Effects of Inactivated Mycobacterium bovis Vaccination on Molokai-Origin Wild Pigs Experimentally Infected with Virulent M. bovis.

Pauline Nol,Ramón A Juste,Suelee Robbe-Austerman,Christian Gortázar,Jack C Rhyan,Morgan E Wehtje,Joaquín Vicente,Laurie A Baeten,Richard A Bowen,Kristina Lantz,Iker A Sevilla,Tyler C Thacker

doi:10.3390/pathogens9030199

Abstract

The wild pig population on Molokai, Hawaii, USA is a possible reservoir for bovine tuberculosis, caused by Mycobacterium bovis, and has been implicated in decades past as the source of disease for the island’s domestic cattle. Heat-inactivated vaccines have been effective for reducing disease prevalence in wild boar in Spain and could prove useful for managing M. bovis in Molokai wild pigs. We designed an experiment to test this vaccine in wild pigs of Molokai genetics. Fifteen 3–4-month-old pigs were orally administered 106–107 colony forming units (cfu) of heat-inactivated M. bovis (Vaccinates; n = 8; 0.2 mL) or phosphate buffered saline (Controls; n = 7; 0.2 mL). Each dose was administered in a 0.5 mL tube embedded in a fruit candy/cracked corn mix. Boosters were given seven weeks post-prime in the same manner and dose. Nineteen weeks post-prime, pigs were orally challenged with 1 × 106 cfu of virulent M. bovis. Twelve weeks post-challenge, pigs were euthanized and necropsied, at which time 23 different tissues from the head, thorax, and abdomen were collected and examined. Each tissue was assigned a lesion score. Ordinal lesion score data were analyzed using non-parametric Wilcoxon Signed Rank test. Effect size was calculated using Cohen’s d. Four of eight Vaccinates and four of seven Controls had gross and microscopic lesions, as well as culture-positive tissues. Vaccinates had statistically lower lesion scores than Controls in the following areas: gross thoracic lesion scores (p = 0.013 Cohen’s d = 0.33) and microscopic thoracic lesion scores (p = 0.002, Cohen’s d = 0.39). There were no differences in head lesion scores alone, both gross and microscopic, nor were there differences when comparing combined gross and microscopic head and thoracic lesion scores. These results are indicative that this vaccination protocol affords a modest degree of infection containment with this vaccine in Molokai wild pigs.

Highlights

Animal tuberculosis (TB), caused primarily by Mycobacterium bovis, a member of the Mycobacterium tuberculosis Complex (MTBC), is a globally significant disease that affects numerous livestock and wildlife species, as well as humans
An effective vaccine against TB would be useful for managing M. bovis in the Molokai wild pig population, if the disease were determined to be endemic
Oral vaccination and boost by heat-inactivated Mycobacterium bovis shows moderate efficacy in controlling disease severity in wild pigs of Molokai genetics when experimentally infected with virulent M. bovis nineteen weeks after prime and twelve weeks after boost vaccination

Summary

Introduction

Animal tuberculosis (TB), caused primarily by Mycobacterium bovis, a member of the Mycobacterium tuberculosis Complex (MTBC), is a globally significant disease that affects numerous livestock and wildlife species, as well as humans. The presence of TB in free-ranging wildlife populations has impeded eradication efforts in the United States, as well as worldwide. In addition to the disease being endemic in white-tailed deer (Odocoileus virginianus) in the northeastern part of the state of Michigan, it is possible, but not established, that wild pigs (sometimes referred to as feral swine; Sus scrofa) on the Hawaiian island of Molokai may maintain the infection as well. Mycobacterium bovis-infected swine have periodically been detected on the island, and were thought to be the source of spillover to livestock in previous decades [3,4]. An effective vaccine against TB would be useful for managing M. bovis in the Molokai wild pig population, if the disease were determined to be endemic

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