In vivo and ex vivo studies of the immune system in relation to infectious disease that are carried out in the natural target species provide data that are relevant to understanding the biology of the immune cells and immunity to infection. This is particularly the case for diseases that show host specificity. Ex vivo studies that exploit the surgical cannulation of lymphatic ducts have allowed access to natural dendritic cells. Investigations of these cells have revealed the presence of subpopulations that differ in their ability to stimulate T cells and differ in the range of cytokines synthesized. These differences would be forecast to have major effects on the bias and type of immune response that are induced. Studies in vivo of the effect of depleting T-cell populations with monoclonal antibodies (mAbs) have shown how different T-cell populations have differing critical roles for different infectious diseases, and how they may contribute to the immune response and pathology after infection. Here the case is made for how studies in cattle have aided our understanding of immunity to several infections that can be exploited for the rational design of effective vaccination and control strategies.

Transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative disorders of humans and animals associated with an accumulation of abnormal isoforms of prion protein (PrP) in nerve cells. The pathogenesis of TSEs involves conformational conversions of normal cellular PrP (PrPc) to abnormal isoforms of PrP (PrPSc). While the protein-only hypothesis has been widely accepted as a causal mechanism of prion diseases, evidence from more recent research suggests a possible involvement of other cellular component(s) or as yet undefined infectious agent(s) in PrP pathogenesis. Although the underlying mechanisms of PrP strain variation and the determinants of interspecies transmissibility have not been fully elucidated, biochemical and molecular findings indicate that bovine spongiform encephalopathy in cattle and new-variant Creutzfeldt–Jakob disease in humans are caused by indistinguishable etiological agent(s). Cumulative evidence suggests that there may be risks of humans acquiring TSEs via a variety of exposures to infected material. The development of highly precise ligands is warranted to detect and differentiate strains, allelic variants and infectious isoforms of these PrPs. This article describes the general features of TSEs and PrP, the current understanding of their pathogenesis, recent advances in prion disease diagnostics, and PrP inactivation.

Bovine immunodeficiency virus (BIV) was first isolated in 1969 from a cow, R-29, with a wasting syndrome. The virus isolated induced the formation of syncytia in cell cultures and was structurally similar to maedi-visna virus. Twenty years later, it was demonstrated that the bovine R-29 isolate was indeed a lentivirus with striking similarity to the human immunodeficiency virus. Like other lentiviruses, BIV has a complex genomic structure characterized by the presence of several regulatory/accessory genes that encode proteins, some of which are involved in the regulation of virus gene expression. This manuscript aims to review biological and, more particularly, molecular aspects of BIV, with emphasis on regulatory/accessory viral genes/proteins, in comparison with those of other lentiviruses.

Cattle are a major reservoir of Escherichia coli O157:H7, an important zoonotic pathogen that causes hemorrhagic colitis and hemolytic uremic syndrome (HUS). Colonization of cattle occurs predominantly in the large intestine, and may especially target follicle-associated epithelium (FAE) in the terminal rectum. Bacterial colonization involves induction of attaching–effacing (A/E) lesions, mediated by type III secreted proteins and an outer membrane protein called intimin. ToxB, encoded on plasmid pO157, contributes to adherence of E. coli O157:H7 through promotion of the production and/or secretion of type III secreted proteins. Production of type III secreted proteins and intestinal colonization appear to involve quorum-sensing mechanisms. In the human host, E. coli O157:H7 may have a preference for FAE in the distal small intestine. The H7 flagellum induces production of chemokines such as interleukin 8, and neutrophilic infiltration of the intestinal mucosa, which in turn may enhance Shiga toxin (Stx) uptake across the intestinal epithelium. Both Stx and cytokine responses play critical roles in the induction of the vascular lesions that underlie hemorrhagic colitis and HUS. In cattle, Stx binds to intestinal crypt cells and submucosal lymphocytes but not vascular endothelium. The role played by Stx in cattle may be to suppress mucosal immunity, yet enhance other effects that promote intestinal colonization.

Food-borne bacterial illnesses strike more than 76 million North Americans each year. Many of these illnesses are caused by animal-derived foodstuffs. Slaughter and processing plants do an outstanding job in reducing bacterial contamination after slaughter and during further processing, yet food-borne illnesses still occur at an unacceptable frequency. Thus, it is imperative to widen the window of action against pathogenic bacteria. Attacking pathogens on the farm or in the feedlot will improve food safety all the way to the consumer’s fork. Because of the potential improvement in overall food safety that pre-harvest intervention strategies can provide, a broad range of preslaughter intervention strategies are currently under investigation. Potential interventions include direct anti-pathogen strategies, competitive enhancement strategies and animal management strategies. Included in these strategies are competitive exclusion, probiotics, prebiotics, antibiotics, antibacterial proteins, vaccination, bacteriophage, diet, and water trough interventions. The parallel and simultaneous application of one or more preslaughter strategies has the potential to synergistically reduce the incidence of human food-borne illnesses by erecting multiple hurdles, thus preventing entry of pathogens into the food chain. This review emphasizes work with Escherichia coli O157:H7 to illustrate the various strategies.

Hepatitis E virus (HEV) is an emerging pathogen belonging to a newly recognized family of RNA viruses (Hepeviridae). HEV is an important enterically transmitted human pathogen with a worldwide distribution. It can cause sporadic cases as well as large epidemics of acute hepatitis. Epidemics are primarily waterborne in areas where water supplies are contaminated with HEV of human origin. There is increasing evidence, however, that many animal species are infected with an antigenically similar virus. A recently isolated swine virus is the best candidate for causing a zoonotic form of hepatitis E. The virus is serologically cross-reactive with human HEV and genetically very similar, and the human and swine strains seem to be cross-infective. Very recent evidence has also shown that swine HEV, and possibly a deer strain of HEV, can be transmitted to humans by consumption of contaminated meat. In this review, we discuss the prevalence, pathogenicity, diagnosis and control of human HEV, swine HEV, the related avian HEV and HEV in other hosts and potential reservoirs.

The regulation of gene expression is a fundamental cellular process that is controlled at multiple levels. Abnormal regulation of gene expression has been directly implicated in the pathogenesis of some diseases of animals and humans and may contribute to the disease process in unrecognized ways in many others. Furthermore, novel treatment strategies for a number of different diseases may hinge upon our ability to exploit mechanisms that normally alter the expression of endogenous genes. While the study of gene regulation has traditionally focused on transcription as a major regulator of gene expression, it has recently become apparent that the post-transcriptional control of gene expression may play an equally important role. In particular, rapid, context-specific regulation of the stability of mRNA transcripts encoding highly active proteins, such as cytokines, growth factors, oncogenes and cell-cycle regulators, appears to play a key role in the control of these molecules and the processes they mediate. Many of the known regulatory pathways for mRNA stability involve proteins that interact with specific AU-rich elements in the 3′-untranslated region of the transcript. This review will address some important aspects of the normal regulation of mRNA stability and known or potential contributions of RNA stability regulation to health and disease.

Bovine torovirus (BoTV) is a pleomorphic virus with a spike-bearing envelope and a linear, non-segmented, positive-sense single-stranded RNA genome. This kidney-shaped virus is associated with diarrhea in calves and apparently has a worldwide distribution. This review provides details of the history and taxonomy of BoTV since its discovery in 1979. Information about virion morphology and architecture, antigenic and biological properties, viral genome, protein composition, thermal and chemical stability, and pH and proteolytic enzymes resistance is also summarized. A major focus of this review is to postulate a possible epidemiological cycle for BoTV, based on epidemiological data obtained in our studies and other published data, and progressing from the newborn calf to the adult animal. The distribution, host range, pathogenesis, disease and clinical signs (under experimental and natural exposure), pathology, diagnosis, prevention, treatment and control of BoTV infections are also described. In addition, a discussion of the zoonotic implications of torovirus-like particles detected in patients with gastroenteritis that resemble and cross-react with BoTV is presented. Hopefully, the findings described here will alert others to the existence of BoTV in cattle and its contribution to the diarrheal disease complex. This review also highlights the need for continual vigilance for potential zoonotic viruses belonging to the order Nidovirales, such as the SARS coronavirus.

Infectious salmon anemia (ISA) virus (ISAV), an economically important new pathogen in marine aquaculture, is classified in the family Orthomyxoviridae, genus Isavirus. The main structural properties of this genus include enveloped virions 90–140 nm in diameter with surface projections of a combined receptor-binding hemagglutinin and receptor-destroying enzyme activity demonstrated to be an esterase, hence recently designated HE, and a genome composed of eight segments of linear, single-stranded, negative sense RNA ranging in length from 1.0 to 2.4 kb, with a total size of approximately 14.3 kb. The viral genome encodes at least ten proteins, of which nine are structural and one is non-structural. Examination of more than 160 ISAV isolates has led to the identification of two hemagglutinin subtypes of ISAV, one North American and one European. The immune response against ISAV after infection or vaccination does not provide full protection against the infection. The recent discovery of antibody-mediated uptake and replication of ISAV in macrophage-like fish cell lines suggests that Fc receptor-mediated antibody-dependent enhancement of the ISA virus infection might also occur in vivo, as the virus in Atlantic salmon (Salmo salar) targets endothelial cells lining blood vessels and macrophage-like cells. Cumulative mortalities in Atlantic salmon during natural ISA outbreaks and experimental infections range from 0 to 100%. ISAV causes fatal systemic infections in marine-farmed Atlantic salmon and asymptomatic infections in feral fish. Experimentally induced fatal clinical disease in rainbow trout (Oncorhynchus mykiss) has identified a correlate of virulence of ISAV that may explain its emergence as a fish pathogen.

Many bursate nematodes are of major importance to animal health. Animals are often parasitized by multiple species that differ in their prevalence, relative abundance and/or pathogenicity. Implementation of effective management strategies for these parasites requires reliable methods for their detection in hosts, identification to the species level and measurement of intensity of infection. One major problem is the difficulty of accurately identifying and distinguishing many species of bursate nematode because of the remarkable morphological similarity of their eggs and larvae. The inability to identify, with confidence, individual nematodes (irrespective of their life-cycle stage) to the species level by morphological methods has often led to a search for species-specific genetic markers. Studies over the past 15 years have shown that sequences of the internal transcribed spacers of ribosomal DNA provide useful genetic markers, providing the basis for the development of PCR-based diagnostic tools. Such molecular methods represent powerful tools for studying the systematics, epidemiology and ecology of bursate nematodes and, importantly, for the specific diagnosis of infections in animals and humans, thus contributing to improved control and prevention strategies for these parasites.

Haemophilus somnus (‘Histophilus somni’) is a Gram-negative bacterium and opportunistic pathogen associated with multisystemic diseases of bovines. Some strains of H. somnus are relatively avirulent, but are biochemically and antigenically similar to the virulent strains. Several virulence factors have been identified in H. somnus, of which lipo-oligosaccharide phase variation, induction of apoptosis, intraphagocytic survival and immunoglobulin Fc binding proteins have been well studied. The sequencing of the genomes of two strains of H. somnus has facilitated the identification of genes responsible for distinctive attributes within this species and related bacteria. The genetic characterization of this bacterium will enhance our understanding of H. somnus virulence factors and facilitate the development of new and improved vaccines. This review presents an overview of H. somnus virulence factors, their role in resistance to host defenses and pathogenesis, and the host immune response to this bacterium. Current areas of investigation in the post-genomic era and recent insights into new pathogenic mechanisms are also discussed.

Molecular epidemiology allows us to trace specific microorganisms and mobile genetic elements and to assess their epidemiological and evolutionary relationships. Examples of molecular epidemiology investigations in veterinary hospitals are discussed. They demonstrate the great similarities with the situation in human medicine and the potential usefulness of molecular epidemiology in our fight against antimicrobial resistance and nosocomial infections in veterinary hospitals. A broad knowledge of the diversity of antimicrobial resistance determinants in some major groups of pathogens and commensals from animals such as Enterobacteriaceae, Pasteurellaceae, enterococci and staphylococci is emerging. However, there are important gaps in this knowledge, which are discussed here. Many more molecular epidemiology studies will be necessary to understand and follow the evolution of the problem in veterinary medicine and agriculture on a global scale. To be able to build useful surveillance programs and reliable epidemiological models, and to identify critical intervention points, we need to improve our understanding of antimicrobial resistance at the animal and farm levels. Studies assessing the dynamics of bacterial populations and of resistance determinants at these levels are desperately needed. Understanding the relationships between antimicrobial resistance, colonization factors, and virulence also represents a major issue for which molecular epidemiology investigations will be needed.

Dendritic cells are central to the initiation of primary immune responses. They are the only antigen-presenting cell capable of stimulating naive T cells, and hence they are pivotal in the generation of adaptive immunity. Dendritic cells also interact with and influence the response of cells of the innate immune system. The manner in which dendritic cells influence the responses in cells of both the innate and adaptive immune systems has consequences for the bias of the adaptive response that mediates immunity to infection after vaccination or infection. It also provides an opportunity to intervene and to influence the response, allowing ways of developing appropriate vaccination strategies. Mouse and human studies have identified myeloid, lymphoid and plasmacytoid dendritic cells. Studies in domesticated animals with agents of specific infectious diseases have confirmed the applicability of certain of the generic models developed from mice or from in vitro studies on human cells. In vivo and ex vivo studies in cattle have demonstrated the existence of a number of subpopulations of myeloid dendritic cells. These cells differ in their ability to stimulate T cells and in the cytokines that they produce, observations clearly having important implications for the bias of the T-cell response. Dendritic cells also interact with the innate immune system, inducing responses that potentially bias the subsequent adaptive response.

Disease is a major source of economic loss to the livestock industry. Understanding the role of genetic factors in immune responsiveness and disease resistance should provide new approaches to the control of disease through development of safe synthetic subunit vaccines and breeding for disease resistance. The major histocompatibility complex (MHC) has been an important candidate locus for immune responsiveness studies. However, it is clear that other loci play an important role. Identifying these and quantifying the relative importance of MHC and non-MHC genes should result in new insights into host–pathogen interactions, and information that can be exploited by vaccine designers. The rapidly increasing information available about the bovine genome and the identification of polymorphisms in immune-related genes will offer potential candidates that control immune responses to vaccines. The bovine MHC, BoLA, encodes two distinct isotypes of class II molecules, DR and DQ, and in about half the common haplotypes the DQ genes are duplicated and expressed. DQ molecules are composed of two polymorphic chains whereas DR consists of one polymorphic and one non-polymorphic chain. Although, it is clear that MHC polymorphism is related to immune responsiveness, it is less clear how different allelic and locus products influence the outcome of an immune response in terms of generating protective immunity in outbred animals. A peptide derived from foot-and-mouth disease virus (FMDV) was used as a probe for BoLA class II function. Both DR and DQ are involved in antigen presentation. In an analysis of T-cell clones specific for the peptide, distinct biases to particular restriction elements were observed. In addition inter-haplotype pairings of DQA and DQB molecules produced functional molecules, which greatly increases the numbers of possible restriction elements, compared with the number of genes, particularly in cattle with duplicated DQ genes. In a vaccine trial with several peptides derived from FMDV, BoLA class II DRB3 polymorphisms were correlated with both protection and non-protection. Although variation in immune responsiveness to the FMDV peptide between different individuals is partly explainable by BoLA class II alleles, other genetic factors play an important role. In a quantitative trait locus project, employing a second-generation cross between Charolais and Holstein cattle, significant sire and breed effects were also observed in T-cell, cytokine and antibody responses to the FMDV peptide. These results suggest that both MHC and non-MHC genes play a role in regulating bovine immune traits of relevance to vaccine design. Identifying these genes and quantifying their relative contributions is the subject of further studies.

Pathogens that enter the body via mucosal surfaces face unique defense mechanisms that combine the innate barrier provided by the mucus layer with an adaptive response typified by the production and transepithelial secretion of pathogen-specific IgA. Both the measurement and induction of mucosal responses pose significant challenges for experimental and practical application and may need to be adapted to the species under study. In particular, for livestock, immunization procedures developed in small rodent models are not always effective in large animals or compatible with management practices. This paper reviews the latest advances in our understanding of the processes that lead to secretory IgA responses and how this relates to the development of mucosal immunization procedures and adjuvants for veterinary vaccines. In addition, it highlights the complex interactions that can take place between the pathogen and the host's immune response, with specific reference to Chlamydia/Chlamydophila infections in sheep.

The largely unanticipated difficulties of parasite vaccine development have led us to a renewed awareness of the survival strategies evolutionarily embedded within parasites over hundreds of millions of years. We have grown to appreciate that efforts to disrupt parasite–host relationships are substantially compounded by our incomplete understanding of the complex immune responses that occur in the naturally infected host. Given the inability to transfer laboratory successes to field trials, research is leading us to conclude that genetically defined animal models may not be good predictors of the unique and disparate protective immune responses one can expect from the genetically heterogeneous populations of animals that represent the parasite's natural environment. This is further compounded by the abundance of mechanisms parasites have created for themselves to defend against immune intervention. Thus, in the never-ending saga of vaccine development, it is only appropriate that pitfalls and advancements be critiqued as they apply across parasite groups, with a look towards promising technologies that may propel this field to the level of scientific achievement once envisaged.

The most widely used vaccines for the control of classical swine fever (CSF) in countries where it is endemic are live attenuated virus strains, which are highly efficacious, inducing virtually complete protection against challenge with pathogenic virus. In the European Union (EU), the combination of prophylactic mass vaccination and culling of infected pigs in endemic regions has made it possible to almost eradicate the disease. However, it is not possible to discriminate between infected and vaccinated animals, thus hampering disease control measures that rely on serology. Therefore, vaccination was banned at the end of 1990 before the internal common market was established in the EU. Vaccination is allowed only in severe emergencies. In addition, there are strict restrictions on the international trade in pig products from countries using vaccination. To circumvent these problems, marker vaccines which allow differentiation of infected from vaccinated animals (DIVA) have been developed. There are several approaches, ranging from protective peptides, single expressed proteins, naked DNA and chimeric viruses. To date, two subunit vaccines based on the E2 glycoprotein are commercially available and have been tested extensively for their efficacy. The accompanying discriminatory tests are based on an ELISA detecting another viral glycoprotein, the Erns. The subunit vaccines were found to be less efficacious than live attenuated vaccines. In addition, the currently available discriminatory tests do not provide high enough specificity and sensitivity. Although there is an urgent need for more advanced marker vaccines and better discriminatory tests, the development of new DIVA vaccines against CSF is hampered by the small market potential for these products.

This review examines the mechanisms involved in anti-tumor immunity and how peptides present in many tumor types (tumor-associated antigens) are recognized by T cells from tumor-bearing cancer patients. Tumor-associated antigens are derived from proteins that are also expressed in normal cells. It is predicted that immune responses to such peptides will be compromised by self-tolerance or that stimulation of effective immune responses will be accompanied by autoimmunity. We also consider that the immunity induced against two autoantigens, which are highly conserved in vertebrates, involve qualitatively different mechanisms, such as the production of antibodies and cell-mediated immune responses. However, both pathways lead to tumor immunity and identical phenotypic manifestations of autoimmunity. Appropriate selection of the optimal tumor antigen is critical for the induction of an anti-tumor immune response. Thus, we stress that the methods for antigen presentation using dendritic cells play a critical role in the development of tumor vaccines, to break immune tolerance and induce a strong immune response against them. The viability and feasibility of expansion of canine dendritic cells from bone marrow and peripheral blood ex vivo for the treatment of spontaneous cancers in dogs is also discussed.

Randomized controlled trials of veterinary vaccines are essential if we are to have a reasonable understanding of how those vaccines can be expected to perform when used in the field. This manuscript reviews a few (but certainly not all) of the key elements that need to be considered in the design of veterinary vaccine trials. The first step in the design of such a trial is to have a clear statement of the objective of the trial that reflects what is expected of the vaccine (e.g. should it minimize clinical disease or does it need to prevent infection?). Because domestic animals are often managed in groups, the ‘unit of concern’ used in a vaccine trial becomes of great importance. Whether the trial should be carried out at the individual or group level will depend on the objectives of the trial and the extent of concern about ‘group effects’ affecting the trial. Sample sizes will also be influenced heavily by the choice of unit of concern and the nature of the primary outcome being assessed. Finally, while there is no easy solution (except to conduct group-level trials, which may be logistically impossible), the potential for group effects to influence the trial outcome must be considered.