Replacing the TB skin test with mandatory blood testing across all herds is not the solution to worsening TB trends, the Department of Agriculture has said.
TDs and senators sitting on the Joint Oireachtas Committee on Agriculture queried top Department officials on the reasons for gamma interferon blood testing not having replaced the traditional skin test.
The Department’s senior superintending veterinary inspector Damien Barrett warned that a test switch from skin to blood would “balloon” reactor numbers, as there would be a jump in the number of cattle slaughtered on foot of false positive results.
The skin test picks up a lower proportion of TB-infected cattle, but turns up fewer false positives, Barrett explained.
False positives
“On the point of specificity – that is showing up false positives – the skin test is superior. It has a specificity of about 99.5%, which means that about one in every 5,000 is a false positive,” he said.
“The difficulty with the skin test though is that the sensitivity is no better than 80%, which means that out of every 10 infected cattle, it is not detecting two, when it is performed to a high standard.”
The official said that where it suspected that there is cattle-to-cattle transmission occurring or where there has been undetected infection, blood testing is used.
“The trouble is that if you roll that out over 7m cattle, you are going to get an awful lot of false positives on that blood test and that is the big issue on the blood test – the lack of specificity and false positives,” Barrett continued.
“The net result of that, if we deemed animals that failed the blood test as reactors, would be that reactor numbers would balloon if we carried out the blood test on a wide scale.”
That is the big issue on the blood test – the lack of specificity and false positives
Numerous committee members questioned how New Zealand has made such significant progress with TB eradication, while Ireland’s disease metrics are on an upwards curve.
Barrett stated that risk-based trading - only allowing herds to buy in cattle from herds of equivalent or superior TB status – and blood testing to “quite a substantial level” are among the measures that contributed to New Zealand’s success.
Badgers
Wildlife proved itself another hot topic for the committee members, many of whom pinned the uptick in reactor numbers and herd incidence witnessed since around 2018 on badgers and deer.
“For the most part, our evidence would show that badgers are the main reservoir of infection in wildlife, but we have a multi-host system where it is a combination of infections from badgers and cattle and we cannot cherry pick between either,” Barrett said.
“They are the main [wildlife] drivers and given that we have about 7m cattle in the country and about 120,000 to 150,000 badgers, based on those figures the driver of infection is coming from cattle.”
Read more
Minister proposes sweeping new TB measures
Department to scrap pre-movement TB testing proposal for suckler cows
Spiralling TB costs could cost Department €180m next year
Replacing the TB skin test with mandatory blood testing across all herds is not the solution to worsening TB trends, the Department of Agriculture has said.
TDs and senators sitting on the Joint Oireachtas Committee on Agriculture queried top Department officials on the reasons for gamma interferon blood testing not having replaced the traditional skin test.
The Department’s senior superintending veterinary inspector Damien Barrett warned that a test switch from skin to blood would “balloon” reactor numbers, as there would be a jump in the number of cattle slaughtered on foot of false positive results.
The skin test picks up a lower proportion of TB-infected cattle, but turns up fewer false positives, Barrett explained.
False positives
“On the point of specificity – that is showing up false positives – the skin test is superior. It has a specificity of about 99.5%, which means that about one in every 5,000 is a false positive,” he said.
“The difficulty with the skin test though is that the sensitivity is no better than 80%, which means that out of every 10 infected cattle, it is not detecting two, when it is performed to a high standard.”
The official said that where it suspected that there is cattle-to-cattle transmission occurring or where there has been undetected infection, blood testing is used.
“The trouble is that if you roll that out over 7m cattle, you are going to get an awful lot of false positives on that blood test and that is the big issue on the blood test – the lack of specificity and false positives,” Barrett continued.
“The net result of that, if we deemed animals that failed the blood test as reactors, would be that reactor numbers would balloon if we carried out the blood test on a wide scale.”
That is the big issue on the blood test – the lack of specificity and false positives
Numerous committee members questioned how New Zealand has made such significant progress with TB eradication, while Ireland’s disease metrics are on an upwards curve.
Barrett stated that risk-based trading - only allowing herds to buy in cattle from herds of equivalent or superior TB status – and blood testing to “quite a substantial level” are among the measures that contributed to New Zealand’s success.
Badgers
Wildlife proved itself another hot topic for the committee members, many of whom pinned the uptick in reactor numbers and herd incidence witnessed since around 2018 on badgers and deer.
“For the most part, our evidence would show that badgers are the main reservoir of infection in wildlife, but we have a multi-host system where it is a combination of infections from badgers and cattle and we cannot cherry pick between either,” Barrett said.
“They are the main [wildlife] drivers and given that we have about 7m cattle in the country and about 120,000 to 150,000 badgers, based on those figures the driver of infection is coming from cattle.”
Read more
Minister proposes sweeping new TB measures
Department to scrap pre-movement TB testing proposal for suckler cows
Spiralling TB costs could cost Department €180m next year
This is a subscriber-only article
SHARING OPTIONS: