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

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 10 6 -10 7 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 × 10 6 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

A proof-of-concept study to investigate the efficacy of heat-inactivated autovaccines in Mycobacterium caprae experimentally challenged goats

“…Nowadays, there is a certain degree of background regarding the inactivated vaccine candidate based on M. bovis (HIMB), that have been reported to protect goats against TB in laboratory and field conditions 18,20 , as well as cattle 21 , wild boar 22,23 , red deer 24 , badgers 25 and wild pigs 26 . HIMB represents an interesting alternative to live-attenuated BCG vaccine since the deployment logistics are simpler and the strain survival is not required 27 .…”

Development and Challenges in Animal Tuberculosis Vaccination

“…Further studies are needed for evaluating whether this vaccination scheme could reduce the TB lesion prevalence in adult boars. The effectiveness of HIMB was also proved in Molokai wild pigs, where the oral vaccination induced a modest degree of infection containment [16].…”

“…Inactivated vaccines are expected to be more stable in baits (i.e., under high environmental temperatures) and safer as they are based on dead bacteria [14]. Recent studies have begun to address these concerns by demonstrating that oral vaccination with a HIMB successfully protects captive wild boar (Sus scrofa) [15], Molokai-origin wild pigs [16], red deer (Cervus elaphus) [17] and badgers (Meles meles) [18] against progressive disease. Field studies have also shown protective effect of oral HIMB vaccination in piglets against TB in endemic free-ranging wild boar populations [14].…”

“…Greater false positives with a higher vaccine dose [30] All studies included a non-vaccinated group (control); * Infection at different times upon vaccination (6, 26 and 52 weeks post-vaccination); BCG-Bacillus Calmette-Guérin; HIMB-heat-inactivated Mycobacterium bovis vaccine; DXM-dexamethasone; SC-subcutaneous; IT-intratonsillar, ITC-intratracheal; CFU-colony forming units; PBMC-peripheral blood mononuclear cells; LST-lymphocyte stimulation test; TBL-tuberculosis-like macroscopic lesions; IGRA-interferon gamma (IFNγ) release assay; MAPIA-multi-antigen printing immunoassay; IB-immunoblot; IL-interleukins; C3-complement factor 3. Partial vaccine efficacy [16] All studies included a non-vaccinated group (control). BCG-Bacillus Calmette-Guérin (all studies used the Danish strain); HIMB-heat-inactivated Mycobacterium bovis vaccine; IM-intramuscular; CFU-colony forming units; OF-oropharyngeal; *-Assays performed under field conditions, so infective dose cannot be determined; TBL-tuberculosis-like macroscopic lesions; IGRA-interferon gamma (IFNγ) release assay; ILs-interleukins; C3-Complement factor 3; MUT-methylmalonyl-CoA mutase.…”

Local Lung Immune Response to Mycobacterium bovis Challenge after BCG and M. bovis Heat-Inactivated Vaccination in European Badger (Meles meles)

“…A live vaccine (BCG) for badgers is only licensed in the UK for intramuscular delivery, and its use in oral baits is promising, although the use of a live vaccine at a sufficient protective dose complicates the delivery in the field, especially at high environmental temperatures, impacting the survival of the vaccine [16]. The efficacy of a heat-inactivated M. bovis (HIMB) vaccine has also been assessed in different species, such as wild pigs [17], red deer [18], wild boar [19][20][21], goats [22,23], sheep [24] and zebrafish [25]. A recent experimental study in Spain showed a similar reduction in bacterial loads and lesion scores in BCG and HIMB orally vaccinated badgers [26] compared with non-vaccinated controls.…”