge for quality assurance of the post-hatch vaccination process.
Bringing it together
Managing respiratory disease is difficult. Numerous contributing factors are tough to control, including the strain and challenge level of disease-producing viruses, the quality of facilities and management across farms and seasonality. Tightening up post-hatch vaccination at the hatchery may not be a silver bullet, but it’s an item we can control and measure and one that impacts all chicks at little additional cost.
Editor’s note: The views expressed in this article are solely those of the author.
