Aspergillosis

Poultry Diseases

Aspergillosis

Fungi and yeasts may affect poultry by direct infection or via infection of feedstuffs, which may also alter the nutritional composition of the feeds. Direct infection of birds is normally associated with the respiratory tract, with nervous system and eye problems the sequelae of the condition.

Apergillosis is by far the most common mycosis of birds and is an economically important respiratory disease in poultry (Charlton et al., 2013). Losses are incurred through bird mortality and carcass condemnation. Other fungi and yeasts associated conditions include dactylariosis and candidiasis.

Respiratory aspergillosis is not commonly found in modern, clean commercial poultry systems. Infection is by the inhalation of spores, normally associated with litter, bedding and feed. Infection can also originate from infected eggs hatching during incubation and releasing a large number of spores, thus infecting other chicks (Pattison et al, 2008). Infection, particularly in young birds often results in brooder pneumonia (see box below). Although aspergillosis is predominantly a disease of the respiratory tract, other organs can be infected (Beernaert et al., 2010). Systemic infection usually to the brain or eyes, sometimes occurs (Wallace, 1976). Clinical signs are often, non-specific and dependent on the organ involved (Charlton et al., 2013).

Newly hatched chicks have an immature immune system making them more vulnerable to mycosis infection.

The incidence and severity of disease is increased in warm and wet/humid environments, as well as in dusty conditions and in situations resulting in high levels of ammonia production. Contaminated poultry bedding is often the source of infection.

Newly hatched chicks are most susceptible to aspergillosis, which manifests as brooder pneumonia. Although older birds may be constantly exposed, an acquired immunity means that new infections rarely develop unless individuals receive excessive exposure. Stress resulting in immunosuppression may also be a factor in adult infections.

In decreasing order of incidence, spores produced by A. fumigatus, A. flavus, A. niger, A. glaucus and A. terreus are infective to poultry. Aspergillus spores are widely distributed in the environment, found in soil, grains and decaying plant matter and mycosis arises from inhalation of these spores from sources such as equipment, air ducts, contaminated feeds and poultry litter. Wet litter may produce large quantities of spores that are released as litter dries out. The optimum temperature for growth of A. fumigas is 40°C, however few environments impede growth, as it is reported to occur at 9°C and 55°C at a range of 11-96% relative humidity (Charlton et al., 2013).

Chronic mycosis is often seen in older birds and often associated with a suppressed immune system linked to poor nutrition, poor hygiene, bad ventilation and dust. Photo copyright of Mike Suarez, Animal Welfare Approved.

Avian aspergillosis is often classified as either acute or chronic. Acute aspergillosis is thought to be the result of inhaling an overwhelming number of spores, and results in high morbidity and mortality, especially in young birds (Charlton et al., 2013) while chronic aspergillosis is generally associated with situations where the bird has lower levels of immunity (Beernaert et al., 2010) and is often reported in adult breeder birds, particularly in turkeys (Charlton et al., 2013). At post mortem, white foci will be seen in the lungs.

In acute aspergillosis difficult and open-mouthed breathing of hatchery infected chicks can be observed within the first five days. These “gaspers” suffer from obstruction of the respiratory airways. Similar symptoms can  be confused with infectious bronchitis and infectious laryngotracheitis, although these are accompanied by gurgling and rattling noises and there is usually no noise associated with aspergillosis. At post-mortem, gross lesions are observed, particularly in the lungs.

Chronic infection may result in failing respiratory function, blindness, neurological dysfunction or duodenal dropsy. In adults especially, an occasional bird may become affected whilst the remainder of the flock remains apparently healthy. These individual cases arise because of a lowered immunity, or as a consequence of a severe exposure. Immunity can be impaired by factors such as administration of tetracyclines, vaccinations, metabolic disease, inadequate nutrition, overcrowding, starvation, toxicosis, thermal discomfort and traumatic injuries.

The pathological symptoms of an outbreak of aspergillosis in 6.5 week old pullets are described by Barton et al., (1992). Other than a sudden increase in mortality to 0.5% per day, there was no evidence of disease in the flock.

Treating Aspergillosis

There is no effective treatment for this condition.

Good Practice Based on Current Knowledge

• Eggs destined for hatching should be stored so as to reduce exposure to dust that may contain spores
• Handling, transport and storage should aim to avoid the sweating of eggs
• Conditions that promote fungal growth, such as wet litter, should be avoided
• It is also important to clean and disinfect equipment and air ducts in the hatchery and brooder area on a regular basis
• Frequent moving of feed troughs and water dispensers is advised, as these can develop moulds