Content on this page requires a newer version of Adobe Flash Player.

Get Adobe Flash player

Section 3. Cattle TB Eradication Part 1


Badgers : - Mistakenly getting the Blame as the intractable low point supposedly self-maintaining hidden reservoir of TB is best understood by a reappraisal of how TB schemes succeed or fail in Part 1; then a more detailed overview of the mistakes made in failing to tackle the True "Hidden Cattle Reservoir" in Part 2.

Part 1 . A Historical Overview Of Cattle TB Schemes

Rather ironically, badgers have been given the blame; Both initially for stalling the cattle TB eradication scheme at the low point in Figure 1, Map 2 .. and now with cattle TB at Crisis point and "Out of control", with c. 7000 herds under TB restriction, and up to 40,000 reactors/yr in an area of half GB, since farmers leaders still claim that badgers are the main problem and cattle -to-cattle transmission is unimportant :- the Crisis has arisen precisely because TB has exploded within the cattle population ! With such confusion widespread, it is important to re-discover how TB Eradication Schemes actually work ! GB Had a textbook Area Eradication Scheme, in which tight cattle controls alone did Eradicate TB in Scotland, almost all of Wales (see Maps, & Figure 2), and even in such intractable hotspots as West Penwith/Lands End, and the Isle of Wight and Anglesey ..without any badger culls. Amusingly it has been suggested that cattle controls alone may be enough to eradicate cattle TB ( ISG pp. 5, 147-9, 175 ; Kao 1997).

Relaxing cattle controls historically has always led to an upswing and resumption of the inexorable spread amongst the cattle population. Dozens of countries during both world wars, and the "U-Shaped" graph of Figure 1, was replicated repeatedly eg. as noted in Introduction in Michigan, long before white-tailed deer suddenly discovered as a problem (Myers 1969), Germany had most herds infected post-WWII, but got TB down dramatically by 1965 allowing a premature switch to 2 year testing, then 1973-1983 to 3 year testing, with an upswing in reactors, so annual testing reimposed leading to eradication (who 1992 vaccine review). Ulster too achieved a low point of just 174 reactors in 1971, prematurely going to triennial tests, so a rise to 1800 reactors in 1983, mirroring 1962.. the post-FMD upsurge in 2002, was reduced by 40 % within 4 years .. badgers have until recently always been regarded as a spillover host and not culled (Abernethy 2011). Many accounts of country TB eradication schemes Fischer 2005, Francis 1947 & 1958, ICMB 1-5, Myers 1940 & 1969, More 2006 (various studies) , Morris 1994, Rua-Domenech 2006, O'Connor 1986 & 1989, Pritchard 1988.

Cattle are the ideal long lived social herd natural host for bovine TB, beautifully co-adapted so that the disease is self-maintaining with spread across generations and within the population. Spread occurs both in the individual ; then within and between herds . TB is a progressive bronchopneumonia, and as explained later in Part 2 (see Figures 3 & 4 ); can be likened in the individual to a slow but malignant metastatic cancer, with simplistically there intergrading stages :- proliferation of bacilli, is seen outwardly as progression from the 1 or 2 droplet nuclei penetrating thin walled lung alveoli, with initial granuloma or incipient tubercle lesion foci, to an increase in size and number of tubercle lesions , stage 1 being with Non Visible or NVL lesions with too few bacilli to be detectable so unconfirmed cases ; then stage 2 Visible Lesions VL cases , at abattoir inspection (or human x-ray) , and lesions later merging to show cavitation with uncontrolled proliferation of bacilli (no blood supply); and lastly :-stage 3 with spread to over 50 body sites, notably udder and uterus with potential for transmission in milk or in utero to unborn calf in "Clinically affected" TB cases, removed under early Tuberculosis Orders, with emaciation, tubercular cough, bacilli in sputum and milk in stained smears microscopically (Steward 1941).

Cattle TB Eradication Schemes are hence based very pragmatically on the identification and removal of skin test reactor cases before they can spread TB within the herd and to new herds via exported new cases ; a test and slaughter policy as with most animal disease controls. In essence this is simply a reversal of the three stages described above ;or effectively a draining of the "cattle TB reservoir" by skimming off stage 3 clinical cases, then gradually reducing the proportion of stage 2 visible lesion VL cases VS stage 1 NVL /unconfirmed cases. critically Important to realise that when schemes near eradication, there is still a stage 1 "hidden self-sustaining reservoir" of cattle TB (wrongly assumed to replaced by a badger one !). Significantly, with elimination of Stage 3 advanced cases .. all transmission reverts back to entirely aerogenous respiratory cattle-to-cattle transmission.

Eradication schemes hence pragmatically comprise three elements :
1. Annual Testing

It takes around a year to reach the more infectious Stage 2 Visible Lesion status (SEE details later, in Part 2 , Figure 3 & 4); so annual testing is the optimum timing for tests .. 6 monthly is too short a period for most new cases to become reactors , although 6 month check tests are a vital follow up to herds which have tested clear. Allowing the test interval to slip to 2 years simply allows greater within-herd spread, and since they are not under TB restriction, to export cases widely ! :- amply demonstrated after foot & mouth 2001 below. Abattoir inspections have always been a "safety net" for finding any cases missed by routine testing .. during the low point, a few dozen a year... but now several hundred. Data from Eire with up to 46 % of herds found by abattoir inspection suggests annual testing /no pre-movement testing is NOT a rigorous enough control (More 2006).

2. Movement Restrictions

Herds with reactors are put under movement restriction , so preventing any export to new herds . EC Directive (EC 64/432) suggest 2 clear tests needed to de-restrict both confirmed AND unconfirmed breakdowns.. belatedly applied in Wales, then by DEFRA.

Figure 1

Figure 2