Mar 2002 Page 11

       nfectious disease is one of a number of
       factors that can affect performance in
       broiler flocks. Modern industry practices such as stringent hatchery sanitation programs, widespread vaccinations, use of feed additives and bio-security precautions have evolved to provide in most cases an effective shield against serious losses caused by disease.
However, growers must still be on guard
against outbreaks of infectious diseases. The purpose of this article is to outline the basic facts Alabama growers need to know about the most common broiler flock diseases and the management practices that are most helpful in preventing disease problems.

Types of Diseases
     Four basic categories of infectious diseases are seen on broiler farms. They are classified according to the organisms that cause the diseases, as viral, bacterial, fungal, or parasitic. Some of the categories are represented by only a few significant species or strains, others by several to many. Viruses can only reproduce in the chicken, whereas most fungal and bacterial species may reproduce in the environment as well as in the chicken. Parasites must leave the host in order to complete their life cycle in the litter and then may re-infect other chickens. The viruses in particular, and most bacteria, are contagious and may move directly from bird to bird. This is not true of the fungal and parasitic infections. Fungal infections are seen primarily in young chicks, whereas all other categories may infect broilers at any age. Only laboratory analysis can provide reliable
diagnosis of specific diseases. Some infectious diseases result in unique

signs or symptoms, but many do not. Further, some symptoms may arise from multiple causes. What growers can do and need to do is maintain careful observation of the flock and of bird behavior. If any abnormalities do arise, the general category of the disease problem usually can be identified. Steps then can be taken to suppress or at least limit the spread of the disease problem.

Viral Infections
     A typical broiler flock will be exposed to many viruses. Planned immunization has been very successful in reducing their impact in broilers.
     Vaccines for the most significant viral diseases are administered to the parent flocks and their protective antibodies are passed in the egg to the hatchling broiler chick. The chicks are then usually hyper-immunized at the hatchery and also again in the field.      “Airsacculitis” is one of the more common conditions, and may be caused by a combination of viral and bacterial causes. Watery eyes, coughing, and generalized respiratory distress are common symptoms. Severe cases may spread to the gut and result in condemnations and mortalities. Although improper vaccine selection or administration may be involved in triggering airsacculitis, it is usually the result of poor air quality, improper temperatures, poor drinker management, and/or severe environmental stress.
      Some viral diseases also can have the
effect of suppressing the bird's immune system. As a result, a flock may develop other
unrelated infections. A damaged immune
system can also make vaccinations ineffective.
Preventing this serious situation from developing starts with the vaccination program.
      However, good flock management to
eliminate stress is also extremely important.

Adequate ventilation in terms of air quality
and quantity dilutes virus particles and
reduces litter moisture for controlling the
secondary bacterial infections. Removal of
dust and feather follicles from house equipment between grow-outs also helps reduce viral causes.
Growers should be aware that viral infections tend to be highly contagious and may move from bird to bird, house to house, or farm to farm. Bio-security and sanitation precautions are very important in combating viral diseases. Continued poor performance in successive flocks, lack of bird uniformity, and excessive disease problems from a variety of infections are signs of immune-suppressive viruses. The integrator usually conducts testing to assess the immune status of representative flocks on a complex basis.

Bacterial Infections
Most bacteria are opportunists by nature and may infect the chicken through any point of entry and then invade a variety of tissues or organs. Often the symptoms produced by bacterial infections are not unique and their exact identification requires lab diagnosis. Massive infections, which are rare, may produce generalized infection among different tissues and organs, resulting in mortalities, often with an extremely putrid odor. Good litter management and removing dead birds as soon as possible are effective in avoiding or limiting problems from these bacteria. Bacterial infections that originate in the hatchery are most commonly found in the navel area or respiratory system (lungs). Navel infections in day-old chicks are usually characterized by a foul odor, excessive culls, and occasionally high mortality in the first three days. Respiratory tract infections may result in