THE term ‘milk drop’ is now in common usage to describe an acute fall in the milk yield of an individual cow or group of cows, often in the absence of other conspicuous clinical signs. An epidemic of milk drop may follow the introduction of a specific infectious agent, most notably Leptospira hardjo or infectious bovine rhinotracheitis virus, into an immunologically naive herd. These agents may also be responsible for sporadic cases in herds where some degree of immunity already exists. Epidemic or sporadic milk drop may also have a non-infectious cause, such as acetonaemia or ruminal acidosis. The list of potential infectious and non-infectious causes of milk drop is seemingly endless but, in most cases, the fall in milk yield will be accompanied by at least one other obvious clinical sign, such as diarrhoea or respiratory distress. Careful clinical examination, supported where necessary by ancillary laboratory tests, is therefore required to establish a clear-cut diagnosis. This article discusses recent evidence that has pinpointed influenza A viruses as being possible causes of sporadic milk drop syndrome in dairy cows.
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