Week-three mortality in a broiler shed with a borrowed labourer
A poultry unit houses 2,000 day-old broiler chicks in a 300 m² shed with a thatched roof, 5 cm of litter, one feeder per 40 birds and one drinker per 60 birds. Only Marek's vaccine was given, at the hatchery. In week 3, mortality climbs from 8 to 65 birds a day. Affected birds are dull with ruffled feathers, pass greenish watery droppings, gasp, and a few twist their necks. Post-mortem on two birds shows haemorrhages on the caecal tonsils, a mottled spleen and whitish plaques in the intestine. The litter is damp near the drinkers, the ammonia smell is strong, and the labourer who cleans this shed also works every morning on a neighbouring layer farm. The owner plans to treat with a sulpha drug and to stop all vaccination because, he says, vaccines caused the disease. Feed has been stored in the shed for five weeks and the bagged maize at the bottom smells musty.
1. Give a reasoned differential diagnosis of the outbreak, state which lesions are decisive and which are missing from the report, and say what you would examine or submit before committing to a treatment.
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What the signs point to
- Week 3, acute rise in mortality, greenish watery droppings, gasping and twisted necks: this is the clinical face of a neurotropic/respiratory paramyxovirus, i.e. Newcastle disease (Ranikhet). Greenish droppings come from bile-stained gut contents after caecal damage.
- Caecal tonsil haemorrhage and whitish intestinal plaques are not typical of ND alone; they point to caecal and intestinal coccidiosis, common at 3-5 weeks with damp litter.
- A mottled spleen is non-specific: ND, septicaemia or E. coli can all produce it.
What argues against the alternatives
- IBD (Gumboro) causes watery whitish droppings and vent picking at 3-6 weeks; its decisive lesion is a swollen, oedematous or atrophied bursa of Fabricius. The bursa is not mentioned, so IBD cannot be confirmed or excluded.
- Mycotoxin from the musty maize causes poor growth, a pale liver and immunosuppression, but rarely twisted necks or greenish droppings. It may, however, be why the flock is susceptible.
The missing observations Examine the bursa, the caecal cores, the trachea and the proventriculus. Submit tracheal and cloacal swabs plus a blood sample for HI testing against ND, and a feed sample for aflatoxin. Two post-mortems in a 2,000-bird flock is a very small sample.
Most likely judgement A mixed outbreak: Newcastle disease in a flock with no ND vaccination beyond hatchery Marek's, with coccidiosis and E. coli secondary to damp litter and poor ventilation, possibly on a background of mycotoxin exposure. Treating only with a sulpha drug would suppress the coccidia and leave the virus to run through the flock.
2. Correct the owner's misconceptions and set out what should be done in the next 72 hours and what the vaccination schedule should have been.
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Corrections
- ‘Vaccines caused the disease’ is wrong. Marek's at the hatchery protects against Marek's only; it gives nothing against ND or IBD. The flock was unprotected against the two diseases that matter most at three weeks.
- Stopping vaccination would leave survivors and the next flock equally exposed.
- Sulphonamides act on coccidia, not on a virus, and at the wrong dose in young chicks they can damage the kidneys and leave residues.
Next 72 hours
- Confirm the diagnosis: bursa, caecal cores, trachea, and swabs for HI/RT-PCR; send a feed sample for aflatoxin.
- If ND is confirmed, discuss ring vaccination of survivors with LaSota (eye drop or drinking water) with the veterinary officer, keeping the vaccine cold and handling it correctly; timing matters and it must not be done blindly without a diagnosis.
- Treat coccidiosis with amprolium or toltrazuril under direction; give electrolytes and vitamins A and E in the water, and a glucose-electrolyte drink.
- Replace the damp litter, correct the feeders to about one per 25-30 birds and the drinkers to one per 50, improve cross-ventilation until the ammonia smell disappears, and cull birds that cannot reach feed or water.
- Record daily mortality and culls; if weekly mortality passes about 5-10% with no response, depopulate, clean out and rest the shed.
The schedule that should have been used Marek's at day 1 in the hatchery; RD F-strain at day 5-7; IBD at day 14; LaSota at day 21-28, with IBD repeated in endemic areas. Record every vaccination with date, batch and route.
The principle In a viral outbreak, medication buys time and controls secondary infection; it does not stop the virus. Prevention is vaccination plus litter and ventilation management.
3. The labourer works on two farms daily. Analyse his role in the outbreak, list the biosecurity measures that would have prevented it, and advise whether the owner should treat or cull this flock.
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The labourer as a fomite Newcastle disease virus is carried on clothes, boots, hands and crates. One person working on a layer farm every morning and then on this broiler shed, without a change of clothes and a footbath, is an efficient carrier. It explains how a virus entered a flock that never left the shed. Layer farms are often endemic for ND, so the risk is not hypothetical.
Biosecurity that should have been in place
- All-in/all-out with a 10-14 day clean-out: remove litter, wash, disinfect (sodium hypochlorite or formalin fumigation), rest the shed.
- One entrance, a footbath at it, boots and overalls used only on this farm, hand washing, no visitors in the last week.
- No worker moving between poultry farms; if unavoidable, work from the youngest flock to the oldest and shower and change in between.
- Feed stored dry, off the floor, away from the shed wall; no wild birds or rodents; mesh the sides.
- Vaccination as scheduled, with records.
Treat or cull? At 3 weeks with 65 deaths a day, a mixed infection and an unprotected flock, treating is unlikely to pay: the survivors will be uneven, feed conversion will be poor, and the virus stays in the shed and threatens neighbours. With a 3-week-old flock the economically defensible decision is usually to cull the affected shed, disinfect, rest it and restock, provided the diagnosis is confirmed first; partial medication and continued rearing mostly converts feed into losses and prolongs the outbreak.
Monitoring that triggers the decision Daily mortality curve, water intake (a fall of more than 10% in a day is the earliest alarm), and flock uniformity. Set the cull threshold in advance rather than deciding in the middle of the outbreak.