Assessing the potential of plains zebra to maintain African horse sickness in the Western Cape Province, South Africa

African horse sickness (AHS) is a disease of equids that results in a non-tariff barrier to the trade of live equids from affected countries. AHS is endemic in South Africa except for a controlled area in the Western Cape Province (WCP) where sporadic outbreaks have occurred in the past 2 decades. There is potential that the presence of zebra populations, thought to be the natural reservoir hosts for AHS, in the WCP could maintain AHS virus circulation in the area and act as a year-round source of infection for horses. However, it remains unclear whether the epidemiology or the ecological conditions present in the WCP would enable persistent circulation of AHS in the local zebra populations. Here we developed a hybrid deterministic-stochastic vector-host compartmental model of AHS transmission in plains zebra (Equus quagga), where host populations are age- and sex-structured and for which population and AHS transmission dynamics are modulated by rainfall and temperature conditions. Using this model, we showed that populations of plains zebra present in the WCP are not sufficiently large for AHS introduction events to become endemic and that coastal populations of zebra need to be >2500 individuals for AHS to persist >2 years, even if zebras are infectious for more than 50 days. AHS cannot become endemic in the coastal population of the WCP unless the zebra population involves at least 50,000 individuals. Finally, inland populations of plains zebra in the WCP may represent a risk for AHS to persist but would require populations of at least 500 zebras or show unrealistic duration of infectiousness for AHS introduction events to become endemic. Our results provide evidence that the risk of AHS persistence from a single introduction event in a given plains zebra population in the WCP is extremely low and it is unlikely to represent a long-term source of infection for local horses.

Characterising Non-Structural Protein NS4 of African Horse Sickness Virus

African horse sickness is a serious equid disease caused by the orbivirus African horse sickness virus (AHSV). The virus has ten double-stranded RNA genome segments encoding seven structural and three non-structural proteins. Recently, an additional protein was predicted to be encoded by genome segment 9 (Seg-9), which also encodes VP6, of most orbiviruses. This has since been confirmed in bluetongue virus and Great Island virus, and the non-structural protein was named NS4. In this study, in silico analysis of AHSV Seg-9 sequences revealed the existence of two main types of AHSV NS4, designated NS4-I and NS4-II, with different lengths and amino acid sequences. The AHSV NS4 coding sequences were in the +1 reading frame relative to that of VP6. Both types of AHSV NS4 were expressed in cultured mammalian cells, with sizes close to the predicted 17–20 kDa. Fluorescence microscopy of these cells revealed a dual cytoplasmic and nuclear, but not nucleolar, distribution that was very similar for NS4-I and NS4-II. Immunohistochemistry on heart, spleen, and lung tissues from AHSV-infected horses showed that NS4 occurs in microvascular endothelial cells and mononuclear phagocytes in all of these tissues, localising to the both the cytoplasm and the nucleus. Interestingly, NS4 was also detected in stellate-shaped dendritic macrophage-like cells with long cytoplasmic processes in the red pulp of the spleen. Finally, nucleic acid protection assays using bacterially expressed recombinant AHSV NS4 showed that both types of AHSV NS4 bind dsDNA, but not dsRNA. Further studies will be required to determine the exact function of AHSV NS4 during viral replication.

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.

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

African horse sickness (AHS) was induced in five horses by inoculation, to determine the ultrastructural changes in endothelial cells of capillaries in the myocardium, lung, spleen and liver. The animals developed cardiac and mixed forms of the disease. Alterations detected in the endothelial cells of the vessels of infected animals included: the presence of structures associated with viral infection, hypertrophy, degenerative changes, appearance of cytoplasmic projections, changes in permeability, alteration of intercellular junctions, loss of endothelium, subendothelial deposition of cell debris and fibrin, and vascular repair. In association with these changes, oedema, haemorrhages and microthromboses were detected, particularly in the myocardium and lung. This study showed that infection of, and changes to, the capillary endothelial cells of the organs under study was independent of the form in which the disease manifested itself but was dependent on the organ and blood vessel type. Thus, different levels of viral tropism were observed for the endothelial cells of the vessels in different organs. Viral infection was commonest in the endothelial cells of myocardial vessels, followed by those in the lung, whereas in the spleen and liver, endothelial cell infection was rare and, in the case of the liver, limited to the interstitial capillaries.

The role of pulmonary intravascular macrophages in the pathogenesis of African horse sickness

African horse sickness (AHS) is a disease of equids, characterized by severe pulmonary oedema and caused by an orbivirus. To determine the role of pulmonary intravascular macrophages (PIMs) in the development of pulmonary microvascular changes in this disease, five horses were given an intravenous inoculation of 10(6)TCID50of serotype 4 of AHS virus. Viral replication was detected in endothelial cells, PIMs, interstitial macrophages and fibroblasts. Alveolar and interstitial oedema, and changes in pulmonary microvasculature, consisting mainly of the sequestration of neutrophils and the formation of platelet aggregates and fibrinous microthrombi, were related to endothelial changes and to a high degree of PIM activation. This suggested that the PIMs, once activated, contributed to these vascular changes by releasing chemical inflammatory mediators.

Donkeys as reservoirs of African horse sickness virus

Investigations have been carried out to elucidate the possible role of the donkey in the epidemiology of African horse sickness (AHS). These studies have shown that despite the absence of pyrexia or other observable clinical signs, donkeys become infected with virulent AHS virus serotype 4 (AHSV 4) and that they develop a viraemia which can persist for at least 12 days, albeit at a comparatively lower titre than that recorded for similarly infected ponies. AHSV 4 showed a similar tissue tropism in the pony and donkey but the virus appeared to replicate less efficiently in donkey tissues. The only gross pathological changes observed in the donkeys post mortem were increased fluid accumulation in the serosal lined compartments, particularly the peritoneal cavity, and petechial and ecchymotic haemorrhages on the left hepatic ligament. The absence of infectious virus or viral antigens in any of the tissues collected at 14 and 19 days post inoculation (dpi) from 6 experimental donkeys suggest that, though susceptible to infection, the donkey is unlikely to be a long term reservoir for AHSV. Although AHSV 4 was detected in all 6 donkeys following the primary inoculation, no virus could be isolated from blood collected from two donkeys subsequently challenged with a second virulent virus, AHSV 5. Data generated from virus neutralisation tests showed a second primary antibody response, against AHSV 5, in these donkeys at 12 dpi. In contrast, the boost in antibody levels detected from 5 dpi, as measured by ELISA, was probably due to an anamnestic response against the AHSV group-specific viral proteins. Homogenised spleen tissue, collected post mortem from a donkey 7 dpi with AHSV 4, caused a lethal, cardiac form of AHS when inoculated into a susceptible pony.

Immunohistochemical demonstration of African horse sickness viral antigen in formalin-fixed equine tissues

The distribution of viral antigen was studied in various tissues of three ponies, aged 3-4 years, infected experimentally with a virulent strain of African horse sickness virus (AHSV) serotype 4. Tissues were collected from the animals in the terminal stage of the peracute form of the disease and from one noninfected horse, included as a control. A polyclonal antibody with specificity for AHSV, plus the nonstructural protein NS2, was used in a sensitive avidin-biotin-peroxidase-complex (ABC) method performed on formalin-fixed, paraffin-embedded tissue sections. AHSV antigen was located primarily in endothelial cells of capillaries and small venous and arteriolar vessels, particularly of cardiopulmonary tissues. Viral antigen was also identified in cells resembling macrophages and in reticular cells of spleen and lymph nodes. The pattern of viral antigen labeling in some lymph nodes along the mantle zone of lymphoid follicles was compatible with the morphology of cellular processes of follicular dendritic cells. In some tissues, viral antigen was detected occasionally in circulating cells, probably monocytes, within the larger vessels. These findings suggest that endothelial cells, and to a lesser extent mononuclear cells, are the main target cells of AHSV infection during the late stage of the peracute form of the disease.

Clinical pathology and hemostatic abnormalities in experimental African horsesickness

Infection of naive North American horses with 10(4) cell culture infectious doses (CCID50) of virulence variants of African horsesickness virus (AHSV), designated AHSV/4SP, AHSV/9PI, and AHSV/4PI, reproduced three classical forms of African horsesickness: acute (pulmonary), subacute (cardiac), and febrile, respectively. Distinct clinicopathologic and hemostatic abnormalities were associated with each form of disease. Hemostatic abnormalities included increased concentration of fibrin degradation products and prolongation of prothrombin, activated partial thromboplastin, and thrombin clotting times. Hemostatic findings indicated activation of the coagulation and fibrinolytic systems with clotting factor consumption in acute and subacute cases of African horsesickness. Hematologic abnormalities in acute and subacute cases of African horsesickness included leukopenia, decreased platelet counts, elevated hematocrit, and increased erythrocyte counts and hemoglobin concentration. Leukopenia was characterized by lymphopenia, neutropenia, and a left shift. Increased levels of serum creatine kinase, lactate dehydrogenase, aspartate aminotransferase, and alkaline phosphatase, hypocalcemia, hypoalbuminemia, hypoproteinemia, and elevated creatinine, phosphorus, and total bilirubin levels were present in some but not all horses. Metabolic acidosis, indicated by decreased total bicarbonate and increased lactate and anion gap, was present in horses with the acute form of disease. Mild thrombocytopenia and leukopenia were occasionally associated with the febrile form of disease. These results suggest a role for intravascular coagulation in the pathogenesis of African horsesickness.

Presence of African horse sickness virus in equine tissues, as determined by in situ hybridization

In a retrospective study, a negative-sense digoxigenin-labeled RNA probe, corresponding to the gene encoding nonstructural protein-1 of African horse sickness virus (AHSV) serotype 4, was applied to formalin-fixed, paraffin-embedded tissue taken from horses in the terminal stages of infection with AHSV. Fifteen infected ponies and one noninfected control were studied. Ponies exhibited a range of clinical signs and lesions. Thirteen ponies were infected with serotype 4, one with serotype 1, and one with serotype 2. Ponies were monitored clinically and euthanatized when severely clinically ill. The following tissues were available for study by in situ hybridization and histopathology: lung, heart, spleen, neck muscle, and supraorbital fat. Histologically, the most striking changes were pulmonary edema and, in some, acute myocardial necrosis. In situ hybridization revealed virus distributed widely in sections of lung and heart examined, with relatively less in spleen, neck muscle, or supraorbital fat. Virus was localized to target cells with morphologic features compatible with endothelium in all organs except spleen, where it was found in both endotheliumlike cells and large mononuclear cells.

Characterization of virulence variants of African horsesickness virus

There are three clinicopathologic syndromes associated with African horsesickness (AHS) virus infection in horses. These different forms of AHS (pulmonary, cardiac, and fever forms) vary in the organs affected, the severity of lesions, time of onset of clinical signs and mortality rates. We have studied the effects of infection with three cell culture passaged variants of AHS virus in naive North American horses. One of these viruses, AHS/4SP, consistently caused the pulmonary form of AHS with rapid onset of severe pulmonary edema and 100% mortality. A second variant, AHS/9PI, resulted in signs and lesions typical of the cardiac form of AHS: pericardial effusion, subendocardial hemorrhage and widespread subcutaneous edema. Mortality was approximately 70%. The third variant, AHS/4PI, produced mild to subclinical disease in horses, usually expressed only as transient mild fever. No mortality occurred in horses due to infection with AHS/4PI. All surviving infected animals did, however, seroconvert with both neutralizing and ELISA-reactive antibodies. The results of these studies indicate clearly that in naive horses the form of disease expressed is a property of the AHS virus inoculum.

A morphological study of the lesions of African horsesickness

Gross, histological and ultrastructural findings are described in 6 natural cases and in 2 experimental cases of African horsesickness. From the gross lesions the cases were divisible into 2 groups which represented the previously described pulmonary and mixed forms of the disease. Histologically, abundance of fibrin and inflammatory cells in oedematous lung suggests that the pulmonary lesion is an exudative pneumonia. Lymphoid depletion and necrosis in germinal centres were consistently present. Electron microscopy failed to demonstrate virus particles or virus-associated structures in the tissues. Ultrastructural evidence of vascular injury was not apparent in oedematous tissues. Possible mechanisms in the development of the lung oedema are considered and a comparison is made with oedema induced by alpha-naphthyl-thiourea. Lack of structural evidence of vascular injury revealed by this study extends some hope for therapy in African horsesickness.

An outbreak of African horsesickness in dogs

Several dogs died from African horsesickness which was contracted by the consumption of uncooked meat from the carcase of a horse that had died from the disease. Respiratory embarrassment was the main clinical sign, while marked hydrothorax and pulmonary congestion and oedema were the major post mortem findings. Serotype 6 horsesickness virus was isolated from 2 of these dogs.

The reaction of imported British Shire horses to African horse sickness: a case report

Twelve (12) heavy horses of the Shire breed imported into Nigeria in 1974 died within two months after importation. This was because of inclement weather and non-availability of AHS vaccine.