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Bovine respiratory syncytial virus infection

Bovine respiratory syncytial virus infection

Bovine respiratory syncytial virus infection

Previous authors: M VAN VUUREN

Current authors:
J L MCGILL - Assistant Professor, MS, PhD, Department of Veterinary Microbiology and Preventive Medicine, Iowa State University, 1907 Christensen Drive, VMRI Building 5, Iowa, 50011, United States of America
RE SACCO - Research Microbiologist/Immunologist /Lead Scientist, PhD, Ruminant Diseases and Immunology Research Unit, National Animal Disease Center/USDA/ARS, 1920 Dayton Avenue, Ames, Iowa, IA 50010, United States of America

GENERAL INTRODUCTION: PARAMYXOVIRIDAE AND PNEUMOVIRIDAE

Introduction

Infection with bovine respiratory syncytial virus (BRSV) is inapparent in the majority of animals, but in some it does cause mild to severe23 respiratory tract disease characterized by fever, coughing, serous nasal and ocular discharges, dyspnoea and, in some animals, subcutaneous emphysema. It is one of several viruses that are primary pathogens in the bovine respiratory disease complex (see Pneumonic pasteurellosis in cattle). Respiratory syncytial virus also infects sheep and goats and may cause rhinitis in sheep.25 Evidence for the existence of a respiratory syncytial virus (RSV) affecting horses has been reported.24

The virus was first isolated from cattle in Switzerland in 197040 and has subsequently been found worldwide. Its prevalence and importance in Africa is largely unknown, although antibody to it is widespread in feedlot cattle in South Africa.61

Several reviews on BRSV have been published.3, 7, 28, 36, 44, 46, 59

Aetiology

The virus is classified as a member of the genus Pneumovirus in the family Paramyxoviridae. Virions are pleomorphic, enveloped and measure 80 to 500 nm in diameter for the round and pleomorphic forms (Figure 1), and up to 5 microns in length for filamentous forms (Figure 2). The elongated nucleocapsid is helically coiled and tortuous and contains single-stranded RNA.

The envelope contains transmembrane surface glyoproteins, but, in contrast to ortho- and paramyxoviruses, pneumoviruses lack neuraminidase and haemagglutinin. Virions mature by budding from the cytoplasmic membrane.

A wide range of cell cultures are susceptible to infection with BRSV, including those prepared from most bovine tissue types, as well as kidney cells of pigs, hamsters, monkeys and humans.38 Cytopathic effects are characterized by multinucleate syncytial cell formation, intracytoplasmic inclusions and necrosis.

Bovine respiratory syncytial virus is antigenically related to human respiratory syncytial virus (HRSV),40 but the two viruses are distinct.48 It has been proposed that ruminant RSVs can be divided into 2 subgroups, BRSV and ovine RSV, with caprine RSV more closely related to BRSV.20

The genome of respiratory syncytial viruses is approximately 15.2 kb and is transcribed into 10 major subgenomic mRNAs encoding 11 proteins, because M2 mRNA contains two overlapping reading frames, which encode 2 proteins, M2-1 and M2-2.16

Bovine respiratory syncytial virus infection

Figure 1 Pleomorphic form of bovine respiratory syncytial virus

Bovine respiratory syncytial virus infection

Figure 2 Filamentous form of bovine respiratory syncytial virus

Epidemiology

Cattle are probably the principal reservoirs of the virus. Sheep and goats may also be infected but their role in the epidemiology is not clear, and inapparent infections are common in these species.2, 15, 19, 20, 32, 37, 43

Since the initial isolation of BRSV in Switzerland in 1970, the virus has been detected worldwide in cattle herds, with rates of infection impacted by multiple factors. In some countries, it has been estimated that frequency of BRSV exposure exceeds 50 per cent in some dairy and beef herds.28 Respiratory disease due to BRSV is most severe in calves less than 6 months of age, and infection can occur in the presence of maternal antibody. In some geographic areas, by 12 months of age, exposure of calves to BRSV may exceed 70 per cent.58 Reinfection in calves is common, likely as a result of reintroduction of BRSV into herds.36 Spread of the infection may result from direct contact with or aerosol droplets from infected animals, or indirectly from contaminated surfaces.29 Respiratory disease in cattle in the northern hemisphere caused by BRSV occurs mainly in late autumn, winter and early spring, although outbreaks have also been reported in the summer. In addition to a role for fluctuations in temperature, risk of BRSV infections can be impacted by stresses associated with weaning, transport, handling, crowding, and mixing of cattle from different sources.

Pathogenesis

The severity of clinical disease caused by BRSV is dependent on the age and immune status of the calf, the route of infection, and the strain of the virus.8 Some strains are significant primary pathogens of young cattle because, acting alone, they are capable of causing severe damage to the lower respiratory tract.12, 39, 45 However, the virus more frequently predisposes the respiratory tract of cattle to the pathogenic effects of secondary bacterial infections.6

The virus replicates in epithelial cells of the nasal cavity, pharynx, trachea, bronchi and bronchioli, and in type II pneumocytes and alveolar macrophages. It induces cytopathic changes characterized by loss of cilia and/or necrosis of bronchial and bronchiolar epithelial cells.11 The virus also alters opsonization and phagocytosis by alveolar macrophages.38, 56  Reduced mucociliary clearance resulting from a loss of cilia is responsible for the accumulation of fluid and tissue debris in the airways and alveoli, which provides an ideal environment for the growth of bacterial opportunists.

Studies conducted over the last two decades have enhanced our knowledge of the mechanisms whereby BSRV is able to establish respiratory tract infection and induce inflammation and subsequent respiratory disease. In particular, roles have been shown for non-structural proteins NS1 and NS2, as well as surface-expressed fusion and attachment glycoproteins.59 For example, NS1 and NS2 are known to regulate host type I interferons and calves infected with BRSV NS deletion mutants had no microscopic and macroscopic lung lesions.59 Likewise, it was shown that calves challenged with BRSV fusion protein deletion mutants exhibited significantly reduced pulmonary inflammation.

Clinical signs

Although most infections are subclinical, some infected animals show mild to severe disease13, 23 after an incubation period of five to seven days. In naturally infected dairy calves, early clinical signs are characterized by fever and coughing with nasal, oral and sometimes ocular discharges followed within two to three days by rapid abdominal breathing.26 Severely affected calves may show signs of respiratory distress, such as breathing through an open mouth and forced grunting expiration.60 The clinical signs that result from experimental BRSV infection mirror those observed during  natural outbreaks. Upper respiratory tract signs such as nasal discharge and coughing develop on days three to five after infection, progressing to severe lower respiratory tract signs on days seven to nine after infection, including fever, anorexia, dyspnoea and hypoxia.5, 22, 55, 64 Animals with subclinical infections may have some reduction in feed and water intake, with subtle reductions in weight gain; however, severe disease can lead to rapid weight loss and dehydration. Animals that survive severe disease lag in weight gain and growth, and are susceptible to secondary bacterial infections.44

Pathology

In calves that die from disease, irregular areas of atelectasis and pneumonia, particularly of the cranioventral parts of the lungs, together with lung oedema and interstitial emphysema are usually evident. A mucopurulent exudate may be present in the bronchi of atelectatic and pneumonic areas. The interstitial emphysema may affect all the lobes of the lungs but is often more pronounced in the caudal lobes. In severe cases, emphysematous lesions may develop subpleurally, resulting in pneumothorax, pneumomediastinum or pneumopericardium.9, 33 Subcutaneous emphysematous bulla formation may be prominent. Bronchial and mediastinal lymph nodes are enlarged and oedematous.6, 41

Microscopic changes associated with BRSV infection are characterized by an acute necrotizing bronchiolitis with the development in many bronchioli of syncytial cells formed by the proliferation of epithelial cells, some of which may contain eosinophilic intracytoplasmic inclusion bodies. The lumens of the bronchioli contain neutrophils, plasma cells, macrophages, mucus and cellular debris.

Evidence of atelectasis and pneumonia is present, the exudate in alveoli containing neutrophils, macrophages and cellular debris. Epithelialization may be evident in some alveoli, even with a tendency to form syncytia, some of which may contain intracytoplasmic inclusion bodies.12, 39, 60

Microscopically, tracheobronchial lymph nodes have enlarged cortical areas with prominent follicles, expanded parafollicular areas, and enlarged medullary cords due to lymphocytic hyperplasia. The medullary sinuses commonly contain macrophages, variable numbers of lymphocytes and plasma cells, and sporadic neutrophils and eosinophils.44

As viral infection progresses, an attempt to repair necrotic airway epithelium may result in epithelial hyperplasia and bronchiolitis obliterans.14 Persistent fibrinous and exudative inflammation is followed by fibroblast infiltration and neovascularization in an attempt at healing. Fibrous polyps covered by respiratory epithelium may extend into the airway lumen, often resulting in permanent decreases in airflow and alveolar ventilation.14, 44

Diagnosis

Since respiratory infections in cattle cannot be differentiated clinically, a diagnosis of disease caused by BRSV infection can only be made with laboratory support including  post mortem examination, histopathology, immunohistochemistry, real-time reverse transcription PCR (RT-PCR) and occasionally virus isolation.44

The gold standard for BRSV diagnosis is RT-PCR. It is rapid, specific and highly sensitive compared to other techniques.  A commercial RT-PCR kit detects all BRSV strains assessed with 99.3 per cent efficiency and a detection limit of 0.1 TCID50.54 Several multiplex RT-PCR assays have also been developed that can detect multiple viral and bacterial pathogens that are commonly associated with bovine respiratory disease, including BRSV.35, 50, 52 For ante mortem testing, tracheal washes and bronchoalveolar lavage samples offer the highest sensitivity, although guarded nasopharyngeal swabs are also acceptable for detecting BRSV.18 Nasal swabs are the least sensitive sample for BRSV diagnosis. Intranasal vaccination with modified live viruses can result in BRSV detection by RT-PCR, underlining the importance of case history when interpreting diagnostic test results.27, 53

Antigen detection tests, such as fluorescent antibody and immunoperoxidase-staining techniques applied to cells recovered either from the nasal passages or tissue sections, are commonly used for the detection of the virus.30, 34, 47, 51, 62, 63 Due to challenges with endogenous (background) fluorescence, immunofluorescence stains are less commonly used and often more difficult to interpret.7, 44 Immunohistochemistry on formalin-fixed tissues is generally more successful, particularly when assessing tissues that have undergone some degree of autolysis during transport to the laboratory. 1-step enzyme-linked immunosorbent assays are generally more sensitive than traditional antigen-capture enzyme immunoassays.42, 44 Because of the close antigenic similarity between bovine and human respiratory syncytial viruses, a rapid, patient-side strip test designed for detection of human RSV has been shown to be effective for detecting BRSV in nasal and tracheal swabs taken from cattle.49, 57

Bovine respiratory syncytial virus is labile and difficult to recover by the use of cell cultures.12, 31, 48  For this reason, virus isolation is less commonly used. When attempting virus isolation, it is imperative to use a sucrose-containing transport medium to preserve the virus if any delay during transport is anticipated, and to inoculate cell cultures as soon as possible after the specimens have been collected.1 Tracheal washes and bronchoalveolar lavage yield cells from deeper parts of the respiratory tract than do nasopharyngeal swabs,4 and therefore offer the possibility of more efficient detection of BRSV.

Retrospective detection of BRSV infections can be accomplished by antibody detection using indirect immunofluorescence, virus neutralization and complement fixation tests. However, since most infections are inapparent, and vaccination programmes are widely used, care should be exercised when attributing respiratory disease in cattle to BRSV based on serology.

Control

Prevention of BRSV infection is the optimal approach for controlling the disease in the individual and at the herd level. Multiple modified-live and killed vaccines are commercially available generally marketed as multivalent products that also target other common pathogens of the respiratory disease complex. Given the susceptibility of young calves to BRSV infection, use of a maternal vaccination programme and attention to colostrum intake are highly encouraged. A modified-live, multivalent vaccine containing BRSV, BHV-1 and PI3 has been approved for intranasal administration and has shown some efficacy against BRSV infection in calves as young as three days of age.17, 21 However, protection induced by intranasal vaccination appears to wane within a few months22 and should be followed by a second dose around 60 days later.17

Treatment of sick animals should be based on early detection of disease and focused on supportive care, including administration of NSAIDS and correction of dehydration.10 Antibiotics are commonly administered to limit secondary bacterial infections. There are few data available to support the use of antihistamines or corticosteroids, although this approach has been suggested for animals demonstrating severe respiratory signs associated with BRSV infection.6 Some clinicians believe that a change of feed or a 48-hour fast is an important part of the early treatment of acute respiratory distress in calves.26 Most animals will recover in several days without treatment.

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