1. REOVIRIDAE: ROTAVIRUSES AND ORBIVIRUSES
PETER H. RUSSELL, BVSc, PhD, FRCPath, MRCVS
Department of Pathology and Infectious Diseases, The Royal Veterinary College,
Royal College Street,
London NW1 OTU.
Web site
This lecture is the part of THE VIROLOGY COURSE, (The aim of this foundation course is to link the structure, diagnosis and control of the different families of veterinary viruses in an up-to-date and well-paced manner). This lecture has been updated by Dr Janice Bridger.
COURSE OBJECTIVES - hyperlinks to course.htm
BOOK LIST - hyperlink to book.htm
USEFUL WEBSITES- hyperlink to websites.htm
INDEX OF ABBREVIATIONS - hyperlink to index.htm

2. LEARNING OBJECTIVES Students should be able to:
Relate the pathogenesis of rotaviruses to the methods used for their control.
Discuss the difficulties of making an accurate rotavirus diagnosis.
Explain the control measures adopted for African horse Sickness Virus.

3. FAMILY CHARACTERISTICS:
- Non-enveloped, spherical, medium sized (60-80 nm) virions composed of concentric rings of capsomeres giving characteristic appearances in the electron microscope.
- Segmented genome (10 or 11 segments), double-stranded RNA; each segment codes for a separate viral protein.
- Resistant viruses which can persist in the environment.

4. DISEASE & PATHOGENESIS
ROTAVIRUSES (RVs)
DISEASE & PATHOGENESIS
- Acute, self-limiting, malaborption diarrhoea (calf/piglet scour)
with anorexia, vomiting (pigs), weight loss, depression, dehydration, death. Incubation period 1-4 days.
- Usually occurs in the neonatal period.
- Target tissue is the small intestinal enterocyte. Infected enterocytes are shed. Villi shorten and become covered with immature, undifferentiated cuboidal epithelium. The ability to digest food decreases. A malabsoption diarrhoea "white scour" results.
- Strains vary in virulence - some replicate without causing disease.

5. ROTAVIRUSES (RVs) ANTIGENICITY:
- Complex. At least 5 groups established (A to E) which are antigenically distinct. Rotaviruses within each group are strongly related by common group antigens which can form the basis of diagnostic methods.
- Group A RVs separated into serotypes based on neutralization antigens on two of the outer capsid proteins.

6. ROTAVIRUSES (RVs) HOST RANGE
Group A: present in all mammals examined. Important role in calf and piglet diarrhoea; role in lambs & foal diarrhoea less certain; not associated with diarrhoea in cats & dogs.
Groups B to E cause diarrhoea experimentally but importance in natural disease unknown.
Group B: pigs, sheep, cattle, rats, man. Group C: pigs, man, ferrets Group D: avian species Group E: pigs.
RVs from different animal species thought of as distinct but some transmission between species may occur naturally.

7. ROTAVIRUSES (RVs) EPIZOOTIOLOGY:
Faecal-oral transmission. Highly infectious; spreads rapidly.
Group A RV: enzootic in domestic animals throughout the world. All animals become infected sooner or later & all have antibody. The young excrete RV in faeces, clinically or sub-clinically, as passive maternal antibody wanes in the gut (commonly in the second week of life in calves; before or after weaning in piglets). Adult cows and sows can act as reservoirs: excrete virus in faeces at parturition and a source of RV for the young.
Other groups: Less known but common in their respective species

8. ROTAVIRUSES (RVs) DIAGNOSIS
Care is needed to interpret positive results as RVs are commonly found in the faeces of clinically normal neonatal animals. Compare the number of RV+ve diseased animals with the number of RV+ve unaffected, age-matched animals on the farm. Diagnosis of outbreaks should not be based on single faecal samples.
Methods: On faeces:
ELISAs available commercially for group A RVs. ELISAs for human RV OK for animal RV.
Latex agglutination: commercially available for group A; calf-side test.
Electron microscopy: RVs have a distinctive morphology. Also allows visualization of other enteropathogenic viruses Eg. coronaviruses & caliciviruses.
PAGE (Polyacrylamide gel electrophoresis) of segmented viral nucleic acid: Used in some Veterinary Investigation Centres. Distinctive band patterns.
On tissues: Immunocytostaining - allows intestinal damage & RV infection to be correlated. Useful for smaller farm animals (pigs); early tissues samples at the beginning of diarrhoea essential before infected enterocytes lost. Single animal can give diagnosis.
Rising antibody titres: not useful because majority of animals have high levels of rotavirus antibody in serum.

9. ROTAVIRUSES (RVs) CONTROL:
The following relates to group A RVs but may also relate to the other groups.
By an inter-relationship between passive immunity & virus exposure. It is impossible to prevent infection of the young but disease can be supressed/eliminated by keeping the infection pressure low and ensuring neonatal animals receive adequate levels of passive antibody orally while at risk.
Remove dirty bedding, sanitize hands, gloves, boots, clothing after dealing with sick animals; deal with sick animals first; steam clean/fumigate premises. Practise all-in, all-out system where possible.
Antibody must be present in the gut lumen to neutralize the infectivity of the virus and this requires daily feeding of colostrum. Disease occurs when passive immunity wanes (either naturally or by weaning).
Diseased animals usually have good levels of circulating antibody derived from the early ingestion of maternal antibody. These are NOT protective because the pathogenesis is confined to the gut.
Bovine RV: A dam vaccine is available in the UK to boost & prolong secretion of RV neutralizing antibody in colostrum/milk. Primarily for suckler beef herds but stored colostrum from vaccinated cows can be fed to dairy calves.
Porcine RV: No vaccine marketed in the UK; bovine colostrum will protect piglets.
Canine & Feline RVs: No vaccines thought necessary.

10. AFRICAN HORSE SICKNESS VIRUS (AN ORBIVIRUS)
"orbis" = ring (latin) - ring-shaped capsomeres are seen on the surface of virions by electron microscopy. Recognized in Africa since It is one of the most feared viral diseases of horses but does not occur in the UK.
DISEASE & PATHOGENESIS:
Pantropic, fatal disease. Predominantly infects endothelial cells and myocardium. In the acute pneumonic form, interlobular pulmonary oedema, pericarditis, haemorrhages and oedema of the viscera occur; death within 5 days. A more prolonged form of the disease has cardiac involvement. Mortality can be 90% depending on the virulence of the isolate and host susceptibility. Subclinical disease can occur in donkeys, vaccinated horses and native horses.

11. AFRICAN HORSE SICKNESS VIRUS (AN ORBIVIRUS)
ANTIGENICITY: Nine serotypes.
DIAGNOSIS:
Clinical diagnosis is possible. Intracerebral inoculation from suspected cases causes the death of suckling mice from which the virus can be identified by complement fixation tests and haemagglutination inhibition tests.

12. AFRICAN HORSE SICKNESS VIRUS (AN ORBIVIRUS)
HOST RANGE: Horses; mules, donkeys & zebras appear to be more resistant to the disease.
EPIZOOTIOLOGY:
The virus replicates in the horse and salivary glands of the vector, the midge Culicoides imicola . Native horses harbour the virus and "fuel" Culicoides. Once, the virus was found only in Africa. Since 1987 it has re-appeared in Spain (as far north as Madrid) & Portugal where the vector occurs. The virus may be endemic there. In the recent Spanish outbreaks hundreds of horses have died or been destroyed. The presence of the virus in Spain is highly relevant to British veterinary surgeons because of the international traffic of horses to affected countries, for example, to Spain for the Barcelona Olympics.

13. AFRICAN HORSE SICKNESS VIRUS (AN ORBIVIRUS)
CONTROL:
Strong preventative control measures are required - bans on movements of all susceptible animals; protection against vectors; slaughter or immediate isolation of sick animals; mandatory & immediate vaccination of the entire equine population with the appropriate serotype; monitoring for the presence of the virus and the vector. Protection zones (where animals are vaccinated to contain the spread of the disease) and Surveillance zones (no vaccination but animals monitored for disease) are set up around outbreaks.

14. OTHER MEMBERS
Many REOVIRUSES infect the alimentary and/or respiratory tract of cattle, sheep, dogs and cats without causing disease (REO= Respiratory-Enteric-Orphan).
BLUETONGUE VIRUS, another Orbivirus, causes high mortality in sheep. Transmitted between animals by Culicoides midges. Outbreaks occur when and where the Culicoides breed. Found in Africa, N. America, Portugal & Spain. Controlled by import controls, spraying to kill insect vectors,polyvalent live vaccines with the relevant local serotypes.

15. Summary
Rotaviruses pathogenesis is confined to the gut; passive maternal antibody in the gut lumen is important to control the disease.
Care is required in RV diagnosis as RVs are also found commonly in sub-clinical infections
Control of African Horse Sickness Virus centres around vaccination and removal of infected horses and vectors.