Diseases in Poultry

Poultry provide globally important sources of animal protein and are amongst the most intensively reared of all livestock species. Diseases of poultry are therefore of major concern, both locally and on an international scale. It is also important to consider the concept of, and the need for, biosecurity - or the range of management procedures designed to protect livestock from infection. However, in many scavenging or village poultry systems, predation may be a more important cause of mortality than disease, for example in parts of India (see research project R7633)

What is Disease? Disease as a Cause of Mortality in Poultry Reservoirs of Infection Important Diseases of Poultry   Bacterial Diseases   Viral Diseases   Fungal Diseases   Parasitic and Protozoan Diseases Management-related Problems References and Further Reading

Disease is defined as a departure from health, and includes any condition that impairs normal body functions. Disease results from a combination of indirect causes that reduce resistance or predispose an animal to catching a disease, as well as the direct causes that produce the disease (Damerow, 1994). Direct causes can be divided into two main categories, infectious and non-infectious. Some of the non-infectious conditions may result in reduced immune response (e.g. problems with nutrition), or increased contact with infectious organisms (e.g. poor housing and management), and will lead to increased incidences of infectious diseases.

In contrast to modern poultry production, village-based poultry production is often characterised by a range of diseases occurring at the same time. Most often free-range poultry have sub-clinical infections with a high number of endoparasites and ectoparasites (Permin and Pedersen, 2002).

The low productivity in traditional systems is mainly due to high mortality, which is caused by mismanagement, diseases, lack of nutritional feeding and predators. In traditional systems the mortality has been estimated to be in the range of 80 - 90% within the first year after hatching (Permin and Hansen, 1998). In most family poultry flocks, disease is an important problem. Diagnosis, treatment and/or prevention of diseases are of major importance to any attempts at increasing productivity. In commercial production systems chickens are therefore routinely vaccinated against major diseases such as Newcastle Disease, Mareks Disease, Infectious bronchitis, Avian Influenza and others, depending on the specific situations and recommendations in each country.

In order to cause an infection in an animal, diseases need to be introduced from a reservoir of infection where the disease causing organism survives, and where it may also multiply. Transmission may be direct, or may be indirect via a vector, or other intermediate host. Reservoirs may themselves be other living organisms, including other poultry. Reservoirs may also be inanimate, such as water or the soil.

Animate or live reservoirs include:

		Other domestic poultry - chickens, ducks, guinea fowl, turkeys, geese. As well as live birds, this includes dead poultry that are not properly disposed of
		Wild birds (including caged birds)
		Wild animals, including rats and other rodents
		Other domestic livestock
		Humans
		Snails, slugs and earthworms
		Arthropods, including fleas, mites, lice, mosquitoes and other biting flies and also, in some cases, insects that may be eaten by the poultry

Inanimate reservoirs of infection include:

		Feed supplied to the poultry, which may contain fungi, bacteria or toxins
		Water. This is essential, but must be supplied clean. Stagnant or dirty water that includes organic matter may include large numbers of bacteria, fungi or protozoa
		Soil, litter and dust - which may contain spore bearing organisms (e.g. fungi or bacteria) that can invade the body through a wound
		Housing or other structures used by the poultry that are not kept sufficiently clean, or not adequately ventilated.

Poultry diseases can be divided into five groups, namely those caused by bacteria, by viruses, by fungal infections, by parasites, and those resulting from nutritional deficiency. To this list should also be added problems caused by or made worse as result of, poor housing and management and by stress. The tables below are adapted from Permin and Bisgaard (1999), who summarised those disease that were most likely to cause problems under free range or village (scavenging) conditions.

*immunosuppressive disease

• Nutritional Problems

Birds that are fed an adequate diet made up from a good commercial feed are unlikely to suffer from nutritional problems unless there are additional factors involved, such as diseases that result in diarrhoea or otherwise interfering with the digestive system.

Village, backyard or scavenging poultry on the other hand are subject to the availability of food in their normal environment, which may or may not be deficient in vitamins, minerals or other important dietary component.

For a general discussion of Vitamins and Minerals in Smallstock nutrition
For more information on nutritional problems in chickens

• Stress

Any form of stress may result in a disease outbreak, or diseases in individual birds, as a result of a disease causing organism that is not normally a problem. Any disease will also result in stress, and may therefore increase the potential for other diseases. Stress may result in the immune system not being able to cope with the disease challenge. Common forms of stress in commercial poultry systems include:

	Heat stress (any extremities in temperature, both hot and cold).
	Overcrowding, perhaps due to poorly designed housing or inadequate facilities. Fighting for space at a feeder or drinker will result in injury, and in reduced growth or productivity. Overcrowding increases the exposure of disease causing organisms and often increases the opportunity for the growth and spread of these organisms.
	Moving, e.g. from a hatchery to farm or from one building to another.
	Vaccination. Negative reactions from some vaccines may occur up to 10 days after inoculation.
	Temporary, or longer term, shortages in feed or water.
	Changes in feeds or feeding methods, or changes to a daily routine.
	Poor nutrition. Inadequate diets may result in deficiency diseases and general lowering of resistance to infection.
	Parasites (both external and internal). They weaken the bird and provide additional opportunities for the introduction of other disease causing organisms.
	Physical disturbances such as excessive noise or other unnecessary activity.

For more on Stress in Poultry

• Heat Stress

A condition caused by high temperatures, especially associated with high humidity and low air speed. Symptoms include: Panting, legs and wings outstretched, increased thirst, reduced feed consumption, and reduced egg production.

Birds regulate their body temperature by controlling heat loss through their skin and feather cover, and through evaporation by panting. A mature chicken starts panting when temperatures reach about 29 or 30°C, with chicks being affected at higher temperatures. Panting causes a chicken to exhale large quantities of carbon dioxide and this increases the pH of the blood. This causes physiological changes that result in stress, and chickens may stop eating, lay fewer and smaller eggs (with thinner shells). Young birds may stop growing.

Increased air circulation in poultry housing is important, and the provision of adequate shade from the sun is vital. Provide extra water supplies, and make sure that the water is clean.

Cold may also be a serious problem, particularly for chickens.

• Poorly designed poultry houses

• Cannibalism and feather picking

Cannibalism is a problem that is particularly associated with large poultry flocks where birds kept in close confinement peck at each other. This can produce significant mortality in the flock when injury results. It will also cause a decrease in egg production as the hen-pecked birds become stressed.

Some chickens are more likely to engage in cannibalism than others. The problem has a range of causes, which are summarised by Damerow (1994), and include:

• Heat without adequate ventilation.
• Nests and nesting areas not dark enough.
• Crowding, and high densities of birds in areas where they are unable to get away from each other.
• Boredom or lack of exercise (this is more likely in cage birds than in free range birds).
• Feed and water troughs too few or too close together (crowding at the feeding stations).
• Feed that is too high in calories and too low in fibre (they quickly satisfy their nutritional needs and get bored).
• Feed that is deficient in nutrients may result in the birds becoming irritated which can subsequently lead to cannibalism. This is likely to be in the protein levels or an imbalance of sodium.
• External parasites may cause a chicken to pull out its own feathers, and draw blood. This may attract other birds to peck at the area.
• Injury or bleeding from other causes.
  

• Injury

Injuries, when left untreated, are likely to result in infections. Injuries to the birds therefore need to be avoided, and good management practices coupled with well designed housing is an important preventative measure. Overcrowding will commonly lead to aggression between birds - with a high probability of injuries to one or both birds. Similarly, a lack of space at feeders and drinkers will also lead to aggression and injury.

Overcrowding will also lead to stress, and stress can lead poultry to "pick" at one another - eventually resulting in cannibalism (see above).

Free range, scavenging and backyard poultry are much less likely to suffer from problems of overcrowding than intensively reared broilers or layers. However, poultry housing still needs to be constructed to to provide a good environment for the birds - both while roosting at night and when nesting and egg laying. Poorly constructed housing may result in stress and injury.

• Poisoning

Poisoning should not be a problem with chickens, but common sense needs to be used in keeping the birds away from pesticides, herbicides, rodenticides, fungicides, and also from treated seed intended for planting. Other than accidental poisoning with such chemicals, the most likely cause of poisoning is probably from mycotoxins produced by fungi or moulds growing in feed.

• Rodents

Rodents are attracted to poultry houses by the availability of feed, in feeders or spilled on the ground, as well as by the availability of water and by protection and shelter. Rodents can spread diseases, for example through droppings left in feed troughs. Rodents can be a particular problem where there is a raised floor that provides a dark and protected space offering nesting sites. Quite apart from the risk of disease, a rodent problem represents a financial loss through wasted feed.

Chickens are natural predators and will catch and eat smaller mice, but the more agile species and the larger species (e.g. rats) are not controlled by chickens and where there are problems, rodent control measures will be required.