Giulio De Leo Dipartimento di Scienze Ambientali Università degli Studi di Parma - Italy

1. EEE - ICTP Trieste April 13-15 2005 “To cull or not to cull, this is the problem”:undersired effects of animals removal to eraticate diseases in widlife populations(the adaptive dynamics of CSF in wild boars) Giulio De LeoDipartimento di Scienze Ambientali Università degli Studi di Parma - Italy Thanks to A.Dobson and M. Pascualand to the NCEAS WG on Seasonality and Infectious diseases

2. c :hunting rate [t-1] If the case of no culling (c=0): It can be proven that it is possible to eradicate the disease if: c > r [1- 1/Ro] Let’s take the Classical Swine Fever (CSF) as a reference disease

3. Basics of Classical swine fever (CSF) or Hog Cholera • A highly contagious disease due to a RNA virus, Family TOGAVIRIDAE, Genus Pestivirus; • It is a List A disease in the OIE Classification of Diseases • Suidae are the sole natural hosts;

4. Basics of Classical swine fever (CSF) or Hog Cholera • Infected animals may shed large amounts of virus for 20 - 40 days through oronasal and lacrimal secretions, urine and feces • The direct contact between infected and susceptible animals is the principle means of viral transmission;

6. Basics of Classical swine fever (CSF) or Hog CholeraEpidemiology • CSF causes high morbidity and mortality (up to 90%) during the first epidemic wave… but low virulent strains can be isolated in wild boars in the following endemic phase; • Acute infections • Chronic infections /endemic phases

7. CSF distribution(OEI 1995-1997)

8. CSF outbreaks in wild boars1990-2001

9. Why people care about CSF in EU? • Wild boars are blamed to be the reservoir of CSF

10. Country N. Swines removed (       = 1 milion specimen) NL 10 Germany 2 Spaain 1 Belgium < 1 Italy < 1 UE Damages from CSF between 1993 and 2000 A 100 kg pig ~ €150,00

11. which is easier to say than to do it… • The EU supports a program to eradicate the virus from wild boar mainly based on reducing population density through culling • What to cull? • Where to cull? • When to cull? • How to cull?

12. b Ro Culling rate [y-1] 1 Culling rate [y-1] Drawbacks of culling (1/2)as reported by the Italian Wildlife National Service (INFS) • It may push hosts out of their natural home range, thus fostering disease spread ~ 20% increase of culling rate ~ 60% reduction of population density with respect to constant b

13. Drawbacks of culling (2/2) • It may push hosts out of their natural home range, thus fostering disease spread • If culling is focused mainly on old (low susceptible) hosts, it may change population age structure in favor of more susceptible yearlings • Given the existence of multiple strains of CSF (Biagetti et al. 2001), a change in host density may foster the selection of less virulent but more persistent strains, thus making culling more costly and ineffective • Research questions: • Is it possible? • If positive, under which conditions? • Which are the consequences?

14. m c m m c c I2 I1 a1 a2 A simple two-strains competition model S • Hosts I1 infected with a Low Virulent Strain (LVS)  smalla1 • Hosts I2 infected with a High Virulent Strain (HVS)  largea2 b2 b1 b12 highly virulent strain (large a2) low virulent strain (small a1) b12is the super-infection coefficient

15. The new equations (LVS vs. HVS): S' =G(S) -b1I1S - b2I2S- cS I1' =b1I1S- (a1+ m + c)I1-b12I1I2 I2' = b2I2S- (a2+ m+ c) I2+ b12I1I2 • Assumptions on the LVS vs. HVS • Disease induced mortality (virulence) • a1 << a2 • Transmission rate • b1b2b12 0

16. Basic epidemiological implications of the above assumptions • Basic Reproductive rate (LVS vs. HVS) • Threshold density for disease eradication • KT1<KT2  LVS can persist in a very sparse population

17. Further epidemiological implications • If there is no super-infection(b12=0) • LVS always outcompetes HVS

18. If LVS outcompetes HVS • If LVS and HVS coexist • If HVSoutcompetes LVS where If there is super-infection (b12 >0)

19. Further epidemiological implications • If hog population density K (as well b12)is sufficiently high, thenHVS can coexist with, or even outcompete LVS  decreasing population density by culling migh increase the chance ofLVSto outcompeteHVS

20. Epidemiology of classical swine fever in wild boars of Eastern SardiniaLVSHVSfrom a field survey by Guberti (1998)and Artois et al. (2002)

21. Prevalence at the equilibrium as a function of culling rate, when the two strains are isolated Removal rate

22. Prevalence at equilibrium as a function of harvesting effort for two competing strains

23. Total Prevalence

24. Number of infected individuals

25. m c S m m b2 b1 I2 I1 c c a1 a2 b12 LVS (small a1) HVS (large a2) g1 g2 R m c The SI2R model(Susceptible-Infected-Recovered)

27. Conclusions • It is possible that the reduction of host density by culling may indeed foster the selection of less virulent strains • This in turn would reduce the threshold host density for disease eradication • If this happens, the harvesting effort required to completely eradicate the disease will be higher than initially expected • If culling effort is not large enough, the net effect of this policy is to increase both prevalence and the number of infected hosts • The harvesting effort required for the eradication of the least virulent strains may be unrealistically high (or too costly)

28. Further developments • Analyse pop.dynamics by using a stochastic (possibly spatially explicit, seasonal) version of the model • Introduce age structure and age-depedent epidemiological parameters