R-10577: Part 1 of a two-part project Pigs as Hosts of Bovine Tuberculosis in New Zealand – a review G. Nugent, B. Reddiex, J. Whitford, and I. Yockney Landcare Research PO Box 69, Lincoln 8152 New Zealand Landcare Research Contract Report: LC0203/098 PREPARED FOR: Animal Health Board PO Box 3412, Wellington DATE: May 2003 Reviewed by: Approved for release by: Tony Arthur Phil Cowan Scientist Science Manager Landcare Research Biosecurity and Pest Management DOI: https://doi.org/10.7931/rd25-2860 © Landcare Research New Zealand Ltd 2003 No part of this work covered by copyright may be reproduced or copied in any form or by any means (graphic, electronic or mechanical, including photocopying, recording, taping, information retrieval systems, or otherwise) without the written permission of the publisher. 3 Contents Summary .............................................................................................................................................. 4 1. Introduction ................................................................................................................................ 7 2. Background................................................................................................................................. 7 3. Objectives ................................................................................................................................... 8 4. Sources of Information ............................................................................................................... 8 5. Population Status and Resource Value of Feral Pigs ................................................................. 9 5.1 Distribution, population size, and densities ...................................................................... 9 5.2 Pig liberations ................................................................................................................... 9 5.3 Population dynamics ....................................................................................................... 11 5.4 Resource value ................................................................................................................ 11 6. Home Range Size and Movement Patterns of Feral Pigs ......................................................... 12 6.1 Home range sizes ............................................................................................................ 12 6.2 Dispersal and maximum movement distances ................................................................ 12 7. Feral Pigs and Bovine Tuberculosis ......................................................................................... 15 7.1 Pathogenesis .................................................................................................................... 15 7.2 Patterns of prevalence and persistence ............................................................................ 16 7.3 Geographic distribution of Tb in feral pigs in New Zealand .......................................... 20 7.4 Susceptibility to Tb and use of pigs as sentinels ............................................................. 22 7.5 Transmission from feral pigs .......................................................................................... 23 8. Communicating with the pig hunters ....................................................................................... 25 8.1 Pig hunting clubs, competitions and magazines ............................................................. 25 8.2 Individual hunters ........................................................................................................... 26 9. Control of feral pigs.................................................................................................................. 27 9.1 Current control ................................................................................................................ 27 9.2 Effect of recreational hunting on pig density .................................................................. 27 9.3 Control methods .............................................................................................................. 27 10. Discussion................................................................................................................................. 30 11. Recommendations .................................................................................................................... 33 12. Acknowledgements .................................................................................................................. 34 13. References ................................................................................................................................ 34 14. Appendices ............................................................................................................................... 41 14.1 Partial list of pig hunting clubs in New Zealand (postal addresses as at November 2002) ............................................................................................................................... 41 14.2 Postal survey sent to pig hunting clubs in New Zealand ................................................ 42 14.3 Phone survey of pig hunters ............................................................................................ 44 Landcare Research 4 Summary Project and Client The role of feral pigs as hosts and vectors of bovine tuberculosis (Tb) in New Zealand and the consequent need for pig control was reviewed by Landcare Research, Lincoln, for the Animal Health Board (Phase 1 of the two-part project R-10577) during August 2002 – March 2003. Objectives Review the role of feral pigs and pig hunting in the maintenance and spread of bovine tuberculosis, by: • updating 1990s reviews of pig ecology, Tb-host status and control methods; • providing an initial assessment of whether and, if so, how pigs and pig hunters might be managed as part of vector control operations. Main Findings Feral pigs occupy 35% of New Zealand (c. 93 000 km2). Most New Zealanders, and Māori in particular, view feral pigs more as resources than as pests. In 1988, an estimated 23 000 hunters hunted pigs on c. 300 000 days and killed about 100 000 pigs. Pigs have a high reproductive rate, and unhunted populations can potentially double within a year. They are widespread within Tb-infected areas in New Zealand, but are usually held at low numbers by recreational hunting with dogs. The prevalence of Tb in pigs in some areas is high (>90%). Despite this, hunters currently have a limited knowledge of Tb. Hunters translocate pigs to boost hunting opportunities, and also transport potentially infected carcasses long distances. A few dispose of carcass parts in uninfected areas where they could be scavenged by other wildlife. The best routes for providing hunters with information are presentations at hunting competitions and club meetings, and via hunting magazines and newsletters. Pigs occupy large home ranges. In good habitat, most range sizes are likely to be less than 10 km2, at least for sows, and a 6-km radius around a pig kill site is likely to encompass most of its home range. However, shifts of up to 35 km have been recorded in New Zealand. Pigs appear to be much more susceptible to becoming infected with Tb than cattle, deer, or possums in the same area. This underpins use of pigs as sentinels for detecting the presence of Tb. Most recent data on prevalence, lesion distribution, and pathogenesis are consistent with pigs becoming infected through scavenging. Some field evidence suggests that possum control substantially reduces the force of infection in pigs. There are no new data that contradict the long-standing inference that pigs are not maintenance hosts in New Zealand. The rate of intraspecific transmission in the wild appears insufficient to independently maintain Tb in pigs. However, we speculate scavenging of pigs by pigs could help maintain Tb for several generations when both pig density and their Tb prevalence are initially high. Landcare Research 5 Despite the weight of evidence pointing to spillover-end-host status, some circumstantial evidence suggests pigs may play a role in the maintenance and spread of Tb in New Zealand. Infected pig carcasses may transmit Tb to scavengers such as ferrets, cats, and possibly also possums, but there is no evidence indicating direct transmission from live pigs to possums, deer, or cattle. Grossly infected pigs sometimes have large suppurating lesions, so spread via environmental contamination is also possible, but the rarity of infection in susceptible and commonly sympatric species such as sheep and rabbits suggests such transmission is rare. Overall, our assessment is that there is no strong new imperative for pig control within areas of established infection provided both possums and ferrets are reduced to low densities. Where pig and ferret densities are both high, it is possible that pig control could speed the reduction in reactor rates by at least removing some of the reservoir of infection in the area. At the margins of infected areas, pig control might reduce the risk of Tb spread, but such control would be costly and be opposed by hunters. It is likely to be more cost-effective to detect totally new outbreaks as they occur rather than to undertake prophylactic vector control to prevent them. Annual maintenance control is required to maintain a consistently low density of pigs. There is a wide range of affordable