Reoviridae is a family of viruses that includes orthoreoviruses, rotaviruses, orbiviruses, and coltiviruses. They are non-enveloped viruses with double-layered protein capsids and segmented double-stranded RNA genomes. Rotaviruses are the most common cause of severe diarrhea in infants and young children worldwide. They replicate in the cytoplasm of intestinal cells. Orbiviruses commonly infect insects and can be transmitted to vertebrates by insects, causing diseases like bluetongue in sheep. While reoviruses are ubiquitous, their role in human disease is unclear.

1. Reoviridae
By- SanjuSah
St. Xavier’sCollege, Maitighar, Kathmandu
Department of Microbiology

2. Reoviruses, Rotaviruses, Orbiviruses and Coltiviruses
6 hrs

4. Introduction
•Reo: from Respiratory enteric orphan
•In 1959, Sabin proposed the name reovirus to reflect the fact
that viruses of this group had been isolated from the
respiratory and enteric tracts and were orphan (reo) viruses
without known associated disease.
•The family Reoviridae is divided into several genera.
•Four of the genera are able to infect humans and animals:
Orthoreovirus, Rotavirus, Coltivirus, and Orbivirus.
• Other genera infect only the plants, insects, and fish.

5. • These genera can be divided into two groups; one group contains
viruses with large spikes at the 12 vertices on the particle (eg,
Orthoreovirus) whereas members of the second group appear
more smooth, lacking the large surface projections (eg,
Rotavirus).
• The Reoviridae viruses are non-enveloped viruses with double-
layered protein capsids.
• The genome consists of a 10–12 segmented double-stranded
RNA with a total genome size of 16–27 kbp depending on the
genus
• Rotaviruses contain 11 genome segments, whereas
orthoreoviruses and orbiviruses each possess 10 segments and
coltiviruses have 12 segments.

6. •The individual RNA segments vary in size from 680 bp
(rotavirus) to 3900 bp (orthoreovirus).
• The virus core contains many enzymes essential for
transcription and capping of viral RNA.
• These viruses are unusually resistant to heat, a wide pH (3.0–
9.0), and to lipid solvents but are sensitive to 95% ethanol,
phenol, and chlorine

7. Important Properties of Reoviruses

8. *Biliary atresia is a rare
condition in newborn
infants in which the
common bile duct between
the liver and the small
intestine is blocked or
absent

9. Structure and composition
•The virions of Reoviridae family viruses measure 60-80 nm in
diameter and possess 2 concentric capsid shells, each of which
is icosahedral.
•No envelope is present.
•The inner capsids of all genera display sharply defined
subunits; the outer capsids of rotaviruses and orbiviruses lack
well-defined subunit structures.
•The genome consists of double-stranded RNA in 10-12
discrete segments, with a total genome size of 16-27 kilobase
pair (kbp), depending on the genus.

10. •The double-shelled particle is the complete infectious form
of the virus.
•The individual RNA segments vary in size from 680 base
pair (bp), i.e., rotavirus, to 3900 bp, i.e., orthoreovirus.
•The virion core contains several enzymes needed for
transcription and capping of viral RNA.

12. Structure
of
family
Reoviridae

14. •REPLICATION:CYTOPLASMIC
• Attachement to host receptors probably mediates endocytosis
of virus into host cell.
•Particles are partially uncoated in endolysosomes, but not
entirely, and penetrate in the cytoplasm.
• Early transcription of the dsRNA genome by viral
polymerase occurs inside this sub-viral particle (naked core),
so that dsRNA is never exposed to the cytoplasm.
• Transcription from each of the dsRNA segments produces
mRNA templates for translation.
• Viral proteins and genomic RNAs aggregates in cytoplasmic
viral factories.

15. • (+)RNAs are encapsidated in a sub-viral particle, in which
they are transcribed to give RNA (-) molecules with which
they become base-paired to produce dsRNA genomes.
•The capsid is assembled on the sub-viral particle.
• Mature virions are released presumably following cell death
and associated breakdown of host plasma membrane.

16. Viral
replication

17. •Rotaviruses
• Rotaviruses are a major cause of diarrheal illness in human
infants and young animals, including calves and piglets.
•Infections in adult humans and animals are also common

18. •Morphology
•Rotavirus shows following features:
•The virus measures 70 nm in diameter and possesses three-
layer icosahedral capsid without an envelope.
•The sharply defined circular outline of the outer capsid gives
the appearance of the rim of a wheel placed on short pokes
radiating from a wide arm.
•This appearance gives the virus the name “rota” (Latin word
which means wheel).
•The genome consists of a 11 segmented double-stranded
RNA genome.

19. •The outer shell is composed of a major glycoprotein with a
molecular weight of 34,000 Da and this protein is known as
viral protein (VP7).
•It also consists of a minor trypsin-sensitive protein with a
molecular weight of 84,000 Da and this protein is designated
as VP4
•The minor shell or core protein of the virus core consists of
four proteins: VP1, VP2, VP3, and VP6.
• The virus also consists of six nonstructural proteins

20. Schematic representation of a rotavirus virion. The virus is composed of three
protein shells, an outer capsid, an inner capsid, and an internal core, that
surround the 11 segments of double-stranded RNA. The outer capsid proteins
VP4 and VP7 are neutralization antigens and define the P and G serotypes,
respectively. VP6, the inner capsid structural protein, is the subgroup antigen.

21. Rotaviruses: serotypes, groups, and subgroups
•HRVs belonging to 11 G serotypes have been isolated, but
the vast majority have been identified as G1, G2, G3, or G4,
and strains belonging to these G types have commonly been
designated as serotype 1, 2, 3, or 4, respectively.
•The severity of illness caused by viruses that belong to these
4 serotypes varies little, if at all.
•At least 6 different HRV P types have been identified. P type
1a, the most common, is usually associated with G types 1,
3, or 4; whereas, P type 1b is usually associated with G type
2.

22. •Rotaviruses are classified into seven groups (from A to G)
based on the antigenicity of VP6 and the electrophoretic
mobility of the genomic segments.
•Human disease is caused mostly by group A and
occasionally by group B and C rotaviruses

23. •Viral replication
•Rotaviruses replicate in cytoplasm of the host cell.
•Infection of the cell begins by attachment to the cell surface
at the site of beta-adrenergic receptor via VP4 protein and
occurs by clathrin-mediated endocytosis
•Particles are partially uncoated in endolysosomes (loss of the
VP4-VP7 outermost layer), and penetrate into the cytoplasm
via permeabilization of host endosomal membrane.
•Early transcription of the dsRNA genome by viral polymerase
(RNA dependent RNA polymerase) occurs inside these now
double-layered particles (DLPs), so that dsRNA is never
exposed to the cytoplasm.

24. • The nascent (+)RNAs are extruded into the cytoplasm and
serve as template for viral proteins synthesis.
•The transcribed mRNAs are extruded into the cytoplasm
and encode a variety of structural and nonstructural
proteins.
• RNA polymerase is one of those proteins that synthesizes
minus strands, which become a part of progeny virus.
• Progeny cores with replicase activity are produced in
cytoplasmic virus factories (also called viroplasms).
•This implies synthesis of complementary (-)RNA and initial
steps of viral morphogenesis.
• Late transcription occurs in these progeny cores.

25. •At the periphery of virus factories, these core are coated with
VP6, forming immature DLPs that bud across the membrane
of the endoplasmic reticulum, acquiring a transient lipid
membrane which is modified with the ER resident viral
glycoproteins NSP4 and VP7; these enveloped particles also
contain VP4.
•As the particles move towards the interior of the ER
cisternae, the transient lipid membrane and the nonstructural
protein NSP4 are lost, while the virus surface proteins VP4
and VP7 rearrange to form the outermost virus protein layer,
yielding mature infectious triple-layered particles.

26. •Mature virions are released presumably following cell death
and associated breakdown of host plasma membrane.

27. • Pathogenesis and Immunity
• Rotavirus is transmitted by feco–oral transmission.
• Rotavirus survives the acidic environment in the stomach and
initiates infection in the mucosal cells of the small intestine.
• After absorption, the viruses replicate in the cytoplasm of the
enterocytes and damage their transport mechanism.
• Non-structural protein 4 (NSP4) of the rotavirus may act as a
viral enterotoxin, which causes secretion of fluids by
stimulating a signal transduction pathway*.
*Signal transduction (also known as cell signaling) is the
transmission of molecular signals from a cell's exterior to its
interior

28. • The toxin induces influx of calcium ion into enterocytes and
release of neuronal activators, and alters sodium and glucose
absorption.
• The resulting diarrhea is due to impaired sodium and glucose
absorption, as damaged cells on the villi are replaced by non-
absorbing immature crypt cells
• The rotavirus after replicating in the cell causes damage to
the cell.
• The damaged cells are released into the lumen of intestine,
releasing large quantities of viruses in the diarrheic stool.

29. •It takes around 3–8 weeks for restoring the normal function
of the cell.
•Hence, the rotavirus produces watery diarrhea similar to
that seen in cholera

30. •Host immunity
•Rotavirus infection is characterized by the presence of high
quantity of immunoglobulin A (IgA) in the intestinal
secretions.
•The IgA plays an important role in conferring the gut
immunity against rotavirus.
•It protects newborns up to the age of 6 months.
•The serum antibodies usually reduce the severity of the
disease, but not necessarily prevent reinfection.
•Even a small quantity of a virus may cause infection and
diarrhea in the absence of specific antibodies in the serum.

31. •Clinical Symptoms
•Rotavirus is a major cause of diarrhea in infants and young
children.
•Diarrhea
•The incubation period is short being less than 48 hours.
•The condition manifests commonly as fever, vomiting,
diarrhea, and occasionally dehydration. Vomiting is usually
of short duration and can occur before or after the onset of
diarrhea.
•The diarrheic stool may be watery, green, or yellow but
does not contain mucus.

32. •Rotavirus diarrhea is a self-limiting disease, and patients
recover completely within 5–10 days without any
complications on sequelae.
•Also, rotavirus has been reported as an agent of traveler’s
diarrhea in adults.
•The virus has also been reported to cause gastroenteritis in
adults.

33. •Laboratory Diagnosis
• Specimen
•Diarrheic stool is the specimen of choice for demonstration
of rotavirus and viral antigens.
•Microscopy
•Rotavirus can be demonstrated in stool by direct electron
microscopy (EM) and by immunoelectron microscopy
(IEM)

34. •Direct antigen detection
•Enzyme immunoassay are useful tools to detect rotavirus
antigen directly in the stool for diagnosis of diarrheal illness.
•This is a method used for routine diagnosis of rotavirus
diarrhea.
• Isolation of the virus
•Rotaviruses are difficult to grow in cell culture, hence are
not routinely used for diagnosis of rotavirus diarrhea

35. • Treatment
• No specific antiviral therapy is available for rotavirus
infection.
• The treatment of the condition is mostly supportive.
• It consists of restoring the fluid loss in dehydrated patients.
• Oral rehydration fluid using glucose and electrolyte
solution is currently preferred than the intravenous fluids to
restore the hydration.
• Intravenous fluids are used only in severely dehydrated
children.

36. Prevention
• A rhesus–human reassortant rotavirus tetravalent
vaccine (RRV-TV; Rota shield) is a being used since
1998.
• The vaccine consists of a polyvalent preparation
consisting of the VP7 protein of each of the four
clinically important serotypes with the attenuation
phenotype of the rhesus rotavirus.
• This has been successful in preventing severe rotavirus
diarrhea

37. •A pentavalent human-bovine reassortant live-attenuated, oral
vaccine (RotaTeq) has been developed
•This vaccine contains five live reassortant rotaviruses
•Four reassortant rotaviruses express the VP7 protein (G1, G2,
G3, or G4) from the human rotavirus parent strain and the
attachment protein (P7[5]) from bovine rotavirus parent strain
WC3.
•The fifth reassortant virus expresses the attachment protein
(P1A[8] from the human rotavirus parent strain and the outer
capsid protein G6 from the bovine rotavirus parent strain.
•RotaTeq is administered in three oral doses at 1- to 2-month
intervals beginning at 6 to 12 weeks of age.

38. •Orthoreovirus
•Reoviruses (which also are called orthoreoviruses to avoid
confusion with the family Reoviridae) are nonenveloped
viruses.
•Reovirus particles are composed of an inner protein shell (ie,
core) of a diameter of 60 nm, which is surrounded by an outer
protein shell (ie, outer capsid) that measures 80 nm in
diameter.
•The core is composed of 3 major (ie, lambda-1, lambda-2,
sigma-2) and several minor proteins that surround 10
segments of double-stranded RNA.
•The virions also contain 3 nonstructural proteins.

39. • Reoviruses are ubiquitous, with a very wide host range.
• Three distinct but related types of reovirus have been recovered
from many species and are demonstrable by Nt and HI tests.
• Reoviruses contain a hemagglutinin for human O or bovine
erythrocytes
• Reoviruses are moderately heat-stable, stable through a wide pH
range, and stable in aerosols, particularly when the relative
humidity is high.
• For recovery from clinical material, Reoviruses can be cultured in
monkey kidney cells, HeLa cells, etc
• Because of their widespread prevalence in nature and the ease of
infection in laboratory animals, reoviruses have been used widely
in pathogenicity studies

40. • Human volunteers’ studies have failed to establish a clear
cause-and-effect relationship between reoviruses and human
ill-ness.
• So far, reoviruses have been linked with upper respiratory
infection, fever, enteritis, and febrile exanthema in children.
• All three serotypes of the virus have been recovered from
healthy children and from children with minor febrile illness,
diarrhea, or enteritis.
• The exact method of transmission of reoviruses is not known.
• Since viruses are isolated most frequently from the feces, the
infection appears to be transmitted by the fecal–oral route.

41. • The laboratory diagnosis of the human orthoreovirus infection
can be made by:
• The isolation of the virus and detection of the viral anti-gen
and RNA genome in various clinical specimens, such as
feces, throat swabs, and nasopharyngeal specimens.
• The serological tests, such as hemagglutination inhibition,
complement fixation, or virus neutralization to demonstrate
antibodies.
• These serological tests are used primarily for epidemiological
studies.
• No specific treatment is available for orthoreovirus infection.
• No preventive measures have been suggested due to the lack of
definitive association of orthoreovirus with human disease.

42. • Orbivirus
• Orbiviruses commonly infect insects, and many can be transmitted
by insects to vertebrates.
• More than 100 serotypes have been identified.
• Serious animal pathogens include bluetongue virus of sheep and
African horse sickness virus.
• Antibodies are found in many vertebrates, including humans.
• The genome consists of 10 segments of double-stranded RNA.
• The replicative cycle is similar to that of reoviruses.
• In contrast with the general stability of other reoviruses,
orbiviruses are inactivated by low pH.

43. •Orungo virus, isolated in Africa, has been implicated in an
acute illness with myalgias and headache.
•Lebombo virus is another orbivirus isolated from humans in
Africa.
•Kemerovo virus has been implicated in neurologic
infections in central Europe and Russia.
•Serologic evidence of infection with Lipovnik or Tribec
virus has been demonstrated in patients with polyradiculitis
in the former Czechoslovakia.
•Changuinola virus has been isolated from humans in
Panama.

44. • Coltivirus
• Coltiviruses resemble the orbiviruses in size and in having 2 capsids.
• These viruses contain 12 RNA segments.
• Colorado tick fever (CTF), Salmon River virus (in Idaho), Eyach
virus (in Germany and France), isolate S6-14-03 (in California),
Banna virus, Beijing virus, and Gansu virus (in China) either are
proved or suspected of causing human disease.

45. •Colorado tick fever is an acute viral infection transmitted by
the bite of wood tick (Dermacentor andersoni) caused by
Colorado tick fever virus.
•Colorado fever was so named because the illness was
believed to occur predominantly in Colorado and was used to
distinguish this clinical illness from that of Rocky Mountain
spotted fever caused by Rickettsia species.
•The causative agent of this fever was recognized as a virus in
1946
•Colorado tick fever virus is a double-stranded RNA virus
surrounded by two capsids.

46. • The virus contains 12 RNA segments.
• The virus infects and replicates in the bone marrow, lymph nodes,
spleen, and liver of rhesus monkey, but without producing any
histological abnormalities.
• The virus has been shown to replicate in erythroid precursor cells
without severely damaging them and is present in mature red blood
cells.
• Colorado tick fever occurs almost exclusively in the Western United
States and in Canada.
• The disease is transmitted by the bite of wood tick, D. andersoni.
• Larval and nymphal stages of D. andersoni ticks usually transmit the
virus among all rodents, but only adult ticks transmit the virus to
humans.

47. • The ticks acquire the infection on feeding an infected viremic
host.
• The infected ticks subsequently transmit the virus through their
saliva during act of feeding on a new susceptible host.
• Squirrels, rabbits, and deer are the natural animal hosts for the
virus.
• The virus generally causes a nonspecific febrile illness.
• The incubation period is short and varies from 3 to 6 days.
• The clinical manifestations of the acute condition are
characterized by the sudden onset of fever, chills, headache with
retro-orbital pain, malaise, nausea, and occasionally vomiting.

48. •A rash is generally absent by which Colorado tick fever is
differentiated from the Rocky Mountain spotted fever.
•The fever is typically biphasic (i.e., with two episodes of
fever), each of which lasts 2–3 days, separated by a remission
of approximately equal duration. In most cases, the febrile
period is followed by moderate to marked weakness and
malaise.
•Complications are rare, but may include hepatitis,
pericarditis, epididymoorchitis, atypical pneumonitis,
encephalitis, and aseptic meningitis.
•Viruses are present in the erythrocytes during the first 2
weeks of disease.

49. •This is followed by a period during which the viruses infect
and replicate within the erythropoietic cells.
•In the infected red blood cells, the viruses can live for the life
of the cells, which is nearly 120 days.
•A single attack of infection usually produces lifelong
immunity.
•No specific antiviral treatment is available for Colorado tick
fever.
•The condition is usually self-limited and can be prevented by
avoiding contact with the wood tick