Pathogenesis of African horse sickness: ultrastructural study of the capillaries in experimental infection

Pathological features of African horse sickness virus infection in IFNAR-/- mice

African Horse Sickness (AHS) is a vector-borne viral disease of equids. The disease can be highly lethal with mortality rates of up to 90% in non-immune equine populations. The clinical presentation in the equine host varies, but the pathogenesis underlying this variation remains incompletely understood. Various small animal models of AHS have been developed over the years to overcome the financial, bio-safety and logistical constraints of studying the pathology of this disease in the target species. One of the most successful small animal models is based on the use of interferon-alpha gene knock-out (IFNAR-/-) mice. In order to increase our understanding of African Horse Sickness virus (AHSV) pathogenesis, we characterised the pathology lesions of AHSV infection in IFNAR-/- mice using a strain of AHSV serotype 4 (AHSV-4). We found AHSV-4 infection was correlated with lesions in various organs; necrosis in the spleen and lymphoid tissues, inflammatory infiltration in the liver and brain, and pneumonia. Significant viral antigen staining was only detected in the spleen and brain, however. Together these results confirm the value of the IFNAR-/- mouse model for the study of the immuno-biology of AHSV infections in this particular in vivo system, and its usefulness for evaluating protective efficacy of candidate vaccines in preclinical studies.

Experimental infection with African Horse Sickness Virus in horses induces only mild temporal hematologic changes and acute phase reactant response

OBJECTIVE

African Horse Sickness (AHS) is a vector-borne disease endemic to sub-Saharan Africa caused by African Horse Sickness Virus (AHVS). Infections in naïve horses have high morbidity and mortality rates. AHS pathogenesis is not well understood; neither the hematologic changes nor acute phase response occurring during infection has been fully evaluated. The study's objective was to characterize the hematologic changes and acute phase response during experimental infection with AHSV.

ANIMALS

4 horses negative for AHSV group-specific antibodies.

PROCEDURES

In this prospective, longitudinal study conducted between November 23 and December 2, 2020, horses were experimentally infected with AHSV, and blood samples were obtained before inoculation and then every 12 hours until euthanasia. Hematologic changes and changes for serum amyloid A (SAA) and iron concentration were evaluated over time using a general linear model including natural logarithm of sampling time.

RESULTS

All horses were humanely euthanized due to severe clinical signs typical of AHS. Median Hct increased significantly, and the median WBC count, monocyte count, eosinophil count, and myeloperoxidase index changed significantly in all horses over time. Horses developed marked thrombocytopenia (median, 48 X 103 cells/µL; range, 21 X 103 to 58 X 103 cells/µL) while markers of platelet activation also changed significantly. Median SAA increased and serum iron concentration decreased significantly over time.

CLINICAL RELEVANCE

Results indicated severe thrombocytopenia with platelet activation occurs during infection with AHSV. Changes in acute phase reactants SAA and iron, while significant, were unexpectedly mild and might not be useful clinical markers.

African horse sickness: The potential for an outbreak in disease-free regions and current disease control and elimination techniques

African horse sickness (AHS) is an arboviral disease of equids transmitted by Culicoides biting midges. The virus is endemic in parts of sub-Saharan Africa and official AHS disease-free status can be obtained from the World Organization for Animal Health on fulfilment of a number of criteria. AHS is associated with case fatality rates of up to 95%, making an outbreak among naïve horses both a welfare and economic disaster. The worldwide distributions of similar vector-borne diseases (particularly bluetongue disease of ruminants) are changing rapidly, probably due to a combination of globalisation and climate change. There is extensive evidence that the requisite conditions for an AHS epizootic currently exist in disease-free countries. In particular, although the stringent regulations enforced upon competition horses make them extremely unlikely to redistribute the virus, there are great concerns over the effects of illegal equid movement. An outbreak of AHS in a disease free region would have catastrophic effects on equine welfare and industry, particularly for international events such as the Olympic Games. While many regions have contingency plans in place to manage an outbreak of AHS, further research is urgently required if the equine industry is to avoid or effectively contain an AHS epizootic in disease-free regions. This review describes the key aspects of AHS as a global issue and discusses the evidence supporting concerns that an epizootic may occur in AHS free countries, the planned government responses, and the roles and responsibilities of equine veterinarians.

Comparative ultrastructural characterization of African horse sickness virus-infected mammalian and insect cells reveals a novel potential virus release mechanism from insect cells

African horse sickness virus (AHSV) is an arbovirus capable of successfully replicating in both its mammalian host and insect vector. Where mammalian cells show a severe cytopathic effect (CPE) following AHSV infection, insect cells display no CPE. These differences in cell death could be linked to the method of viral release, i.e. lytic or non-lytic, that predominates in a specific cell type. Active release of AHSV, or any related orbivirus, has, however, not yet been documented from insect cells. We applied an integrated microscopy approach to compare the nanomechanical and morphological response of mammalian and insect cells to AHSV infection. Atomic force microscopy revealed plasma membrane destabilization, integrity loss and structural deformation of the entire surface of infected mammalian cells. Infected insect cells, in contrast, showed no morphological differences from mock-infected cells other than an increased incidence of circular cavities present on the cell surface. Transmission electron microscopy imaging identified a novel large vesicle-like compartment within infected insect cells, not present in mammalian cells, containing viral proteins and virus particles. Extracellular clusters of aggregated virus particles were visualized adjacent to infected insect cells with intact plasma membranes. We propose that foreign material is accumulated within these vesicles and that their subsequent fusion with the cell membrane releases entrapped viruses, thereby facilitating a non-lytic virus release mechanism different from the budding previously observed in mammalian cells. This insect cell-specific defence mechanism contributes to the lack of cell damage observed in AHSV-infected insect cells.

Epizootics of sudden death in tammar wallabies (Macropus eugenii) associated with an orbivirus infection

Epizootics of sudden death in tammar wallabies (Macropus eugenii) occurred at six research facilities and zoological gardens in New South Wales, Australia, in late 1998 and at one Queensland research facility in March 1999. There were 120 confirmed tammar wallaby deaths during this period; however, population censuses indicated that up to 230 tammar wallabies may have died. The majority of animals died without premonitory signs. A small proportion of wallabies exhibited increased respiratory rate, sat with a lowered head shortly before death or were discovered in lateral recumbency, moribund and with muscle fasciculations. Gross postmortem findings consistently included massive pulmonary congestion, mottled hepatic parenchyma and subcutaneous oedema throughout the hindlimbs and inguinal region. Approximately 30% of the animals examined also had extensive haemorrhage within the fascial planes and skeletal muscle of the hindlimb adductors, inguinal region, ventral thorax, dorsal cervical region and perirenal retroperitoneal area. The tissues of affected animals became autolytic within a short period after death. Bacteriological examination of tissues from 14 animals did not provide any significant findings. Toxicological examination of the gastric and colonic contents of four animals did not reveal evidence of brodifacoume or other rodenticides. Viruses from the Eubenangee serogroup of the Orbivirus genus were isolated from the cerebral cortex of nine, and the myocardium of two, tammar wallabies and the liver and intestine of another tammar wallaby. A similar orbivirus was also isolated from the cerebrospinal fluid of another tammar wallaby that died suddenly. The disease agent appears to be a previously unrecognised orbivirus in the Eubenangee serogroup. This is the first report of epizootics of sudden deaths in tammar wallabies apparently associated with an orbivirus infection.

Involvement of the skin during bluetongue virus infection and replication in the ruminant host

Bluetongue virus (BTV) is a double stranded (ds) RNA virus (genus Orbivirus; family Reoviridae), which is considered capable of infecting all species of domestic and wild ruminants, although clinical signs are seen mostly in sheep. BTV is arthropod-borne (“arbovirus”) and able to productively infect and replicate in many different cell types of both insects and mammalian hosts. Although the organ and cellular tropism of BTV in ruminants has been the subject of several studies, many aspects of its pathogenesis are still poorly understood, partly because of inherent problems in distinguishing between “virus replication” and “virus presence”.BTV replication and organ tropism were studied in a wide range of infected sheep tissues, by immuno-fluorescence-labeling of non-structural or structural proteins (NS2 or VP7 and core proteins, respectively) using confocal microscopy to distinguish between virus presence and replication. These results are compared to gross and microscopic pathological findings in selected organs from infected sheep. Replication was demonstrated in two major cell types: vascular endothelial cells, and agranular leukocytes which morphologically resemble lymphocytes, monocytes/macrophages and/or dendritic cells. Two organs (the skin and tonsils) were shown to support relatively high levels of BTV replication, although they have not previously been proposed as important replication sites during BTV infection. The high level of BTV replication in the skin is thought to be of major significance for the pathogenesis and transmission of BTV (via biting insects) and a refinement of our current model of BTV pathogenesis is discussed.

African horse sickness virus induces apoptosis in cultured mammalian cells

Infection of mammalian cell cultures with African horse sickness virus (AHSV) is known to result in dramatic cytopathic effects (CPE), but no CPE is observed in infected insect cell cultures despite productive virus replication. The basis for this phenomenon has not yet been investigated, but is suggestive of apoptosis being induced following virus infection of the mammalian cells. To investigate whether AHSV can induce apoptosis in infected mammalian cells, Culicoides variipennis (KC) insect cells and BHK-21 mammalian cells were infected with AHSV-9 and analyzed for morphological and biochemical hallmarks of apoptosis. In contrast to KC cells, infection of BHK-21 cells with AHSV-9 resulted in ultrastructural changes and nuclear DNA fragmentation, both of which are associated with the induction of apoptosis. Results also indicated that AHSV-9 infection of BHK-21 cells resulted in activation of caspase-3, a key agent in apoptosis, and in mitochondrial membrane depolarization. Cumulatively, the data indicate that the intrinsic pathway is activated in AHSV-induced apoptosis.

Tissue tropism of African horsesickness virus in the chicken embryo demonstrated with the avidin-biotin complex immunoperoxidase method

In horses, African horsesickness virus (AHSV) exhibits marked tropism for certain microvascular endothelia and components of the mononuclear phagocyte system. In this study, the tropism of a field isolate of AHSV serotype 5 was studied in 24 chicken embryos. Histopathology on embryonic tissues harvested with 12 hour intervals revealed progressive changes associated with endothelial damage. Immunolabeling demonstrated viral antigens in the microvascular endothelium of the spleen, lungs, and the mesenchymal connective tissue at the base of the neck, from 24 hours post inoculation. Subsequently, specific immunolabeling increased steadily in endothelia of these and other tissues such as skeletal and cardiac muscle, gastrointestinal smooth muscle, mesonephric glomeruli, liver, subcutis and feathers. Positive immunolabeling was also occasionally observed in circulating mononuclear cells and in Kupffer cells in the liver. It was concluded, that this isolate of AHSV displayed similar tissue tropism in the chicken embryo as in the horse.

Tissue and Cell Tropism of African Horse Sickness Virus Demonstrated by Immunoperoxidase Labeling in Natural and Experimental Infection in Horses in South Africa

Tissues from 196 experimental and confirmed natural cases of African horse sickness (all 9 serotypes) were examined with a standardized and validated immunohistochemical assay for detection of the causative virus. The study confirmed that heart and lung are the main target tissues for African horse sickness virus (across all serotypes), followed closely by spleen. It also indicated that microvascular endothelial cells and monocyte–macrophages are the main target cells for virus replication. The importance of monocytes as target cells was emphasized, with relatively few tissue macrophages containing antigen in the lung and spleen, respectively. The results were largely in agreement with those of previous studies, but the large number of cases examined permitted more precise description of the location and distribution of antigen in different tissues. Comparison with descriptions of tissue and cell tropism of other orbiviruses indicated similarity with African horse sickness. Immunohistochemistry was shown to be a useful and consistent technique for demonstrating target cells, but the difficulty of identifying cell types—in particular, different types of monocyte–macrophages—is a limitation.

Bluetongue virus infection alters the impedance of monolayers of bovine endothelial cells as a result of cell death

Bluetongue virus (BTV) is the cause of bluetongue, an emerging, arthropod-transmitted disease of ungulates. Bluetongue is characterized by vascular injury with hemorrhage, tissue infarction and widespread edema, lesions that are consistent with those of the so-called viral hemorrhagic fevers. To further investigate the pathogenesis of vascular injury in bluetongue, we utilized an electrical impedance assay and immunofluorescence staining to compare the effects of BTV infection on cultured bovine endothelial cells (bPAEC) with those of inducers of cell death (Triton X-100) and interendothelial gap formation (tissue necrosis factor [TNF]). The data confirm that the adherens junctions of BTV-infected bPAECs remained intact until 24h post-infection, and that loss of monolayer impedance precisely coincided with onset of virus-induced cell death. In contrast, recombinant bovine TNF-alpha caused rapid loss of bPAEC monolayer impedance that was associated with interendothelial gap formation and redistribution of VE-cadherin, but without early cell death. The data from these in vitro studies are consistent with a pathogenesis of bluetongue that involves virus-induced vascular injury leading to thrombosis, hemorrhage and tissue necrosis. However, the contribution of cytokine-induced interendothelial gap formation with subsequent edema and hypovolemic shock contributes to the pathogenesis of bluetongue remains to be fully characterized.

Standardization and Validation of an Immunoperoxidase Assay for the Detection of African Horse Sickness Virus in Formalin-Fixed, Paraffin-Embedded Tissues

An immunoperoxidase assay for the detection of African horse sickness virus (AHSV) in formalin-fixed tissues is a valuable tool in the study of the pathogenesis of the disease, as well as a useful addition to existing diagnostic tests when only preserved tissues are available. An assay that uses Hamblin antiserum in a basic avidin-biotin complex detection system was standardized and validated in accordance with the guidelines of the American Association of Veterinary Laboratory Diagnosticians Subcommittee on Standardization of Immunohistochemistry. Using 128 positive cases of African horse sickness confirmed by viral isolation and serotyping and 119 negative cases from countries where the disease has never occurred, diagnostic sensitivity and diagnostic specificity were 100% in the prime target tissues of heart and lung. There was no variation in the ability of the assay to detect all 9 serotypes of AHSV, and there was no cross-reactivity with other orbiviruses in formalin-fixed tissues. The only cross-reactivity observed was in the lungs of 2 negative cases infected with Rhodococcus equi. The assay gave good results on tissues that had been fixed in formalin for up to 365 days. Nonspecific staining was minimal provided that the standard procedures for processing and staining tissues were followed. Good immunohistochemical results were also obtained on samples fixed as long as 24 hr after death. The assay, therefore, provides a robust diagnostic tool for detection of AHSV in formalin-fixed tissues, provided the analysis is done by an experienced pathologist.

Endothelial cells as active participants in veterinary infections and inflammatory disorders

Endothelial cells were once viewed as relatively inert cells lining the vasculature. They are now recognized as active and responsive regulators of coagulation, platelet adhesion, fluid homeostasis, wound healing, leukocyte extravasation and vascular tone. Endothelial cells play a key role in the host response to infectious agents by regulating leukocyte trafficking, producing inflammatory cytokines and presenting antigen in association with major histocompatibility class II (MHC II) molecules. A number of infectious agents have a tropism for endothelial cells. Infection of endothelial cells can promote thrombosis, vascular leakage, and increased adherence and emigration of leukocytes. Furthermore, activation of a systemic inflammatory response, in the absence of direct endothelial cell infection, can also lead to endothelial cell dysfunction. The purpose of this review is to highlight the interactions between endothelial cells and infectious or inflammatory agents that contribute to coagulation disturbances, vasculitis and edema. A select group of viral and bacterial pathogens will be used as examples to demonstrate how endothelial cell dysfunction contributes to the pathogenesis of infectious and inflammatory disorders.

Comparative susceptibility of selected avian and mammalian species to a Hong Kong-origin H5N1 high-pathogenicity avian influenza virus

Seventeen avian species and two mammalian species were intranasally inoculated with the zoonotic A/chicken/Hong Kong/220/97 (chicken/HK) (H5N1) avian influenza (AI) virus in order to ascertain a relative range of susceptible hosts and the pathobiology of the resultant disease. A direct association was demonstrated between viral replication and the severity of disease, with four general gradations being observed among these species. These gradations included the following: 1) widespread dissemination with rapid and high mortality, 2) neurological disease relative to viral neurotropism, 3) asymptomatic infection or only mild transient depression associated with minor viral replication, and 4) absence of disease relative to minimal to no viral replication. This investigation not only demonstrates that the chicken/HK virus could infect multiple avian species, but also that the virulence of the chicken/HK virus varied significantly among avian species, including those species that are members of the same order.

Characterization of equine E-selectin

Expression of E-selectin on activated endothelium is a critical initial step that leads to extravasation of leucocytes during inflammation, yet E-selectin is largely uncharacterized in several animal species including the horse. We have sequenced and compared E-selectin genes derived from activated cultures of purified equine (horse), cervid (black-tailed deer) and ovine (sheep) pulmonary artery endothelial cells (ECs). Phylogenetic and amino acid sequence comparisons indicate that bovine, cervid and ovine E-selectin are similar, whereas human and equine E-selectin are more closely related to each other than to the ruminant molecules. Human E- and P-selectin-specific monoclonal antibodies that also recognize equine E-selectin were identified and used to characterize its expression. Expression of E-selectin was more readily induced by lipopolysaccharide treatment in equine ECs than in human ECs and supported adhesion and activation of neutrophils, consistent with the extreme sensitivity of horses to endotoxaemia and septic shock.

Infection kinetics, prostacyclin release and cytokine-mediated modulation of the mechanism of cell death during bluetongue virus infection of cultured ovine and bovine pulmonary artery and lung microvascular endothelial cells

Bluetongue virus (BTV) infection causes a haemorrhagic disease in sheep, whereas BTV infection typically is asymptomatic in cattle. Injury to the endothelium of small blood vessels is responsible for the manifestations of disease in BTV-infected sheep. The lungs are central to the pathogenesis of BTV infection of ruminants; thus endothelial cells (ECs) cultured from the pulmonary artery and lung microvasculature of sheep and cattle were used to investigate the basis for the disparate expression of bluetongue disease in the two species. Ovine and bovine microvascular ECs infected at low multiplicity with partially purified BTV were equally susceptible to BTV-induced cell death, yet ovine microvascular ECs had a lower incidence of infection and produced significantly less virus than did bovine microvascular ECs. Importantly, the relative proportions of apoptotic and necrotic cells were significantly different in BTV-infected EC cultures depending on the species of EC origin and the presence of inflammatory mediators in the virus inoculum. Furthermore, BTV-infected ovine lung microvascular ECs released markedly less prostacyclin than the other types of ECs. Results of these in vitro studies are consistent with the marked pulmonary oedema and microvascular thrombosis that characterize bluetongue disease of sheep but which rarely, if ever, occur in BTV-infected cattle.

Pathobiology of A/chicken/Hong Kong/220/97 (H5N1) avian influenza virus in seven gallinaceous species

Direct bird-to-human transmission, with the production of severe respiratory disease and human mortality, is unique to the Hong Kong-origin H5N1 highly pathogenic avian influenza (HPAI) virus, which was originally isolated from a disease outbreak in chickens. The pathobiology of the A/chicken/Hong Kong/220/97 (H5N1) (HK/220) HPAI virus was investigated in chickens, turkeys, Japanese and Bobwhite quail, guinea fowl, pheasants, and partridges, where it produced 75-100% mortality within 10 days. Depression, mucoid diarrhea, and neurologic dysfunction were common clinical manifestations of disease. Grossly, the most severe and consistent lesions included splenomegaly, pulmonary edema and congestion, and hemorrhages in enteric lymphoid areas, on serosal surfaces, and in skeletal muscle. Histologic lesions were observed in multiple organs and were characterized by exudation, hemorrhage, necrosis, inflammation, or a combination of these features. The lung, heart, brain, spleen, and adrenal glands were the most consistently affected, and viral antigen was most often detected by immunohistochemistry in the parenchyma of these organs. The pathogenesis of infection with the HK/220 HPAI virus in these species was twofold. Early mortality occurring at 1-2 days postinoculation (DPI) corresponded to severe pulmonary edema and congestion and virus localization within the vascular endothelium. Mortality occurring after 2 DPI was related to systemic biochemical imbalance, multiorgan failure, or a combination of these factors. The pathobiologic features were analogous to those experimentally induced with other HPAI viruses in domestic poultry.