Themes:Immune evasion by virusesZoonoses: transmission to humansViruses as prophylactics

Themes

1.     Cell, tissue and host tropism of viruses2.     Arthropod transmitted viruses3.     Cancer-causing (oncogenic) viruses4.     Antigenic variation5. Persistent infections6.    Pathogenic variants generated in situ7.    Zoonotic infections8.    Emerging and re-emerging viral diseases9. Immune evasion by viruses10. Viruses as prophylactics/therapeutics

Poxviridae

Key features

1. Poxviruses are the largest animal viruses and have a complex structure and mode of assembly. Unlike most other DNA viruses, poxviruses assemble in the cytoplasm. 2. Poxviruses affect many species of animals. Although generally species-specific, some poxviruses of domestic animals are zoonotic. 3. All poxviruses cause skin lesions and some additionally cause serious systemic disease. 4. Poxvirus infections are readily diagnosed by clinical signs and electron microscopy. 5. Poxviruses are very resistant to the environment and can persist for many months on dried lesion material.6. Used as vectors for the delivery of new vaccines.

Poxviruses

• The poxviruses are very large, about 200-300 nm in length.

• The genome is double stranded DNA of 130-280 kbp, depending on the genus, which encodes around 200 genes.

• The parapoxviruses can be readily distinguished morphologically from the other genera since they are narrower and have a more oval shape.

• The viral core contains an RNA polymerase which, immediately following infection, transcribes several viral genes necessary for viral DNA replication and protein synthesis.

• Different viral proteins are translated before DNA replication (early proteins, mainly enzymes), or after DNA replication (late proteins, mainly structural).

• Viral particles are constructed in the cytoplasm within viral factories. These factories can be visualised in stained cells as inclusion bodies.

Mechanisms of immune evasion

• All poxviruses produce lesions on the skin •Viruses replicate in the face of a host immune response • Poxviruses produce many gene products that impair host immune responses

• Genes may be direct homologues of host genes

• Novel genes are also encoded

Theme : immune evasion by viruses

Mechanisms of immune evasion

Target many primary host immune mediatorscomplement proteinscytokines e.g. Interferon-gamma binding protein

Enhance the viral environmentEpidermal growth factor homologueVascular endothelial growth factor homologue

Pathogenesis and immunity

• All poxviruses cause lesions on the skin.• In most cases initial replication at the site of infection (skin, respiratory tract) results in virus being taken to local lymph nodes and via the thoracic duct to the blood.• This viraemia spreads the virus throughout the body and particularly to the skin where lesions develop.• Immunity to many poxviruses is very strong and lifelong. The exception is the parapoxviruses.• Following recovery from natural infection with orf virus, immunity is short lived and sheep become susceptible to re-infection within a few months. Clearly this feature is advantageous to the virus, permitting its survival in flocks.

Diseases caused by poxvirusesOrthopoxviruses

Cowpox (cats), ectromelia (mice), camel pox,(variola, vaccinia)

ParapoxvirusesOrf (sheep), bovine papular stomatitis,paravaccinia (pseudocowpox) (cattle)

CapripoxvirusesSheep pox, goat pox, lumpyskin disease (cattle)

CamelpoxvirusesCamel pox

LeporipoxvirusesMyxomatosis, Shope fibroma

AvipoxvirusesFowl pox, pigeon pox, canarypox, wild birds

Orthopoxviruses

Smallpox:The beginnings of vaccination

Smallpox (variola): 1000 BC - 1977 AD

"The smallpox was always present, filling the churchyards with corpses, tormenting with constant fears all whom it had stricken, leaving on those whose lives it spared the hideous traces of its power, turning the babe into a changeling at which the mother shuddered, and making the eyes and cheeks of the betrothed maiden objects of horror to the lover.”

Smallpox and vaccination

In the 18th century Edward Jenner, an English country doctor, combined two observations to produce a theory that changed the world.

Dairy workers had a very much reduced incidence of smallpox compared to other people who were exposed

Often dairy workers were afflicted with the lesions of cowpox, the common disease of dairy cattle

Therefore, infection with cowpox prevents smallpox

Then Jenner carried out the appropriate experiment

Smallpox vaccination

Edward Jenner used cowpox from a lesion on the hand of the milkmaid Sarah Nemes to inoculate 8 year old James Phipps.

Jenner later challenged James with live smallpox virus and showed that he was protected.

Sarah

Jenner

Smallpox: nomenclature

The classical name for smallpox is variola

The old name for cowpox was vaccinia

Therefore the process of immunisation against smallpox was called vaccination

The agent that was continually passaged in people (then cattle) for the purposes of vaccination continued to be called vaccinia

Smallpox vaccination

Smallpox was eradicated in 1977 due to surveillance and vaccination: last case occurred in eastern Africa

Now stocks only held in two secure laboratories in Russia and the USA (Centers for Disease Control, Atlanta)

Potential for bioterrorism!!

Where is cowpox today?

The virus produces lesions on the teats and the contiguous parts of the udder of cows and is spread through herds by the process of milking. However, Cowpox has almost disappeared from domestic cattle.

Cowpox is zoonotic and the lesions that it produces in man resemble those of smallpox, but are localised and transient

Here it is!

Rodents are reservoirs. Also infects zoo

animals

Infects felids, anteaters, okapi, elephants

Particularly severe in cheetahs

Cowpox infection in cats

Highest risk of infection

• Free range cats in rural or suburban areas

Cat-to-cat transmission

• Occurs, but disease is uncommon

Cat-to-human transmission must be prevented

• Immunodeficient people are at high risk

Theme : zoonotic disease

N.B. Lesions in humans usually appear as single maculopapular eruptions on the hands or the face; infection is accompanied by systemic signs such as nausea, fever, and lymphadenopathy

Cowpox infection in catsHow is the virus maintained?

Likely to be transmitted from small wild mammals. Bank vole, field voles and wood mice

But 40% of bank voles have anti-cowpox antibodies

May be maintained in burrows by insect transmission

Skin inoculation Original single skin lesionon head, neck or forelimb

Secondaryskin lesions

Local growthof virus

Lymph nodes

Viraemia

Dermal nodulesUlcerated papulesVesicles (tongue)ScabbingRecovery (6-8 weeks)

Cowpox infection in catsCowpox infection in cats

Rarelyfatal in cats

Accurate diagnosis is importantVirus isolation - sensitiveElectron microscopy – rapid

PCR

Camel pox

• Camelpox virus is responsible for a very important systemic disease of camels in Africa and Asia, with extensive skin lesions.

• Up to 25% of affected animals may die.

Parapoxviruses

Orf, bovine papular stomatitis, pseudocowpoxViruses very closely relatedAll are potential zoonoses

Orf

• Orf, or contagious pustular dermatitis, is a very common infection of sheep and goats.

• The name is derived from an old word for ‘rough’, referring to the lesions.

• The virus is generally spread to lambs from their mothers and causes large proliferative lesions on the lips & nose, particularly at muco-cutaneous junctions.

• Although often extensive, the lesions usually resolve within a few weeks and the animal recovers completely. However, immunity is short lived and sheep can become re-infected.

Orf in sheep and goatsContagious pustular dermatitis

Typically affects mouth and nasal planum, eyelids, feet, teats.Crusty lesions which can bleed* May prevent lambs from suckling *Live, virulent vaccine virus. Only use in affected flocksUse in ewes pre-lambing. Short-lived immunity

Orf as a zoonosis

OedemaPapule Crust

Spontaneous recovery from orf in 3 to 6 weeks

Bovine papular stomatitis

Little clinical significanceWorldwide distribution

BPS is endemic in cattle causing vesicular lesions in the mouth and lips. The condition is not usually serious but on occasion, when the lesions are more severe, has to be distinguished from other vesicular diseases such as foot-and-mouth disease or mucosal disease.

Pseudocowpox

Common in cattlePseudocowpox (paravaccinia) in cattleCharacteristic “horseshoe” scabsMilker’s nodules in humans

Capripoxviruses

Early lesions of sheep pox

Economically the most impt. pox disease of domestic spp.

Endemic in Africa, Middle East, India

Notifiable Disease

Morbidity rate: Endemic areas 70-90%

Mortality rate: Endemic areas 5-10%

Extensive lung lesions

Skin lesions

Sheep pox

These are by far the most severe poxvirus conditions of domestic animals. In outbreaks in fully susceptible animals the mortality rate may reach 50%. The disease is particularly severe in young lambs or kids.Sheeppox is a systemic disease in which widespread nodules develop in the skin, particularly in areas with little hair. Lesions may extend to lungs

Lumpy skin disease

Lumpyskin disease is a condition similar to sheepox but affects cattle and buffalo in some parts of Africa. The mortality rate is very much less than in sheeppox infection. A vaccine against sheeppox also protects against lumpyskin disease

Lumpy skin disease

High morbidity, low mortality

Prolonged recovery affects productivity

Transmitted by insect vectors

Leporipoxviruses

Two closely related virusesMyxoma virus - myxomatosis

Shope fibroma virus - vaccine

Myxomatosis

Skin ‘tumours’ result from an initial proliferation of

undifferentiated mesenchymal cells in the

dermis which become large stellate cells termed

‘myxoma cells’

Only mild disease in cottontail rabbitsSpread by fleas/mosquitos

Myxomatosis

Transmitted by the rabbit flea

Mechanical vector

The characteristic signs of myxomatosis are:Swollen head, eyelids, genitaliaPurulent conjunctivitis

Live, attenuated myxoma virus vaccine

Shope fibroma

Shope fibroma virus causes benign, self-limiting tumours

The virus is antigenically related to myxoma virus and can be used as a vaccine against myxomatosis

Myxomatosis was the first agent used for the biological control

Australia in the 1950s

Myxomatosis vaccines are not available in Australia

Avipoxvirus :Fowlpox

•Lesions on non-feathered areas of the skin (nodules develop on the head, comb and wattles).•Transmission by biting arthropods.•Also diphtheritic form•Birds with the other more serious form, which spreads by aerosol, have proliferative lesions in the epithelium of the upper respiratory tract, which frequently leads to asphyxia and death

Fowlpox is now uncommon in commercial poultry flocksVirus elimination is difficult

Poxviruses are used as vectorsfor genetically modified vaccines

Foreign gene inserted into

poxvirus genome

Protein expressed when infecting cells

i.e. following vaccination

RecombinantPoxvirus

Theme: virusesAs prophylactics

Canarypox as a vaccine vector

• Because of their large content of DNA, poxviruses are good candidates as vectors for the delivery of genes of interest into cells.

• Several poxvirus genes have been identified which are not essential for viral growth and can be deleted without affecting the growth of the virus. Therefore any gene of interest can be swapped for one of these non-essential genes.

• The resulting recombinant poxvirus will then deliver that gene into cells by infection and will promote the expression of the gene and the production of the encoded protein, as well as poxvirus proteins.

• Avipoxviruses, which complete their full replication cycle only in avian cells, go through an abortive replication cycle in mammalian cells, expressing only their early genes.