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EPIDEMIOLOGY

Dangers of moving cowsMark E. J. Woolhouse

The movement of cattle around the country, and the presence of badgers, are both implicated in the high incidence of bovine tuberculosis in Britain. The problem may get even worse in the near future.

Every year in Britain, as in many other coun-tries, millions of cattle are moved between livestock farms, markets and abattoirs. Thispractice is known to contribute to the spreadof infectious diseases1, and a study by Gilbertet al. (page 491 of this issue)2 suggests that it isimplicated specifically in recent increases inthe incidence of bovine tuberculosis (BTB).

Gilbert and colleagues provide the mostcomprehensive analysis yet of the role of cattlemovements (Fig. 1) in the epidemiology ofBTB. Their work is a tour de force in the inte-grated statistical analysis of a set of complexdatabases, including several million recordsfrom the recently created British Cattle Move-ment Service. This is one of the first timesthese data have been used for epidemiologicalresearch, reflecting the increasing willingnessof UK government services to share suchresources with the scientific community.

Using a statistical technique known as mul-tiple logistic regression, Gilbert and colleaguesshowed that the geographical distribution of BTB was most closely associated with thenumber of recent cattle imports from infectedareas. They then used the results from theirstatistical analysis as inputs into a computermodel (in technical jargon, a spatially explicit,stochastic simulation model) of the changingdistribution of BTB in Britain. The model predicted the presence or absence of BTB inany given 5-km�5-km cell in 2003 with morethan 80% accuracy, and was similarly success-ful in predicting the spread of BTB to newcells. Looking further ahead to 2005, they pre-dicted a continued increase in the number

of cases annually (Fig. 2, overleaf), and a risk of further spread from the disease’s currentstrongholds in the southwest and parts of central England to regions such as Wales,Cumbria and the Scottish Borders.

Unsurprisingly, cattle movements were notthe only determinant of BTB incidence. Otherpredictors identified (from a set of 100 possi-bilities investigated) included the recent pres-ence of BTB locally and measures of climate,land use and vegetation. In a further analysis,Gilbert et al. found that movements played amore obvious role outside ‘core’ areas whereBTB is already established, implying thatmovements are more important for the spreadof infection than for its persistence. Indeed,there are regions where BTB occurs only spo-radically despite regular imports of cattle frominfected areas. Some other necessary factorseems to be missing — which brings the discussion round to wildlife reservoirs, andespecially the badger.

Badgers are known to carry BTB, but Gilbertand colleagues’ study is equivocal on the ani-mals’ role in the epidemiology of the disease inBritish cattle. Proximity to known badger loca-tions does appear as a predictor in their analy-ses, but it is not prominent. Interpreting thisresult is difficult because the available badgerdata are of patchy quality and the presence ofbadgers is likely to be correlated with other fac-tors. To better understand the role of badgers in the persistence of BTB, we need to turn to experimental studies, such as the ‘Four Counties’ trial in Ireland, the results fromwhich were published earlier this year3.

This study compared the rate of detection ofBTB in cattle herds in each of four pairings of a reference area (where few badgers wereculled) and a removal area (where badgerswere culled proactively, regardless of infectionin the cattle). By the end of the study periodthe chances of a confirmed BTB case in a herdin the removal areas had fallen by between62% and 95% relative to the reference areas.This is the clearest demonstration to date of alink between badgers and BTB in cattle.

It is worth noting that the success of theFour Counties trial was not immediate. Overthe five-year study period, in every removalarea the number of herds affected sometimeswent up, not down, from one year to the next,although a reduction was always achievedeventually. This is no surprise: such variabilityhad already been predicted by mathematicalmodels of the impact of badger culling4.

All this provides pointers for a major ongoingexperimental study in southwest England, theRandomised Badger Culling Trial. This trialoriginally had three arms: no badger culling,proactive culling and reactive culling (under-taken only after confirmation of BTB in localcattle). The reactive-culling arm was aban-doned because of a reported 27% increase inherds with BTB (relative to the no-cullingarm) rather than the anticipated decrease5.However, the increase was not statistically sig-nificant, indicating that it could simply havebeen a blip, and that it was arguably still toosoon in the course of the trial to expect muchof an effect anyway6. So the aborted arm tellsus little. At least the proactive arm of the trial

Figure 1 | Transport connection.Gilbert et al.2 find that the

geographical distribution ofbovine tuberculosis is associated

with the number of cattleimported from infected areas.

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survives, but definitive results are not expectedfor some years.

The experiences of these trials illustrate anepidemiological principle. The dynamics ofchronic infections — such as BTB and BSE incattle, scrapie in sheep, or tuberculosis andHIV/AIDS in humans — are inherently slow,and it may be years, or even decades, beforethe effects of any interventions (or any otherchanges in their epidemiologies) are fully real-ized. We should not expect a quick fix.

That said, given that the incidence of BTB in Britain is expected to remain high or evenincrease further (Fig. 2), the sooner that moreeffective control measures are introduced, thebetter. The UK government is reviewing itsoptions for tackling BTB7; these include both badger culling and the statutory pre- andpost-movement testing of cattle (which

would benefit from improved diagnostics),and might conceivably extend to greaterrestrictions on cattle movements. The research discussed here suggests that all of theseapproaches merit serious consideration. ■

Mark E. J. Woolhouse is at the Centre forInfectious Diseases, University of Edinburgh,Easter Bush, Roslin, Midlothian EH25 9RG, UK.e-mail: mark.woolhouse@ed.ac.uk

1. Woolhouse, M. E. J. et al. Biol. Lett.doi:10.1098/rsbl.2005.0331 (in the press).

2. Gilbert, M. et al. Nature 435, 491–496 (2005).3. Griffin, J. M. et al. Prev. Vet. Med. 67, 237–266

(2005).4. Smith, G. C. et al. J. Appl. Ecol. 38, 509–519

(2001).5. Donnelly, C. A. et al. Nature 426, 834–837 (2003).6. Roper, T. J. Nature 426, 782–783 (2003).7. DEFRA Government Strategic Framework for the Sustainable

Control of Bovine Tuberculosis (bTB) in Great Britain (Dept forEnvironment, Food & Rural Affairs, London, 2005).

currently rather far from the 1:2 resonance —the ratio of their current orbital periods is near1:2.5 — so the implication here is that these planets have since migrated through 2:1 totheir present positions. This is a remarkableconcept, because we usually think of the plan-ets’ orbits as being rather static and changinglittle over time.

There is, however, good reason to believethat the giant planets’ orbits did undergo amarked readjustment early in the Solar Sys-tem’s history. The best evidence for planetarymigration is preserved in the Kuiper belt,which is a swarm of comets orbiting beyondNeptune. Several of these objects are also ineccentric orbits at Neptune’s 3:2 MMR, wherethey circle the Sun twice for every three orbitsof Neptune. However, these bodies areunlikely to have formed in such unusualorbits; rather, the prevailing thinking is thatthey were gravitationally trapped at this reso-nance while Neptune migrated outwards4

early in the Solar System’s history.But what could have caused the planets’

orbits to migrate? During this early epoch ofplanet formation, interplanetary space was stillfilled with numerous bits of debris — planet-esimals — that had not yet been accreted bythe newly formed planets. But as the giantplanets grew to their final sizes, their greatmasses made them very effective at tossing thisplanetesimal debris around the Solar System,an interaction that also caused the planets’orbits to shift in response. In the model of Tsiganis et al., Neptune’s orbit doubles in sizewhen it is gravitationally scattered by Saturninto a more distant and eccentric orbit aboutthe Sun (Fig. 1). Neptune’s subsequent gravita-tional interactions with the numerous plan-etesimals then makes its orbit more circular asthat planet migrates farther outwards. How-ever, this orbital evolution requires a lot ofdebris — a mass equivalent to about two Nep-tunes, all of which must ultimately be scatteredout of the Solar System by the migrating plan-ets. Evidently, planet formation was a messyand inefficient business.

The scheme sketched by Tsiganis et al.1 isplausible in the sense that their model doesindeed excite the eccentricities and inclinationsof the giant planets to the requisite values. Butcaution is required: the fact that a simulation ofplanet formation produces an end state in goodagreement with the observed Solar Systemdoes not prove that the simulated events actu-ally happened. Rather, the eccentricities andinclinations of the giant planets could insteadbe relics of another process — perhaps arisingfrom gravitational perturbations from otherlarge protoplanets that might also once haveroamed the early Solar System. Indeed,Uranus’s very large obliquity (its rotational axisis tipped 98° away from its orbital axis) is usu-ally interpreted as evidence for a collision withanother protoplanet that had a mass compara-ble to that of Earth5. Nonetheless, the authors’model deserves credit for at least having

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Figure 2 | Trends in the numbers of reported cases of bovine tuberculosis in British cattle 1995–2005.Red squares, observed values, including provisional 2004 data accessed on 8 April 2005. The dip in2001 is probably due to decreased testing during the epidemic of foot-and-mouth disease at that time.Red line, exponential trend up to 2003. Blue line, extrapolation of exponential trend to 2005; bluesquares, average predicted values for 2003–05 from the model of Gilbert et al.2.

PLANETARY SCIENCE

When giants roamedJoe Hahn

An early epoch of planetary migration could explain the current orbits of thegiant planets, the origin of Jupiter’s Trojans, and an intense bombardmentof the early Solar System with a shower of asteroids and comets.

In a triplet of papers in this issue1–3, H. F. Levi-son and colleagues contend that the orbits ofJupiter, Saturn, Uranus and Neptune have beendisturbed in a small but significant way. Theyargue that the giant planets’ eccentricities (thedeviation of their orbits from a true circle) andinclinations (the tilt of their orbital planes) aremuch larger than those predicted by theories ofplanet formation — which implies that someprocess has disturbed the orbits of the giantplanets since the time of their formation.

In the first paper (Tsiganis et al., page 459)1,the authors show that the passage of Jupiter andSaturn through a 1:2 mean-motion resonance

(MMR) can account for the orbital spacings,eccentricities and inclinations of all four giantplanets. An MMR is a ‘sweet spot’ in the SolarSystem when the orbital periods of two bodiesare ratios of whole numbers, and this is also asite where a planet’s periodic gravitational perturbations can ‘pump-up’ another body’seccentricity and/or inclination. Thus Jupiter’s1:2 MMR is the site where Saturn completesone orbit about the Sun for every two orbits ofJupiter. The authors’ find that the passage ofJupiter and Saturn through this resonance canexcite their eccentricities and inclinations tocurrent levels. However, Jupiter and Saturn are

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