Chickens are generally hardy animals and when kept in clean and dry conditions are not overly prone to diseases. However, when an outbreak occurs, it is important that the disease be identified and treated quickly. Firstly, we will take a look at the most common causes of a disease outbreak.

Weather conditions: Obviously, we cannot change the weather, however we can minimise the shock extreme weather can have on the birds. In hot weather, misting the birds and sheds with cool water, providing air movement with fans (or even clever ventilation), supplying reliable and easily accessible cool drinking water will ease the stress. In cold weather, covering open sections of the shed with clear plastic, providing high carbohydrate food and closing sources of drafts all help to keep the birds comfortable.

Parasites: External and internal parasites such as mites, lice and worms all help to weaken the birds' constitutions and make them more susceptible to disease. Such parasites can also be important vectors of diseases themselves. Keeping your birds free from lice, mites and fleas is an important step in preventing blood-borne virii from entering your birds' systems. Regular worming with a variety of drugs keeps their intestinal systems in tiptop shape and allows them to fully utilise their food.

Overcrowding: Over-stocking the hen house is a common problem with novice poultry keepers. Birds kept in overcrowded conditions are far more prone to disease than those kept in less close proximity.

Infection from outside sources: The most common source of diseases is new birds brought in from other places. Obviously, chickens are the main carriers of chicken diseases, so it is important to quarantine for a few weeks, all new stock before introducing them to your main flock. Even taking all these precautions, sometimes disease occurs anyway, perhaps carried by wild birds such as sparrows or starlings, or on the wind or even on your own shoes.

Diseases are broken down into groups. These groups are classified by the various infective agents and we shall look now at the more common and important ones.

Viral Diseases:

Newcastle Disease is an extremely virulent disease which causes high mortality in flocks. It is spread primarily on chicken carcases and chicken meat products, however it can also be carried on clothing, by wild birds and be wind-borne. Affected birds tend to gape and salivate heavily, but without apparent respiratory distress. They usually have a fetid diarrhea of yellowish-white colour, sometimes tinged with blood. Other symptoms are twisted necks, paralysis, loss of balance and circling movements of the head. Vaccines are available, however many countries insist upon a complete slaughter of birds in affected yards to control any further outbreak. Newcastle Disease is particularly dangerous as it can affect nearly all kinds of birds.

Mareks Disease affects young birds between the ages of 9 to 16 weeks. It is caused by a strain of herpes virus. Birds develop drooping wings, lameness and paralysis, particularly of the legs. Another strain of the virus causes internal cancerous growths resulting in quick death, often with no prior warning signs. Mortality in both strains is very high. Birds are usually infected at day old, the virus being transmitted on feather dander (dust), often from the broody hen. Birds which survive the disease become carriers and once Mareks has appeared in your flock, it becomes virtually useless to hatch chicks under hens, an incubator being the only safe way to avoid chick losses. There is no cure for Mareks Disease however day old chicks can be vaccinated.

Fowl Pox is usually transmitted by mosquitoes. There are 2 main strains of the disease, the first appearing as greyish warty scabs on the comb, wattles and faces of the birds. The can result in serious disfigurement making such birds useless for showing. The second strain is more serious, causing cheesy substances to form in the respiratory passages, particularly the throat, which can cause death by asphyxiation. Birds often recover from Fowl Pox but can remain carriers. There is no sure cure, other than treating the scabs with iodine and scraping the muck out of the birds' throats and painting with iodine. A vaccine is available which is applied with a two prong needle into the flap of skin on the wing. Unlike Mareks Disease where the vaccine must be given at day old, Fowl Pox vaccine can be given to birds of all ages.

Infectious Laryngo-Tracheitis (ILT) is a serious respiratory disease. It shows as an acute inflammation of the trachea. Affected birds cough and throw their heads back as if trying to clear their throats. If left to their own devices, death of the affected birds often results. While this disease appears to be viral, some types of antibiotics have been found to help the birds recover more or less with no further recurrence (however this may be due to misdiagnosis).

Infectious Bronchitis (IB) is another respiratory disease which tends to affect younger birds. Vaccination by eyedrop is recommended for both ILT and IB. Hens which recover often lay misshapen eggs for the rest of their lives.

Bacterial Diseases:

Pullorum is an acute infectious disease which once ran rampant. It affects young chicks and can result in 100% mortality. It is caused by a strain of salmonella. Strangely, it seems to affect heavy breed fowls more than light breeds. Pullorum is spread in a wide variety of ways. Feather dander, flies, cohabitation with infected chicks, on clothing and shoes and even infection passed through the ovary and into the egg, where the chick is infected before birth. This makes the disease very serious, particularly on commercial poultry farms. Infected chicks often die without any prior warning, but some may have white, pasty droppings which stick to the vent and which the chicks strain to pass. Others may gasp as if suffocating. Chicks which survive fail to thrive and are not generally useful. Another strain affects older birds which causes eggs to be infected in the ovaries. Such birds are useless as breeders. The disease can be treated by the use of certain antibiotics given in the feed. In Australia, this disease is notifiable, but does not have the same impact on poultry as it once did, due to vigorous testing and veterinary intervention.

Chronic Respiratory Disease (CRD) shows as flu-like symptoms and is often caused by a number of different bacteria. It is found where birds are overcrowded in cold, drafty conditions. Antibiotics are used to clear up this problem.

Coryza is another respiratory disease. Symptoms are swollen eyes, runny nostrils and eyes, with a particularly foul smell. Birds often show stained wings where they repeatedly wipe their eyes. This disease usually occurs when there is cold weather, (change of seasons is a high risk time) dusty conditions and in high pollen seasons. Antibiotics are used to cure this disease, however recovered birds remain carriers and will become continually reinfected. Some breeds and strains of chickens seem to be more susceptible than others.

Protozoan diseases:

Coccidiosis is an important disease of young chicks. It is caused by a number of nematodal coccidia which all chickens are normally in contact with. However when conditions become damp and musty, excessive levels of these coccidia infect the caecal tubes if the chicks resulting in sudden death. An important symptom is blood around the vent or bloody diarrhea. Once noticed, swift action is needed to prevent high losses. Treatment is by administration of sulpha drugs. Fast recovery occurs after treatment begins.

Obviously there are many more diseases which can infect chickens, but these are the more common ones the backyard chicken breeder will encounter. Unfortunately, many different disease of chickens show identical symptoms which makes accurate diagnosis very difficult. Basic management of birds showing disease symptoms, such as separation (quarantine) from other birds, correct disposal of dead birds (burning is best) and veterinary advice all help to minimise the losses a disease outbreak may normally cause. Sometimes, unavoidably, all birds in a flock may have to be destroyed and restocking not carried out for many months.