Viral Diseases of Poultry

Viruses are responsible for some of the major and most virulent diseases in poultry, a well known example being Newcastle Disease. Some viruses capable of infecting poultry are of particular concern since they may also be able to cross the species barrier and infect other livestock, as well as man. An example of the latter is Avian Flu. Viruses can cause disease in several different ways: by disrupting or destroying cells; by invading and disrupting the immune system, or activating the immune system, resulting in fatigue and fever; by triggering the development of antibodies that lead to inflammation and tissue damage; and by interacting with chromosomes to cause a tumour. After a bird is infected by a virus, the immune system attempts to fight the virus. This may take a few days or several weeks and the bird is then immune to further infection by that particular strain of the virus. The bird may, however, continue as a carrier and infect other birds that have not been exposed. Some viruses rapidly cause major damage and death occurs before the immune system is able to cope with the infection. Some other viruses weaken the immune system to the extent that the bird is then open to secondary infection.

Marek's disease Avian Leukosis Newcastle disease Fowl Pox Infectious Laryngotracheitis Infectious Bursal Disease, "Gumboro" Avian Flu / Avian Influenza References and Further Reading

The virus responsible for Marek's disease is thought to be responsible for greater mortality in chickens than any other disease. Every flock, except for those maintained under strict pathogen-free conditions, can be assumed to be infected. Flocks that do not show any symptoms may still be infected. Stress may weaken the immune systems and result in an outbreak of this disease. Although symptoms may not be apparent, there may be a reduced growth rate and a decrease in egg production.

The virus is concentrated in the feather follicles and shed in sloughed skin and feather cells. It has a long survival time and viable virus can be isolated from houses that have not been occupied for many months. Transmission is by aerosols containing infected dust. Younger birds are most susceptible to infection.

There is no treatment for Marek's disease. The main method of control is by vaccination of day-old chickens as a subcutaneous injection. Results are improved by strict sanitation to reduce or delay exposure and also by breeding for genetic resistance.

Marek's disease may produce a variety of clinical responses: visceral, neural, ocular, skin or combinations of these.

	Visceral	Marek's of the visceral type is characterized by widespread involvement with lesions commonly seen in gonads, liver, spleen, kidney and occasionally heart, lungs and muscles. The disease is often acute, with apparently healthy birds dying very rapidly with massive internal tumours.
	Neural	The neural type is characterised by progressive paralysis of the wings, legs and neck. Loss of body weight, anaemia, laboured breathing and diarrhoea are common symptoms.
	Ocular	Ocular leukosis (gray-eye) is usually seen in early maturity. Morbidity and mortality are usually low but may approach twenty-five percent in some flocks. It is characterized by the spotty depigmentation or diffuse graying of the iris in the eye. The pupil develops an irregular shape and fails to react to light. Emaciation diarrhoea and death follow.
	Skin	Skin leukosis produces the most severe losses in intensively reared broilers, as a result of carcasses being condemned at the processing stage. Enlargement of the feather follicles due to accumulations of lymphocytes is the typical lesion. This is potentially the most infective version of Marek's because it is produced around the feather follicles and is shed with the skin particles and with feathers.

A widespread infection in chicken flocks, Avian, or Lymphoid Leukosis is caused by members of the leukosis/sarcoma group of avian retroviruses and is normally a disease of adult chickens. Transmission of the virus from infected birds is via both the eggs (vertical transmission) and via the faeces (horizontal transmission). The virus may also be transmitted mechanically by blood-sucking parasites.

Leukosis characteristically produces lymphoid tumours, particularly in the liver and spleen. The tumours may also affect other organs such as ovary and lungs. Affected birds show loss of appetite, progressive emaciation and diarrhoea, and usually die. There is no treatment or vaccination, but there are a number of precautionary measures that can be taken to help control the levels of infection within a flock:

• Purchase resistant breeds or strains of birds,
• Brood in isolation and do not mix birds of different ages, especially up to six weeks of age,
• Keep the facilities clean and disinfected,
• Control blood-sucking parasites,
• Limit stress to a minimum,
• Provide adequate rations.
  

A major constraint to production of village chickens in many developing countries is Newcastle disease. Various strains of the Newcastle disease virus are capable of causing 100 % mortality in unprotected flocks. Outbreaks of Newcastle disease are unpredictable and effectively discourage villagers from paying proper attention to the husbandry and welfare of their chickens - with consequent impacts on human health, and the household economy.

However, in areas where Newcastle disease is endemic, control through vaccination is generally a very cost-effective intervention.

For more on Newcastle disease, see the following publications:
Alders, R. and Spradbrow, P. (2001). Controlling Newcastle Disease in Village Chickens: A Field Manual. ACIAR.
Alders, R., dos Anjos, F., Bagnol, B., Fumo, A., Mata, B. and Young, M. (2002). Controlling Newcastle Disease in Village Chickens: A training Manual. ACIAR Monograph No. 86 128 pp.
Young, M., Alders, R., Grimes, S., Spradbrow, P., Dias, P., da Silva, A. and Lobo, Q. (2002). Controlling Newcastle Disease in Village Chickens: A Laboratory Manual. ACIAR Monograph No. 87 142 pp.
Epidemiology of Newcastle Disease and the Economics of its Control

Fowl pox is a viral disease which is spread by the bites of mosquitoes and other bloodsucking insects. The incubation period is between 1 and 2 weeks. The infection leads to the formation of wart-like nodules on the non-feathered parts of the head and legs and occasionally to similar lesions or canker in the mouth, nose and throat. In addition to the wart-like nodules, the disease is characterised by fever, listlessness, reduced egg production, and late growth of young birds. Fowl pox can also be spread directly by infection of small wounds in the skin and mouth of the bird. Overcrowding birds increases the risk of injury and may result in fighting and pecking. Correcting this aspect of management will reduce the risk of spread. The virus cannot enter intact skin but can survive for a long time in infected material such as scabs and litter.

Treatment is considered to be of  little value. Lesions normally heal within a few weeks, but in severe cases it may be necessary to remove scabs and treat with antiseptics. Prevention is firstly by reducing exposure to mosquitoes.  This can be done by screening sheds and by dealing with mosquito habitats. In commercial production systems where Fowl Pox is a problem, vaccination is a possible and effective measure. Precautions should be taken when administering the pox vaccine as it is a live type of virus vaccine. Because the pox vaccine produces a mild form of the disease, only healthy birds should be vaccinated. For further details see Butcher and Rossi 1990).

Infectious laryngotracheitis is a contagious respiratory disease caused by a virus of the herpes family, and which is characterised by gasping, neck extension and inflammation of the membrane around the eye. The virus may live for 8 to 10 days in droppings and up to 70 days in carcasses, but is not spread through the eggs. People and contaminated equipment can also introduce infection into a flock. There may be rapid airborne transmission among birds in close contact, but the virus normally requires mechanical transfer to cover greater distances such as between buildings.

The main symptoms are gasping, coughing and sticking the neck forward and upwards with each breath in an attempt to try and clear the mucous which builds up in the trachea. Some birds may die as a result of suffocation.

Vaccination keeps the disease from spreading, but causes the vaccinated birds to become carriers (vaccinated birds will infect any unvaccinated birds they come into contact with). As a result, vaccination is not recommended unless the disease is particularly prevalent in the area.

Infectious Bursal Disease (IBDV) is an immuno-suppressive disease of domestic poultry and occurs when the virus invades lymph tissue, especially the cloacal bursa. It results in atrophy of the cloacal bursa, reduced immunity and increased susceptibility to other infections. Another result of infection is a failure to develop an immune response to vaccines. Immunosuppression resulting from an IBDV infection is the underlying cause of many cases of respiratory and enteric disease in chickens.

Chickens infected with IBDV shed the virus in their faeces, and the feed, water, and litter become contaminated. Infection can also be through contaminated equipment. Other chickens in the house typically become infected by ingesting the virus. The disease tends to appear suddenly in large flocks of broilers, and when it does most of the flock is involved. Symptoms include whitish diarrhoea that sticks to the vent feathers - causing birds to pick at their own vents. Infected birds may not move much, suffer a loss of appetite and will become dehydrated. Deaths may occur after about a week, but survivors recover quickly. Chicks exposed to the virus before they are two weeks old develop an immunity. Immunisation is the principal method to control IBD. Immunisation of breeder flocks is especially important, to confer maternal antibodies that will protect chickens from early infection.

Avian influenza was first identified over 100 years ago during an outbreak in Italy. Since then, the disease has cropped up at irregular intervals in all regions of the world. Once domestic birds are infected, avian influenza outbreaks can be difficult to control and often cause major economic impacts for poultry farmers in affected countries, since mortality rates are high and infected fowl generally must be destroyed in order to prevent the spread of the disease.

Avian influenza is caused by type A influenza virus. The symptoms can vary from a mild disease with little or no mortality to a highly fatal and rapidly spreading epidemic depending on the strain of the infecting virus, the host that is infected, as well as environmental and other stress factors. More strains of avian influenza virus have been isolated from ducks than from any other species although most free-flying birds can be infected. Waterfowl, including for example wild ducks, are generally more resistant to avian influenza than are domestic poultry. Viruses that cause no obvious disease in waterfowl can be highly pathogenic in domestic poultry. Turkeys are more commonly infected than are chickens.

Waterfowl act as a reservoir of avian influenza virus by carrying the virus in their intestinal tract and shedding it in their faeces. The viruses are spread to susceptible birds by inhalation of infective particles and from contact with the faeces of infected birds.

Symptoms include coughing, sneezing, ruffled feathers, swollen heads, nervous signs such as depression, and diarrhoea. These may occur together or singly. In some cases, birds die rapidly without clinical signs of disease. In other cases, there may be no symptoms and the birds will develop antibodies to the particular strain of the virus.

Wild birds and their excreta should be considered a major potential source of avian influenza. Preventing direct contact with free-flying birds and preventing contact with the faeces of wild birds are important precautionary measures.

In most cases, the influenza viruses that infect birds do not infect humans. Similarly, those that infect humans generally do not infect poultry. However, the possibility of the avian influenza virus crossing the species barrier and infecting humans does exist and in such cases the resulting infection in humans may be highly pathogenic.

Once domestic birds are infected, avian influenza outbreaks can be difficult to control and often cause major economic impacts for poultry farmers in affected countries, since mortality rates are high and infected fowl generally must be destroyed -- the technical term is "culled" -- in order to prevent the spread of the disease.

Asian Bird Flu

Asian Bird Flu is the name commonly used in the media to describe a deadly form of the avian influenza virus that is causing a potentially serious outbreak in a number of countries in Southeast Asia, and which is spreading to other parts of the world. This strain of avian influenza is known as H5N1 avian influenza virus and is a type of highly pathogenic avian influenza virus that causes a severe disease in poultry. The Asian Bird Flu is unusual because it is extremely deadly for poultry, it has spread to many different countries in Asia, has caused disease outbreaks in wild birds, and has also caused a number of human infections and deaths. These features, especially the ability to cross the species-barrier and infect humans, make this strain of avian influenza different from and potentially more dangerous than any other outbreak of avian influenza that has been recorded in the last 60 years.

For more information on Avian Influenza, see the FAO web site, and a manual prepared by the FAO on Prevention and Control of Avian Flu in small scale poultry, and the FAO's Avian Influenza disease card.