Respiratory Viral Infections
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In general
- Some viruses are thought to induce modifications of the pulmonary defences by:
- Damaging the upper respiratory tract, thereby facilitating bacterial attachment and colonisation, with reduced mucociliary clearance
- Decreasing surfactant levels by destroying Type 2 pneuMonocytes
- Impairing the phagocytic ability of alveolar macrophages
In Dogs
Canine Respiratory Coronavirus
In Cats
Feline viral rhinotracheitis
Feline Calicivirus
In Horses
Equine rhinovirus
Equine influenza
Equine Rhinopneumonitis
Equine viral arteritis (EVA)
- Causative agent: equine arterivirus
- Rhinitis, peripheral oedema, bronchitis/bronchiolitis, conjunctivitis, periorbital oedema
- Replicates in macrophages and endothelial cells
- Disseminates via the circulatory system causing necrotising arteritis
- Interstitial pneumonia
- Transmitted by respiratory and venereal routes through direct contact with infected horse or its secretions
- Stallion are a reservoir of infection as they are chronic shedders
Equine adenovirus
- Adenoviridae
- May cause necrotising bronchiolitis in immune-deficient foals (Arabian foals)
- Grossly:
- Atelectasis and consolidation of lobules in cranioventral region
- Mucopurulent exudate in airways
- Histologically:
- Severe bronchiolitis, necrotising -> proliferative
- Bronchiolar obstruction by sloughed debri and neutrophils -> alveolar atelectasis
- May lead to secondary bacterial infections
African horse sickness
- Caused by orbivirus, family reoviridae
- Respiratory distress or cardiovascular failure
- Rapid death due to massive pulmonary oedema
- Hydrothorax may also develop
- Large amounts of froth present in airways
Hendra Virus
In Cattle
Infectious bovine rhinotracheitis (IBR)
Parainfluenza- 3
- Causative agent: parainfluenza- 3 virus (PI3)
- On its own causes rhinitis
- Often part of multi-aetiology disease complex (e.g. Enzootic pneumonia of calves), often followed by Pasteurella sp. obscuring viral origin
- Replicates in airway epithelial cells and results in an initial bronchitis -> bronchiolitis -> extension into alveoli, causing bronchointerstitial pneumonia
- Early stages may show intracytoplasmic inclusions
- The resulting exudate is predominantly neutrophilic
- Positive confirmation lies in a Fluorescent Antibody Test (FAT) to the specific virus on frozen sections of tissue
Bovine adenovirus
Respiratory syncytial virus
- Causative agent Respiratory syncytial virus (RSV), synonym: bovine RSV (BRSV)
- Outbreaks of RSV associated disease usually occur associated with winter housing
- Gross pathology in severe cases
- Cranioventral atelectasis and consolidation
- Interstitial emphysema
- More prominent in the caudal lung lobes
- Results from bronchoconstriction which results in airway obstruction - this constriction is thought to arise from mast cell degranulation and histamine release
- Histologically
- Acute bronchiolitis, characteristic of the bronchiolar response is the formation of syncytial giant cells (formed by proliferating bronchiolar epithelial cells which may contain intracytoplasmic inclusion bodies), alveolar epithelium sometimes affected
- Obstruction of bronchioles by exudate - these may later become obliterated by the fibrous tissue of organisation
- May contribute to Enzootic pneumonia of calves
Bovine rhinovirus
In Sheep
Maedi Visna
- Caused by a retrovirus
- The respiratory from of the disease caused by maedi-visna virus (Maedi) is also called lymphoid interstitial pneumonia
- Transmitted by close contact and via milk
- The pulmonary lesions develop very slowly hence this disease is uncommon in sheep < 2 years old
- Increased respiratory rate upon exertion, loss of weight
- Remains in Monocytes and macrophages
- Gross findings
- Severe interstitial pneumonia
- Lungs fail to collapse properly on opening the chest and can weigh more than twice the normal weight
- Impressions of the ribs remain on the visceral pleura
- Lungs are a mottled grey/ tan colour - the lesions can vary from irregular grey speckling to homogeneous grey consolidation
- Rubbery in consistence
- Diaphragmatic lobes most affected
- Associated bronchial and mediastinal lymph nodes are often enlarged
- Histologically
- Major features are extensive lymphoid proliferation around perivascular, peribronchial and peribronchiolar sheaths associated with pulmonary lymphatics
- Many of these areas contain germinal centres and smooth muscle hyperplasia (in walls of terminal bronchioles and alveoli)
Parainfluenza -3
- As in cattle
Pulmonary adenomatosis
- See neoplasia
In Goats
Caprine Arthritis-Encephalitis (CAE)
- Caused by retrovirus (lentivirus) similar to Maedi Visna in sheep described above
- Two forms:
- Non-suppurative leukoencephalomyelitis in young goats and kids
- Chronic, non-suppurative arthritis-synovitis in adult goats
- Also causes interstitial pneumonia which tends to be obscured by other clinical signs
- Gross pathology:
- Mainly caudal lobes
- Lungs are firm, grey-pink with grey-white focal lesions on cut surface
- Micro pathology:
- Thickened alveolar wall
- Lymphocyte infiltration and type II pneumocyte hyperplasia
- Can be confused with or coexisting with Parasitic pneumonia
In Pigs
Inclusion body rhinitis
- Herpesviridae, porcine cytomegalovirus
- Disease of suckling piglets 1-5 wks of age
- Clinical signs: those associated with acute/subacute rhinitis (ie: serous nasal discharge, progressing to catarrhal or purulent discharge with time and secondary bacterial infections; sneezing; pyrexia), fever in young piglets (3-8wks old)
- May progress to sinusitis, otitis media or pneumonia
- Morbitity high, mortality low
- Gross pathology - catarrhal discharge becoming purulent (secondary infection)
- Histology:
- Large basophilic intranuclear inclusion bodies in the surface and subepithelium of nasal and sinus glandular epithelium with lymphocytic infiltration of the mucosa
- Bursting of nucleus with cell necrosis and sloughing of necrotic epithelium
- Can develop viraemic stage, with inclusions and focal necrotising lesions in other organs eg: renal tubular epithelium
- Usually younger piglets, can die during this phase
- Usually resolves if uncomplicated but rhinitis may persist if secondary infection is present
- May persist in pulmonary macrophages
Swine influenza
Porcine reproductive and respiratory syndrome
- The syndrome is caused by a small enveloped RNA virus which belongs to the new Arteriviridae group
- Replicates in and destroys macrophages and endothelial cells causing vasculitis -> viraemia -> virus shedding (nasal secretions, faeces)
- Clinical signs: respiratory and reproductive failure, weaned pigs, tachypnoea, eyelid oedema, conjunctivitis
- Moderate to severe interstitial pneumonia in the cranial lobe
- Superimposed bacterial infections are common
- Infectious disease in swine that emerged 10 years ago
- Today, PRRS is endemic in many if not all the pig-producing countries
Postweaning multisystemic wasting syndrome (PMWS)
- Caused by a porcine circovirus alone or in combination with porcine parvovirus
- May cause mild interstitial pneumonia, failure of lungs to collpse on opening the thoracic cavity
- Microscopically: thickening of alveolar wall due to type 2 pneumocyte hyperplasia