Saturday, July 28, 2007

£2.8 million - The Pathman project.

In October 2000 Defra instigated a project to investigate the 'Pathogenisis and diagnosis of tuberculosis in cattle - complementary field studies". Known as the 'Pathman project', this work was designed to investigate the undiscovered reservoir of cattle Tb missed by the skin test - or so it seems to us. The correct term, is " to advance the understanding of cattle-to cattle transmission of bovine tuberculosis in Great Britian'. But its core result seems to have escaped notice.

200 tuberculin reactor cattle at standard interpretation of the skin test, were collected ex farm and paired with 200 in contact animals. Blood samples were drawn and nasal mucous samples were collected; then the animals were slaughtered and subject to a two hour (at least) thorough postmortem examination. The bloods were sent for trace element testing, as were liver samples which are much more accurate, and also immunological assays to check past disease exposure.

The in-contact animals were kept on farm at the Boxworth research establishment for seven weeks, in isolation. During this time several bloods were drawn, and up to seven nasal swabs taken to "detect early stage disease and m.bovis excretion". At the end of the holding period, another skin test was performed and then the animals slaughtered under the same procedure as the reactors.

(3.4 TB50)The results were only slightly better than those performed in 8 minutes by SVS postmortems, in that just over half the reactors had VL (55 per cent) and thus half did not and would record NVL. The in-contacts recorded 14 per cent macrospopic lesions and on the 7 week skin retest, 15 of these 200 animals had become reactors at standard interpretation. The site of lesions most commonly found were in the thorax of the cattle, with head lesions second. Very few were found in the abdominal cavity, (3.2) and even less if lesions were not apparent elsewhere.

32 animals in the study were found to have lung lesions, and the range of postmortem VLs was much higher (89 per cent) in yearlings and calves than in other classes. Dairy cattle had the lowest confirmation rate at 34 per cent, a point noted by the investigative team. (Table 5)

Investigation into other inputs such as vaccinations or exposure to other diseases gave mixed results. (table 10) Having antibodies to Johnes disease (m.avium paratuberculosis) was associated with confirmed bTb in both dairy and non dairy reactors but in the in contact animals it was thought to be "not statistically significant". Antibodies to Leptospira, were strongly associated with bTb in in-contact dairy cattle - but the opposite in non dairy contacts. In reactors there was no association. Having antibodies to liver fluke was associated with lower risk in all categories.

Selenium and copper were investigated and it was found that levels of selenium tended to be higher in dairy animals than non-dairy. The conclusion was that a lower liver level was associated with an increased risk of bTb, but that the association 'was not statistically significant'. (table 12) Other categories found a slight correlation. Dairy animals were significantly and consistently "less likely to have bTb confirmed by culture, than beef or other production classes" involved in the study. (4.8)

Considering the amount of emphasis placed on cattle to cattle transmission by the ISG, it is interesting to note that in the Pathman project, not a single one of the 1543 nasal mucosal samples of which 1006 proved clear of contamination - including those from the 32 reactors found to have lung lesions - proved positive for m.bovis, a point missed by the ISG when describing the project in their final report. How could John Bourne have missed that, one might ask? It was mentioned at least four times (4.6) (3.6)the executive summary and the conclusion.

"M.bovis was not detected by bacterial culture in any of the nasal mucus samples." and "The results suggest that large concentrations of M.bovis are not present in the nasal passages, and the shedding of M.bovis, if it occurs, is rare in naturally infected GB cattle."

To be fair (and why should we be?) the paper points out that the researchers 'have not looked at transmission through mouth respiration and coughing', and they conclude, rather strangely we think, 'that these could remain potential sources of disease transmission'. We think they would like to do another trial; but think about it. Cattle graze and eat with their heads lower than their gut, thus any mucous in the lungs passes towards the exit - which in all the cattle we've known is the nose and not the mouth. Cattle with pneumonia will pant for sure, but any discharge from an excess of mucous in the lungs exits from the nose and not the mouth. Hence, as cattle keepers, not scientists, our cryptic comments.

But also making a cryptic observation in this week's Veterinary Record, are R.M.Q.Sainsbury and Dr. John Gallagher, both remarking on the lack of onward transmission from these 1006 samples, and others in a complimentray study not noted by the ISG. They point out that the final report of the ISG "used data somewhat inconsistent with those in the recently published 'Pathogenisis and diagnosis of tuberculosis in cattle - complementary field studies'".

"In total over 1500 nasal mucus samples were taken in order to ascertain whether tuberculosis (bTB) was passed from cattle to cattle via the respiratory tract. Micobacterium bovis was not isolated from any of these samples despite lesions being found in the lungs of 32 of the cattle. Also quoted in the study are the results of an ongoing, separate longitudinal study of reactor animals [SE3033]where no bacilli have been detected in approximately 1000 nasal mucus samples from a further 40 reactors [ taken at varying time intervals]"

Returning to the ISG final report, the letter continues:
"In appendix 1, the ISG report does mention the pathogenisis and longitudinal studies, but appears to play down the significance of their results despite the fact that both studies confirmed 'on the ground' experience of many working in this field, that is, that cattle-to-cattle spread is uncommon on infected farms."

The writers emphasise their point thus;
"This point is crucial. In the ISG report it is stressed that infected cattle have 'serious implications for the maintenance and persistence of the disease in infected herds, and for the spread of the disease to neighbouring herds and to other parts of the country'. This is used to justify the recommended further rigorous movement controls and additional testing (recommendations 21 - 29) and would inevitably lead to the slaughter of many more cattle."

And they conclude:
"The overall conclusions of this Defra published report have increased our grave concerns regarding the ISG recommendation that controls on cattle alone would be relied upon to reduce the incidence and the spread of bovine Tb. We believe the ISG has seriously overstressed the importance of cattle-to-cattle spread, and the adoption of this recommendation will not control the disease. Progress will only be made when the original source of cattle infections is addressed and that means facing the reality of the large reservoir of Tb in wild badgers."

We agree with that conclusion, and have already pointed out the crass futility of repeating such cattle-only measures, attempted by greater men than John Bourne and with such stunning success

And after £2.8 million, Professor Bourne missed the histology conclusion on those mucus samples completely. They were all negative. Every one.

It may be churlish to point out the obvious, but we will point it out nevertheless; that is, if micobacterium bovis wasn't plastering the pastures and cattle feed troughs of GB, then the disease status of those cattle unfortunate enough to fall over it would not be called into question. But as it has been, surely the negative-for-onward-transmission results of every mucosal sample taken, deserves a higher profile?
See SE3013 here

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