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Domestic Poultry Diseases

In this section there is a list of the more common diseases of domestic poultry together with notes on notifiable diseases (diseases that must be notified to the authorities) 

ASPERGILLOSIS Caused by the inhalation of fungal spores. The disease affects the respiratory tract and may occur in day-old chicks due to incubator contamination. It can also occur in older birds due to spores on mouldy straw and hay. There is no treatment for this condition.


AVIAN INFLUENZA A viral disease causing very high mortality (Fowl Plague) - a notifiable disease.
There is no treatment and no vaccine is available in the UK for this condition.


BUMBLEFOOT Seen as swelling of the foot pad due to Staphylococcal bacterial infection. Treatment of individual birds is often not effective.

COCCIDIOSIS There are at least seven species seen in domestic poultry, all affecting the intestinal tract. It may be controlled by feeding coccidiostats to younger birds. Disease occurs as a result of decreased feed intake, overwhelming infection or management failures. The incidence may be reduced by disinfecting houses between batches with a suitable disinfectant.

Severe coccidiosis under the microscope

COLIBACILLOSIS E.coli causes generalized bacterial infections, particularly in younger birds. It can result in poor growth rates and can cause high mortality. May be treated with antibiotics. In older birds generalized infections are usually secondary to some initiating stress.

CORONAVIRUS (Infectious Bronchitis) A viral disease that can cause respiratory disease or lowered egg production and poor shell quality in laying birds. The disease may be prevented with a suitable vaccination programme

EGG PERITONITIS Caused by egg yolks entering the body cavity. It can result in high mortality in early lay and is more common in free-range birds. The exact cause of this condition is not fully understood. It may also be due to a specific infection such as Salmonella or Pasteurella. In these cases treatment may be beneficial.

ERYSIPELAS More commonly seen in adult birds paricularly if they have access to land on which pigs or sheep have been kept. Can cause sudden high mortality.  It may be treated with antibiotics and prevented by vaccination. Can be caught by humans so owners should not open any carcasses that potentially may be infected.

FLEAS Blood sucking insects. In heavy infestations they can cause severe irritation and can affect a bird's performance.

GAPES Caused by the worm Syngamus trachea that lives in the windpipe. The lifecycle of the worm is either direct or via earthworms so land can remain contaminated for years.  The origin of infection may be wild birds such as pheasants. Routine control of worms in small flocks is advised.



GUT WORMS A number of species may be present including Heterakis, Capillaria and Ascaridia.
Heterakis can carry the Histomonas (Blackhead) parasite. Capillaria can cause severe disease with loss of condition and egg production. Routine control of worms is advised.

HISTOMONIASIS(Blackhead) Blackhead is caused by a protozoan parasite. It affects the caecal lining and causes scour, loss of condition and deaths. It may be be carried in Heterakis worms. No specific treatment available for blackhead but it may be prevented by worm control and good management practices.

LEUCOSIS A viral disease that causes tumours in older birds. Relatively rare. No treatment is available.

Leucosis lesions in an avian liver

LICE AND MITES Biting and blood-sucking insects affecting the skin. Control can prove difficult.

MAREK'S A viral disease of chickens that results in tumour formation and which is caused by a Herpes virus. Birds are infected in the first few weeks of life although clinical signs are usually seen between 14 and 30 weeks of age but sometimes later than this. Two forms of the disease are commonly seen in small flocks - paralysis of limbs and loss of condition caused by internal tumours. It can be controlled by vaccination and rearing chicks in isolation. 

Chicken with Marek's disease

MYCOPLASMOSIS A bacterial disease that causes a respiratory infection and reduced egg output. It spreads directly between birds and via the egg. Clinical disease is often started by stress. It may be controlled and treated with antibiotics. Vaccination of limited use in small flocks. 

NEWCASTLE DISEASE (Fowl Pest) A viral disease that may cause nervous signs and high mortality. It is a notifiable disease and it can be prevented by vaccination. There is no treatment for affected birds.

PASTEURELLOSIS A bacterial disease that causes high mortality in birds of all ages. Rats act as a reservoir of infection. Can cause egg peritonitis in laying birds. It may be treated with antibiotics and prevented by vaccination.

RICKETS Soft bones in young birds. May be nutritional in origin. 

SALMONELLOSIS There are many types of Salmonella bacteria and most do not cause disease except in young birds under stress. Its importance is because it can cause disease in humans. Birds can be monitored for the presence of disease. (see the separate article on the control of Salmonella in small flocks)

SCALY LEG MITE  The Scaly Leg Mite, Knemidocoptes, causes mange lesions on the legs with thickened scales and also lesions on unfeathered parts of the body. Affected birds may be treated but the results are often disappointing. Good cleaning of empty accomodation is essential if the mite is not to affect subsequent flocks.

Chicken Scaly Leg

TUBERCULOSIS Birds affected with Avian TB lose condition and eventually die. It is usually a disease of birds two years old or more. No treatment is available.                                                                                                                                                             

YOLK SAC INFECTION Causes mortality in chicks up to seven days old. It is caused by a variety of bacteria that affect the chicks at hatching.

Salmonella Control in Small Flocks

Flocks of less than 350 laying hens and which supply eggs direct to the consumer or through local retailers do not have to comply with the testing regime of the National Control Programme for Salmonella in Laying Flocks. However for the safety of all who eat the eggs it is important that action is taken to reduce the likelihood of salmonella infection in all laying birds.  This rule must not be confused with the need for flocks of 50 birds or more to be entered on the GB Poultry Register.

Neither of these regulations precludes either small flocks being tested for Salmonella or flocks of less than 50 birds being entered on the register.

As replacement stock are one of the major ways of introducing disease into a flock it is important that care is taken when replacement birds are purchased.

If birds are homebred to produce replacement stock they may carry disease even if reared in isolation as some diseases are transmitted through the egg.

If birds are bought in, it is important to be sure of the health status of the flock of origin and its vaccination history. I would advise that birds are not bought from auctions if the vendor and their flock is not known to the buyer. Even if you are satisfied with the health status of the flock of origin, new birds should always be isolated from the main flock for a period of time as the stress of moving can cause them to exhibit signs of any diseases that they may be carrying.  Not only do new birds need to be kept isolated but measures need to be taken to prevent the spread of disease by mechanical means between birds on hands and footwear.

The term ‘Biosecurity’ is used to describe the measures taken to prevent or reduce exposure of stock to disease causing organisms. To be effective a Biosecurity policy needs to be based on an understanding of the possible sources of disease and the measures that can be taken to combat them.

For Salmonella, possible sources of infection are introduced stock, rodents (mice and rats), wild birds and humans (on contaminated footwear).

baited mousetrap and length of pipe



Rat trap next to shed wall with bricks used to protect domestic animals

NOTE: Traps should be checked at least twice daily

If contaminated by rodents or wild birds then feed and water are other possible sources of disease.

vermin proof containers

To combat these sources it is important that rodents around poultry houses are effectively controlled and wild birds are not attracted to the site (avoid spilling feed and feed the birds indoors).

food waste carelessly disposed

Waste food carelessly disposed of will atract vermin and wild birds


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