Course objectives

• To promote Islamic values through an appreciation of the role of microbes in the well-being of human life.

• To familiarise students with structure-function relationship of microbe and parasite in diseases.

• To acquire basic skills in the identification and isolation of microbial pathogens.

Learning outcomes

• Demonstrate understanding of the principles of diagnostic medical microbiology and the clinical correlations.

• Make observations, understand the fundamental elements of experimental design, generate and analyse data on pathogenesis and control of disease.

• Demonstrate proficiency in collection, interpretation, and presentation of scientific data in medical microbiology when conducting a research.

• Communicate about case studies using appropriate oral and written means.

Parvovirus and Herpes virus

SBM 2044

Medical Microbiology

Parvoviruses

• Are the simplest DNA animal viruses – host dependent for viral replication.

• The only pathogenic parvovirus to humans, B19 of genus Erythrovirus – cause of erythema infectiosum “slapped-cheek disease” or “fifth disease”.

• Common in children age 5-15.

Structure & composition

• Icosahedral, non-enveloped.• Virions are extremely resistant to inactivation.

Stable at pH3-pH9; 56°C for 1hr.• Virions contain 2 coat proteins. The major

capsid protein , VP2, represents ~ 90% of virion protein. The non-structural protein – for virus replication and may modulate host cell genes and cause diseases.

• Genome is ~5kb, linear, single-stranded DNA.

• A negatively stained preparation of parvovirus as seen by transmission electron microscopy. The individual virions have a diameter of only 22nm. (nm=one-millionth of a millimeter) www.wadsworth.org

Parvovirus replication

• Highly tropic for human erythroid cells. Cellular receptor for B19 is blood group P antigen (globoside) – expressed in mature erythrocytes, erythroid progenitors, endothelial cells, placenta.

• Viral replication in nucleus.• Major sites of viral replication are assumed to be

the adult marrow and fetal liver, causing cell death by inhibiting RBC production.

• Both virus-specific IgM and IgG Abs are produced following B19 infection.

Pathogenesis

• Transmission presumably via the resp route.

• B19 can be found in blood and resp secretions. Some may replicate in intestinal mucosal cells and cause enteritis.

Fifth disease

• Most common in school children.

• Symptoms: rash, pain in joints (arthropathy) in adult cases which mimic rheumatoid arthritis.

• Viraemia 1 week after infection, virus present in nasal washes, gargle specimens.

• Patients are treated symptomatically.

Herpesviruses

• Everyone will get herpesviruses at some time.

• Large viruses; enveloped; all have a core of ds DNA in the form of a toroid, surrounded by a protein coat that exhibits icosahedral symmetry and 160 capsomeres.

Structure and composition

• Envelope is derived from nuclear membrane of infected cell and contains viral glycoprotein spikes.

• Genome is a linear ds DNA of 124-235kbp.• Sequence arrangement – herpesvirus possess

terminal and internal repeated sequences.• Large genome and encodes >100 proteins.

• Genomes of herpes viruses. HSV, VZV and CMV have inverted repeat sequences. This results in the formation of more than one isomer by recombination. Because VZV has only two inverted repeats, it can only form two isomeric forms. Direct repeats do not allow recombination and so  EBV and HHV6 have only one isoform.

Common name Sub-

family

Clinical features

1 Herpes simplex virus-1 α Gingivostomatitis, cold sores, encephalitis

2 Herpes simplex virus-2 α Genital herpes, meningo-encephalitis

3 Varicella-zoster virus α Varicella (chickenpox), zoster (shingles)

4 Epstein-Barr virus γ Mononucleosis (glandular fever), Burkitt’s Lymphoma, nasopharyngeal carcinoma

5 Cytomegalovirus β Mononucleosis, congenital CMV disease, hepatitis, retinitis in the immunocompromised

6 Human herpes virus-6 β Exanthum subitum/Roseola infantum, febrile childhood illness7 Human herpes virus-7 β

8 Kaposi’s sarcoma-associated

γ

Herpes Simplex Viruses

• HSV-1 and HSV-2 exhibit substantial sequence homology. They differ in their mode of transmission: HSV-1 by contact (saliva), whereas HSV-2 is transmitted sexually or from a maternal genital infection to a newborn.

• HSV have rapid growth cycle, 8-16hr.• Large genome 150kbp which encodes >70

polypeptides.

HSV: Pathogenesis• 1) Primary infection:• HSV-1 infections are usually limited oropharynx;

HSV-2 is usually transmitted by genital routes. Viral replication occurs at the site of infection.

• Virus then invades local nerve endings and is transported to dorsal root ganglia.

• 2) Latent infection:• HSV-1 infects the trigeminal ganglia, whereas

HSV-2 in sacral ganglia. Both in non-replicating states.

HSV: Pathogenesis

• 3) Reactivation:• Provocative stimuli (fever,

stress, pneumococcal infection) can trigger recurrence – many asymptomatic. Symptomatic recurrences are usually manifested as cold sores near the lip.

http://www.sumanasinc.com/webcontent/anisamples/microbiology/herpessimplex.html

• Expression of herpesviruses genes.• http://pathmicro.med.sc.edu/

HSV: Immunity

• Newborns acquire passively transferred maternal Abs up to 6-month old.

• 6-month to 2 years : highly susceptible• HSV-1 Abs begin to develop in early childhood.• HSV-2 Abs rise during the age of adolescence.• During primary infection, IgM followed by IgG

and IgA for long periods. NK cells and interferons are important in controlling both primary and recurrent HSV infections.

• After recovery, the virus is carried in a latent state in the presence of Abs.

HSV- Diagnosis• Cytopathology: to stain scrapings from the base

of lesions (of mucous membranes of the mouth/genitalia) and look for multinucleated giant cells and intranuclear eosinophilic inclusions bodies.

• Tissue specimen or fluid is introduced into a primary cell line such human embryonic kidney TC and is then observed for cytopathic effects within 24-48h – essential for immunocompromised and neonatal patients.

• Serological analysis is useful for primary infection but not recurrence, because the Ab titre usually does not increase.

Varicella-Zoster Virus

• Varicella (chickenpox) is a mild, highly contagious disease mainly in children, characterized clinically by a generalized vesicular eruption of the skin and mucous membranes.

• Zoster (shingles) is characterised by a rash limited in distribution to the skin innervated by a single sensory ganglion.

• Varicella is the acute disease, from primary contact with the virus, whereas zooster is the response of partially immune host to reactivation of varicella virus present in latent form in neurons in sensory ganglia.

VZV - Pathogenesis

• Varicella: • Route of infection is the mucosa of the Upper

Resp T or the conjunctiva.• Replication in regional lymph nodes → 1°

viraemia which leads to replication in liver and spleen.

• 2° viraemia involving infected mono-nuclear cells transports virus to the skin → rash.

• Swelling of epith cells, ballooning degeneration and accumulation of tissue fluids.

• Host humoral and cellular immune responses are important immune defence.

VZV- Pathogenesis

• Zoster:• Similar skin lesions to those of varicella.• Often only a single ganglion may be

involved. Distribution of lesions corresponds closely to the areas of innervation from an individual dorsal root ganglion.

• Cell-mediated immunity is probably the most important host defense.

VZV- Clinical findings

• Incubation period is 10-21 days. • Malaise and fever are the earliest symptoms, followed by

rash (first on trunk and then on the face, the limbs and buccal and pharyngeal mucosa in the mouth).

• Rash lasts for 5 days.• Complications are rare in children. Eg. Encephalitis,

varicella pneumonia.• Zoster – usually occurs in immunocompromised. Starts

with severe pain in the area of skin supplied by one or more groups of sensory nerves and ganglia.

• Trunk, head and neck are commonly affected.• Immunity: Abs induced by varicella vaccine can persist

for at least 20 years.

Cytomegaloviruses

• The name derives from the massive enlargement of CMV-infected cells.

• DNA genome 240kbp (largest of HHV)-only a few proteins encoded by the virus (>200) have been characterised.

• CMV are very species-specific and cell-specific – all attempts to infect animals with human CMV have failed. CMV replicates only in human fibroblasts and replicates very slowly in cultured cells.

CMV - Pathogenesis

• A) Normal hosts: • CMV is transmitted by close contact from

person-to-person.• 4 to 8 weeks of incubation period.• Virus causes systemic infection and is an

infectious mononucleosis-like syndrome.• Lifelong latent infections, like all herpesviruses.• Cell-mediated immunity is depressed with

primary infections and may take months to recover.

CMV - Pathogenesis

• B) Immunocompromised hosts:• Immunocompromised such as those receiving

organ transplants, on chemotherapy, AIDS are at greatest risk.

• Severe infection and pneumonia is the most common complication.

• C) Congenital and perinatal infections:• Virus can be transmitted in uterus with both

primary and reactivated maternal infections.• Babies suffer with cytomegalic inclusion disease

and will exhibit developmental defects and mental retardation.

Epstein-Barr virus

• The major target cell for EBV is the B lymphocytes, and infected B lymphocytes are immortalised by the virus.

• Discovered in African children with unusual malignant tumour (Burkitt lymphoma).

• The epithelium of oropharynx is the portal entry for EBV during 1° infection. Virus then moves to parotid gland, to replicate and undergoes latency state.

• Infectious mononucleosis are presented with sore throat, hig fever, cervical lymphadenopathy – which develop after 30-50 days.

Burkitt Lymphoma

Human Herpesvirus 6

• HHV-6 appears to be unrelated antigenically to the other known human herpesviruses.

• Virus grows well in CD4 T lymphocytes.• HHV-6 infections usually occur in early

infancy, causing roseola infantum characterised by high fever and skin rash.

• Mode of transmission is possibly via oral secretions.

Human Herpesvirus 7

• A T-lymphotropic human herpesvirus.

• Immunologically distinct from HHV-6.

Human Herpesvirus 8

• Also called Kaposi’s sarcoma-associated herpesvirus (KSHV).

• Virus can be transmitted through organ transplants.

• KSHV is the cause of Kaposi’s sarcomas, vascular tumours of mixed cellular composition.

Treatment

• Acyclovir for HSV and VZV.

• Gancilovir for CMV

• Famcyclovir for VZV

• Varicella-zoster immunoglobulin is used to prevent disease when eg. Immuno-compromised are exposed to infection.