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Commercial Poultry Diseases

Hygiene and Biosecurity

Biosecurity is the science of reducing the spread of potentially harmful micro-organisms onto and within a site. Biosecurity practices are required in addition to vaccination programmes and planned antimicrobial treatments to reduce the levels of potential pathogenic organisms on a farm.

Disease Transmission 

Pathogenic organisms are transmitted by a variety of routes and an understanding of the ways organisms spread can be helpful in designing strategies to limit disease spread. 

Respiratory viruses               - spread by the birds coughing and sneezing

                                                 - carried by aerosol

Intestinal organisms              - spread via faeces

Oviduct infections                  - spread vertically via the egg 

Pathogenic organisms live for variable lengths of time in the environment and this also has implications in designing strategies to limit disease spread. eg 

Coccidia                    )

Many Bacteria           )           live for long periods outside the host in the environment

Aspergillus                 )

Respiratory viruses – generally live for short periods outside the host but can be carried several miles on wind currents

Methods of Spread of Infection


Wind                                                  - Respiratory viruses

Faeces                                               - Coccidia, Many Bacteria

Vermin                                                - Pasteurella, Salmonella

Human intervention                           - Newcastle Disease on lorries, Bacteria on footwear

Feed                                                   - Salmonella organisms

Equipment                                         - E.coli

Litter                                                  - Parasites


The quality of the housing available to livestock can have a significant effect on the effectiveness of Bio security programmes.


            Siting buildings near ponds or lakes utilised by migratory water fowl

            Poorly drained areas with standing water

            Areas with high concentrations of wild birds/vermin

            Major roads with poultry traffic 

Ensure there is the facility to dispose of litter effectively and well away from the site.

Concrete around houses will reduce surrounding vegetation to keep down vermin, and reduce contamination of boots and tyres with mud, litter etc. 


Keep vehicles entering the site to a minimum and consider spray disinfection of vehicle wheels at the entrance to the site.

Regularly refill foot-dips with a DEFRA approved disinfectant (ideally one which is effective in the face of organic soiling).

Keep overalls and boots for visitors on-site and insist that they are worn.

Supply hand-washing facilities for staff and visitors on arrival and departure

Maintain a record of ALL visitors to the site 

Carcase Disposal 

Ensure all carcasses are either removed or incinerated as soon as possible. Use containers with well-fitting lids to reduce vermin numbers.

Factors affecting Range Use

It is important for their wellbeing that free-range laying hens make good use of the range provided for them. In some surveys the average number of hens using a range were estimated at just 20% meaning that on some farms the number of hens using the range was much lower than this. 

Factors that have been observed to affect this figure include:           

1) Wind speed and the wind chill factor. This effect can be modified by providing the birds with wind breaks. It seems that temperature per se does not have an effect on a flock’s use of the range 

2) The indoor stocking density. The higher the density indoors the fewer birds use the range 

3) The age birds are first allowed onto the range. The earlier birds are let out the better is their range use 

4) Area of shelters on the range. The greater the area the better the range is used by the birds 

5) Bird’s confidence. The more fearful birds are the less they use the range 

6) Providing grit improves range usage 

7) Earth floor litter areas improve range use

Salmonella prevention in Laying Flocks

Salmonella infection in laying hens rarely leads to disease in the birds but it is a health threat to farm workers, visitors and consumers of eggs and poultry meat. It is this final threat that has made it so politically important.

Salmonellae are intestinal bacteria that can affect all animals and birds. The proper cooking of food will destroy the bacteria but in poultry it can also be transmitted in intact eggs so increasing the risk of human food poisoning infections.

More than 2500 types of Salmonella have been identified but only a very small proportion of these have been recorded in poultry.

Two types of Salmonella found in laying birds are of real concern in human health. They are Salmonella enteritidis and Salmonella typhimurium and they are of particular concern as they can be transmitted via the egg.

All commercial laying flocks are now routinely tested.

Transmission from breeding flocks to commercial flocks has been virtually eliminated and although the diseases could be introduced into laying flocks via infected pullets these are routinely vaccinated and tested before moving so should not pose a threat to laying flocks.

Potential sources of infection on layer farms:

  1. Pullets – unlikely if birds come from a reputable source.
  2. Other poultry units nearby – Salmonella may be introduced by wildlife and flies
  3. Human contamination on clothing, boots etc.  To reduce this likelihood:

Keep access restricted to only those who actually need access.

Visitor’s vehicles should be kept well away from the houses and if practicable disinfect the vehicle wheels.

Provide overalls and boots for visitors. (the  minimum provision should be a satisfactory boot wash with a disinfectant bath and brush available).

  1. Domestic animals – dogs and cats may carry salmonella so should not go into sheds or onto the range even if the houses are empty
  2. Wild animals – possibly these are greatest threat. Includes birds, insects and mammals.

Birds. Do everything possible to discourage wild birds particularly clean up any feed spillages promptly

Insects. Flies and mites can carry Salmonella so ensure these are effectively controlled.

Mammals. Especially rats and mice. An effective rodent control programme is essential. Note that there is a requirement in testing schedules for rodent droppings or swabs from bait boxes.

Other mammals such as foxes also need to be discouraged – correct disposal of carcasses necessary.

  1. Feedstuffs. Should be free of Salmonella if obtained from a mill who operates under the relevant DEFRA codes of practice for the control of Salmonella. Food can become subsequently contaminated if vermin have access to feed.
  2. Water. Should be safe if from a mains source. If from other sources have water tested and chlorinate it. It can become subsequently contaminated especially in the header tank if it is not enclosed.
  3. Catching gangs, cleaning gangs, equipment etc.

Routine testing of laying flocks: Designed to test for the actual presence of infection on the farm – must at all costs avoid false positives.

Sources of false positives:

  1. Testing kits / equipment – use kits from a known source
  2. Contamination of kits in storage – ensure that they are stored correctly
  3. Contamination of kits when in use / incorrect collection technique for samples – follow the guidelines for collecting samples.
  4. Laboratory error – all testing laboratories have to be DEFRA approved so there should be little likelihood of laboratory errors occurring. 


Always consult the relevant codes of practice for full details of sampling procedures.



  Sandhill Veterinary Services 14 Long Street, Topcliffe YO7 3RW telephone 01845 578710 fax 01845 577685 email