Coronavirus

Coronavirus

1. Replication

   • 1. Virus attachment to plasma membrane
     • Attachment - animopeptidase N
       • Digestive enzyme
   • 2. Entry to cytoplasm/nucleus
     • Endocytosis
       • Viral membrane and endosome fuse together
   • 3. Un-coating of virus particle
     • Fusion and nucleocapsid release
   • 4. Synthesis of mRNA and protein
     • Transcription of polymerase
     • Nester RNA transcripts
     • 5’ cap, leader sequence and poly A tail
       • Could be translated directly to protein
         • But uses 3 different reading frames
     • Produces 2 polyproteins
       • Cleaved before assembling into one large RNA polymerase complex
   • 5. Replication of nucleic acid
     • Polymerase makes full length +ve sense copy of genome
       • Plus set of nested transcripts
         • Same leader sequence
         • Dissociates and reattaches at one of 7 downstream complementary sequences
         • Only 5' end of each mRNA is translated
   • 6. Virus assembly
     • In Golgi membrane
     • Budding from ER
       • Budding from an intracellular site
         • No need to kill cell, exposing peplomers
           • Avoids antibodies binding and mediating complement fixation or ADCC
             • Thus activating cascade, NK cells
   • 7. Exit of virus from cell
     • Exocytosis

2. Characteristics

   • Genome
     • +ve sense RNA
     • Helical nucleocapsid
   • Structure
     • Enveloped virus with characteristic large peplomers

3. Porcine Transmissable Gastroenteritis

   • Protection
     • Secretory IgA is protective
     • Systemic IgA is not
     • TGE does not survive long out of host
       • Control by depopulation and disinfection
       • Lipid envelope not robust
     • Remove seropositive adults
     • Carrier animals can shed virus for >100 days
   • Pathogenesis
     • Entry to host
       • Ingestion
       • Virus is acid labile
         • Protection by milk or food allows passage through stomach
     • Targeted cells
       • Attachment to aminopeptidase-N on villous enterocytes
         • Other cells e.g. macrophages
       • Enterocytes normal function
         • Digestion
         • Absorption
     • Effect on cell function
       • Virus is rapidly cytotoxic
       • Infects villus
         • Destroys those involved with absorption
       • Loss of digestive function
         • Neonatal enterocytes also express lactase
         • Bacteria in colon break down milk
         • Osmotic effect
     • Effect on host
       • Loss of fluid absorption
       • Undigested lactose with secondary bacterial fermentation in colon
       • Osmotic effects compound diarrhoea
         • Yellow, milky intestinal contants
       • Translucent
         • Expanded
         • Loss of villi
         • Full of gas and fluid
       • Basal crypt cells less affected
         • Quick recovery given animal has not died from dehydration
   • Introduction to a naïve herd
     • Vomiting, diarrhoea, fever throughout herd
     • All pigs affected, near 100% mortality
       • Adults may recover and become persistently infected carriers
     • May become endemic in large units
     • Outbreaks in post weaning piglets
       • Withdrawal of IgA from milk
         • More protective than antiserum in blood
         • Virus is in gut lumen
           • Antibodies are on other side of basement membrane
             • Hides from immune system
       • Weaning is a high risk time
   • Porcine epidemic diarrhoea
     • New infection, transient vomiting and diarrhoea in sows and weaners
     • May cause mortality in younger pigs
     • Endemic when introduced to a new herd
       • Herds >300 sows
     • Differentiation from TGE by lab testing (e.g. rapid capture ELISA on faeces
   • Respiratory variant
     • Enterotropism lost, gained tropism for respiratory tract
     • Mild replication in URT possible, can't cause severe clinical disease
     • Clinical signs
       • 1-2 days transient cough
     • Rapid spread between herds by aerosol
     • Recovered pigs seroconvert to TGE
       • Protected from enteric TGE
     • TGE has become rare in Uk and mainland Europe (not Ireland or USA)

4. Canine Coronavirus

   • Mild enteric disease in puppies
   • Can persist in GI tract for months, periodic shedding in faeces
   • More severe disease can occur in association with CPV
   • Canine respiratory coronavirus
     • Associated with outbreaks of (severe) respiratory disease in kennels
     • Sequencing shows a difference to enteric virus
     • Can make parvovirus worse than it already is, but unlikely to cause disease on its own

5. Feline Enteric Coronavirus

   • Immunity
     • IgG to S-protein by both active and passive immunisation
       • Enhanced monocyte infection
         • Accelerated pathogenesis
     • T helper cells exacerbate disease
       • Recruiting monocytes to foci of infection
       • Cytotoxic T-cells may be important in controlling initial infection
         • Remove infected cells
     • Temperature attenuated vaccine and nasal inoculation may offer some protection from primary infection
   • Diarrhoea in kittens
   • Chronic infection in some cats
   • Ubiquitous 'avirulent' coronavirus
   • 2 types
     • Type 1
       • Feline Coronavirus distinct from other Coronaviruses
       • Primary cellular receptor unknown
       • Hard to grow
       • Most prevalent
     • Type 2
       • High degree of similarity with Canine Coronavirus
       • Cellular receptor APN
       • Grows well but less common
     • Persistent infection with either type can lead to Feline Infectious Peritonitis
   • Evolution of new biotypes
     • Non proof reading polymerase and cut/paste recombination of coronavirus
     • FIP variants have enhanced replication in macrophages
       • Fc receptor mediated uptake
       • Antibodies enhance infection
       • Escapes endosome into cytoplasm - infection spreads
   • Pathology
     • Infection of macrophages, ↑ inflammatory cytokines, ↑ vascular permeability
       • Immune cells to area of infection
     • immune response
       • Helper T-cells
       • Antiviral CTL
       • Invasion by neutrophils
       • Perivascular necrosis
     • Chemotaxis of monocytes
       • Become virus infected
       • Antibody enhanced infection via Fc receptors
     • Pro-granulomatous foci containing neutrophils, lymphocytes, macrophages, and deposits of IgG-Virus-C3 complexes
     • Clinical signs
       • Variable depending on affected organ
       • Malaise sickness due to elevated inflammatory mediators
       • Peritoneum, Liver, kidneys, GI signs
         • May have peritoneal effusion organ enlargement
         • Peritonitis peritoneum common site of infection
         • Liquid or dry form
       • Lungs and pleura
         • Respiratory signs, pleural or pericardial effusion
       • CNS, skin, ocular lesions
       • If a patient presents with general localised effusion and swelling, this virus should be a differential
   • Virus peplomers retained in macrophage Golgi
     • Not exposed on cell surface of infected macrophages
       • Protection from ADCC and NK cells
   • Diagnosis typically by PM
     • Perivascular infiltrate in FIP seen through an opthalmoscope

6. Bovine coronavirus

   • Diarrhoea in calves
     • Cryptosporidia in older cattle
       • Winter dysentry
   • Monoclonal antibody to spike protein of bovine coronavirus
     • S-protein binds sialic acid
   • Transient, self limiting
   • Affects villi, less of an effect on crypts
     • Net fluid secretion, diarrhoea, malabsorption of sugars and fermentation
   • Can also be detected in respiratory tract and lymph nodes
     • Can cause respiratory disease

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