Virology 4: Double Stranded RNA Viruses

What are double-stranded RNA viruses?

• genetic material composed of 2 complementary RNA strands

• group 3 BCS

Describe the stability of dsRNA viruses

inherently stable due to ds nature of RNA, which forms a helical structure

How do dsRNA viruses replicate?

1. inside host cell, viral capsid removed, dsRNA genome released into cytoplasm

2. viral polymerase enzyme transcribes the dsRNA genome into mRNA inside the viral particle

3. viral mRNAs released into cytoplasm & translated by host’s ribosomes to produce viral proteins

4. viral polymerase synthesises new dsRNA genomes using mRNA as a template

5. viral proteins & genomes assemble into new viral particles

List examples of dsRNA viruses

reoviridae (rotavirus, bluetongue virus, african horse sickness virus)

What are the characteristics of the reoviridae genome?

• genetic reassortment (genetic shift)

• high mutation rate (genetic drift)

• numerous serotypes & strains of each virus species

What are the characteristics of Reoviridae viruses?

• non-enveloped

• icosahedral capsid

• segmented genomes

What species are affected by rotavirus?

young animals (calves, piglets, foals & lambs)

What are the clinical signs of rotavirus?

• severe gastroenteritis → diarrhoea, dehydration, death

• depression and lethargy

• weight loss

• fever

How is rotavirus transmitted?

faecal-oral route, highly contagious

What impact does rotavirus have on the livestock industry?

significant economic losses due to decreased productivity, treatment costs & mortality

What does this image show?

villi atrophy due to rotavirus

What cells does rotavirus infect?

• mature enterocytes at middle & top of intestinal villi

• prominent vacuolisation

What does damaged villi due to rotavirus infection cause?

short, so reduced nutrient adsorption

How can rotavirus be diagnosed?

• white scours

• point of care testing (antigen capture test for Rotavirus in faeces)

• viral RNA extraction & testing by RT-PCR

How can rotavirus be prevented?

inactivated & live attenuated vaccines in cattle and pigs

What species are primarily affected by bluetongue virus?

ruminants (sheep most severely affected)

What are the clinical signs of bluetongue virus in sheep?

• high fever

• swelling of face and tongue

• lameness

• nasal discharge

• cyanosis of tongue

What are the clinical signs of bluetongue virus?

• often subclinical

• fever

• swelling

• erosions of mouth and nose

• reduced milk production

How is bluetongue virus transmitted?

biting midges - seasonal outbreaks typically in late summer & early autumn

Why is bluetongue virus relevant to the livestock industry?

• economic losses due to reduced productivity, treatment costs & mortality

• outbreaks lead to restrictions on livestock movement

Describe the pathogenesis of bluetongue virus

• replicates in local areas of bite site (dendritic cells, macrophages, lymphocytes & endothelial cells that line walls of blood vessels)

• viremia (interacts with platelets & erythrocytes, causing damage to endothelial cells → vascular thrombosis, infarction, necrosis & haemorrhage)

What is primarily the cause of death from bluetongue virus?

pulmonary oedema

Why is it thought that there’s overwintering of bluetongue virus?

• continuous low-level transmission

• midge and virus stability

How can bluetongue virus be diagnosed?

• clinical findings

• RT-PCR (most common method for detecting BTV RNA in blood, tissue or midge samples)

• ELISA (detect antibodies against BTV in blood samples - good for identifying past infections & for surveillance)

Which reovirus is a notifiable disease?

bluetongue virus

What are the reporting and control measures of bluetongue virus?

• notifiable disease

• movement restrictions

How can bluetongue virus be prevented?

live attenuated vaccines for cattle and sheep

What species are affected by African Horse Sickness virus?

equines

What are the different forms of African Horse Sickness in horses?

• pulmonary form

• cardiac form

• mixed form

What are the clinical signs of the pulmonary form of African Horse Sickness virus in horses?

• severe respiratory distress

• frothy nasal discharge

• high fever

• sudden death within hours

What are the clinical signs of the cardiac form of African Horse Sickness Virus in horses?

• swelling of head, neck, and chest

• subcutaneous oedema

• severe cardiovascular symptoms through endothelial damage & fluid accumulation in tissues

• death in days

What are the clinical signs of the mixed form of African Horse Sickness in horses?

combination of pulmonary and cardiac symptoms

What are the clinical signs of African Horse Sickness in mules and donkeys?

mild symptoms - including fever & mild respiratory signs

How is African Horse Sickness virus transmitted?

vector borne (primarily biting midges - Culicoides) outbreaks seasonal, in warm, wet seasons

What are the replication sites of African Horse Sickness?

• initial replication in macrophages & dendritic

• replicates extensively in endothelial cells (leads to vascular damage & CS oedema & haemorrhage)

• epithelial cells

How can African Horse Sickness Virus be prevented?

vaccination in endemic countries annually

How can African Horse Sickness be controlled?

• stabling (keep indoors during peak vector activity)

• stable protection (fine-gauge, insecticide-treated shade cloth, double-door systems & positive pressure ventilation)

• topical measures (insect repellents and insecticides)