RESPIRATORY VIRUSES

Viruses that initiate infection via respiratory tract

Site of infection Family Viruses

Local respiratory infection

Orthomyxoviridae Influenza А and В viruses

Paramyxoviridae Parainfluenza viruses (4 types), respiratory-syncytial virus RSV (3 types)

Picornaviridae Rinoviruses (113 types)

Reoviridae Reoviruses (3 types)

Coronaviridae Types 1-4

Adenoviridae Types 1-7, 14, 21

Generalized diseases, usually with initial respiratory symptoms

Herpesviridae Varicella virus, Epstain-Barr virus (EBV), cytomegalovirus

Paramyxoviridae Mumps and measles viruses

Togaviridae Rubella virus

Picornaviridae Some enteroviruses

Bunyaviridae Hantaviruses

Arenaviridae Lassa fever virus

Family Orthomyxoviridae. Influenza virus

• (-) RNA enveloped virus with haemagglutinin and neuraminidase spikes

• Pleomorphic

• Helical symmetry

• Diameter 80-120 nm

• 3 types: A, B and C

Structure of influenza virus• The internal antigens (matrix and RNP) are the type-

specific proteins and are used to determine whether a particular virus is A, B, or C.

• The external antigens (hemagglutinin and neuraminidase) are

subtype and strain-

specific antigens of

influenza A virus:

H1N1, H2N2, H3N2

Comparison of influenza A, B and C viruses

A B CSeverity of illness ++++ ++ +

Animal reservoir yes no noHuman pandemics

yes no no

Human epidemics

yes yes no

Surface antigens 2 2 1Antigenic changes

shift, drift drift drift

Amantadine, rimantadine

sensitive no effect no effect

Zanamivir sensitive sensitive

Influenza virus replication

• Hemagglutinin (HA) protein is involved in attachment and membrane fusion in the endosome of the infected cell.

The neuraminidase protein

• digests sialic acid (neuraminic acid) on the surface of cells, when the virus binds to the cell, and it is internalized (endocytosed).

• Promotes the release of virions from infected cells.

Types of hemagglutinin and neuraminidase

Antigenic variations• Antigenic drift is due to mutation. Both HA and NA after a few

years may accumulate changes in genome that an individual immune to the original strain is not immune to the drifted one. Antigenic drift results in sporadic outbreaks and limited epidemics.

• Antigenic shift is due to reassortment. It is the process by which at least two different strains of a virus combine to form a new subtype having a mixture of the surface antigens of the two original strains. As a result, the population has no immunity against the new strain. Antigenic shift results inan epidemic or pandemic.

Antigenic drift creates influenza viruses with slightly modified antigens, while antigenic shift generates viruses with entirely novel antigens.

Past antigenic shift1918 H1N1 “Spanish Influenza” 20-40 million

deaths

1957 H2N2 “Asian Flu” 1-2 million deaths

1968 H3N2 “Hong Kong Flu” 700,000 deaths

1977 H1N1 Re-emergence No pandemic

2009 H1N1 “Swine Flu Mild Pandemic

At least 15 HA subtypes and 9 NA subtypes occur in nature. Up until 1997, only viruses of H1, H2, and H3 are known to infect

and cause disease in humans.

Theories behind antigenic shift• 1. Reassortment of the H and N genes between

human and avian influenza viruses trough a third host.

• 2. Recycling of pre-existing strains (this probably occurred in 1977 when H1N1 re-surfaced).

• 3. Gradual adaptation of avian influenza viruses to human transmission.

Coinfection with human and animal strains of virus can generate very different virus strains by genetic reassortment

Reassortment of H and N genes between human and avian influenza virusesSometimes pigs can be infected with more than one virus type at a time, which can allow the genes from these viruses to mix. This can result in an influenza virus containing genes from a number of sources, called a “reassortant” virus.

Bird or avian flu (2006)

• The outbreaks affecting some Asian countries was caused by influenza A/H5N1 virus.

• It caused severe infection in humans.

• While avian influenza spreads rapidly among birds, it does not infect humans easily, and there is no confirmed evidence of human-to-human transmission.

2009 Influenza A (H1N1) Virus • The 2009 Swine flu

outbreak in humans was due to a new strain of influenza A virus subtype H1N1 that derived in part from human influenza, avian influenza, and two separate strains of swine influenza (American swine and Eurasian swine viruses).

• Swine influenza viruses are most commonly of the H1N1 subtype, but other subtypes are also circulating in pigs (e.g., H1N2, H3N1, H3N2).

Epidemiology and pathogenesis

Spread: via small particle aerosols.

• The incubation period is about 18 to 96 hours.

• Site of infection: the epithelial cells of the respiratory tract:

trachea and bronchi

Pathogenesis• Infection of mucosal cells

results in cellular destruction and desquamation of the superficial mucosa.

• The resulting edema and mononuclear cell infiltration are accompanied by local symptoms: nonproductive cough, sore throat, and nasal discharge.

• Systemic symptoms: fever, muscle aches, headache, and general prostration.

Symptoms

Pathogenesis and immunity

Immunity to an influenza virus is type-specific and lasts for many years. Recurrent cases of influenza are caused primarily by antigenically different strains.

LABORATORY DIAGNOSIS OF INFLUENZA

Specimen: the nose and the throat swab.

1. Detection of antigen by immunofluorescence (IF) and ELISA.

2. The virus isolation in cell culture or chicken eggs.

Identification of influenza strain and type: hemagglutination inhibition (HAI) and hemadsorption inhibition (HadsI).

3. Serology: HAI, neutralization test (NtT), ELISA, CFT in pared sera.

Immunofluorescence (IF) to detect virus into host cells

direct IF

indirect IF

Hemadsorption (Hads)• Virus growth in cell cultures is detected by testing for

hemadsorption: red cells are added to the culture and adhere to virus budding from infected cells.

• If the culture tests positive, hemadsorption inhibition test with specific antisera is used to identify the virus.

positive Hadscell culture

Chicken embryo structure

Chick embryo culture method

• Fluid from the amniotic or allantoic cavity of chick embryos is tested for the presence of newly formed viruses by haemagglutination test;

• the virus in positive fluids is then identified by haemagglutination inhibition test with specific antisera.

Removing allantoic fluid Inoculation of chick embryo

Haemagglutination inhibition test (HAI)Influenza viruses bind to red blood cells using the haemagglutinin causing the formation of a lattice. HA: two-fold serial dilutions of a virus are prepared, mixed with red blood cells, and added to the wells of a plastic plate. The red blood cells that are attached to virus particles form a lattice that coats the well. The red blood cells that are not bound by virus sink to the bottom of a well and form a button.

The basis of the HAI assay is that antibodies to influenza virus will prevent attachment of the virus to red blood cells. By adding specific antibodies to the virus it is possible to block this interaction and detect the virus. If antibodies to the virus are specific, hemagglutination will not be observed.

negative HA positive HA

negative HAI positive HAI

TREATMENT

Class

Effective against Drug name

M2 inhibitors (adamantane derivatives)

Influenza A Amantadine, rimantadine

Neuraminidase inhibitors

Influenza A & B Zanamivir, oseltamivir

Mechanism of Action of Neuraminidase Inhibitors

Interferon• The innate immune system forms the first line of

defence against viruses - Interferons (IFNs), glycoproteins known as cytokines.

• Interferons are produced by: – the cells of the immune system (leukocytes, fibroblasts, or

lymphocytes) in response to challenges by foreign agents such as viruses, parasites and tumor cells;

– cells infected with a virus.

• Interferons assist the immune response by: – inhibiting viral replication within host cells, – activating natural killer cells and macrophages, – inducing the resistance of host cells to viral

infection.

VACCINES

• The vaccine is multivalent and the current one is to two strains of influenza A and one of influenza B.

• Inactivated preparation of egg-grown virus.

• Live, attenuated vaccine.

• Split-vaccine consists of all viral antigenes.

• Sub-united vaccine consists of hemagglutinines and neuraminidases (H1N1, H3N2 and B).

Unique features of Adenoviruses• Double-stranded linear(+)DNA.• Naked icosahedral capsid has

fibers (viral attachment proteins) and vertices.

• 47 human serotypes• Viruses cause:

– lytic, – persistent, – latent infections in humans, – some strain can immortalize

certain animal cells.

Life cycle of adenoviruses

Pathogenesis of adenoviruses infections

• Virus is spread:– by aerosol, – direct contact, – fecal-oral.

• Virus infects: epithelial cells in respiratory and gastrointestinal tract, conjunctiva and cornea.

• Virus persists in lymphoid tissue (tonsils, adenoids, Peyer’s patches).

Mechanism of adenovirus spread within the body

Time course of adenovirus respiratory infection

ILLNESS ASSOCIATED WITH ADENOVIRUSES• Incubation period is 2-14 days.

Clinical syndromes:• Eye

Epidemic keratoconjunctivitis, acute follicular conjunctivitis, pharyngoconjunctival fever.

• Respiratory systemCommon cold (rhinitis), pharyngitis, tonsillitis, bronchitis, pneumonia.

• GenitourinaryAcute hemorrhagic cystitis, orchitis, nephritis.

• GastrointestinalGastroenteritis, mesenteric adenitis, appendicitis.

• Rare results of adenovirus infections: Meningitis, encephalitis, arthritis, myocarditis, hepatitis.Fatal disease may occur in immunocompromised patients, as a result of a new infection or reactivation of latent virus

Respiratory consequences of adenovirus infections

IMMUNITYStrong, type-specific

PREVENTIONA vaccine is available against Adult Respiratory Distress Syndrome.

Oral, live-attenuated vaccines against serotypes 4 and 7 of adenovirus is administered in tablet form.

It is given to new recruits into various arm forces around the world.

LABORATORY DIAGNOSIS OF ADENOVIRUSES INFECTION

1. Detection of antigen from pharyngeal and eye secretions and feces by: IF, ELISA and polymerase chain reaction.

2. The virus isolation in cell culture. Cytopathic effects (CPE) include swelling and rounding of cells. Identification: HAI, CFT; type virus – by NtT.

3. Serology (rise in antibody titer): HAI, NtT, CFT.

Family Paramyxoviridae• Genus Respirovirus (parainfluenza viruses types1 and 3) • Genus Rubulavirus (parainfluenza viruses types 2 and 4,

mumps virus)• Genus Morbillivirus (measles virus)• Genus Pneumovirus (respiratory syncytial virus)

Parainfluenza viruses• ss (-) RNA viruses• Enveloped viruses with hemagglutinin and

neuraminidase spikes and fusion (F) protein• Helical symmetry• 4 types: 1, 2, 3, 4a, 4b

Clinical features of parainfluenza (PIV)• Incubation period is 2 to 6 days.

• Clinical symptoms:• Rhinitis, pharyngitis, cough, fever, croup

(laryngotracheobronchitis), bronchiolitis, and pneumonia.• Croup - the subglottic region becomes narrower and

results in difficulty with breathing, a seal bark-like cough and hoarseness.

• There is clinical variation between the different PIV types.

• PIV-1 and 2: croup in children ages 2-6 years in autumn/early winter.

• PIV-3: bronchiolitis and pneumonia, and croup sporadically, without a particular seasonal occurrence.

• PIV-4: mild upper respiratory infections.

LABORATORY DIAGNOSIS OF PARAINFLUENZA

1. Detection of antigen from nasopharyngeal aspirates and throat swab by IF and PCR.

2. The virus isolation in cell culture. Indication: Haemadsorption of erythrocytes

on the surface of cells infected with virus.

Identification: HadsI, HAI, NtT, CFT.

3. Serology – detection of rise in titer

of IgG in pared sera:

NtT, ELISA, CFT, HAI.

Respiratory Syncytial Virus (RSV)

• ss RNA enveloped virus• Considerable strain variation exists,

may be classified into subgroup A and B. • CPE - the formation of multi-nucleate syncytium

Clinical features:

MUMPS• Mumps is a viral infection that primarily affects the salivary

glands causing them to become inflamed, resulting in the characteristic “chipmuck cheeks”

• Transmission by: – salivary gland secretions, mainly just before and shortly

after clinical onset – direct and close person-to-person contact and – airborne route.

The virus enters the body through the pharynx or the conjunctiva.

Clinical symptoms of mumps• The incubation period usually is 18 to 21 days, but

may extend from 12 to 35 days.

• Prodromal phase: fever, anorexia, malaise, myalgia.

The symptoms: Fever.Parotitis. Pain from parotitis swelling persists for 7 - 10 days. It may be unilateral or bilateral.

Up to 20% infections result in no symptoms.

Complications of mumps

• Orchitis 20-50 %• Meningitis and

meningoencephalitis 15 %• Ovaritis 5 %• Pancreatitis 2-5 %• Rare complications: polyarthritis,

diabetes, nephritis, thyroiditis, deafness, myocarditis.

LABORATORY DIAGNOSIS OF MUMPS

1. The virus isolation from the saliva, liquor or urine in cell culture (or chicken eggs).

CPE: giant multinucleated

cells formation.

Identification:

HAI, NtT, IF, CFT.

2. Serology: HAI, NtT, ELISA, CFT (demonstrating IgM in the first serum and detecting IgG rise in paired sera).

Measles virus• Family Paramyxoviridae • Genus Morbillivirus• enveloped RNA virus which is composed of a helical nucleocapsid, and

matrix protein. The envelope surface is covered with hemagglutinin and fusion proteins which are embedded in a lipid bilayer.

Electron Micrograph

Age distribution of measles cases• Measles is one of the most contagious of all human viruses,

with about 40 million infections world wide each year, and one to two million deaths.

Measles• a systemic infection, disseminated by viremia, with acute

disease manifestations involving the lymphatic and respiratory systems, the skin, and sometimes the brain.

• Measles virus enters through the oropharynx and the conjunctiva.

• Incubation period is 10 days until the onset of fever and 14 days to rash.

• The clinical symptoms:– Fever, cough, laryngitis, coryza, conjunctivitis, Koplick’s spots,

and maculopapular rash.

• Complications:– Pneumonia, otitis media, optic neuritis– Encephalitis, subacute sclerosing panencephalitis (SSPE)– Hemorrhagic measles

Measles pathogenesis

Koplick’s spots

Conjunctivitis. Maculopapular rash

Subacute Sclerosing Panencephalitis

LABORATORY DIAGNOSIS OF MEASLES

Clinically Koplick’s spots are pathognomonic.

1.Detection of antigen from nasopharyngeal aspirates and throat swab by IF.

2. The virus isolation in cell culture.

CPE: giant multinucleated

cells formation.

Identification: HAI, IF, NtT.

3. Serology: HAI, NtT, CFT.

Family Togaviridae. Rubella virus• It is an enveloped (toga=cloak),

RNA virus with icosahedral symmetry.

• There is only one major antigenic type.

Pathogenesis and clinical features of rubella

• Spread via an aerosol route through infected droplets.

• Replication: in the epithelium and lymph nodes of the upper respiratory tract leading to viremia and  spread to other tissues.

• Incubation period is 12 to 23 days (the average 18 days).

• Enlarged tender lymph nodes (post-auricular, post-cervical, and suboccipital) usually develop 4-6 days before rash.

• Maculopapular rash begins on the face and lasts from 12 hours to 5 days.

Rubella rash

Complications of rubella

• Encephalitis

• Arthritis

• Thrombocytopenic purpura

• Congenital syndromes:

Laboratory diagnosis of rubella• Serology:• Rising Ig G antibody titres - 4-fold or greater:

HAI, CFT, NtT, IF.• Presence of rubella-specific IgM: ELISA.• Isolation the virus in cell cultures from

respiratory tract secretions and, in infants with congenital infection, from urine, cerebrospinal fluid, and blood. Identification:– viral interference, – HAI, – IF, – ELISA.

Vaccination• Combined measles-mumps-rubella live-virus

vaccine (MMR) is recommended for all healthy 12 to 18-month-old children.

• The second dose of:– rubella vaccine is given either at school entry or at

entry to high school. – mumps vaccine is recommended either at 6 or 12

to 13 years of age.– Live measles vaccine should be given to anyone

who does not have a history of measles or has not received vaccine after the age of 15 months.