Given the recent rise in temperatures, the threshold at which the bluetongue virus can become active in midges, has likely been reached in the warmer parts of the country such as the southeast, the Department of Agriculture has warned.

As a result, infected midges which survived the winter and spring will have the potential to become infective and can transmit the virus to a naïve ruminant host – such as cattle or sheep.

For other parts of the country, especially more northern parts and those at higher altitudes, it is unlikely that the cumulative temperature threshold has yet been reached, the Department said.

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However, with temperatures to rise this week, this will likely change.

Threshold

For a midge to become infective with bluetongue virus and capable of transmitting to a susceptible ruminant host, the Department has said that there must be an average daily temperature of greater than 12°C over a period of time.

A “degree-day” is a 24-hour period where the temperature is 1°C above 12°C.

Approximately 50 “degree-days” are required for the viral replication phase of bluetongue within the midge host.

For example, a 24-hour period where the temperature is 20°C would equate to eight “degree days”. Therefore, approximately six full days at a consistent temperature of 20°C would provide the 50 “degree day” threshold, it said.

Increasing risk

As the risk of bluetongue transmission is increasing with the warmer weather and the possibility of an incursion from England and Wales, the Department of Agriculture is now encouraging any farmers who have not yet vaccinated stock to consider doing so.

Since bluetongue was first detected in January in county Wexford, cases have been identified in herds in the following counties: Wicklow, Louth, Laois, Monaghan, Kildare, Cork, and Tipperary.

“Investigations indicate these cases are historic cases from last autumn and not active infections,” the Department said.