Will we ever eradicate animal tuberculosis?

Two characteristics of the Mycobacterium tuberculosis complex (MTC) are particularly relevant for tuberculosis (TB) epidemiology and control, namely the ability of this group of pathogens to survive in the environment and thereby facilitate indirect transmission via water or feed, and the capacity to infect multiple host species including human beings, cattle, wildlife, and domestic animals other than cattle. As a consequence, rather than keeping the focus on certain animal species regarded as maintenance hosts, we postulate that it is time to think of complex and dynamic multi-host MTC maintenance communities where several wild and domestic species and the environment contribute to pathogen maintenance. Regarding the global situation of animal TB, many industrialized countries have reached the Officially Tuberculosis Free status. However, infection of cattle with M. bovis still occurs in most countries around the world. In low- and middle-income countries, human and animal TB infection is endemic and bovine TB control programs are often not implemented because standard TB control through testing and culling, movement control and slaughterhouse inspection is too expensive or ethically unacceptable. In facing increasingly complex epidemiological scenarios, modern integrated disease control should rely on three main pillars: (1) a close involvement of farmers including collaborative decision making, (2) expanding the surveillance and control targets to all three host categories, the environment, and their interactions, and (3) setting up new control schemes or upgrading established ones switching from single tool test and cull approaches to integrated ones including farm biosafety and vaccination.

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

This study aimed to assess the efficacy of a heat-inactivated Mycobacterium caprae (HIMC) vaccine in goats experimentally challenged with the same strain of M. caprae. Twenty-one goats were divided into three groups of seven: vaccinated with heat-inactivated Mycobacterium bovis (HIMB), with HIMC and unvaccinated. At 7 weeks post-vaccination all animals were endobronchially challenged with M. caprae. Blood samples were collected for immunological assays and clinical signs were recorded throughout the experiment. All goats were euthanized at 9 weeks post-challenge. Gross pathological examination, analysis of lung pathology using computed tomography, and bacterial load quantification in pulmonary lymph nodes (LN) by qPCR were carried out. Only HIMC vaccinated goats showed a significant reduction of lung lesions volume and mycobacterial DNA load in LN compared to unvaccinated controls. Both vaccinated groups showed also a significant reduction of the other pathological parameters, an improved clinical outcome and a higher proportion of IFN-γ-producing central memory T cells after vaccination. The results indicated that homologous vaccination of goats with HIMC induced enhanced protection against M. caprae challenge by reducing lung pathology and bacterial load compared to the heterologous vaccine (HIMB). Further large-scale trials are necessary to assess the efficacy of autovaccines under field conditions.

Advance in strategies to build efficient vaccines against tuberculosis

Tuberculosis is a chronic consumptive infectious disease, which can cause great damage to human and animal health all over the world. The emergence of multi-drug resistant strains, the unstable protective effect of Bacillus Calmette-Guérin (BCG) vaccine on adults, and the mixed infection with HIV all warn people to exploit new approaches for conquering tuberculosis. At present, there has been significant progress in developing tuberculosis vaccines, such as improved BCG vaccine, subunit vaccine, DNA vaccine, live attenuated vaccine and inactivated vaccine. Among these candidate vaccines, there are some promising vaccines to improve or replace BCG vaccine effect. Meanwhile, the application of adjuvants, prime-boost strategy, immunoinformatic tools and targeting components have been studied concentratedly, and verified as valid means of raising the efficiency of tuberculosis vaccines as well. In this paper, the latest advance in tuberculosis vaccines in recent years is reviewed to provide reliable information for future tuberculosis prevention and treatment.

Development and Challenges in Animal Tuberculosis Vaccination

Vaccination with Bacillus Calmette-Guérin (BCG) constituted a major advance in the prevention of human tuberculosis (TB) in the beginning of the past century. BCG has also a clear potential for use in animals and, in particular, in the main domestic species subjected to TB control programs, cattle. Nowadays, the use of BCG vaccination against TB in cattle is not permitted by European Union legislation because BCG can induce a cellular immune response producing diagnostic interference in the eradication programs based on tuberculin single and comparative intradermal tests imposed worldwide. In this review we recall the history of TB vaccination as well as different vaccine trials and the response to vaccination in both domestic and wild animals. Promising potential inactivated vaccines are also reviewed. Research studies are mainly focused to improve vaccine efficacy, and at the same time to ensure its easy administration, safety and stability in the environment. Great challenges remain, particularly in terms of vaccine candidates and also in the acceptance of vaccination. Vaccination should be included in a strategic plan for integrated control of TB under a "one health" perspective, which also includes other measures such as improved biosafety on farms to avoid or decrease contact between domestic and wild animals or control of wildlife reservoirs to avoid overabundance that may favor infection maintenance.

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

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.

Tuberculosis vaccination sequence effect on protection in wild boar

The Eurasian wild boar (Sus scrofa) is a reservoir for tuberculosis (TB) in which vaccination is a valuable tool for control. We evaluated the protection and immune response achieved by homologous and heterologous regimes administering BCG and heat-inactivated Mycobacterium bovis (IV). Twenty-one wild boar piglets were randomly allocated in five groups: Control, homologous BCG, homologous IV, heterologous IV-BCG, heterologous BCG-IV. Significant 67% and 66% total lesion score reductions were detected in homologous IV (IVx2) and heterologous IV-BCG groups when compared with Control group (F_4,16 = 6.393, p = 0.003; Bonferroni _{Control vs IVx2} p = 0.026, Tukey _{Control vs IV-BCG} p = 0.021). No significant differences were found for homologous BCG (although a 48% reduction in total lesion score was recorded) and BCG-IV (3% reduction). Heterologous regimes did not improve protection over homologous regimes in the wild boar model and showed variable results from no protection to similar protection as homologous regimes. Therefore, homologous regimes remain the best option to vaccinate wild boar against TB. Moreover, vaccine sequence dramatically influenced the outcome underlining the relevance of studying the effects of prior sensitization in the outcome of vaccination.

Use of the Human Vaccine, Mycobacterium bovis Bacillus Calmette Guérin in Deer

The only vaccine ever approved for human tuberculosis was developed a century ago from an isolate of Mycobacterium bovis derived from a tuberculous cow. Initial safety and efficacy studies of an attenuated version of this isolate were conducted in cattle and other animals. In 1921 the first human, an infant, was orally dosed with this attenuated strain that came to be known as M. bovis bacillus Calmette-Guérin (BCG); named for Albert Calmette and Camille Guérin, the two French scientists that developed the strain. Since 1921, billions of people have been vaccinated with BCG making it the oldest, most widely used, and safest vaccine in use today. It is also the tuberculosis vaccine most studied for use in wildlife, including deer. While BCG vaccination of deer may not reliably prevent infection, it consistently decreases lesion severity, minimizing large, necrotic lesions, which often contain large numbers of bacilli. It is believed that decreased lesion severity correlates with decreased disease transmission; however, this hypothesis remains to be proven. Safety studies in white-tailed deer show BCG may persist in lymphoid tissues for up to 12 months; a factor to be considered in deer used for food. Beyond efficacy and safety, methods of vaccine delivery to free-ranging deer are also under investigation, both in the laboratory and in the field. The ideal delivery method is effective, efficient and safe for non-target species, including livestock. Ingestion of BCG by cattle is of special concern as such cattle may present as "false positives" using currently approved diagnostic methods, thus interfering with efforts by animal health agencies to monitor cattle for tuberculosis. An effective BCG vaccine for deer would be of value in regions where free-ranging deer represent a potential source of M. bovis for livestock. Such a vaccine would also be beneficial to farmed deer where M. bovis represents a serious threat to trade and productivity.

Response of goats to intramuscular vaccination with heat-killed Mycobacterium bovis and natural challenge

Caprine TB causes chronic disease with severe economic and health consequences. We assessed the effect of intramuscularly administered heat-inactivated Mycobacterium bovis (M. bovis) on 20 kid goats (10 vaccinated, 10 controls), naturally exposed to M. caprae through close contact with infected goats. At necropsy, visible TB-compatible lesions were recorded in all animals with the exception of 1 control and 2 vaccinated goats. The median of the total lesion score was 9 (IQR 3-13.5) and 5 (IQR 3-6.75) in control and vaccinated goats, respectively (median lesion reduction 44.4%, p =  0.224). The lung lesions of the vaccinated goats were restricted to the caudal lobes, while 6 controls had additional lung lobes affected (p =  0.01). The median lung lesion score reduction in vaccinated goats was 100%; however, this reduction was not significant (p =  0.124), possibly due to the low sample size. Regarding the abdomen, only one vaccinated goat presented visible lesions compared to three goats in the control group. The results provide further evidence of the potential of heat-inactivated M. bovis for controlling TB in different host species, including ruminants.

Efficacy of parenteral vaccination against tuberculosis with heat-inactivated Mycobacterium bovis in experimentally challenged goats

Tuberculosis (TB) in animals is a re-emerging disease with a wide range of hosts that causes large economic losses in livestock. Goats are particularly susceptible to TB and, in endemic areas, vaccination may be a valuable measure to control the disease. The main aim of this study was to evaluate the efficacy of parenteral vaccination of goats with a heat-inactivated Mycobacterium bovis (HIMB) vaccine, and compare it to M. bovis Bacille Calmette–Guérin (BCG) vaccine. Twenty-four goat kids were divided in 3 groups as following: HIMB vaccinated group (n = 8), BCG vaccinated group (n = 8) and unvaccinated group (n = 8). Afterwards, goats were experimentally challenged with Mycobacterium caprae by the endobronchial route. Antigen specific interferon-γ release assays and serology were performed after vaccination and challenge. Pathological and bacteriological parameters were evaluated after necropsy at 9 weeks post-challenge (p.c.). HIMB vaccine showed similar levels of protection to BCG in terms of volume reduction of thoracic TB lesions, presence of extra-pulmonary lesions, as well as a slight reduction of bacterial load in pulmonary lymph nodes. Moreover, HIMB vaccine did not induce interferences on the interferon-γ release assay based on reagents previously developed to differentiate infected from BCG vaccinated individuals. The results indicate that HIMB is a suitable vaccine candidate for further larger-scale trials under field conditions in goats.

Management of hunting waste as control measure for tuberculosis in wild ungulates in south-central Spain

In recent decades, habitat change and the intensive management of wild ungulates for hunting have led to an increase in their populations in south-central Spain. This implies a higher generation of hunting waste, which can favour the transmission of infectious diseases, including tuberculosis (TB). The aim of this study was to assess the usefulness of the proper disposal of hunting waste as TB control measure in wild boar (Sus scrofa) and red deer (Cervus elaphus) during the 2008/2009 to 2016/2017 hunting seasons. Blood samples from 664 wild boar and 934 red deer were obtained in 14 game estates in two provinces in Andalusia (Area 1), where the disposal of hunting waste was implemented since the 2012/2013 hunting season. Besides, six game estates in the province of Ciudad Real, in Castilla-La Mancha (Area 2), an adjacent region where this management measure was not implemented during the studied period, were used as controls, sampling 277 wild boar and 427 red deer sera. The Mycobacterium tuberculosis complex (MTC), seroprevalence detected in wild boar from Area 1, was significantly higher before the disposal of big game hunting by-products (82.8%; 2008/2009-2012/2013) compared to the second period (61.8%; 2013/2014-2016/2017) (p < .001), after this control measure became established. By contrast, no significant differences between periods were found in wild boar (41.3% versus 44.8%; p = .33) and red deer (14.9% versus 11.6%; p = .19) from Area 2 as well as in red deer (10.8% versus 10.5%; p = .48) from Area 1. The proper disposal of hunting waste contributed to achieve a 25% reduction in MTC seroprevalence in wild boar. These results are of particular relevance regarding wild boar in the current context of re-emerging and emerging diseases such as TB and African Swine Fever in Europe. Further studies are needed to assess the effect of this measure on the health status of livestock and other wildlife species.

Oral Vaccination with Heat-Inactivated Mycobacterium bovis Does Not Interfere with the Antemortem Diagnostic Techniques for Tuberculosis in Goats

Vaccination against tuberculosis (TB) is prohibited in cattle or other species subjected to specific TB eradication campaigns, due to the interference that it may cause with the official diagnostic tests. However, immunization with a heat-inactivated (HI) Mycobacterium bovis vaccine via the oral route has been suggested to overcome this issue. In this study, the main goal was to assess the interference of the HI vaccine by different routes of administration using a previous vaccination and re-vaccination (boosting) protocol. TB-free kid goats were divided into three groups: oral (n = 16), intramuscular (IM; n = 16), and control (n = 16). Results showed that there was a significant difference in the percentage of animals positive to the single intradermal test (SIT) and blood based interferon-gamma release assay (IGRA) caused by vaccination when performed in the IM group compared to the oral group (p < 0.001). Nevertheless, no positivity to the SIT or IGRA test was observed in orally vaccinated goats regardless of the different interpretation criteria applied. None of the groups presented positive antibody titers using an in-house ELISA and samples collected 2 months after the boost. These results suggest the potential usefulness of the HI vaccine by the oral route in goats to minimize the interference on diagnostic tests (skin and IGRA tests) and reducing the necessity of defined antigens to replace the traditional purified protein derivatives for diagnosis. Finally, the results pave the way to future efficacy studies in goats using different routes of HI vaccination.