Wednesday, April 18, 2007

Is that all?

Last month, Defra published figures for the source of the various outbreaks of bTb and this seems to have upset the Badger Trust somewhat, who then used it as a stick with which to beat the intradermal skin test. But a letter in this week's Veterinary Times - the sight of which we are most grateful - puts an entirely different interpretation on the figures.

Andrew Proud, BVSc, DVSM, MRCVS writes:

It is clear that the Badger Trust thinks we should all be horrified by the figures [....] but any informed veterinary surgeon who pauses to think, will respond otherwise. Twelve percent of new herd incidents were disclosed by a combination of routine meat inspection, and tracing from other herd incidents: my first response is so what?
My second is .. only twelve per cent?

When I read on (from our Trevor's outraged outpourings) and found that "the situation in three and four year parishes is even more serious, with 18 percent of new incidents..." detected at MHO inspection, I responded "No more than 18 percent? What a striking vindication of Government policy!"

This represents no inadequacy in the tuberculin test; clearly, the Badger Trust does not know that in three and four year testing areas, only adult breeding animals are (skin) tested. Cattle being reared for slaughter are screened for Tb only by meat inspection unless they are in a herd in which reactors have been disclosed in breeding animals.

But my third reaction, following some crude arithmetic, is to observe that the Badger Trust has, unwittingly, produced powerful evidence against one of its favourite theses. If the 87 new herd incidents detected at meat inspection, and the 33 detected following tracing from reactor herds represent 12 percent, then the latter category accounts for only 3.3 percent of new incidents.

What the Badger Trust does not seem to understand is that tracing of animals moved off is a key part of the investigation of all herd incidents. A little allowance must be made for the recording periods, but essentially these figures suggest that tracing from the 96 percent of herds where reactors were found not to have been purchased in animals, disclosed reactors in no more than five percent of herds to which these cattle had been moved.

Even ignoring the fact that in most cases where the traced animals react, all other animals in the recipient herd test clear and continue to do so. This is powerful evidence that the spread of bovine tuberculosis by cattle movement is not significant."

The Badger Trust seem to have got in a pretty froth about these figures, but by disecting them in rather more detail and with an overview of the testing / surveillance situation as it exists, Andrew Proud's reaction (and ours) is "Is that all?"


Anonymous said...

From Trevor Lawson, Badger Trust

Nothing in the Veterinary Times. Some other publication, perhaps?

Andrew Proud has previously argued in the Government Veterinary Journal that "‘there is no reason to suppose that in the absence of a protected wildlife reservoir population [TB testing measures] would no longer work". This view does not take account of the Government's own advice from its vets - that TB testing measures, as they stand, are inadequate and cannot control the spread of bovine TB amongst cattle.

Proud suggests that the figures we have quoted should not give cause for alarm. But herds exempt from testing are finishing herds from which stock are supposed to go direct to slaughter. Our data clearly show that some of the herds escaping the TB test are selling cattle on, so they are not just exempt finishing herds.

Mr Proud might think that a testing regime which allows the export of infected cattle to 33 new herds is acceptable. I doubt that the farmers who bought the infected livestock would agree. As for the other 87 new herds detected through slaughter, these will also probably have been infected as a result of purchased cattle or contact with neighbouring outbreaks arising from purchased cattle.

In short, we are looking at 120 new herd incidents in three and four yearly areas that are likely have been created by cattle movements. Try telling the affected farmers: "Is that all?"

What are the implications for neighbouring herds, from nose to nose contact? And where is the evidence that "in most cases where the traced animals react, all other animals in the recipient herd test clear and continue to do so"?

Mr Proud does not support his claim with data and large-scale cohort studies suggest the opposite. Carrique-Mas et al, 2005, point out that "infection can be imported on to farms through movement of infected cattle" and "the distribution of numbers of reactors after a period without testing is highly suggestive of on-farm transmission between cattle".

One other thing. Andrew Proud ws a veterinary officer in Gloucestershire until his retirement in 2005. This is the same county where, whilst the DVM was telling the NFU that badgers were to blame for the disease, trading standards were prosecuting farmers for moving cattle to market and other farms whilst under TB restriction. Right under the DVM's nose.

Matthew said...

Veterinary Times. April 16th. p.54

The point is Trevor, that the surveillance system, including MHO at slaughterhouses, (and Trading Standards with access to CTS database) are picking up any straglers that the intradermal test misses - for whatever reason. And the number picked up is very few, compared with those found by the test.

As we have pointed out before, nose-to-nose contact with infected cattle, as shown by the Irish work (Costello et al) took twelve months to make a dent in transmission. And that was pairs of cattle housed and sharing feed and water 24/7 with an infected animal. Easy it is not, so the 'implications' are minimal.

The work of Carrique-Mas et al, says it all; "after a period without testing...". so, yes the potential is there,- even with the comfort blanket of pre-movement testing - for an infected yearling to travel to a 3/4 year testing area and be missed. If that animal subsequently becomes an open lung lesion case, then of course onward transmission is possible. Testing post movement of breeding cattle is one thing I think we can agree on.

There are bad apples in every barrel Trevor. Can't defend farmers who try and move cattle when they are under Tb restriction, and wouldn't try. But isn't the point that Trading Standards officers' access to the CTS data base found them?

Andrew Proud's letter was entitled "Badger Trust Drew Wrong Conclusions", by the way.

Anonymous said...

From Trevor Lawson, Badger Trust

Glad to see you can see the logic of post-movement testing. Unfortunately, the farming unions have made it politically impossible to implement such a measure, even though it is recommended by vets and TB will remain uncontrolled without it.

Defra has just published further work by the VLA on cattle susceptibility to bovine TB. It takes an incredibly low dose to cause full blown infection - just a few bacteria.

Low dose TB infection in cattle: disease dynamics and diagnostic strategies (SE3024) reports:

"Predominant disease phenotype seen in GB is likely to be caused by aerosol infection of small numbers of bacilli delivered by small aerosol particles (3-5 organisms) to the lung."

The research has been published in peer-reviewed scientific journals.

It also reveals that animals testing postive with the skin test, but particularly gamma interferon, without showing clinical symptoms "are therefore not ‘false-positives’ but infected sub-clinically and could pose future infection risks to herds".

No doubt you will argue that this shows how hazardous an infected badger is to cattle. But another study, Bovine TB transmission in restocked herds: risk factors and dynamics (SE3026), reports that 6% of a cohort of restocked herds in the SW suffered herd breakdowns (HBDs) at their first test after restocking.

"The associated risks for this were a farm history of bTB before 2001, purchasing cattle from high frequency testing herds (> 6 tests in 12 years) and the herd size at testing. The risk associated with history of HBD on the farm reduced the greater the time since the last HBD. If the last HBD was five years previously the risk was not increased."

If local badgers are to blame for infection, why does the risk decrease over time? The evidence points to latent infection in the herd.

"Approximately 50% of the 148 cohort farms broke down during the study. Associated risks with bTB exposure were farm history of bTB, storing manure or slurry in a closed container compared with storing in the open or spreading immediately, purchasing cattle from market and purchasing steers."

The slurry storage is an interesting issue. Bovine TB is vulnerable to UV light. Does storing the slurry in closed containers protect M. bovis and create the right dark, anaerobic and warm conditions for it to multiply?

"Approximately 1000 / 55,000 cattle that were tested on the farms between 2001 and 2004 were reactors. The strongest associated risk for being a reactor was the number of reactors cattle were exposed to before the test. bTB history of the farm and herd size were also associated with reactors."

We've said it all before.

"Considering the GB cattle population as a whole, and combining bTB testing data (VetNet) and cattle movement data (BCMS) showed that in 3 randomly selected cohorts 72-84% of animals died prior to being tested for bTB."

Does Mr Proud still think that this is a vindication of the Government's TB testing policy? Only 16-28% of cattle are ever tested for TB.

"Mathematical models indicate that the qualitative patterns of changes in bTB incidence can be explained by the dynamic consequences of changing testing interval."

Any comment, Mr Proud?

"There is an associated (stationary) risk of HBD from persistence of bTB in the farm environment, outside of cattle on the farm. This risk decays with time since HBD."

Here's that decay factor again. Woodchester Park and other studies show stability in badger populations over time. So are we looking at a different factor - the time it takes for bovine TB infection in the wider farm environment, slurry etc - to die off under the influence of sunlight. Other ISG studies have shown that covered yards are associated with increased risk. Could this, again, be linked to the rate of death under sunlight of M. bovis?

"The frequency of testing for bTB determines the success of both control and surveillance. We hypothesise that increasing testing coverage / effort will result in disclosure of more infection and greater control."

Mr Proud?

Another study also comes to interesting conclusions. The Risk to Cattle from Mycobacterium bovis Infection in Wildlife Species other than Badgers (SE3009) reports that:

"Habitat variables were better predictors of [TB]risk [than small mammal populations]. The incidence of bTB declined with increasing hedgerow density and with the presence of headlands –areas adjacent to boundaries which cattle are prevented from grazing – but increased with rising numbers of gaps in hedgerows. No relationship with landcover variables was detected.

"Thus the risk of bTB was lowest in farms with small fields and stock-proof hedging, in which cattle were prevented from close contact with hedgerows, and their associated wildlife (including many badger latrines), by means of conservation headlands. ... What is more certain is that farm habitat management compatible with maintaining good small mammal populations, are widely associated with lowered bTB risk in cattle. It is possible that these associations may be due to unmeasured confounding factors such as stocking density. Nevertheless, given the importance of bTB, we suggest that detailed consideration be given to farm habitat management as means of controlling the disease."

18 years ago, I reported on the rate at which contractors were ripping out hedgerows on farms in the West Country. No we find that they may have been preventing cattle infection from badgers. Not discussed by the authors, but also possible, is that good hedgerows (without gaps) minimise nose to nose contact with cattle...

Matthew said...

My, doesn't time fly? We suggested post movement testing - particularly and especially for cattle moveing to 3/4 year testing regimes - 10 long years ago.
Your predecessor, Dr. King, the fragrant Elaine also suggested grant aided isolation facilities for this - with which we agree.
That's 2 things.

Defra are correct that 'just a few bacteria' will provoke a skin reaction to the test - if not full blown Tb in cattle. PQ's told us just 70 units of CFO (colony forming organisms) were needed.

A badger with kidney lesions excretes up to 300 units in just 1 ml of urine - and he will void 30ml at a time indiscriminately over grassland. He will also use this wonderful source of m. bovis as a scent marker, and he will spray it - and spit from the other end - if cornered, frightened or surrounded. How's that for a transmission opportunity?

We are aware of VLA's seduction to gamma interferon, and we are quite well aware of its blanket cover of other types of Tb, including avian, johnnes and skin Tb. Specific it is not.

On the SW restocks, we would ask if the presence of a wildlife reservoir had been discounted. and if 6 per cent suffered a breakdown, (presumably found by the intradermal skin test) does this not mean that 94 per cent did not?

The survival of m.bovis was the subject of your predecessor's thesis which is entitled "Factors influencing the risk to cattle of infection with bovine tuberculosis (mycobacterium bovis)from Badgers(meles meles)"

We hope you have better luck locating it than we did. But yup, UV is a very quick death, but its survival underground (in setts?) is much more prolonged. Although why the ancestral home of an infected badger should be preserved to infect the next incomer is beyond comprehension. Badger urine on partially dried forage may pose problems too. Bristol (Harris et al) did work on that in 2001.

For cattle slurry to pose a problem, it must first be contaminated; from a bovine with gut lesions? How common is that? The bulk of cattle lesions (we are told by the post mortem chaps) are in the throat and lungs with the majority being in the very, very early stages. A grain of sand, is how it was described.

Trevor, we cannot comment on what Mr. Proud would or would not say. However the figure you quote of 72 - 84 percent cattle which 'died' before testing is quite extraordinary. Is this in 3/4 year testing, where as Andrew Proud pointed out, beef animals reared for slaughter are screened at slaughter and not subject to skin tests unless the breeding herd was found to have reactors?

Certainly in annual and two year testing zones, every animal over 42 days is tested. Prior to this pre movement testing cut off, even new born calves were jabbed. We feel that your basic arithmetic on this one is skewed.

Having completed a TB 99 for the ISG, I have to say - it was a waste of time. All questions except the one that mattered. Yes we had covered yards, and yes we had silage and yes we had cattle. Also several dead ones. We also had badgers. And no purchased cattle. And Tb for a very long time. Next question?

We covered the 'research' about hedges in March last year:

The one thing the research missed is that a hedge is a natural boundary for badgers as well as for cattle. And while it does prevent nose to nose contact of cattle, it is often the barrier to another badger social group's territory. We were grateful for sight of a BSc study paper on just this phenomenon which we covered in April 2005;