Dynamics of Aujeszky's disease virus infection in wild boar in enzootic scenarios

Aujeszky's disease (AD) virus is enzootic in Iberian wild boar, thus posing a threat to the official eradication of AD on extensive domestic pig farms in Spain. Understanding the dynamics and drivers of ADV infection in wild boar will help prevent viral transmission at the wild boar-pig interface. This study analyses the dynamics of ADV infection in wild boar and tests relevant hypotheses in order to identify drivers of ADV infection dynamics. Wild boar sera (N = 971) and oropharyngeal tonsils (TN, N = 549) collected over 11 consecutive years in south-western Spain were tested for ADV antibodies and DNA, respectively. We tested the hypotheses that population immunity modulates the risk of ADV infection (H₁ ) and that detecting ADV DNA in TN is a good proxy of the annual ADV infection pressure (H₂ ). This was done by building logistic regression models that were subsequently employed to test the influence of a series of host population and host individual factors-including predictors of ADV immunity in the population-on the annual risk of new ADV infections and on the presence of ADV DNA in TN. The premise of H₁ was that there would be a negative association between the proportion of ADV antibody-positive wild boar in a given year and the risk of ADV infection of naïve individuals. There was, however, a positive association, and H₁ was, therefore, rejected. If detecting ADV in TN had been a good indicator of ADV infection pressure, a positive association with the proportion of ADV antibody-positive wild boar would have been found. However, this was not the case and H₂ was also rejected. We confirmed that ADV infection is a dynamic phenomenon. The risk of infection with ADV can change considerably between consecutive years, and these changes are positively associated with the proportion of infected wild boar in the population.

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.

The Critical Role of Infectious Disease in Compensatory Population Growth in Response to Culling

Despite the ubiquity of disease in nature, the role that disease dynamics play in the compensatory growth response to harvesting has been ignored. We use a mathematical approach to show that harvesting can lead to compensatory growth due to a release from disease-induced mortality. Our findings imply that culling in systems that harbor virulent parasites can reduce disease prevalence and increase population density. Our models predict that this compensation occurs for a broad range of infectious disease characteristics unless the disease induces long-lasting immunity in hosts. Our key insight is that a population can be regulated at a similar density by disease or at reduced prevalence by a combination of culling and disease. We illustrate our predictions with a system-specific model representing wild boar tuberculosis infection, parameterized for central Spain, and find significant compensation to culling. Given that few wildlife diseases are likely to induce long-lived immunity, populations with virulent diseases may often be resilient to harvesting.

Is targeted removal a suitable means for tuberculosis control in wild boar?

We assessed the suitability of targeted removal as a means for tuberculosis (TB) control on an intensely managed Eurasian wild boar (Sus scrofa) hunting estate. The 60km² large study area included one capture (treatment) site, one control site, and one release site. Each site was fenced. In the summers of 2012, 2013 and 2014, 929 wild boar were live-captured on the treatment site. All wild boar were micro-chipped and tested using an animal side lateral flow test immediately after capture in order to detect antibodies to the Mycobacterium tuberculosis complex (MTC). The wild boar were released according to their TB status: Seropositive individuals onto the release site (hunted after summer), and seronegative individuals back onto the treatment site. The annual summer seroprevalence of antibodies to the MTC declined significantly in live-captured wild boar piglets from the treatment site, from 44% in 2012 to 27% in 2013 (a reduction of 39%). However, no significant further reduction was recorded in 2014, during the third capture season. Fall-winter MTC infection prevalence was calculated on the basis of the culture results obtained for hunter-harvested wild boar. No significant changes between hunting seasons were recorded on either the treatment site or the control site, and prevalence trends over time were similar on both sites. The fall-winter MTC infection prevalence on the release site increased significantly from 40% in 2011-2012 to 64% in 2012-2013 and 2013-2014 (60% increase). Recaptures indicated a persistently high infection pressure. This experiment, the first attempt to control TB in wild boar through targeted removal, failed to reduce TB prevalence when compared to the control site. However, it generated valuable knowledge on infection pressure and on the consequences of translocating TB-infected wild boar.

Environmental Presence of Mycobacterium tuberculosis Complex in Aggregation Points at the Wildlife/Livestock Interface

The members of the Mycobacterium tuberculosis complex (MTC) cause tuberculosis (TB). Infection is transmitted within and between livestock and wildlife populations, thus hampering TB control. Indirect transmission might be facilitated if MTC bacteria persist in the environment long enough to represent a risk of exposure to different species sharing the same habitat. We have, for the first time, addressed the relationship between environmental MTC persistence and the use of water resources in two TB endemic areas in southern Spain with the objective of identifying the presence of environmental MTC and its driving factors at ungulates' water aggregation points. Camera-trap monitoring and MTC diagnosis (using a new MTC complex-specific PCR technique) were carried out at watering sites. Overall, 55.8% of the water points tested positive for MTC in mud samples on the shore, while 8.9% of them were positive in the case of water samples. A higher percentage of MTC-positive samples was found at those waterholes where cachectic animals were identified using camera-trap monitoring, and at the smallest waterholes. Our results help to understand the role of indirect routes of cross-species TB transmission and highlight the importance of certain environmental features in maintaining infection in multihost systems. This will help to better target actions and implement control strategies for TB at the wildlife/livestock interface.

Comparative Genomics of Field Isolates of Mycobacterium bovis and M. caprae Provides Evidence for Possible Correlates with Bacterial Viability and Virulence

Mycobacteria of the Mycobacterium tuberculosis complex (MTBC) greatly affect humans and animals worldwide. The life cycle of mycobacteria is complex and the mechanisms resulting in pathogen infection and survival in host cells are not fully understood. Recently, comparative genomics analyses have provided new insights into the evolution and adaptation of the MTBC to survive inside the host. However, most of this information has been obtained using M. tuberculosis but not other members of the MTBC such as M. bovis and M. caprae. In this study, the genome of three M. bovis (MB1, MB3, MB4) and one M. caprae (MB2) field isolates with different lesion score, prevalence and host distribution phenotypes were sequenced. Genome sequence information was used for whole-genome and protein-targeted comparative genomics analysis with the aim of finding correlates with phenotypic variation with potential implications for tuberculosis (TB) disease risk assessment and control. At the whole-genome level the results of the first comparative genomics study of field isolates of M. bovis including M. caprae showed that as previously reported for M. tuberculosis, sequential chromosomal nucleotide substitutions were the main driver of the M. bovis genome evolution. The phylogenetic analysis provided a strong support for the M. bovis/M. caprae clade, but supported M. caprae as a separate species. The comparison of the MB1 and MB4 isolates revealed differences in genome sequence, including gene families that are important for bacterial infection and transmission, thus highlighting differences with functional implications between isolates otherwise classified with the same spoligotype. Strategic protein-targeted analysis using the ESX or type VII secretion system, proteins linking stress response with lipid metabolism, host T cell epitopes of mycobacteria, antigens and peptidoglycan assembly protein identified new genetic markers and candidate vaccine antigens that warrant further study to develop tools to evaluate risks for TB disease caused by M. bovis/M.caprae and for TB control in humans and animals.

Mycobacteria belonging to the Mycobacterium tuberculosis complex infect humans and animals since pre-history and are a serious health problem worldwide. Whole-genome sequencing and comparative genomics generate information on the evolution and molecular basis of pathogenicity and transmissibility. However, while genomic information is increasingly available for the main human pathogens such as Mycobacterium tuberculosis, little is known about closely related bacteria, Mycobacterium bovis and Mycobacterium caprae. These mycobacteria infect humans causing zoonotic tuberculosis and are the main causative agents of animal tuberculosis. Although human-to-human transmission of zoonotic tuberculosis is limited, the infection often causes extra-pulmonary disease in humans and is still a major public health concern in developing countries, causing not only human disease but also severe effects on livelihoods. In this study, whole-genome sequences and targeted comparative genomics of three Mycobacterium bovis and one Mycobacterium caprae field isolates generated new information on the evolution and phenotypic variation of these mycobacteria. The results identified new genetic markers and candidate vaccine antigens that warrant further study to develop tools to evaluate risks for tuberculosis caused by M. bovis/M.caprae and for disease control in humans and animals.

Bacterial membranes enhance the immunogenicity and protective capacity of the surface exposed tick Subolesin-Anaplasma marginale MSP1a chimeric antigen

Ticks are vectors of diseases that affect humans and animals worldwide. Tick vaccines have been proposed as a cost-effective and environmentally sound alternative for tick control. Recently, the Rhipicephalus microplus Subolesin (SUB)-Anaplasma marginale MSP1a chimeric antigen was produced in Escherichia coli as membrane-bound and exposed protein and used to protect vaccinated cattle against tick infestations. In this research, lipidomics and proteomics characterization of the E. coli membrane-bound SUB-MSP1a antigen showed the presence of components with potential adjuvant effect. Furthermore, vaccination with membrane-free SUB-MSP1a and bacterial membranes containing SUB-MSP1a showed that bacterial membranes enhance the immunogenicity of the SUB-MSP1a antigen in animal models. R. microplus female ticks were capillary-fed with sera from pigs orally immunized with membrane-free SUB, membrane bound SUB-MSP1a and saline control. Ticks ingested antibodies added to the blood meal and the effect of these antibodies on reduction of tick weight was shown for membrane bound SUB-MSP1a but not SUB when compared to control. Using the simple and cost-effective process developed for the purification of membrane-bound SUB-MSP1a, endotoxin levels were within limits accepted for recombinant vaccines. These results provide further support for the development of tick vaccines using E. coli membranes exposing chimeric antigens such as SUB-MSP1a.

Do wild ungulates allow improved monitoring of flavivirus circulation in Spain?

As a response to the need for improved and cost-efficient West Nile virus (WNV) and other flavivirus surveillance tools, we tested 887 juvenile free-living red deer, 742 free-living juvenile wild boar, and 327 farmed deer, to detect temporal variability in exposure to these viruses. Thirty of 742 wild boar samples (4%; 95% CI 2.8,5.7) yielded a positive ELISA result. Antibody-positive individuals had been sampled between 2003 and 2011 in localities from central and southern Spain. No wild boar from the northern half of Spain (n=120) tested positive. Regarding juvenile wild red deer, only two out of 887 samples yielded a positive ELISA result (0.2%; 95% CI 0.1,0.8). These two samples came from the same site and sampling year. The likelihood of detecting contact with WNV or cross-reacting flaviviruses was 18 times higher among juvenile wild boar than among juvenile red deer. ELISA positivity among farmed deer increased 10-fold after local flavivirus outbreaks recorded in the summer and autumn of 2010. This survey demonstrated the potential usefulness of juvenile wild ungulates, particularly wild boar, as suitable flavivirus sentinels in southwestern Europe, and that systematic serum banking of samples from hunter-harvested wildlife or from individual farmed ungulates provides valuable material for retrospective epidemiological surveys and future disease monitoring.