Emerging Disease and Epidemiology of Viral Infections

Question 1

Carnivore Protoparvovirus 1

Answer 1

• Linear ssDNA genome (5323 nt), 60 capsomers per particle (VP2)
• Faecal-oral transmission
• Replicates in the nucleus of infected cell & requires cellular DNA polymerase (so only r_eplicates in dividing cells_ e.g. crypts of villi in the small intestine)
• Sub-types: Feline panleukopenia virus, Canine parvovirus, mink enteritis virus, racoon parvovirus

Question 2

Clinical Disease in Dogs

(canine parvovirus)

Answer 2

• Clinical disease associated with virus replication in intestinal epithelium, bone marrow and lymphoid tissues
• Incubation period 3-7 days–> leukopenia + enteric/ myocardial disease
• Enteric form: vomiting, depression, loss of appetite, hemorrhagic diarrhea, dehydration, fever
• Myocarditis seen when neonatal puppies are infected and may show as clinical disease several weeks after exposure

Question 3

Clinical Disease in Cats

(Feline panleukopenia virus)

Answer 3

• Main features similar to dogs (dehydration, depression, leukopenia) however generally self-limiting acute illness (less severe) lasting 5-7 days.
• Most cats develop antibodies to FPV within the first year of life (mostly subclinical disease)
• Main problems due to FPV infection in:
• Pregnant queens- virus passes transplacentally –> abortions/stillbirth
• Very young cats (<5 months) –> Sudden death, infection of cerebellum can neurological signs.

Question 4

Some History

(CPV1 and 1)

Answer 4

• CPV2 was unable to re-infect cats but CPV2a can infect both cats and dogs, as can 2b & 2c
• All known strains of CPV are monophyletic= a single cross species event.
• There is some evidence that FPV infection of wild carnivores (foxes) may have been an intermediate step in adaptation of the virus to dogs.

Question 5

what changed?

(CPV-2)

Answer 5

• Comparison of the genome of FPV and CPV-2 revealed that there were 6 amino acid changes in the major capsid protein VP2
• Three of these (K93N, V103A, D323N) are sufficient to confer cross species infection from cats–> dogs
• These changes –> an ↑ affinity for the dog transferrin receptor (found on many cell types throughout the body & normally involved in iron uptake) instead of the cat transferrin receptor.

Question 6

Steps involved in emrgence of host switching viruses

Answer 6

1. Spillover
2. Limited transmission (outbreak)
3. Sustained epidemic

Factors which affect process of viral emergence:

• Type/ intensity contacts between donor and recipient hosts
• Host barrier to infection
• Viral factors
• Determinants of efficient virus spread in new population

Question 7

WHY ARE VIRUSES PARTICULARLY GOOD AT CROSS SPECIES TRANSMISSION?

Answer 7

• Host immune evasion -serotypes
• Lack of proofreading polymerase (except coronaviruses)–> more likely to mutate (antigenic drift)
• Target conserved (common) receptors
• Antigenic shift
• Rapid life cycle
• Can survive for long periods in the environment

Question 8

Influenza A Virus

Answer 8

• Segmented (negative sense) RNA virus (8 segments) (Orthomyxovirus)
• Enteric pathogen of wild birds (gulls & waterfowl), in birds there are: 16HA types & 9NA types
• Species Influenza A is further subdivided into types based on serology which is determined by the viral envelope proteins Haemaglutinin (H, or HA) and Neuraminidase (N or NA)
• HA : Virus receptor binding protein-binds to sialic acid (a branched carbohydrate present on the glycoproteins on the surface of host cells)- allows the virus to attach to host cell & enter it.
• NA: an enzyme that cleaves sialic acid. During infection, NA cleaves sialic acid present on surface of infected cell to prevent immediate reabsorption of released viruses- enhances the rate of spread

Question 9

Influenza A Replication

Answer 9

• Viral RNA is in complex with RNA dependent RNA polymerase
• Replicate in the nucleus (see exceptions)
• Include many economically important pathogens (e.g. Influenza A, Ebola, Rift Valley Fever virus

Question 10

Antigenic Shift: reassortment

(influenza A)

Answer 10

• Reassortment of genomic RNA segments encoding a H or N subtypes not previously seen in humans results in antigenic shift, this can result in pandemics

E.g. Cross species transmission of avian & human influenza–> pigs

• Influenza HA binds to sialic acid: avian and human influenza HA recognize different shapes of sialic acid (determined by how it is bound to galactose)
• Pigs have both types (shapes): a2,3 and a2,6 linked sialic acid in their upper airways, therefore can be infected with both avian & human influenza, if co-infected can viral reassortment
• Other genes that may be important for host specificity of influenza:
• NA- (neuraminidases) (often NA and HA are inherited as linked genes)
• PB2- has separate human and avian variants (In humans Lys 627 efficient replication, In birds Glu 627 is present)

Question 11

Error Prone Replication

(antigenic drift)

Answer 11

• Viral replicase protein purposefully makes mistakes as it copies the genome gradual change in the sequence of the virus genes (and hence proteins).
• Mutations can changes in AAs of viral surface proteins- to the point where the same virus can no longer be recognized by host’s immunity

Question 12

Drift can cause change in tropism & virulence

(example)

Answer 12

• Low Pathogenic Avian Influenza (LPAI) is generally mild, If the virus infects domestic birds, it can occasionally develop –> High Pathogenic Avian Influenza (fowl plague)
• HPAI severe epizootics, sudden onset, rapid death (within a few hours), up to 100% mortality
• Caused by H5 and H7
• Virus changes from enteric–> to one that can infect every cell in the bird’s body
• HA’s mechanism of action: HA is cleaved–> HA1 + HA2 –> facilitates viral entry by aiding fusion of the virus membrane with vesicle membrane
• Antigenic drift –> insertional mutations at the HA cleavage site so instead of the HA1 + HA2 chains being linked by 2 arginine there’s now several AAs present, this means ubiquitously expressed proteases can now cleave the HA, therefore virus can replicate all over body.
• Nightmare scenario: Highly pathogenic avian influenza infecting humans (shift), then undergoing drift

Question 13

Obiviruses

Answer 13

• Exotic, arthropod- borne diseases- Transmitted by biting midges (Culicoides genus).
• All notifiable diseases

Question 14

Bluetongue Virus

(Obivirus)

Answer 14

• Disease of wild & domestic ruminants. In livestock it mainly affects sheep and cattle
• Mortality in affected sheep herds can be high (up to 70%).
• Pathogenesis: enters via midge bite–> primary replication in lymph node–> spreads in lymph & blood –> lung & spleen –> targets monocytes & endothelial cells –>haemorrhage & oedema

Clinical Signs: In sheep (generally ↓ severe in cows, can be subclinical)

• Fever
• Erosion & ulcers in oral cavity, oesophagus & forestomach
• Bloody, nasal discharge
• Oedema & haemorrhage particularly around head,
• Necrosis of skeletal & cardiac muscle
• Has emerged multiple times in the in Europe in the last decade (particularly BTV-1 & BTV-8), in 2006- 2008 It emerged in the UK
• Control: Transport restrictions in affected areas & vaccination (attenuated vs inactivated)

Question 15

Bluetongue Vaccines

Answer 15

Available:

• Attenuated virus vaccine- can vaccinate against all/ multiple serotypes at once BUT can –> mixing of genes (antigenic shift)–> new forms of virus –> infectious viral strain
• Inactivated viral vaccine- Only type licensed in the UK, safer but (multiple doses required and there are a limited number of serotypes available
• Experimental: Canarypox vectored vaccines & Subunit vaccines

Question 16

Epizootic Haemorrhagic Disease

Answer 16

• EHDV is 73% identical and 82% similar to bluetongue at protein level (comparison of VP1 sequences)
• But mainly causes disease in cattle & deer (generally not sheep)

Question 17

African Horse Sickness Virus

Answer 17

• AHSV–> a disease of short duration (2-4 days) and ↑ mortality (100%) in horses.
• Wildlife reservoir: zebras
• Cell associated viraemia (red and WBCs)–> vasculitis–> ↑ permeability of BV wall
• Clinical Signs:
• Mild forms with fever and oedema of supraorbital fossa
• Cardiac form with additional oedema of head, neck and chest
• Pulmonary form most severe with pulmonary oedema & 95% mortality rate
• Control (In UK)
• Testing of animals imported from areas with AHS
• In case of outbreak slaughter of affected animals & emergency vaccinations (+ vector control in endemic areas)

Question 18

Serological Complexity of BV, EHDV, and AHSV

Answer 18

All three are serologically complex

• Have many serotypes & immune protection is serotype specific
• i.e. if an animal is infected & recovers, they will still only be immune against one serotype
• Bluetongue- 28 serotypes
• AHSV 9- serotypes
• EHDV 8- serotypes

Question 19

Foot and Mouth Disease

Answer 19

• Genus: Aphthovirus (ssRNA, positive sense, non-enveloped)- 7 serotypes
• Affects cloven-hoofed animals (cattle, pigs, sheep, wild ruminants)
• Recovering animals can develop carrier status (persistently infected)

Clinical Signs:

• Vary depending on host and virus strain (–> vesicles on mouths & feet)
• Cattle: clinical signs- drooling, lesions & vesicles on tongue & lamenesss
• Pigs: lameness, vesicles on snout, milder symptoms but shed upto 300x more virus than cows
• Sheep: very mild, mostly lameness, easily overlooked

Question 20

Transmission of Foot & Mouth D.

Answer 20

Secreted in all bodily fluids from affected animals & is aerosolised by coughing/ sneezing

• Quite stable & can remain viable in animal products (milk, cheese, meat, bones) that have been inadequately sterilised (swill feeding of pigs can be a problem)
• Airborne spread via droplet nuclei depending on humidity (~60 km over land & up to 300km over sea)
• Via Fomites- needles, surgical instruments, clothing, bedding, vehicles (tyres)
• Does not infect people but humans can harbour FMDV in their respiratory tract for up to 48 hours

Question 21

Control: Foot and Mouth

Answer 21

• Restrictions on import of food and animals from non-EU countries
• Culling of all affected animals and in-contact animals as soon as possible
• Restriction of movement around premises with infected animals
• No vaccination except for emergencies
• In countries with endemic FMDV: annual vaccination (inactivated vaccine)

Question 22

FMDV: Vaccination Considerations

Answer 22

Why do we not vaccinate?:

• Serotype diversity of the virus (not cross-protective)
• Potential for carrier status
• Inability to distinguish vaccinated and infected animals
• Economically: costly & seriouaffects trade

Problems with not vaccinating:

• Large population of susceptible individuals
• Welfare (culling/movement restrictions on large numbers of animals)

Question 23

Rabies Virus

Answer 23

• Family Rhabdoviridae, Genus Lyssavirus
• Bullet- shaped, enveloped, negative sense ssRNA genome
• Infects all mammals: Uses the conserved acetylcholine receptor in neurones–> NS –>encephalitis
• Replicates in muscle tissue–> motoneurone–> CNS–> salivary gland & replicates–> shedding in saliva
• Reservoir species differ depending on the country:
• Europe: red fox
• Asia, Latin America, Africa: stray dogs
• USA: skunk, raccoon, fox, coyote and bats
• Disease progression is unusually slow
• 14-90days before reaches brain (clinical disease)
• 2-14 days till death after clinical disease begins

Question 24

Control of Rabies

Answer 24

In rabies-free countries (including UK):

Restrictions on import of pets

• Quarantine
• Pet Travel Scheme (PETS): Dogs, cats & ferrets, must be micro-chipped, vaccinated (>12 weeks old &21 days post-vaccination), antibody tested after vaccination and treated against tapeworms 1-5 daysbefore entry. Additional rules apply if travelling with more than 5 pets.

In countries with rabies:

• Vaccination of pets and reservoir hosts
• Control of reservoir host populations

Question 25

Schmallenberg Virus

Answer 25

* Family ***Bunyaviridae*****, genus orthobunyavirus** * **Enveloped**, segmented negative sense **ssRNA viruses-** (Sathuperi) * Infects ruminants, clinical disease mainly in sheep and cattle * Emerging disease in Northern Europe, first described in August 2011 **_Clinical Signs_**: Pyrexia, ↓ milk yield (cattle) - **Abortion & Congenital malformations** (arthrogryposis, scoliosis, hypoplasia of Cerebrum/ cerebellum/ SC) Virus can be detected in the brain of aborted lambs. - Some newborns display **neurological signs** (ataxia, poor sight, ‘swimming’, circular walking) **_Epidemiology_**: Vector- bourne: Virus replicates in **midges** (*Culicoides* genus) and is shed in saliva Has spread rapidly over most of northern Europe (Germany, Netherlands, France, Belgium, UK) **_Control:_** **Inactivated vaccines** available (MSD).

Question 26

Louping Ill Virus

Answer 26

* Family ***Flaviviridae, Genus Flavivirus***- **Enveloped, ssRNA** genome * **Zoonotic,** can --\>mild flu-like symptoms, occasionally severe meningoencephalitis **_Epidemiology_**: Transmitted via **ticks** * Main host: sheep, particularly common in yearlings, ↑ mortality when new to an infested pasture * Occasional infection of cattle, horses, deer & ↑ mortality in grouse * Present in UK particularly hills and moors **_Clinical Signs:_** Outcome is variable and not all infected sheep develop disease - Virus causes **encephalomyelitis** in sheep - Ataxia, tremor, paralysis - Leaping gait **Control:** Natural infection--\> good immunity-**passive immunity protects lambs for 6-8 weeks** * Tick Control * Inactivated vaccine available for use in sheep

Question 27

West Nile Virus

Answer 27

* Arboviral infection: Mosquito- borne *flavivirus* * Developed **positive sense ssRNA** virus with an icosahedral nucleocapsid **_Epidemiology:_** Arthropod vector: Mosquitoes- mainly *Culex* genus, some cases in *Aedes &Anopheles* Transmission cycle is **Bird-Mosquito-Bird**- normally a disease of birds (primarily crows) **Humans & Horses are dead end hosts** (as are bats, chipmunks, skunks, squirrels, llamas & rabbits) 1999- 62 cases of viral encephalitis in humans in New York, 7 deaths, many dead crows were found in the area (due to imported mosquitoes/ imported birds) by 2005 had spread across almost whole USA By 2010 canada **_Clinical Signs_**: **Neurotropic** in mammalian hosts ( --\>encephalitis) - Fever - Weakness/ paralysis of hind limbs, ataxia - Limited vision - Convulsions/ seizures * Horses with clinical signs fatality 33% but (upto 90%) of infected animals show no clear clinical signs * Humans \<1% cases serious illness (encephalitis, meningitis, death) **_Control:_** * Insect repellents/ protective clothing against mosquitoes * Horse- Annual vaccination in spring (before mosquito season begins)

Question 28

Transmissible Spongiform Encephalopathies

Answer 28

**Veterinary:** **Scrapie** in sheep & goats * Chronic Wasting Disease affecting cervid family (deer) * Bovine Spongiform Encephalopathy **Human:** **Sporadic Creutzfeldt-Jakob disease** (CJD). (1 in a million/year) - **Hereditary** forms of (linked to genetic background)- genetic predisposition to infection - **Aquired/Infectious** - Kuru in the Fore tribe in Papua New Guinea (cannibals)-New variant (vCJD)

Question 29

"Prion" theory

Answer 29

* **PrPc**= Normal cellular protein, highly expressed on **neurones,** constantly produced (half-life= 24 hours) - **High α-helix content**, soluble, protease sensitive * **PrPSc** =Disease associated, Transmissible isoform, longer half-life - **High β-sheet content**, mostly insoluble, protease resistant - Acts as a template for converting **PrPC--\> PrPSc** - only change is secondary/ tertiary structure - Forms **self-aggregates** (neurotoxic) / fibrils / plaques - Forms **fibrils**- can be identified in diseased brains - Faciliates conversion * **Can’t destroy infectivity with**: (UV light) / Radiation / Rnase / Heat / pressure, **but can with proteases** * **PrP Knockout mice**- don’t get prion disease, therefore its caused by self- replicating protein

Question 30

Tse's | (clinical Signs and Diagnosis)

Answer 30

**_Clinical Signs_** * Sheep suffering from **scrapie**, scrape themselves due to an itch of CNS origin * Cows with **BSE,** behavioural changes: apprehension then motor changes (lack of coordination) - **Histologically**- **vacuolation** is present when infected which is **reversible** if the prion infection is cured - **Prion plaques** can be seen- similar to those in Alzheimer’s disease **_Diagnosis_** * 3 isoforms of **PrPC / PrPSc molecule**- Same protein but differently glycosylated * Western “Immuno” blot - Detected by antibodies- replaced by 96microwell method/ IHC/ ELISA * Humans: MRI/ EEG/ CSF exam/ tonsil biopsy (vCJD only) * Can measure PrPSc & distinguish from PrPC via their **protease resistance**

Question 31

tse's | (Transmission and Control)

Answer 31

* Originally thought that these diseases didn’t cross species barriers, but **we can transmit prions from scrapie, kuru, vCJD, sCJD, BSE & CWD mice** **_Scrapie:_** * **Transmission**: Scrapie= endemic in Britain but not in NZ, CWD= found in N. America but not Europe - Can persist in environment- infectivity remains in soil * **Control**: Cull infected, Selective breeding for resistant strains of sheep ( --\>codons: 136A, 154R, & 171R) **_Cattle (BSE) transmission:_** * **Transmission**: No evidence of environmental factor - No evidence of horizontal transmission (cow to cow) & little evidence of vertical transmission **Control**: Cull infected, Control diet- i.e Ban meat & bone meal in feed (MBM) & reinforce ban **_Humans_**- Control of diet, blood transfusion, treatment: drugs aimed at alleviating symptoms only, no vaccine