Poxviruses

Introduction largest and most complex viruses Infections: characterized by a rash there is concern that the virus could be reintroduced as a biologic weapon

Table 34–1 Important Properties of Poxviruses Virion: Complex structure, oval or brick-shaped, 300–400 nm in length x 230 nm in diameter; external surface shows ridges; contains core and lateral bodies Composition: DNA (3%), protein (90%), lipid (5%) Genome: Double-stranded DNA, linear; size 130–375 kbp; has terminal loops; has low G + C content (30–40%) except for Parapoxvirus (63%) Proteins: Virions contain more than 100 polypeptides; many enzymes are present in core, including transcriptional system Envelope: Virion assembly involves formation of multiple membranes Replication: Cytoplasmic factories Outstanding characteristics: Largest and most complex viruses; very resistant to inactivation Virus-encoded proteins help evade host immune defense system Smallpox was the first viral disease eradicated from the world

Classification Two subfamilies: -infect vertebrate -infect insect hosts

GenusVirus Primary HostDisease Orthopoxvirus VariolaHumansSmallpox (now eliminated) Vaccinia Humans Localized lesion; used for smallpox vaccination Buffalopox Water buffalo Human infections rare; localized lesion Monkeypox Rodents, monkeys Human infections rare; generalized disease Cowpox Cows Human infections rare; localized ulcerating lesion ParapoxvirusOrf Sheep Human infections rare; localized lesion PseudocowpoxCows Bovine papular stomatitis Cows Molluscipoxvir us Molluscum contagiosum HumansMany benign skin nodules Yatapoxvirus TanapoxMonkeysHuman infections rare; localized lesion YabapoxMonkeysHuman infections very rare and accidental; localized skin tumors

Ancient history – many millions killed Mortality: 25-30% No animal reservoir/vector Stocks still exist by deliberate infection(variolation)→dangerous Edward Jenner introduced vaccination with live cowpox virus in 1798 History (Small pox)

Comparison of Vaccinia & Variola Viruses Vaccinia used for smallpox vaccination may be the product of genetic recombination has a broad host range(includes rabbits and mice, cattle, water buffalo,) Variola has a narrow host range (only humans and monkeys) Both grow in several types of chick and primate cell lines The nucleotide sequences of variola and vaccinia are similar(with the most divergence in terminal regions of the genomes.) - Both vaccinia and variola viruses grow on the chorioallantoic membrane - Variolla have smaller pocks than vaccinia pocks and without necrotic center

Clinical Findings incubation period:10–14 days macule → papule → vesicle → pustule

Immunity An attack of smallpox gave complete protection against reinfection. Vaccination immunity : 5 years and sometimes longer Patients who have defects in both cellular and humeral immune response, usually fatal disease upon vaccination.

Molluscum Contagiosum Molluscum contagiosum is a common, benign viral infection of the skin and mucous membranes caused by the molluscum contagiosum virus. It typically affects children, but can be transmitted sexually in adults. The mature lesion is only papule.

Differential variola – Molluscum Contagiosum Variola (Smallpox)  Lesions (pocks) are widespread  Lesions progress from macules to vesicles to papules to crusts Molluscum Contagiosum  Lesions usually grouped in one or two areas  Lesions are papules only similar to warts

Methisazone, a chemotherapeutic agent Cidofovir, a nucleotide analogue Treatment Laboratory diagnosis -Guarnieri bodies in small pox