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“To cull or not to cull, this is the problem”: undersired effects of animals removal to eraticate diseases in widlife populations (the adaptive dynamics.

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Presentation on theme: "“To cull or not to cull, this is the problem”: undersired effects of animals removal to eraticate diseases in widlife populations (the adaptive dynamics."— Presentation transcript:

1 “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) EEE - ICTP Trieste April Giulio De Leo Dipartimento di Scienze Ambientali Università degli Studi di Parma - Italy Thanks to A.Dobson and M. Pascual and to the NCEAS WG on Seasonality and Infectious diseases

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

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 days through oronasal and lacrimal secretions, urine and feces The direct contact between infected and susceptible animals is the principle means of viral transmission;

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6 Basics of Classical swine fever (CSF) or Hog Cholera Epidemiology 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 )

8 CSF outbreaks in wild boars

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

10 UE Damages from CSF between 1993 and 2000 Country N. Swines removed ( = 1 milion specimen) NL 10 Germany 2 Spaain 1 Belgium < 1 Italy < 1 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  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  RoRo Culling rate [y -1 ] 1

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 A simple two-strains competition model Hosts I 1 infected with a Low Virulent Strain (LVS)  small  1 Hosts I 2 infected with a High Virulent Strain (HVS)  large  2  S I2I2    I1I1     low virulent strain (small  1 ) highly virulent strain (large  2 )  c c c  12 is the super-infection coefficient

15 The new equations (LVS vs. HVS ): S' =G(S) -  1 I 1 S -  2 I 2 S - cS I 1 ' =  1 I 1 S - (  1 +  + c) I 1   12 I 1 I 2 I 2 ' =  2 I 2 S - (  2 +  + c) I 2 +  12 I 1 I 2 Assumptions on the LVS vs. HVS Disease induced mortality (virulence)  1 <<  2 Transmission rate  1   2  12  0

16 Basic epidemiological implications of the above assumptions Basic Reproductive rate (LVS vs. HVS ) Threshold density for disease eradication K T1 < K T2  LVS can persist in a very sparse population

17 Further epidemiological implications If there is no super-infection (  12 =0)  LVS always outcompetes HVS

18 If there is super-infection (  12 >0) If  LVS outcompetes HVS If  HVS outcompetes LVS If  LVS and HVS coexist where

19 If there is super-infection (  12 >0) If  LVS outcompetes HVS If  HVS outcompetes LVS If  LVS and HVS coexist where

20 Further epidemiological implications If hog population density K (as well  12 ) is sufficiently high, then HVS can coexist with, or even outcompete LVS  decreasing population density by culling migh increase the chance of LVS to outcompete HVS

21 Epidemiology of classical swine fever in wild boars of Eastern Sardinia LVS HVS from a field survey by Guberti (1998) and Artois et al. (2002)

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

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

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25 Number of infected individuals

26 The SI 2 R model (Susceptible-Infected-Recovered)  S I2I2    I1I1     LVS (small  1 ) HVS (large  2 ) R    c c c  c

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28 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)

29 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

30 General Information Classical swine fever (CSF) can be found among pigs and wild boar. Transmission takes place through direct contact between animals (secretions, excretions, semen, blood) or indirect contact through vehicles, clothes, instruments, needles, insufficiently cooked waste food fed to pigs; it can also be spread by pig traders and farm visitors. The infection is transplacental. Sources for the virus are blood and all tissues, secretions and excretions of sick and dead animals. Congenitally infected piglets are persistently viraemic and may shed the virus for months. Prevention can be achieved through effective communication between veterinary authorities, veterinary practitioners and pig farmers, effective disease reporting and animal identification system, a strict import policy for live pigs, fresh and cured meat, quarantine of pigs before admission into the herd, sterilisation or prohibition of waste food and serological surveillance. In case of outbreaks in the EU, one needs to resort to the slaughtering of all pigs of affected farms and destruction of cadavers and bedding, disinfection, designation of infected zone, control of pig movement, epidemiological investigation, tracing of sources and spread, surveillance of infected zone. If appropriate, emergency vaccination can also be used. It is a List A disease, according to the OIE Classification of Diseases List A disease, according to the OIE Classification of Diseases. This means it is a transmissible disease that has the potential for very serious and rapid spread, irrespective of national borders, that is of serious socio-economic or public health consequence and that is of major importance in the international trade of animals and animal products

31 Focolai di Peste Suina Classica notificati in Italia dal 1995 al 2004 fonte dati: centro di referenza nazionale per le Pesti Suine (IZS Umbria e Marche) regione Sardegna Toscana Emilia Romagna Lazio Campania Trentino Alto Adige Lombardia Umbria Piemonte Totale

32 Focolai di Peste Suina Classica notificati in UE dal 1995 al 2004 fonte dati: centro di referenza nazionale per le Pesti Suine (IZS Umbria e Marche) nazione Austria Belgio Danimarca Finlandia Francia Germania Grecia Spagna Irlanda Italia Lussemburgo Paesi Bassi Portogallo Slovacchia Svezia GranBretagna totale


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