L e t t e r s

be possible to obtain worms that ale more resistant to drug therapy than is a fully resistant population m wvo. as dru£ toxicity is not invc!ved, An intemat!onally a~reed definition of resistance in ~-Jtistosomo would help avo,~ confusion in the future.

~eferences I Falton. P.G et aL (1996) Pumg,tel Todc;y 12.

316 320 2 Ci0h, D, ~'[ 'a~ (f993) &l~us,~oi. T~kly 9,

162 --166

3 Coles. G.C. et cL (1987) Trans. R. Soc. Trap. Meal Hy.~ 81,782-785

4 Coles, G.C. ant Bruce. JJ. 0990) in ffes,stc;nce of P~rosaes [o Ant~paras~t~c Dlu~s (8oray, ].C., Martin. P.J. and Roush, KT., eds), pp 43-49, MSD AGVE/

5 Pnch,.~. RK. et ol (1980) Austr. Vet.j 56. 239 251

6 Conder. G.A. and Campbell. w.c, (~995) Adv Puras;to,'. 35, 1-84

7 Coles, G,C. and Bruce, J.! (1987) Intl. P~,qs,,ot 17. 767 771

Gerald C. Coles Department of C!inical Vetennary Science University of Bnstol [angford Hollse Bnstol UK BSI8 7DU

George K. Kinotl Department of Zoology Un,versity of" Na,robi PC Box 30197 Nalrob~ Kenya

Schistosorne Resl;tance to Praziquantel: Fact or Fiction?

it is with great interest that we read the the immature eggs in the tissue to mature article by l-allon, Too. Ismaii and Bennett and be killed by the drug. This is also published in a re~ent issue of P~r,.Isr, ology impO~an~ '¢:l~en assessing egg viabilit/ Today ~ and we fully endorse the call for a because the authors' protocol calls for the standard protocol for assessing drug mice to be autopsied. 20 days alter the last resistance, but we fell that there 'aere treatment. During this period, any some areas that were not addressed. The immature eggs deposited in the tissues autho~ focused laq~e~y on the adult worms p!ior to whe~ the majofft> of the worm as the key s~a£e for assessing resistance to a population was kil!ed by th~ o, ug tr'eatrncrlt challenge infection; however, it is the eggs will mature and may therefore give an which will carry, the 8ene(s) and express underestimate of numbers or resistant eggs the phenotype for resistance. In additior~, in these tissues. resistance is unlikely to be pnesent in all For the above reasons, we would eggs. even fi-om a ~in£1e r~sistant worm, surest the following modifications in We 2.~ and others 4 have found that the standard protocol. ( I ) Treat mice immature eggs in the tissues are not 'with two doses of PZQ. I 5 days apart. susceptible to prazieuantel (PZQ). and the (2) Following the second dose, check daily' five-day regime, suggested in the ;3rotocoi, (uniii aui.~p~y) ( ~ t i~ p ~ i ~ . ~ of e~.gs in does not effectively address this problem, faeces, and carry out the e&g-hatch test. We would advocate that two doses of We think that these are important PZQ, 15 days apart, is sufficient to allow considerations and should be

incDrporated into any standard 0rotocol or assessment of PZQ i~sistance in schistosomias;s.

~eferenees [ Fa!lon, e.G. or/Jl 0996) Pc~as~ral T.~day 12,

316-320 2 Giboda, H. and Smith j.M. (1994)] 5@

/Vled. Hyg 97, 98 -102 3 Giboda. H. and Zdarska. Z. (1994) Foh~

Par{~s,tol. 4 I, 55-58 • t Matsuda H. et al (1983)]pnJ Exp. Meal..~.

271 274

Michel Giboda San Juan ~autista School of Med~c,ne PC Box 713656 San Juan Puerto Rico 00936-8465

James M, Smith Institute of Parasitolo~_y Macdonald Campus of McG~II Un.c~:ty 21111 Lakeshoi~ Road Ste-Anne-dc-Bellevue Quebec Canada H9X 3V9

B o o k R e v i e w s ii

Models for Infectious Human Diseases: Their Structure and Relation to Data

edited by Vderie fsham and Graham Medley, Newton Institute and Cambridge Un/vevs~ Press, 1996. £4S.00 (hblc) (490 pages)

ISBN 0 521 45339 9

Tension permeate.~ this book. Although editors claim that c, ne of :he legacies of essentially a conference proceedings (it is the meeting will be the contacts and one of thr~e volumes en,e~ing from the collaborative links it spawned. Certainly. Newton Instltute's 1993 Epidemic Models this volume emphasizes the gulfs that Programme). it ~s far mere interesting than current y exist. the norm. The cor.ferenc~ organizers and The heart of the matter is the familiar the edittJs managed to get under one roof tension between simplicity and complexRy. (~md in1 ,~ or'e volume) fl~eoreticians who As Dye puts it, one school of thought is ~se ,,e~5, different appro tches to human that a model's utility is positive!y correlated d~sease mode!!ing This apparently with the number ofvanables and parameters ger,enated some neat [at one point it ~s it contains; the other believes the correlation asserted that there exist mathematicians is negative, ~Nhat emcees from the 'eve.n le~ ~,nc~..~' than a well-known juxtapos~tlon of pape~ in this volume is that Oxford theoretlrtan (p. 1~5)]. but the it ve!~/much depends on the questions you

I~B

want to ask (eg. understanding fundamental processes versus predicting how long control strategies need to be in place), but both schools evidently still question how good til~ ul', ,~, ~' ,~, ,~w~r s are. 5omewhat orthogonal to the complexity/simplicity divide is the gulf between the use of statistical models fitted to real data, and the use of dynamical theoretical models, which are usually compared qualitatively against data.. For example, pattems of changing CD4 counts in HIV can be treated as a question of better predicting future counts, given past counts (based on stochastic processes) or as a problem of identifying the underlying d~amical phenomena (based on deterministic processes), Again it is clear that t h ~ different approaches address rather different types of questions (often descnption/prediction versus explan:ltion). Many of the participants acknowledged the exciting possibilities greater collaboration would bnng, but one ;lenses that most

Paross{ology T~Ia,A voL 13, no. 4, •997

people amved at the meeting with their preferred approach and !eft with that bias intact. Perhaps that is to be expe_rted, but it is difficult to see how complete understanding can be achieved without more intercourse.

This is e:,sentialiy a buuk by modeller~ rot modellers and, with a few notaale exceptions, there is little attemat to talk to innumme~te biologists (one page contains more differential equations than words). Some empiricists criticize theoreticians for lailing to communicate well ('particularly molecular biologists and immunologists who expe~ othens to use their" language), but it is curiously anti-ir~tellectual to presume that theoreticians should always want to say th in# of interest to empifici!~ts. As this volume illustrates, the field is now so large and last-moving as to be a dscipline in its own nght. Nonetheless, there are important messages in here for non-theoreticians. First, verbal descriptions usually achieve apparent

clanty by fudging the issues (the ngour and clanl.y of thought demanded by mathematical descnption bnng tNngs into sharp focus as the disagreern~ni.~ ~n th,~ volume show). This may be why theoreticians are often reluctant to tFanslate [huir rnathematicai conciuslons back into words. Second, there are empincal que~lons to which theory can make substantial contribution. As Agur points out (p. 348), ,t is now common to use population dynamics to determine optimal vaccine strategies, yet drug protocols for individual patients are largely determined by tnal and error. And Wooihouse. by companng epidemiolog~ca! patterns w~th the output of population dynamical models incorporating vanous immune mechanisms, is able to determine that proteC, ive, nmunity against schistosomes is most likely directed ~ga,nst larvae but elicited by adult antigen. Third. ~t iS inconceivable that full ~xplanations oi what are essentially population-level

Book Reviews i

processes (either within hosts or within host populations) can be achieved by empirical observation alone. ~Mhether empiricists like it or not, immune responses are emergent properties of interacting populations,

So, for those interested in an overview of the range of weaponry theoreticians bnng tO bear on infectious diseases (mostly HIV. malaria and helminth infections) and the sor~ of'questions they can answer, this is one of the best summaries. For those in the field, it provides a very good introduddon to theoretical approaches with which they may be less familiar. It will be interesting to see what a similar conference a few years hence would produce: a hardening of divisions or a productive easing of tension?

InStltute of Cell, Animal and Pop.alat,on Bloloo~y University of Edinburgh Edinburgh, UK EH9 3iT

Olfaction in Mosquito-Host Interactions edited by Gregory R. Book end Gail Cardew, John Wiley & Sons, 1996. £52.50

(x ~ 331 pages) I.~BN 0 471 96362 3

Much of the behaviour of adult mosquitoes is mediated by olfactory cues, including such imporb'lnt features as mating, bloodfeeding, ovipos~tion and nectar feeding, The belief that, with better understanding, we might exploit or modify these behaviours to provide new or improved methods of control has been a major justification for Cib~ Foundation Symposium 200. f~tured in this book By bringing together scientists from disparate disciplines, including beha.,iourai, physiological, biophysical and molecular biology, the organizers hope that new and perhaps provocative ideas about future mosquito research in this field will emerge.

How well are these ambitions achieved? Only time will tell whether the ultimate goal of improved control will be reached, but this volume ce~ainly provides ideas aplenty and would provide anyone interested in the ~eld with a very t 0rough introduction. Although the olfactory mediated interactions between hosts and m(,squitoes are the pnmary focus. olfactory cues for mating ovipositron and nectar feeding are also discussed. Olfactory cues for oviposition are already utilized routinely in samp!ing of mosquito populations (for example in the ovitraps used for the dengue and yellow fever vector Aedes deooypti ) and the ovipos~tion pheromone of the filarias~s vector Culex quinquerc~scletus has been effective in attracttn 8 ovipositing females in field trials where, by including an insect growth regulator, emerging larvae were destroyed. There is also hope of new ways of repelhng mosquitoes. Previous work has shown that cattle differ in their attractiveness to ti;e

P~]ros~tolo~y Todcy, voL 13, no, 4, 1997

head fly and horn fly, There is now evidence that the less attractive cattle produce odorant rr atenais that r:,ask thelr normal attraGiven~ ~s. Such masking semiochem~cals may also be produced by <ome humans, d-,us ralslng th~ posslb~llty t !at thei~ ~ isolation and synthesis could lead to new, rational and highJy potent mosquito repellents.

One consistent theme to emecge from this volume ts the sheer comFJextty of what, superficially, seem to be ~. :npie behaviours. Flosqultoes reclulre subtle control mechanisms to legulate and pnontize behavlour accord,~ to th#." physioiogical state, since ,hey may simultaneously encounter olfactory cues for more than one type of benavlour. Olfactory cues may consist of mlxtures of odours derived from host ammals or p!~nts, or ~om othe: mosqu;toes, the reiabve proportions of which can vary both temporally and spatially. Pulsed exposure to odours may elicit different behaviours than would continuous exposure. Compounds may be ~nh~b~tory at some concentrations and attractant at others. Human sweat Includes some 300-400 compounds released as by- products of metabolism; onb ' . i,-,~cbon of these have been studied. Of the rnore abt.ndant compounds analysed by mass spectroscopy, ten el~oted electrophysiologk d responses t?om the malana vector Arophetes .~umD,oe. Notwithstandin:~ tne ~,c:L!mulat~n,' data. there ~s cie~dy some way to go before we fully understand the conmbut]on of odour to host attractlveness

Those who work with ba~ts for houseflies know the problem~; of running t,~ps containing obnoxious-sme[!in~ substances, Plosq,Jito traps that incorporate human body odour may be no more consumer friendly, However, a sweeter-smellin 8 alternative could be at hand. As Alan Cork,. points out, moscluitoes may feed more frequently on nectar than on blood, and thus floral or honey odours may have potent!J in tmrping systems for population sL p#resslon,

A number of chapters show how new technologies can be used to address c!, 4. problems. Odo,~nt-blnding proteins and pheremone-binding proteins (PBPs) are intimately Involved in olfactory transduction in moths and fruiti~ies, and their study has gwen new insights into the natu-e of set :i',',c specificity and signal transduGion. The Dresophslu homologues of moth PBPs are encoded by two tightly linked genes, and Jt is entirely feasible that mosqurtc PBP genes could be in[roduced into Drosor~:ll.? ~n 3rder tO study electrophysiologlcaily the ode ,"ant response of transfon'nant flies. It is nc : inconcei~,ab~e that ~=,;~-b,~,,~f~c:.~on techniques might soon enable us to produce a mosquito wRh a changed response to odou;: changing an ~,nthropophitic mosqufto into one whrch is zoophlhc, An attempt to achieve :h;s end by more conventional genetic "nears is already under" way.

30. future studies of mosqult9 olfact~on might end up in new low-tech t~Ds for community use but. equally, cou!d form the basis of new rnole,:ular approaches to the control of mosqu~to-bome discos. ~. This fascinating volume will ffave something to say to researchers of either pe:~uasEon.

Harold Townson Liverpool S~hool of Tropic d Pled~one Pembroke Place, Lrveqsool. UK L~ 5OA

159