The best fed and housed stock with the best genetic potential will not grow and produce efficiently if they become diseased or infested with parasites. Therefore good poultry health management is an important component of poultry production. Infectious disease causing agents will spread through a flock very quickly because of the high stocking densities of commercially housed poultry.

For poultry health management to be effective a primary aim must be to prevent the onset of disease or parasites, to recognise at an early stage the presence of disease or parasites, and to treat all flocks that are diseased or infested with parasites as soon as possible and before they develop into a serious condition or spread to other flocks. To be able to do this it is necessary to know how to recognise that the birds are diseased, the action required for preventing or minimising disease and how to monitor for signs that the prevention program is working.

Principles of health management

The key principles of poultry health management are:

  1. Prevention of disease
  2. Early recognition of disease
  3. Early treatment of disease

As much as is possible disease should be prevented. It is easier and less damaging to prevent disease than it is to treat it. However, it must not be assumed that all disease can be prevented. Inevitably, some will get past the defenses, in which case it becomes imperative that the condition be recognised as early as possible to allow treatment or other appropriate action to be implemented as soon as possible to bring the situation under control to limit damage to the flock.

Disease

A disease is any condition that interferes with the normal functioning of the cells, tissues, organs and the whole body systems. Diseases of poultry have many causes and include:

  1. Deficiencies of essential nutrients such as vitamins, minerals or other nutrients.
  2. The consumption of toxic substances such as poisons.
  3. Physical damage e.g. environmental extremes and injury.
  4. Internal and external parasite infestations such as lice and worms.
  5. Infectious disease caused by micro-organisms such as bacteria and viruses.

Diseases that result from nutrient deficiencies, consumption of toxic substances and physical damage are referred to as non-infectious diseases. These diseases cannot be passed from bird to bird and members of the flock must share a common experience for individuals to contract these non-infectious diseases. In the widest sense, infectious diseases are caused by microorganisms that include parasites, fungi, protozoa, bacteria, mycoplasmas, chlamydia and viruses. These diseases are often also called contagious diseases meaning that they can be passed from one bird to another either directly or indirectly.

Direct transmission occurs when one diseased bird passes the cause of the disease via direct contact to a susceptible healthy bird. Such passage may be horizontal transmission (from one bird to another) or vertical transmission (from parent to offspring) via the egg or sperm either inside the egg or on the shell. Indirect transmission occurs when the causal organism is passed from one bird to another via an intermediate host such as insects, earthworms, snails or slugs, wild birds or animals or some other object such as equipment, food or water, vehicles, people, respiratory droplets, litter or faeces.

Causes of infectious disease

Organisms and microorganisms that have the potential to cause harm, such as disease in animals, are called pathogens or disease vectors. There are many different types of pathogens that may be transferred from one bird to another or from one flock to another by many different means. These pathogen types include:

  • Viruses
  • Bacteria
  • Fungi
  • Protozoa
  • Internal parasites
  • External parasites

Viruses

Viruses are the smallest pathogens and can only be seen through an electron microscope. Viruses consist of an outer layer/s surrounding special protein material similar to the genetic material of the cells they invade. They can multiply and do harm only when inside the animal cell and if they invade and damage enough cells, the animal can show signs of that infection.

Antibiotics and other medications as a rule do not affect viruses and, as a consequence, there are very few medications that can treat diseases caused by viruses, although there are times when a drug may be used to control secondary infections. The best way to manage diseases caused by viruses is by quarantine and good hygiene to lower the challenge, and vaccination to maximise the birds’ immunity to future challenges. Some have the ability to survive for very long periods of time in the bird dander and feather debris, litter and manure, insects and rodents.

Bacteria

Bacteria are single cell organisms with a nucleus and multiply by simple fission, which means that one divides into two, and some can do this very quickly inside the host or in a suitable environment. Some are very fragile and do not survive long outside of the host while others may survive for long periods even in a harsh environment. Many have the ability to turn into spores by forming a very tough wall that protects them from most of the materials used to kill them. These types of bacteria are much more susceptible to these compounds when not in the spore form.

Bacteria may be described as being gram positive or gram negative. This characteristic is to do with differences in their cell walls that affects their staining for viewing under the microscope. Whether they are one or the other also influences their response to certain chemicals, including disinfectants.

Different types of bacteria harm the birds in two predominant ways:

  1. Those that attack and damage the birds’ cells or spaces between the cells.
  2. Those that produce toxins or poisons that harm the birds.

There are several antibiotics and other drugs that are effective against different bacteria. However, quarantine and good hygiene that lower the numbers to be targeted by the drugs are the important first lines of defense against these organisms.

Chlamydia

Chlamydia are a little larger than viruses. They live inside the cells they infect particularly in the cells of the respiratory system. They can be treated with antibiotics.

Mycoplasmas

These are single cell organisms slightly larger than chlamydia. They have a cell wall and nucleus. The most commonly known disease caused by this organism is Mycoplasmosis or Chronic Respiratory Disease (CRD) caused by Mycoplasma gallisepticum. Diseases caused by Mycoplasma organisms respond to some antibiotics. These organisms do not survive long outside of the host and good quarantine and hygiene procedures coupled with a suitable house de-population period will provide good control.

Fungi

Fungi are organisms larger than bacteria and are considered to be members of the plant kingdom. They multiply by forming spores that are released and enter the local environment. When conditions are satisfactory the spores start to grow to repeat the cycle.

Fungi harm the birds in two ways:

  1. By being taken into the body e.g. in the respiratory system where they start to grow.
  2. By producing toxins or poisons e.g. in the food. When the birds consume the contaminated food the toxin affects them. A good example of this type of damage is aflatoxin produced by certain moulds or fungi that commonly grow in peanut meal and some litter materials. Moulds or fungi are resistant to nearly all antibiotics.

Protozoa

Protozoa are single cell organisms larger than bacteria. Protozoa have a complex reproduction system that, in many cases, allows them to multiply into extremely high numbers very quickly. A good example of protozoan diseases is coccidiosis of poultry.

Protozoa generally harm the birds by destroying tissue. A number of chemicals have been developed that can be used to treat birds infected by the different protozoans. Others have been developed that interfere with the protozoan life cycle and may be used as preventive treatments while the birds develop a natural immunity. These preventive drugs are often referred to as coccidiastats.

Internal parasites

Parasites are organisms that live off the host. Internal parasites in poultry are multi-celled organisms that live inside the bird usually located in specific organs. Most internal parasites, and particularly those found in Australia, are visible to the naked eye.

While there are many different internal parasites found in poultry, only three are likely to cause harm. These are:

  • Large round worms
  • Caecal worms
  • Tape worms

External parasites

These parasites live outside of the bird. Some spend all of their life on the bird while others spend only some time on the birds. Some cause harm by irritating the bird while others are bloodsuckers that, in sufficient quantity, will cause anaemia. Some of the bloodsuckers often carry organisms called spirochaetes that they inject into the bird while feeding. The spirochaetes may cause harm and tick fever is a good example that can kill many birds.

Prevention of disease

This aspect of poultry management must receive constant, close attention. Failure to maintain a high standard will usually result in an unhealthy flock. The basis of poultry health management is:

  1. The isolation of the flock from disease causing organisms – quarantine.
  2. The destruction of as many harmful organisms as possible – hygiene.
  3. The use of an appropriate vaccination program – trigger the birds’ immune system.
  4. The use of appropriate preventive medication programs – for diseases for which there are no vaccines.
  5. The use of a suitable monitoring program – to monitor for the presence of disease organisms and the success or failure of the hygiene program or the vaccination program.

Quarantine

The principle need is to maintain control over the means of entry by disease causing organisms. These may enter by several routes:

  • Poultry – introducing stock as day old chickens is considered to be the lowest risk method of restocking a poultry farm. Older birds are more likely to be diseased or at least carriers of disease, even if not showing signs.
  • Wild birds/other animals – these often carry the causes of disease and are likely to fly or move from one poultry farm to another if the farms are close enough. The best way to prevent this is to ensure a suitable distance between farms and a minimum of 5 km is recommended. A security fence 2 metres high and with a controlled entry gate should surround the poultry farm and all sheds should be protected from entry by wild birds and all other animals by secure wire netting.
  • Wind – insects and dust carried on the wind from infected to clean farms may also carry the causal organisms of infectious disease. The best way to prevent this is to ensure a suitable distance between farms and a minimum of 5 km is recommended. This distance is influenced by the direction of the prevailing wind. Insects and dust travel further with the wind than against it, and the presence or absence of barriers in the form of hills and high vegetation that catch the dust or insects.
  • People and vehicles – the most common visitors, including vehicles, are very likely to be those that have had contact with other poultry whether they be chicken delivery vehicles, feed delivery vehicles, service people and their vehicles or neighbours in the same business. Entry should only be given to essential visitors and people and vehicles should enter only through a disinfectant wash facility and visitors through a shower/change facility. Disinfectant footbaths and a change of footwear prior to entry to each shed are also recommended. In some circumstances a shower and change of clothing should be required prior to entry to all poultry house. The organisation of staff around the farm is also of importance. Wherever possible, staff should be restricted to one location. However, in some situations there is a need for staff to move from one shed to another. In these cases the principle requirement is to do so in a way that carries the least risk. This means that the normal practice is to move from youngest to oldest flocks on the farm, leaving disease flocks, no matter their age till last.
  • Used equipment – no used equipment should be allowed entry to a poultry farm. If it becomes necessary to allow such entry or to move equipment from one house to another, it should be thoroughly cleaned and disinfected prior to doing so.
  • Food and water – when a diseased bird eats or drinks from a trough it will leave behind contaminated food or water. While it is difficult to prevent this within one pen, if possible, the choice of feeder and drinker may minimise or slow down the transfer of disease from one bird to another. Under no circumstances should open feeders and drinkers extend from one pen to another. All drinkers and feeders should be kept clean even if they have to be cleaned daily.
  • Flies and rodents – in addition to the points raised in relation to distance from other flocks to minimise the movement of insects and animals from one farm to another, all fly and rodent populations should be controlled because they can carry disease causing organisms and pass them on to the stock.

Hygiene

The practice of good hygiene kills microorganisms, including those that cause disease, and all farms carry populations of microorganisms. Therefore, good hygiene practices are an important part of poultry health management. There is an overlapping in the use of the terms quarantine and hygiene.

Good hygiene practices include:

  • The thorough cleaning of poultry houses and equipment after each flock has been removed.
  • The use of vehicle disinfection and wash facilities.
  • The use of foot baths at the entry to each house.
  • The provision of footwear at the entry to each shed.
  • The use of clean litter material after washing the shed and not re-using litter. Litter in the poultry house should be managed to maintain it in a dry friable state without caking or being too wet.
  • Removing all dead birds daily and disposing them in a recommended manner.
  • Maintaining all houses and ancillary buildings and surrounds in a clean and tidy state.

Resisting disease

There are a number of factors that influence whether a bird will succumb to a disease. These include:

  • Genetic resistance of the birds: some genotypes are more resistant than others to infection generally while there are those that are more resistant or susceptible to specific diseases. For example, there are significant differences between at least some genotypes in their resistance to Marek’s Disease.
  • State of well-being of the birds in the flock: birds that are well fed and managed and kept in general good health will have a high level of well-being. Such birds are more likely to fend off an infection than those that have a low level of well-being. The immune system of unthrifty birds is usually significantly weakened.
  • Level of stress in the flock: stress in a poultry flock may be caused by many situations including overcrowding, environment extremes, poor quality food and nutritional deficiencies, harassment and failure by shed staff to react in a timely manner to changing situations in the house. Stress reduces the ability of the bird to fight infection by weakening the immune system.
  • The challenge or numbers of infectious organisms in the bird’s environment: the greater the number or virulence (strength) of the micro-organisms the more likely they are to defeat the birds’ defence and result in a disease. Quarantine and hygiene are the main ways that the number of potentially harmful micro-organisms are kept as low as possible.
  • The level of immunity the birds have: this determines how well the bird can fight invasion by specific infectious organisms. Whether a bird will succumb to an infectious disease depends on the relationship between the number of infectious organisms in the environment and the level of immunity in the bird. The function of the immune system is to defend the bird against invasion by specific infectious organisms.

Many disease outbreaks only occur because there are predisposing circumstances that ensure the success of the invasion by the causal organisms. Stress in the flock is a major factor in this regard. A high level of stress reduces the bird’s ability to fight the invasion by disease causing organisms. Stress, in this regard, may be environment extremes, overcrowding, nutrient deficiencies (even marginal deficiencies), infection, harassment or any other factor with the potential to stress the bird.

In some situations, a primary infection may reduce the ability of the bird to fight invasion by other organisms called secondary invaders or subsequent invaders. In many cases the bird is able to live without harm with the secondary invaders until such time their defences are lowered by the primary infection. It is in this situation that the secondary invaders cause serious harm.

A good example of this situation is the disease, collibacillosis. Collibacillosis is caused by a bacteria called Escherichia coli which is endemic in the environment and, provided normal standards of hygiene are practised and the bird is well nourished and managed, causes no real harm. However, quite often, an invasion by the organism Mycoplasma gallisepticum opens the way for the E. coli bacteria to become virulent, or for more virulent strains to gain entry resulting in the disease collibacillosis.

Disease severity

From the point of view of flock health management, disease in poultry may be one of two levels of severity:

  • Sub-clinical: a sub-clinical disease is one where the signs are not obvious. The birds do not appear to be sick but the infection causes slower growth and/or lower egg production. Sub-clinical disease may predispose to secondary invasion by other organisms. The only evidence that the birds are infected is the lower production efficiency found on an analysis of performance. In many cases this is not found until much of the financial damage has been done.
  • Clinical: a clinical disease is one where the signs that the birds are sick are more obvious. They show the clinical signs typical of the disease with which they are infected. Clinical disease not only affects the performance of the flock but, in many cases, a number of the birds die or never recover to their previous performance level and remain unthrifty.

In each case, affected birds and in many cases recovered birds, are carriers that may be a source of infection for other stock with which they have contact and may transfer the causal organism either directly or indirectly to other stock not involved in this particular outbreak.

Vaccination

Vaccination is aimed at triggering the birds immune system to produce antibodies to fight infection. While not all diseases can be vaccinated against, all potential infectious disease threats should be identified and a suitable vaccination program developed to help combat those that can. Veterinary advice may be necessary to design a suitable vaccination program for each farm.

The keys to effective vaccination are:

  • The potency of the vaccine used and/or its suitability for the disease strain to be controlled.
  • The handling and storage procedures for the vaccine during travel and on the farm.
  • The use of the recommended application techniques.
  • The adherence to the recommended program.

Preventative medication

Vaccines are not available to combat all disease threats. It may be necessary to use a preventive medication to combat infection by some organisms. Veterinary advice may be necessary to determine an appropriate preventive medication program.

Monitoring program

It is not possible to see most infectious agents. Therefore, it is appropriate to have a monitoring program. This may consist of:

  1. Daily checks of the flock.
  2. Regular on-farm and laboratory autopsies.
  3. Blood sampling for laboratory analysis.
  4. Exposing plates and taking swabs for laboratory analysis.

These techniques can be used to monitor the current disease situation including the presence of parasites, the success or failure of cleaning procedures and the success or failure of vaccinating procedures.

Recognising healthy and sick birds

A very important skill for all poultry stock persons to have is the ability to differentiate between healthy and sick birds. It is normal when a flock is diseased to find healthy birds and those with varying degrees of illness. Therefore, it is necessary to be able to tell as early as possible when some of the birds in the pen are sick.

While the manager may be able to identify some diseases from available evidence, it is unlikely that they will be able to identify all. However, the sooner a disease is noticed in the flock, the sooner appropriate action can be taken. This may include initiation of a medication program, send specimens to the laboratory for examination and diagnosis, and/or to call in expert advice. Early action not only gives the manager a chance to cure the condition but it may help prevent it spreading to other stock.

A healthy bird will:

  1. Be active and alert.
  2. Be normal size/weight for the strain, age and sex.
  3. Have no lameness or paralysis.
  4. Have no injuries.
  5. Have no deformities.
  6. Have no discharges from the nostrils or eyes.
  7. Have no stained feathers around the vent.
  8. Have no swellings.
  9. Generally have good plumage related to the whether in a moult or in lay.

A sick bird will show some or all of the following signs:

  • Isolation – sick animals usually seek a quiet place out of the way of others.
  • Hunched stance – sick birds often have a hunched stance with ruffled feathers and eyes partially closed.
  • Diarrhea – usually evidenced by stained feathers in the vent region.
  • Paralysis – of the leg(s), wing(s) or neck.
  • Sneezing and/or coughing; there may be nasal discharge.
  • Blood in the faeces.
  • Swellings – of the joints.
  • Injuries.
  • Loss of weight – this may be pronounced if the condition is a chronic one e.g. Marek’s Disease.
  • Unexpected changes to the food and water consumption – often the first signs of illness.
  • Slower growth or a drop in egg production.

It is normal to find a continuous low level of mortality and a small number of unthrifty birds in the poultry flock. Even though this does occur, attempts should be made to reduce even this “normal” mortality. Normal levels will be in the vicinity of 4% to 40 days for meat chickens, less than 5% to point of lay for layer and breeder replacements and less than 1% per 28 days for layers and breeders.

Source: Health and Welfare, Principles of Health and Welfare: Book 1, NSW Agriculture, MCA, 1995.

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