Respiratory Mortality in a Vaccinated Broiler Flock: Virus, Mycoplasma, or Ammonia?
A 28-day-old broiler flock of 12,000 birds is housed in two identical houses on the same farm. House 2 has had mortality rise from 0.1% to 1.2% per day over four days, with sneezing, tracheal rales, watery eyes, and swelling around the infraorbital sinuses. House 1 remains normal. The flock was vaccinated against Marek's disease at day 1, Newcastle disease (ND) and infectious bronchitis (IB) by spray at day 7, and infectious bursal disease at day 14. House 2 has wet litter near the drinkers and an ammonia level of 35 ppm; house 1 has 10 ppm. Necropsy of six birds shows tracheal congestion, caseous airsacculitis, and lung congestion, but no proventricular haemorrhages. Feed and water intake in house 2 have fallen by 15%.
1. List the differential diagnoses in priority order and the samples you would collect, explaining what each test would change.
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The priority list should include infectious bronchitis, Newcastle disease, Mycoplasma gallisepticum or synoviae, ornithobacterium rhinotracheale, and ammonia-induced respiratory damage with secondary bacterial infection. IB is high on the list because the flock is vaccinated, but spray vaccination does not guarantee protection against a variant, and the swollen infraorbital sinuses fit mycoplasma as well. ND remains possible despite vaccination, and the absence of proventricular haemorrhages does not rule it out. Ammonia at 35 ppm is enough to damage the tracheal epithelium and worsen any infection.
I would collect tracheal and cloacal swabs from live affected birds for IB and ND PCR, tracheal swabs and air-sac tissue for Mycoplasma culture or PCR, and fresh lung and air-sac tissue plus formalin-fixed sections for histopathology. I would also collect paired sera from affected and unaffected birds. PCR results would change control by identifying whether this is a viral flock problem requiring emergency vaccination or culling, a mycoplasma problem requiring targeted antimicrobials and breeder investigation, or primarily an environmental problem requiring ventilation correction.
2. What immediate control measures would you institute, and why would you withhold or use antibiotics?
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Immediately, I would improve house 2 ventilation, remove wet litter, repair drinkers, and reduce ammonia below about 20 ppm. I would increase space and reduce stocking density if possible, and use a mild disinfectant in the water lines after checking compatibility. I would isolate house 2, restrict staff and equipment movement, and enforce footbaths and hand hygiene. I would submit samples before starting any antimicrobial so that the choice is evidence-based.
I would withhold antibiotics unless there is evidence of secondary bacterial infection such as a positive culture from air sacs or lungs, because unnecessary use selects resistance and wastes money. If mycoplasma or ORT is confirmed, I would choose an antimicrobial with a known sensitivity pattern and respect the withdrawal period. If ND or highly pathogenic avian influenza is suspected, I would notify the veterinary authorities immediately and follow statutory control measures rather than treating blindly. Supportive care with electrolytes and vitamins is reasonable but is not a substitute for diagnosis.
3. The owner asks whether the day-7 spray vaccination failed. How would you assess that claim?
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I would assess failure by comparing vaccination technique, storage, and timing with the field evidence. I would check the vaccine cold chain, diluent, spray droplet size, nozzle calibration, and whether birds were evenly exposed. I would also ask whether the vaccine was administered at the correct age and whether house 1 and house 2 received the same batch. A single affected house with poor ammonia control suggests that environmental factors and secondary pathogens may be more important than vaccine failure.
Laboratory evidence would decide the claim. I would compare IB and ND PCR results and, if possible, sequence the IB isolate to see whether it is a vaccine strain or a field variant. Paired serology can show whether the flock responded to vaccination, but a single titre is difficult to interpret. If the same vaccine batch protected house 1 and only house 2 broke down, I would not blame the vaccine alone. The fair conclusion would be that vaccination is only one part of respiratory control; ventilation, ammonia, stocking density, and biosecurity must be corrected together.