Introduction

Poxviruses are among the most easily recognized of all viruses, owing to the lesion by which they have gained their name. Once inside cell, they cause proliferation then lysis, giving way to a characteristic pock with a necrotic center. Poxviruses have risen to fame both for their ability to be eradicated (small pox) as well as their use in fighting other viruses (canarypox vaccines).

Morphology

Huge (up to 450nm), usually enveloped viruses, with a complex capsid symmetry
Up to 30 different structural proteins
Non-structural proteins:
- Viral epidermal growth factor, which stimulates cell growth causing the raised edge of pustule
- Viral tumor necrosis factor, which is non-functioning and acts as an anti-inflammatory by competing with TNF-alpha
- Viral IL-10, which reduces the Th-1 cell mediated response

Therapeutic Use

Recombinant Vaccines

Poxviruses can be used as heat-stable vectors for vaccines against other viruses
Grown in host cell lines or on the surface of chick chorioallantoic membranes in ovo (primordial ectoderm)
This was first accomplished by the recombination of cowpox and variola (smallpox) in the creation of the smallpox vaccine (vaccinia)
More recently, the French used this technique in the creation of the oral rabies vaccine used among the wild fox population:
- Recombinant virus inserts a plasmid encoding rabies gene in place of thymidine kinase gene
Canarypox vaccines now exist for FeLV and Rabies
- Undergoes a single cycle of replication without producing infectious virus in mammals

Virulence and Pathogenesis

Primary replication in abraded squamous epithelium
Viremia followed by multiple epidermal infections
Ballooning then necrosis (hydropic degeneration) of epidermal cells
Concurrent proliferation or adjacent epidermis (GF driven), creating more cells for the virus to infect
All three result in classical sequence of lesions:
- Papule (proliferation)
- Vesicle (fluid filled)
- Pustule (lesion breaks)
- Scab formation (healing begins)
Pock center can succumb to secondary infection
Resolution in 3-4 weeks
Some poxviruses can spread to the upper respiratory tract or viscera, causing more serious pathology

Epidemiology

Spread quickly in unhygienic circumstances
Can survive for years in dust

Diagnosis

Clinical signs
Histology
Electron microscopy
PCR, IIF

Types and Subtypes

6 Genera, all of which produce pox lesions
Subdivided based on external structure by EM

Virus by Species

'''Cow pox''', which also causes cat pox
Small pox (variola)
'''Orf'''
Seal pox
- Skin and flipper infection of wild and captive seals
Pig pox
- Lice as vectors (passive host)
- No vaccine
Capripox (exotic to EU)
- Sheep and goats
- Highly infectious (airborne and vector), with prolonged survival off the host
- Often fatal with visceral necrotic lesions in addition to epidermal
- Prevalent in Asia, Middle East, Africa, and Mediterranean
- ZOONOTIC, vaccination annually
'''Myxomatosis'''
Fowl pox
- Pox form on unfeathered skin
- Disease of poor management
- Scarification can produce partial immunity in chickens
Pigeon pox
Canary pox
Ectromelia
- Affects mouse colonies
- Largely eradicated by the use of specific pathogen free mice and rats
Camel pox
Monkey pox
Lumpy skin disease
- Subsaharan African cattle
Red squirrel parapox
Red/Gray squirrel pox

Poxviridae

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