Ask a producer what disease costs and the answer usually starts with dead birds. The published economics say something else. On the single most widespread parasitic disease in poultry, mortality accounts for a small fraction of the bill, and the overwhelming share is paid quietly, by birds that survive and simply grow less well than they should have.
Key takeaways
- Most of the cost of endemic disease is subclinical: reduced weight gain and degraded feed conversion, not mortality.
- Biosecurity is the only line that also covers agents for which no vaccine exists on your site.
- Vaccination works at population scale when coverage is high and sustained, as national campaign data now shows.
- Any suspicion of a notifiable disease is a veterinary and regulatory matter from the first hour, not an internal decision.
Where the money actually goes
The clearest published figure in this area concerns coccidiosis. Blake and colleagues, in Veterinary Research in 2020, re-estimated the global cost at around £10.4 billion at 2016 prices, in a range of £7.7 to £13.0 billion, equivalent to about £0.16 per chicken produced, using data collected from veterinarians, producers and health experts in eight countries including Brazil, Egypt, India, Nigeria, the United Kingdom and the United States.
The breakdown is what changes practice. Morbidity, meaning reduced weight gain and increased feed conversion in birds that live, represented more than 80% of the total financial cost in India, the UK and the US. In the UK sample specifically, morbidity accounted for 83.1% of the cost, control measures for 16.0%, and mortality for 0.9%.
That single line reframes what a prevention programme is for. A flock can look acceptable on a mortality chart and still be losing money every day through the feed conversion ratio. It also explains why performance data, not just health data, is part of disease surveillance on a commercial site.
Three lines, and what each one cannot do
Biosecurity: the only line with no gaps in coverage
Biosecurity is the only measure that acts on agents you have no vaccine for, including ones nobody has characterised yet. WOAH’s Terrestrial Animal Health Code, chapter 6.5, on biosecurity procedures in poultry production, sets out what that means structurally: physical separation of areas, one-way movement of birds and materials, showers and site-supplied clothing at entry, and cleaning and disinfection of vehicles before each loading, with chapter 4.14 covering disinfection itself.
What biosecurity cannot do is survive being applied selectively. A footbath with exhausted disinfectant, a visitor log completed after the fact and a single uncontrolled door reduce the whole scheme to its weakest crossing point. It is a system property, not a sum of individual measures.
Vaccination: preparation, not a barrier
Vaccination does not stop an agent arriving. It changes what happens when it does, by reducing clinical disease, shedding and spread. It is also the line most sensitive to execution, because a correctly chosen vaccine that is badly stored or unevenly applied produces a flock that is documented as protected and is not.
The handling constraints are unforgiving. Cell-associated Marek’s and HVT products sit in liquid nitrogen at -196 °C and are thawed quickly, never refrozen. Everything else is held at +2 to +8 °C, and a single freezing event separates adjuvant from antigen in an inactivated product. Programme design, product choice and timing are prescribed by the veterinarian responsible for the flock, and local epidemiology is what makes one programme right and its neighbour’s wrong.
Husbandry: it sets the threshold
Feed, water, ventilation, stocking density and litter condition decide how much exposure a bird tolerates before exposure becomes disease. This is the line that fails invisibly, because none of its parameters produce an alarm. The bird’s own behaviour is the instrument, which is why a walk-through with time to actually watch the flock remains the highest-yield surveillance activity on most sites.
| Line of defence | What it acts on | How you know it is working |
|---|---|---|
| Biosecurity | Introduction and spread of any agent, vaccinated against or not | Audited crossings and records, not the absence of an outbreak |
| Vaccination | Clinical expression and shedding of specific agents | Serology, read on spread of titres as well as mean |
| Husbandry | The dose of challenge a bird can absorb | Feed conversion, uniformity and flock behaviour |
What vaccination looks like when it works at scale
France’s duck vaccination campaign against highly pathogenic avian influenza has produced the clearest recent evidence on this. Work published in Emerging Infectious Diseases in July 2025 reported 10 observed H5 poultry outbreaks in 2023 to 2024 against a modelled expectation of 487 outbreaks without vaccination, with a 95% confidence interval of 314 to 756, a reduction estimated at 96% to 99%. As of 1 July 2024, more than 35 million ducks had received two doses and 1.5 million a third.
Two qualifications belong with that figure. The comparison is against a model, not against a real unvaccinated France, and the campaign ran alongside continued biosecurity and surveillance rather than instead of them. The programme has since continued, with a further national campaign running from 1 October 2025 to 30 September 2026 under enhanced surveillance requirements. It is a demonstration of what sustained coverage can achieve, not an argument that vaccination replaces the other two lines.
Detecting it early, and knowing when it stops being your call
Early detection on a commercial site is mostly a records problem. Daily water consumption, feed intake, mortality by house and any change in flock behaviour form a baseline against which a deviation is visible days before a clinical picture is. Water intake in particular tends to move first.
The moment a notifiable disease is suspected, the decision leaves the farm. Avian influenza and Newcastle disease carry statutory reporting obligations in most jurisdictions, and the correct first action is a call to the veterinarian and the competent authority, not a wait-and-see period. Nothing in this article replaces that assessment, and no protocol described here should be applied to a specific flock without veterinary prescription.
Questions from the field
How do I tell one respiratory picture from another?
Clinically, often you cannot. Several viral and bacterial agents produce overlapping signs, which is precisely why laboratory confirmation exists. What you can do reliably is document what you observed, when, and in which house, so the sampling that follows is targeted rather than exploratory.
If the flock is vaccinated, why does disease still appear?
Because vaccination reduces clinical expression rather than blocking infection, and because coverage is rarely as uniform as the records suggest. A wide spread of antibody titres means part of the flock is unprotected while the flock average looks acceptable.
How much does nutrition really contribute?
It contributes to the threshold rather than to immunity as such. A balanced ration and clean water support normal immune function and gut integrity; they do not confer protection against a specific pathogen, and no feed additive should be presented as doing so.
Which environmental factor is most often underestimated?
Litter condition, because it accumulates slowly and nobody sets an alarm for it. Wet litter changes ammonia, changes the respiratory environment and changes parasite pressure at the same time, which makes it one of the few variables that acts on all three lines of defence at once.
Turning the first line into a routine
Biosecurity only protects the flock on the days it is actually applied. Written as a checklist with named responsibilities, it stops depending on who is on shift.
Sources: Blake et al., “Re-calculating the cost of coccidiosis in chickens”, Veterinary Research, 2020, for the global cost of ~£10.4 billion at 2016 prices, the £7.7 to £13.0 billion range, the £0.16 per chicken figure, the eight countries surveyed and the UK breakdown of 83.1% morbidity, 16.0% control and 0.9% mortality; WOAH Terrestrial Animal Health Code, chapter 6.5, on biosecurity procedures in poultry production, and chapter 4.14 on disinfection; “Promising Effects of Duck Vaccination against Highly Pathogenic Avian Influenza, France, 2023-2024”, Emerging Infectious Diseases, July 2025, for the 10 observed outbreaks against 487 modelled (95% CI 314-756), the 96% to 99% reduction and the doses administered; French Ministry of Agriculture national vaccination plan for the 2025-2026 campaign; manufacturer storage instructions for cell-associated Marek’s and HVT vaccines. Consulted August 2026.
Published previously, fully revised on 13 August 2026. General technical guidance for poultry professionals. It does not replace veterinary diagnosis or prescription, and any suspicion of a notifiable disease must be reported to the competent authority without delay.