Disease is the single largest existential risk to a poultry operation. A Newcastle outbreak can wipe out 100% of an unvaccinated flock in seven days. A Marek's wave quietly consumes 15% of your pullets over six months. Coccidiosis takes a slower, more insidious toll on FCR.

The good news is that most of what matters is cheap and preventative. This playbook covers the five diseases that account for roughly 80% of commercial poultry losses — how to recognise them, how to prevent them, and how to respond when prevention fails.

The biosecurity triangle: traffic, cleanliness, isolation

Before we talk about any specific disease, understand that biosecurity is a system of three overlapping ideas. If you have all three, you rarely get sick. If you drop one, you eventually will.

Traffic control

Pathogens move on feet, wheels, tools, dust and birds. Control who and what enters the farm, and in what order. The daily rule is simple: visitor goes to the healthiest flock first, the oldest flock last, and never backwards. Vehicle wheels get sprayed. Boots stay at the farm boundary.

Cleanliness

Clean and disinfect everything, every batch. The standard "all-in, all-out" protocol gives a house 10–14 days to be emptied, washed, disinfected, dried and rested before the next batch arrives. Shortcut this at your peril — a residual coccidiosis load from the previous flock is the gift that keeps on giving.

Isolation

Different species, different ages and different sources never share airspace. Chickens, ducks, geese and turkeys together in one shed is a biosecurity disaster waiting to happen — Newcastle and avian influenza travel between species in ways most farmers do not expect.

Disease 1 — Newcastle (NDV)

Newcastle disease is the single most feared viral disease in tropical poultry. It is highly contagious, often fatal and spread through air, water, feed, bedding and birds. Virulent strains can kill 100% of an unvaccinated flock within 72 hours.

Recognise

Early signs: sudden drop in feed and water intake, respiratory distress, greenish watery diarrhoea, nervous signs (twisted neck, paralysis), rapid death. Egg production collapses, shells become soft or misshapen, mortality rises quickly.

Prevent

Vaccinate. Vaccinate. Vaccinate. The commercial schedule in high-pressure regions is: Day 1 (hatchery, HB1 or Clone 30, by spray or eye drop), Day 10–14 (LaSota, drinking water), Week 6–8 (killed oil adjuvant, injection), repeat killed every 3–4 months in laying flocks.

Respond

If NDV is suspected, do not panic-sell birds (that spreads it). Isolate affected houses immediately, notify your veterinary services (notifiable disease in most countries), and prepare for mass vaccination of unaffected neighbours. Supportive therapy with electrolytes and vitamin AD3E in water may reduce mortality by 10–20% in a confirmed outbreak.

Disease 2 — Marek's disease

Marek's is a herpesvirus-induced lymphoproliferative disease — essentially a cancer caused by a virus. It is universal (every farm has it in the dust), slow-moving and devastating in unvaccinated flocks.

Recognise

Progressive leg paralysis in young birds (8–20 weeks), loss of weight despite normal feed intake, grey eyes, visceral tumours at post-mortem (liver, spleen, ovary). Mortality creeps up from Week 10 onwards.

Prevent

Day-of-hatch vaccination with HVT, with or without Rispens, is non-negotiable. Buy chicks only from hatcheries that vaccinate — insist on seeing the batch record. Once a chick has left the hatchery unvaccinated, Marek's prevention is extremely difficult.

Disease 3 — Gumboro / Infectious bursal disease (IBD)

Gumboro targets the Bursa of Fabricius, destroying immune function in young birds. Even survivors are left immunosuppressed, with poor response to every subsequent vaccine. It is often the hidden hand behind "mysterious" secondary outbreaks.

Recognise

Watery diarrhoea, feather pecking, dehydration, sudden mortality spike at 3–5 weeks of age. Post-mortem: swollen then atrophied Bursa, haemorrhages in thigh muscle.

Prevent

Intermediate-plus or hot IBD vaccine in drinking water around Day 14–18, often repeated at Day 24. Timing matters: too early and maternal antibodies neutralise the vaccine; too late and the field virus wins. Use maternal antibody titres (Deventer formula) if your hatchery can supply them.

Disease 4 — Coccidiosis

Coccidiosis is a protozoal disease caused by Eimeria species. Every farm has it; the question is only whether it is subclinical (silently costing you 10–15% of FCR) or clinical (visibly killing birds).

Recognise

Bloody droppings (E. tenella), chocolate-coloured droppings (E. necatrix), poor weight gain, ruffled feathers, listlessness. Subclinical form: nothing visible, just a stubbornly bad FCR and uneven bodyweights.

Prevent

Two mainstream strategies:

1. Anticoccidials in feed — ionophores (salinomycin, monensin) or chemicals (amprolium, nicarbazin). Rotate between flocks to prevent resistance.
2. Vaccination — live attenuated oocysts given at Day 1 or Day 7. Preferred where organic/no-antibiotic production is required.

Both work; neither works forever. Dry litter is the great equalizer — wet litter is coccidiosis litter.

Disease 5 — Avian influenza (HPAI)

Highly pathogenic avian influenza is the thermonuclear option in poultry disease. It kills nearly 100% within 48 hours, is zoonotic, and triggers movement restrictions that can shut your business down for months.

Recognise

Sudden death without clinical signs. Cyanosis of combs and wattles. Haemorrhages on legs. Massive mortality overnight.

Prevent

Keep wild waterfowl away from your flock. Treat all surface water as suspect. In endemic regions, vaccinate with authorised H5/H9 vaccines. Report suspected cases immediately — your veterinary authority will take over, and if you concealed it, they will take over much more harshly.

A copy-paste commercial layer vaccination calendar

Adapt locally with your veterinarian; this is a typical East African commercial layer schedule.

The daily biosecurity checklist

1. Visitor log signed at the farm gate.
2. Boots dipped on entry and exit of every house.
3. Dedicated clothing for each house (never shared).
4. Rodent bait stations checked, filled if needed.
5. Wild-bird deterrents working (netting intact).
6. Water sanitizer dosing confirmed.
7. Mortality disposed of (incinerated or buried with lime, not left where predators can reach).
8. Visual inspection of flock for ANY abnormal behaviour.

What to do when you suspect a disease outbreak

1. Stop all movement onto and off the farm.
2. Separate affected birds immediately.
3. Submit samples to a diagnostic lab (dead birds < 24 hours, swabs, serum).
4. Document everything: when it started, what you saw, mortality curve, water and feed intake pattern.
5. Contact your veterinarian. Resist the temptation to self-medicate with broad-spectrum antibiotics — most poultry diseases are viral and antibiotics only cost you money while masking the real problem.
6. Notify the regulatory authority if the suspected disease is notifiable in your jurisdiction.
7. Review your biosecurity log. Find the entry point. Fix it before the next flock.

"Every outbreak has a root cause. 'Bad luck' is not a root cause — it is a failure of investigation."

Closing thought

Diseases are not accidents. They are failures of a system that can be designed. Spend an hour a week on biosecurity and you will spend many fewer nights lying awake worrying about a mortality spike that didn't happen.