Common Infectious Diseases in Backyard Poultry

Marek's disease (MD) is a common viral disease of chickens, both in commercial production and backyard flocks (see Marek’s Disease in Poultry). The primary lesions are tumors of the viscera, muscle, skin, and peripheral nerves. Nerve lesions can be an early indicator of the disease and result in a condition termed "range paralysis." Birds with visceral tumors often have cachexia as the only clinical sign. Tumors of the muscles and skin are frequently palpable. Tumors that affect the eyes (ocular Marek’s) could be seen as a grayish color change in the pupils or irregular margins of the pupils, with lack of proper pupillary light reflex. MD cannot be treated but can be prevented by vaccination at hatch. When backyard poultry are acquired or hatched onsite, every attempt should be made to vaccinate for MD. Vaccinations may not be effective if administered to birds >1–2 wk old. Clinical MD generally affects birds 4–14 wk old; however, it is not uncommon in older birds, and death loss is often sporadic rather than explosive. If tumors are found in the viscera of deceased birds, carcasses should be submitted to a diagnostic laboratory for differential diagnosis between MD and avian leukosis (see Lymphoid Leukosis in Poultry), another common lymphoid tumor disease. Avian leukosis is seen in birds >14 wk old, and tumors are similar to those found with MD. Avian leukosis has no treatment or vaccination.

Infectious bronchitis virus (IBV; see Infectious Bronchitis) causes a rapidly spreading respiratory disease in young chicks. Production is reduced and egg shell abnormalities are seen in laying hens. Certain strains of IBV also cause kidney disease. Chicks infected early in life may have permanent damage to the oviduct, so they do not produce eggs or become false layers (see False Layer (Poultry)). IBV is highly transmissible, but most birds recover with supportive treatment. Antibiotics can be administered in the water to prevent secondary infection. Vaccines are available; however, backyard chickens are usually not vaccinated unless they come in contact with other chickens.

Newcastle disease virus (NDV; see Newcastle Disease and Other Paramyxovirus Infections) affects numerous species of birds and is the reason for quarantine regulations for birds entering the USA. Exotic NDV is highly fatal and is not present in the USA at this time. Past outbreaks have resulted in the slaughter of thousands of birds. Milder forms of NDV are present in the USA and are primarily characterized by respiratory disease and a drop in egg production. Mortality is variable and depends on the strain of the virus. As with infectious bronchitis virus, vaccination is available but is generally given to backyard poultry only if exposed to other birds.

Fowlpox virus (see Fowlpox) causes crusty and nodular lesions primarily on the unfeathered portions of the bird. Occasionally, poxvirus can cause lesions in the mouth and trachea, causing death due to suffocation (wet form). If the bird recovers, immunity is generally lifelong. Not all pox outbreaks are caused by fowlpox virus but can be caused by related strains such as turkey pox, psittacine pox, quail pox, etc. Strains are usually species specific but can occasionally affect other species (eg, pigeon pox). One strain may not cross-protect with another. Vaccination is available and should be given to flocks on premises with a previous history of pox or with presence of pox in nearby birds. Poxvirus is transmitted through contact of infected lesions with open wounds and by insect bites (mosquitoes), and insect control is key to prevent spread.

Avian encephalomyelitis (AE; see Avian Encephalomyelitis) is seen in chickens, turkeys, pheasants, and quail. It primarily affects chicks 1–3 wk old. Nearly all commercial flocks are infected, but clinical disease is low because of maternal antibodies. AE can be transmitted vertically in eggs laid between 5 and 13 days after infection and is an enteric infection under natural conditions. The spread is more rapid in floor-raised birds than in cage-raised birds. There is no treatment, and vaccination of breeders (both chicken and turkey) for maternal antibodies to protect the young during early life is critical to prevention. Because many specialty breeders, particularly those that sell stock to an intermediate supplier, do not vaccinate, AE is a fairly common viral disease in backyard poultry. Vaccination should be given after 8 wk of age but by at least 4 wk before production.

In general, Salmonella Pullorum (see Pullorum Disease in Poultry) and S Gallinarum (fowl typhoid; see Fowl Typhoid) cause the greatest problem for poultry, whereas S Typhimurium, S Enteritidis, S Heidelberg, and S Kentucky are important in terms of public health. S Pullorum is egg transmitted, causes a diarrheal disease in young chicks and poults, and results in high mortality. Adult birds are asymptomatic carriers. Diagnosis is based on disease history and isolation of the bacteria. Prevention is achieved by purchasing birds from a breeder flock that is NPIP certified (National Poultry Improvement Plan) clean of S Pullorum and typhoid. Treatment is not recommended, because it can cause birds to become carriers. Fowl typhoid is seen in chickens, turkeys, and many other game and wild birds. Fowl typhoid is similar in disease presentation and diagnosis to S Pullorum, although mature birds can show clinical signs of fowl typhoid. Clinical signs are infrequently observed in poultry infected with S Enteritidis and S Typhimurium, although most paratyphoid Salmonella infections are asymptomatic in most poultry. Flocks can be monitored by obtaining egg samples and environmental samples to culture the organism.

Colibacillosis (see also Colibacillosis) is caused by Escherichia coli and is usually secondary to other infections such as infectious bronchitis virus and mycoplasmosis. E coli is seen in most species and age groups. A wide variety of clinical signs affecting the respiratory, reproductive, and intestinal systems can be seen. Vigorous adherence to biosecurity and sanitation programs can effectively prevent the organism from causing disease. Many antibiotics can be used for treatment, and sensitivity to the antibiotic should be tested. Treatment is usually successful if the disease is in the early stages.

Chronic respiratory disease in poultry (primarily chickens and turkeys) is generally caused by Mycoplasma gallisepticum infection (see Mycoplasmosis). M gallisepticum is a reportable disease in turkeys in select states in the USA. Pathogenicity of M gallisepticum is enhanced by infection with other organisms. Clinical signs of respiratory disease develop slowly in a flock, and feed consumption drops. Infection of the sinuses with purulent exudate (swollen face) is common in turkeys. Serology and isolation and identification of the organism can be used for diagnosis. Prevention, as with the salmonellae, rests with establishment of a clean flock by eliminating the infected flock, completely sanitizing the premises, and obtaining clean stock. Vaccination is available on a state-by-state basis. Treatment is expensive, and the disease often recurs after treatment is stopped. Other important mycoplasmas in poultry include M synoviae (infectious synovitis) and M meleagridis (venereal infection and airsacculitis).

Aspergillosis (see also Aspergillosis), or brooder pneumonia, is seen in many poultry and other species of birds. Birds <3 wk old are most commonly affected, and infection is obtained from hatchers or brooders contaminated with fungal spores. Morbidity is variable, and mortality can be high in clinically affected birds. Culturing the fungus or demonstrating typical fungal hyphae in fresh preparations from lesions are used for diagnosis. Prevention is accomplished by thoroughly cleaning hatchers, incubators, waterers, feeders, and ventilation fans and by keeping litter clean and dry. Treatment is expensive and may not be effective. Ketaconazole and nystatin have been used.

Favus, or ringworm, also known as white comb, is a fungal disease caused by Microsporum gallinae, which is of minor importance in all fowl, especially chickens and turkeys. Affected birds have small, white, chalky deposits on the comb, which can enlarge and coalesce to form a dull white, moldy layer that could be several millimeters thick. The disease is self-limiting, and the comb heals after several months. Typically, if the disease is limited to the comb, the health of the bird is not affected, but if feathered portions are involved, the bird may become emaciated and die. Favus is a public health concern.