Important viral diseases of poultry
Viral diseases are some of the most important infectious diseases affecting poultry. They are characterized by not being able to be treated, but most can be prevented with vaccines. The more important viral diseases are outlined below.
Newcastle Disease (NCD)
This disease spreads rapidly via airborne droplets spread by the coughing or sneezing of infected birds. The virus can be carried by wild birds, through contaminated eggs, and on clothing. As mortality is often 100 percent in young chickens, NCD is probably the most important constraint to family poultry development. Birds of any age can be affected, although young ones are more susceptible. Mortality in older chickens is usually lower, but egg production is usually severely reduced.
The incubation period of three to five days is followed by dullness, coughing, sneezing and gasping. Rapid breathing is accompanied by a gurgling noise in the throat. The respiratory signs usually develop first and are sometimes followed by nervous signs, characterized by twisting of the neck, sometimes combined with dragging of wings and legs. Depending on the environment and the degree of resistance of the birds, not all symptoms may be shown, or they may be in a mild or subclinical form. The twisting of the neck occurs only in birds that survive. Early loss of appetite results in a greenish diarrhoea. The most obvious diagnostic sign of NCD is very sudden, very high mortality, often with few symptoms having had time to develop. Diagnosis of NCD can be difficult from just the symptoms, as they are so varied, and as many other diseases share the same symptoms.
The high incidence of ND among family free-range flocks is due to the following factors:
- The prevalence of virulent strains
- Continuous contact with other domestic and wild species of birds (such as ducks and pigeons) which can carry the virus without showing the disease
- Uncontrolled movement of birds between villages.
There is a seasonal pattern to outbreaks of NCD, influenced by:
- the arrival of migratory birds;
- changes in climatic conditions leading to stress, which predisposes birds to the disease;
- hot, dry and windy periods, which encourage airborne spread of the virus; and
- overuse of the few supply points of water available (during the dry season), which then become heavily contaminated with the virus.
Fowl pox is still prevalent in many poultry flocks for the following reasons:
The fowl pox virus can remain alive in the pox scabs (which have fallen off the birds) for up to ten years, which contaminate the environment.
Mosquitoes and other blood-sucking insects can transmit the virus.
The disease tends to be seasonal, occurring after mosquito breeding times.
Infection occurs early in life, and once a bird is infected, it can shed the virus in skin flakes throughout its life, if it survives. Clinical signs occur in young growing birds in the Acute Marek’s Disease (MD) form, characterized by high mortality from visceral tumours. Another peak of mortality occurs in the Classical MD form, characterized by nerve paralysis in the legs and wings of birds aged from 15 weeks to early in the laying period.
Mycoplasmas are not classified as bacteria or viruses, but as Pleuro-pneumonia-cocci-like organisms (PPLO). These are primarily associated with Chronic Respiratory Disease (CRD), a complex syndrome caused by Mycoplasma gallisepticum in partnership with bacteria (often E. coli), fungi and viruses (often Infectious Bronchitis). M. gallisepticum can be transmitted through the egg. Multi-age flocks, nutritional deficiency and water deprivation are important factors in the epidemiology of the disease in rural poultry flocks.