Farmer's lung

FARMER*S LUNG

Farmer*s Lung is an extrinsic allergy which develops in housed adult cattle following their repeated exposure to mouldy hay dust containing fungal allergens. Two clinical forms have been recognised: a sudden onset form in which only individual animals are ill and an insidious onset form in which several animals within a herd may be affected simultaneously.

INCIDENCE

This is the most common respiratory disease of housed adult cattle in Britain, particularly when climatic conditions mean that bad hay has been made.

AETIOLOGY

The inhalation of mouldy hay dust containing antigens can result in an animal becoming sensitised and developing precipitating antibodies. When a sensitised individual inhales antigens, clinical signs develop after several hours and consequently, a hypersensitivity reaction occurs. Recent evidence has shown that dust particles alone can trigger off the condition.

PATHOGENESIS

When hay with a moisture content of 30-40% is baled, the temperature within the bale can rise to around 65⁰C and the thermophilic microflora which then develops is dominated by the actinomycetes, in particular Mfaeni and T. vulgaris. After bales of mouldy hay have been opened and shaken, up to 1,500 million actinomycete spores may be present per cubic metre of air. These actinomycete spores are able to penetrate down to the lungs. The temperature within the lungs is sufficiently high for germination to begin and the enzyme-like substances produced induce the formation of precipitating antibodies. Afterwards, when these enzyme-like substances are inhaled preformed on plant and dust particles, a hypersensitivity reaction may result in the development of obvious signs of respiratory disease 6-10 hours later.

CLINICAL SIGNS

Sudden onset form - Individual animals. Mild to moderate attacks are unlikely to be noticed. Severe attack - sudden onset difficulty breathing in an alert animal. Milk drop is invariably present in lactating dairy cows and anorexia is a common feature.

Insidious onset form - Individuals or Group Disease. Obvious rapid breathing and frequent coughing in a bright cow. Weight loss. Decreased milk yield.

EPIDEMIOLOGY

This disease is most common m the western mainly upland areas of south-west Scotland, the Lake District of England and Wales; these areas enjoy a regular rainfall during the hay making season. Dairy cows are more often affected because they are housed for longer periods. Older cows tend to develop obvious clinical signs towards the beginning of the winter (November, December, January) whereas younger cows are usually first seen to be ill towards the end of winter (February, March, April).

DIAGNOSIS

Farmer*s lung has only been confirmed in cattle being fed mouldy hay. Even severely affected individuals are bright. After appropriate treatment, recovery time is short.

TREATMENT

Betamethasone for two days. NB. This can produce abortion in pregnant cows. Antibiotics should probably be given. Removal of affected individual from source of antigens. Etamiphylline and frusemide if in cor pulmonale. (N.B. Not all products containing etamiphylline are licensed for food-producing animals).

CONTROL

The only way to avoid the development of this disease in areas where the hay is regularly mouldy is not to make hay.

Change to silage feeding

Artificial drying of baled hay

Install fans - practical and cheap.

If farmers lung is confirmed in cattle then the farmer/cattle-man should be asked whether he himself suffers any respiratory trouble or has developed a flu-like respiratory illness which has proven difficult to shake off. If so, then the seeking of medical advice should be advised.

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