Although this 29-day-old broiler is showing head tremors and poor balance, this is not necessarily indicative of a Newcastle disease outbreak. The bird was observed in a flock with excellent performance and low mortality. No other birds with similar clinical signs were observed in the house or elsewhere on the farm (six houses in total, 104,000 birds on the farm and 13,400 in this house).

The persistent myth that “torticollis = ND” has always been one of the most misleading shortcuts in poultry diagnostics. It ignores the complexity of viral pathogenesis, the variability of lesion patterns, and the fact that neurological signs are shared across multiple unrelated conditions.

A single broiler showing neurological signs (head tremors and poor balance/postural instability), without stargazing, torticollis or increased flock mortality, makes classic virulent Newcastle disease less likely, although it cannot be ruled out without laboratory testing. When only one bird is affected and there is no increase in mortality, isolated causes are generally more likely than a flock-wide infectious disease.

Possible differential diagnoses include:

• Trauma – Head injury, spinal injury or leg injury causing imbalance; this would usually affect only a single bird.
• Bacterial infection of the brain or inner ear – These infections can cause tremors, ataxia and loss of balance, and may occur sporadically.
• Vitamin E deficiency (encephalomalacia) – Causes ataxia, tremors and sometimes retraction of the head ("crazy chick disease"). This condition usually affects multiple birds. It is possible if there is a feed-related issue, although a single affected bird would be unusual.
• Vitamin B deficiency (particularly thiamine deficiency) – Causes tremors, weakness and imbalance, and generally affects more than one bird.
• Marek's disease – May cause paralysis and incoordination, although it is uncommon in commercial broilers because of their young slaughter age.
• Avian encephalomyelitis – Produces fine head tremors and ataxia, but is usually seen in younger chicks (1–3 weeks of age) or in chicks from unvaccinated breeder flocks. It is therefore less likely in a 29-day-old broiler.
• Toxicity – Mycotoxins, salt toxicity, pesticides or ionophore toxicity may produce neurological signs, depending on the nature and extent of exposure.
• Congenital or developmental abnormality – An isolated bird with neurological dysfunction is possible; however, such abnormalities are often associated with poor feed intake and reduced growth. This bird appears to be well developed, making this diagnosis less likely.

Overall, based on the absence of increased mortality, the presence of only a single affected bird, and the observation of conjunctivitis, my tentative diagnosis would be a bacterial upper respiratory tract infection with extension into the inner ear and possibly the brain.

Further observations are recommended:

• Is only one bird affected, or are additional birds beginning to show similar signs?
• Are respiratory signs (gasping, coughing or nasal discharge) developing elsewhere in the flock?
• Is green diarrhoea present?
• Are the birds alert and feeding normally? How is water consumption developing?
• Is there leg weakness or paralysis in other birds, or only imbalance?
• Has there been a recent change in feed, or is there any possibility of feed contamination?

The flock should be monitored closely over the next 24–72 hours. If additional birds develop similar signs, an infectious or nutritional cause becomes more likely.

Recommended approach

• Isolate the affected bird, where practical.
• Observe the remainder of the flock carefully for the development of neurological, respiratory or digestive signs.
• Review feed quality and any recent management changes.
• If the bird dies or is euthanised, submit it for post-mortem examination. Examination of the brain, spinal cord and inner ear, including histopathology where appropriate, together with laboratory testing (such as PCR for Newcastle disease if indicated), provides the most reliable means of distinguishing infectious from non-infectious causes
• Serological testing of blood samples collected on the final day before slaughter, using an ELISA for Newcastle disease antibodies, is recommended.

The absence of increased mortality and the presence of only a single affected bird make severe Newcastle disease less likely, although they do not completely exclude it, particularly in vaccinated flocks, where vaccination may modify the clinical presentation. Thus a single bird with chronic or isolated torticollis in an otherwise stable flock is never ND until proven otherwise.

To conclude that this case represents a Newcastle disease outbreak and to recommend an immediate change to the vaccination programme on the basis of a single case would be premature and scientifically unjustified. Such an approach is not consistent with good veterinary diagnostic practice.