8 Oct 2021
Keith Cutler
Job Title
Image: Grubärin / Adobe Stock
Although risk-based trading is a concept now firmly embedded within herd health management, it remains a fallible process that can be compromised by the quality and accuracy of the information provided about the health status of individual animals and their herds of origin – it can only be successfully engaged with if the health status of the individual animals and herds of origin can be accurately determined and fully declared.
Of course, animals that are being traded will be healthy – what farmer would offer diseased or infirm animals for sale and what farmer would purchase such animals for addition to his or her herd or flock?
Disease, however, is not the only – or perhaps even the paramount – consideration in risk-based trading. The aim of risk-based trading is to be able to accurately assess the impact on herd health that the addition of an imported animal may have, either immediately or over the longer term.
Even if it appears healthy, a purchased animal may be carrying infectious agents; we are all aware of bovine viral diarrhoea persistently infected animals being unwittingly bought and sold, and the potential for replacement breeding stock – particularly bulls – to be infected with venereal Campylobacter if they are not virgin animals.
Where mycobacterial diseases are concerned, apparently healthy animals may be infectious and shedding bacteria that may infect other animals or, at the very least, contaminate the farm environment following a move into a new herd.
Even if they are not diseased or infectious, purchased animals may be infected with the potential to progress to becoming infectious and diseased at any time.
This has been evidenced recently by comments made in the Welsh Senedd by the Welsh first minister, who confirmed more than 70% of the new bTB breakdown herds in the low-risk area of Wales (meaning the probability of any significant disease challenge from wildlife reservoirs is unlikely) had been traced back to the introduction of purchased Mycobacterium bovis-infected animals into the herd.
Some have interpreted the Welsh first minister’s comments as blaming farmers for trading, whether knowingly or not, M bovis‑infected animals. While this is obviously what has happened, to blame the farmers involved is hardly fair or charitable.
For a bovine animal to be legally traded within the UK, it must originate from an Officially bTB Free (OTF) herd and, if necessary, have given a negative “skin” test result within the 60 days preceding the move.
The testing required to define OTF status – the single intradermal comparative cervical tuberculin (SICCT or “skin”) test – and how OTF status is defined is determined by Government in statute and a series of standard operating procedures.
The logical conclusion, therefore, following the Welsh first minister’s statement is that the testing technology being used – and the testing protocols in place and, indeed, statute – are not fit for the purpose intended: the identification of M bovis-infected animals, and to guide herd and individual animal management towards the eradication of bTB, which is the Government’s stated aim.
This is perhaps not a surprise when even the best estimate of the sensitivity of the skin test, made by Goodchild et al (2015) in a Government-sponsored review, is 80%. This means that, on an individual animal basis, the skin test will fail to identify at least one in every five infected animals (other authors have claimed a significantly lower sensitivity for the skin test, meaning even more infected animals will remain undetected).
Despite this, official policy remains the use of the skin test to determine status and an increased frequency of testing to guide management to eliminate M bovis from endemically infected herds; to paraphrase Albert Einstein: “Only a fool persists with the same course of action in the anticipation of a different result.”
“While debate continues to rage, we continue to deny the facts… deny ourselves the advantages of using newer and more sensitive diagnostic technologies and strategies… and deny the presence of infection – claiming that the absence of disease demonstrates this while, in fact, it continues to spread.”
The skin test has been in common use now for 100 years or more and has been the legally required test to define herd bTB status since the 1950s. It has changed little during this time, despite the advance of science, knowledge and diagnostic technology.
Sir Charles Godfray – in his bTB strategy review published in October 2018 – acknowledged the limitations of the skin test, and advised the development and use of novel, more sensitive diagnostic technologies and strategies to identify infected animals. Such novel technologies – some fully validated, others with work yet to do – now exist.
Phage tests, based on the use of an obligate viral pathogen of mycobacterial organisms to detect viable bacterial pathogens, may provide a highly sensitive and more rapid alternative to even the most advanced culture or PCR technologies, but will not resolve the issue of “infected versus infectious”.
The historic perspective – because of the pathology of mycobacterial disease, with the infectious agent invading and persisting within macrophages – is that cell-mediated immunity is the important immunological response to infection, particularly in the case of M bovis, with antibody production being a late-stage response to infection. This is now known not to represent a full picture of the immunological response following infection with mycobacterial pathogens.
Serological tests, developed to detect specific antibodies produced by an infected animal in response to that infection, offer an alternative to the detection of a cell‑mediated immune response – a delayed‑type hypersensitivity response on which the skin test is based, and the possibility of detecting infection prior to the infected animal becoming infectious and progressing to clinical disease (for example, Koo et al, 2005).
One such test is the Enferplex test. This is a multiplex bTB antibody ELISA test based on 15 antigens arranged in 11 “spots”; the use of multiple antigens has been shown to improve diagnostic sensitivity and specificity (Whelan et al, 2008).
This new test has reached the market as a result of collaboration between Enfer Scientific, which develops and produces diagnostic test kits, and provides diagnostic services to the agricultural industry; MV Diagnostics, which provides immunological expertise during the development of diagnostic tests; and vets in practice who provide epidemiological input.
The antigens selected have been carefully chosen to ensure specificity; the selection of alternative antigens could lead to the development of similar testing technology aimed at identifying animals infected with Mycobacterium avium subspecies paratuberculosis (MAP; the cause of Johne’s disease).
Studies in humans and monkeys infected with Mycobacterium tuberculosis have shown the magnitude of the antibody response and the number of antigens recognised increases with progression towards disease (Kunnath‑Velayudhan et al, 2010; 2012).
Similarly, in the Enferplex test, the number of spots giving a positive antibody result – and the magnitude of those responses – can give an indication of progression from infection towards disease and, therefore, of the risk of transmission of infection within the herd.
The specificity of the test has been shown to be 98.4%, while the sensitivity in M bovis culture-positive animals is 94.2% using anamnestic samples collected between 5 days and 30 days after day 1 of a skin test (World Organisation for Animal Health [OIE] validation dossier).
The greatest sensitivity is obtained when both tests are used together to benefit from the “boosting” effect on the immune system of the purified protein derivative (PPD) used in the skin test on an animal already “primed” by infection.
Although OIE‑validated to international standards, the Enferplex test continues to be regarded by Defra as of questionable use (and “not relevant” in Wales), despite the obvious advantages that its superior sensitivity – especially when used in conjunction with the skin test – would bring over current testing technologies and strategies in the detection of currently undetected, M bovis‑infected animals.
The inevitable increase in the number of M bovis‑infected animals that the use of such a novel diagnostic approach – using a combination of skin and Enferplex testing – would bring may also be regarded with suspicion by some in the agricultural industry, with “false” positive results being claimed to be a consequence of the PPD used in the skin test priming the immune system and resulting in the positive result. No evidence exists for this contention.
For the immune system to be primed, the presentation of antigens to T lymphocytes and B lymphocytes – along with multiple “danger” signals – is necessary. This usually requires virulence, or at least significant “irritation”, which is why almost every inactivated and subunit vaccine – whether for human or animal use – includes a potent adjuvant.
PPD without virulence and the potential to cause disease, and in the absence of any effective adjuvant, has not been shown to “prime” an immune response, either cell-mediated or humoral. Infection with M bovis or MAP, however, can prime the immune system, whether that infection is detectable by the commonly used diagnostic technologies and strategies or not. Once primed, PPD is, however, able to boost the immune response of an already infected animal, resulting in an increased efficiency of detection (a greater sensitivity).
A growing body of evidence supports the boosting by PPD of an immune system primed by infection, particularly in dairy herds carrying out quarterly individual animal milk MAP antibody testing. Antibody titres will vary between sequential tests depending on many factors – including the kit used, the conditions in the laboratory where the test is carried out, stage of lactation and yield.
In herds known to be endemically infected with MAP, they may also increase during the period immediately following a whole‑herd SICCT test, with many more cows being defined as “positive”. This is not a surprise; the literature is full of estimates for the number of subclinically infected animals present for every animal clinically affected with Johne’s disease in an endemically infected herd, with a rate of 20 subclinical (infected) animals being present for every clinical (diseased) case often quoted.
We are fooling ourselves if we interpret this increase in the number of animals testing positive for antibodies to MAP during the period after SICCT testing as false positive results – more so if we then carry out faecal testing and interpret a negative result, the absence of identifiable MAP in the faeces, as support for such an interpretation.
A negative faecal test result tells us only that the causative organism has not been identified or is not currently being shed, rather than that the animal is uninfected.
Of course, for those trying to sell high‑health status livestock – and for a Government with the stated aim of eradicating bTB by 2038 – it is important to demonstrate progress and portray the health status of our cattle in the best possible light.
In the meantime, while debate continues to rage, we continue to deny the facts (remember the Welsh first minister’s statement), deny ourselves the advantages of using newer and more sensitive diagnostic technologies and strategies (as recommended by Godfray), and deny the presence of infection – claiming that the absence of disease demonstrates this while, in fact, it continues to spread.
