Major poultry diseases
The main viral, bacterial and parasitic diseases of chickens, from Newcastle disease and avian influenza to Gumboro and coccidiosis, and prevention.
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A poultry house holds thousands of birds of the same age, sharing air, water and feed. When a disease enters, it can spread through the flock in days. In villages, Newcastle disease can kill most of the chickens in a community in a single outbreak, destroying a source of food and cash that women in particular rely on. Knowing the major poultry diseases, their signs and, above all, how to prevent them is essential for anyone keeping or advising on poultry.
Poultry diseases are a standard topic in avian pathology, poultry science and veterinary exams.
Bacterial diseases
- Colibacillosis: infections with pathogenic E. coli, often after viral damage or poor air quality, causing airsacculitis, pericarditis, peritonitis and yolk sac infection.
- Salmonellosis: pullorum disease and fowl typhoid (Salmonella Pullorum and Gallinarum) kill chicks and adults; other Salmonella serovars (Enteritidis, Typhimurium) matter mainly as food-borne infections of people, controlled by testing, vaccination and hygiene.
- Mycoplasmosis: Mycoplasma gallisepticum causes chronic respiratory disease and reduced egg production, spread through eggs and contact.
- Necrotic enteritis: Clostridium perfringens overgrowth in the gut, often after coccidiosis.
- Fowl cholera (Pasteurella multocida) and infectious coryza.
Parasitic diseases
- Coccidiosis: Eimeria protozoa damage the intestine, causing bloody droppings, poor growth and deaths, especially on wet litter. Controlled by coccidiostats in feed, live vaccines, and dry litter.
- Roundworms and tapeworms, especially in free-range and backyard birds.
- Red mites, northern fowl mites and lice, which cause anaemia, irritation and lower egg production.
Village poultry
In backyard and village flocks, Newcastle disease is usually the main killer. Heat-tolerant vaccines such as the I-2 strain, given by eye drop by trained community vaccinators every three to four months, have greatly reduced losses in many African and Asian countries, often paired with simple housing at night and supplementary feed.
Prevention: the foundation
- Biosecurity: fences, single entry with footbaths and clothing change, controlled visitors and vehicles, clean water, rodent and wild bird control.
- All-in, all-out with cleaning, disinfection and rest periods.
- Vaccination programmes matched to local disease risks.
- Good brooding, ventilation, litter and nutrition, which keep immunity strong.
- Disease-free chick sources from tested breeder flocks.
- Prompt diagnosis with post-mortem examination and laboratory tests, and prudent use of antimicrobials under veterinary guidance.
Reading a flock's mortality record
A broiler house is stocked with 10,000 chicks. Weekly deaths are 80, 40, 30, 350, 120 and 50.
- Cumulative mortality = (80 + 40 + 30 + 350 + 120 + 50) ÷ 10,000 = 670 ÷ 10,000 = 6.7 per cent.
- Week 1 mortality of 0.8 per cent reflects normal early losses.
- In week 4, deaths jump to 350 among about 9,850 birds alive: 3.6 per cent in one week, far above the usual weekly 0.1 to 0.2 per cent at that age.
Sudden losses at 3 to 4 weeks, with ruffled feathers, watery droppings and swollen or haemorrhagic bursas at post-mortem, suggest infectious bursal disease. The farmer should confirm with the laboratory, review the vaccination timing against maternal antibody levels, and tighten cleaning and disinfection before the next flock. At 2.8 kg per bird and 1.20 dollars per kg live weight, the 470 extra deaths in weeks 4 and 5 alone cost about 470 × 2.8 × 1.20 ≈ 1,580 dollars in lost sales, plus feed already eaten.
Key points
- Viral: Newcastle disease, avian influenza, infectious bursal disease (immunosuppression), Marek's disease (tumours; vaccinate at hatch), infectious bronchitis.
- Bacterial: colibacillosis, salmonellosis (pullorum, fowl typhoid, food-borne serovars), mycoplasmosis, necrotic enteritis, fowl cholera.
- Parasitic: coccidiosis (Eimeria), worms, mites and lice.
- Prevention rests on biosecurity, all-in all-out, vaccination, good management and clean chick sources.
- Watch weekly mortality: sudden rises signal disease needing diagnosis.
What the exams ask
- School-leaving and entrance papers: names, signs and prevention of common poultry diseases.
- Degree and research-fellowship entrances: aetiology, transmission, lesions, diagnosis and control of major viral, bacterial and parasitic diseases, and vaccination schedules.
- Veterinary examinations: post-mortem diagnosis, flock health programmes, WOAH-listed diseases and zoonotic aspects.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why do my chickens die even though I vaccinated?
Vaccine failure can come from a broken cold chain, wrong timing relative to maternal antibodies, chlorinated water inactivating live vaccines, immunosuppression from Gumboro, or a strain that the vaccine does not cover.
Is coccidiosis always a problem?
Eimeria are present on almost all poultry farms. Disease appears when the challenge is high, as with wet litter, and immunity is low. Good litter management and control programmes keep it in check.
Can people catch poultry diseases?
A few, including some avian influenza strains and Salmonella serovars, can infect people. Most others, such as Marek's disease and Gumboro, cannot.
Sources
- Swayne, D. E. et al. (eds) Diseases of Poultry, 14th edition. Wiley-Blackwell, 2020.
- World Organisation for Animal Health. Manual of Diagnostic Tests and Vaccines for Terrestrial Animals. WOAH, updated annually.
- Pattison, M., McMullin, P., Bradbury, J. M. and Alexander, D. J. (eds) Poultry Diseases, 6th edition. Saunders Elsevier, 2008.
- Alders, R. G. and Pym, R. A. E. "Village poultry: still important to millions, eight thousand years after domestication." World's Poultry Science Journal 65, 181–190 (2009).
- Witter, R. L. "Increased virulence of Marek's disease virus field isolates." Avian Diseases 41, 149–163 (1997).
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