Virology

Virus structure

• genome (RNA vs DNA)

• capsid - protects genome from enzymatic attack (icosahedral vs helical)

• virion associated polymerase (for replication, associated with genome)

• ± envelope & glycoproteins

Virus size

• undetectable by light microscopy, only seen by electron microscopy

DNA genome

• mostly replicate in nucleus

• more stable, more prone to induce latency

Types of capsids

• icosahedral - may be naked or enveloped

• helical - always enveloped

Virus envelope

• lipoprotein bilayer membrane with associated glycoproteins

• connected to the capsid via matrix proteins, strength of interaction determines shape

• more sensitive to heat/drying, detergents, & gastric acid

• often transmitted in droplets/secretions - must stay wet during transmission

• virus does not need to kill cells in order to spread

Naked capsid

• resistant to drying/heat, detergents, & gastric acids

• survive well in the environment, on fomites, & in the GI tract

• must kill host cell to release virus

Viruses depend on the cell for…

• protein synthesis

• energy metabolism

• membrane/lipid biosynthesis

Susceptible cells

• have corresponding receptor to virus

• does not necessarily replicate virus

Permissive cells

• provide intracellular components necessary for replication

• does not necessarily mean the virus can enter the cell

What types of cells can be infected & replicate virus?

• susceptible AND permissive cells (must be both)

Virus life cycle

• attachment/susceptibility

• penetration

• genome uncoating (envelope removed, capsid opened)

• replication

• assembly/maturation

• release

Receptor-binding viral proteins

• always on virus surface (envelope or naked capsid)

• may bind to 1+ cell surface molecules

• binding typically reversible via protein component, irreversible via polysaccharide componet

Types of penetration

• endocytosis, envelope becomes part of cellular membrane, requires viral fusion protein

• via endosomes, taken up & pH change triggers fusion of envelope with endosomal membrane

• naked virus lyses endosome, genome transported to nuclues

Uncoating

• genome made accessible for virus replication via host factors

• after this point, infectious virus particles can’t be detected in the cell (eclipse phase)

Types of transport in cell

• energy dependent using cellular structures

• often use microtubules, some use actin cytoskeleton

Amplification

• RNA: in cytoplasm

• DNA: in nucleus

• viral factories/inclusion bodies (cytoplasmic/nuclear compartments) allow viral and host proteins to come together, increase efficiency

Viral particle release mechanisms

• budding directly from cellular membrane

• exocytosis (transported to membrane via vesicles)

neither method damages cell

• cell lysis (naked viruses), release via cytoplasm

Cell lysis

• disruption of cellular membranes leading to cell death & release of cytoplasmic compounds into extracellular space

• often causes inflammation

Viral classification via disease pattern

• route of infection

• organ system

• acute, latent, or persistent

• localized or systemic

Pathogenicity

• ability to cause disease

Virulence

• a measure of pathogenicity of viruses

• asymptomatic, mild/moderate/severe disease, death

• impacted by infectious dose, route of infection, tissue tropism, virus dissemination in the body, genetic determinants of virulence encoded in virus strains

Infectivity

• measure of ease of transmission

Host resistance

• ability of host to control/eliminate infection or viral-induced disease

Infectious dose

• most accurately determined in laboratory conditions

• LD50 kills 50% of infected animals, ID50 infects 50% of inoculated animals

• can be dependent on host and viral factors + route of infection

Tissue tropism

• a tissue must be susceptible and permissive to support viral infection & replication

Pantropism

• ability to infect multiple cell types/organ systems due to use of multiple receptors

Dissemination

• greater spread of virus leads to greater severity of disease

Antigenic shift

• acquisition of new gene segments during infection of one cell with multiple strains

• creates risk of pandemic

Factors impacting host resistance

• expression of cellular receptors for virus

• cell activity (more susceptible with more activity)

• immune status (pregnancy, stress, therapeutic steroids)

• age (young or old)

• nutrition (deficiency increases incidence, severity, duration)

• husbandry (density, movement, housing, lifestyle)

What is needed to initiate an infection?

• susceptible + permissive cells

• sufficient dose of virions

Viremic spread

• virus leaves organ system of entry via lymph nodes to infect other organs

• can be shed through multiple methods

Respiratory tract

• most common portal of entry for viruses

• lined by epithelial cells - primary target of viruses

• upper vs lower tracts have different temperatures - temperature-dependent viral infections

• defended by mechanical barriers (cilia, mucus-secreting goblet cells) & immune response (IgA in upper, IgG in lower)

Consequences of infection of respiratory tract

• epithelial cell destruction → loss of ciliary activity + mucus lining

• inflammation w/fluid exudation

• increased susceptibility to secondary infections

Respiratory transmission

• direct: droplets/aerosols

• indirect: fomites (food/water bowls)

GI tract

• oral infection (short incubation)

  • infection of epithelium → transcytosis through M cells

  • can be acid/bile resistant or buffered by milk proteins

• hematogenic spread (long incubation)

• defended by acid in stomach, alkaline in intestine, digestive enzymes, bile salts

• mostly infected by non-enveloped viruses

Consequences of GI tract infection

• most commonly diarrhea (destruction of epithelial cells, increased intestinal secretion, increased peristalsis)

  • causes dehydration, hemoconcentration, acidosis

  • watery preferable to bloody

GI tract transmission

• typically fecal-oral

Skin infection

• typically impermeable

• entry possible through abrasions/punctures, arthropod vectors, vertebrate bites, or hematogenic spread

Skin shedding

• contact with small abrasions or lesions

• can shed via feather follicles in birds

• often persist well in environment & are highly contagious

Bloodborne transmission

• precautions necessary to prevent infection via syringes or surgical instruments

Fetal infection

• systemic infection by lympho-hematogenic spread can cross the placenta

• most common in young pregnant animals without prior vaccination

• consequences include fetal death/abortion (depends on virulence + gestational age)

  • abortion more common if pregnancy sustained by fetal progesterone rather than maternal progesterone

• immunocompetence differs by species (ruminants more mature)

CNS infection

• can occur via olfactory nerve, neurons to spinal cord to brain, or hematogenic via leukocytes

• typically dead-end infection

• immunologically privileged site, reduced immune response

Consequences of CNS infection

• often transient

• lytic infection of neurons

• non-inflammatory infection

• progressive demyelination

Rabies

• enveloped RNA rhabdovirus

• spread by bite (domestic + sylvatic cycles)

• virus replicates locally in muscle, enters nerve cells via acetylcholine receptors

• nerves → spinal cord → brain → eyes + salivary glands

Clinical signs of rabies

• prodromal (non-specific symptoms)

• acute excitative phase (anorexia, hyper-excitability, aggression)

• paralytic/end-stage

Rabies treatment

• washing wound with detergent

• post-exposure prophylaxis w/specific IgG + vaccination

  • the further the bite from the brain, the more likely prophylaxis will be successful

Rabies control

• urban: stray dog control, dog vaccination, quarantine of imported animals

• sylvatic: eradication (impractical), vaccination campaigns

Diagnostics during viral infection

• behavior: apathy

• clinical assessment: fever

• WBCs: leukopenia

• antibodies, viral genome

• necropsy: inclusion bodies, IHC

Acute infections

• typical: virus replication, clinical signs, viral clearance (influenza)

• recurrent: repeated typical infection process (BRSV)

Generalized infections

• systemic, occurs in multiple organs

• virus uses multiple receptor types or infect a cell type found in many organs

Canine distemper virus

• clinical signs: fever, URI, V+/D+, neuro signs (seizures, ataxia)

  • severe: primary pneumonia, bacterial superinfections, D+, bone remodeling defects

• often subclinical

• inclusion bodies visible on histo

• has a broad host range - canids, procyonids, mustelids, big cats

• may persist in the body, usually CNS or can induce paw pad hyperkeratosis

Pathogenesis of CDV

• infects respiratory tract → taken up by macrophages + dendritic cells → primary viremia → replication in lung-draining lymph nodes → transported via infected lymphocytes (secondary viremia) → infection of CNS, epithelium (GI), & endothelium → shedding through respiratory secretions, milk, urine, or fece

Newcastle disease virus

• highly contagious, zoonotic

• broad host range across birds, mostly concerned with poultry

  • morbidity/mortality vary by bird species & strain virulence

• clinical signs: decreased egg production, sudden deaths throughout flock, edema of head around eyes, greenish-dark watery diarrhea, resp/neuro signs

  • post-mortem: hemorrhage, necrosis, or ulceration of lymphoid tissue

Pathotypes of NDV

• asymptomatic enteric

• lentogenic: mild form in intestine

fusion precursor protein only cleaved by trypsin

• mesogenic: intermediate form (more severe disease, rarely death), respiratory or neurological

• velogenic: most virulent (high mortality), can be neurotropic or viscerotropic

fusion precursor protein can be cleaved by other proteases

NDV transmission

• contact with feces & respiratory discharge (direct or environmental contamination)

  • virus survives well in the environment

• issues with backyard poultry, feral birds

NDV diagnosis

• may be based on clinical signs but must distinguish from influenza

• virus isolation (PCR) & serology (antibodies, 3-4 weeks post-infection)

Required actions for NDV

• notify federal area vet + state vet

• animals/premises will be quarantined

• samples sent out for diagnosis

• depopulation may be required

Physiological consequences of respiratory infections

• destruction of epithelium, loss of cilia + mucus lining

• inflammation w/fluid exudation (lung damage)

• increased susceptibility to secondary infection

Influenza epidemiology

• antigenic drift: point mutations in gene segments (RNA viruses → epidemic

• antigenic shift: segmented genome → acquisition of new gene segments → pandemic risk

• pandemics w/new strains occur every 20-30 years

Influenza transmission

• sialic acids on host cell glycoproteins bind hemagglutinin on virus

• amino acid changes correspond to changes in binding affinity/receptor specificity - horse virus mutated & can infect dogs

Canine influenza virus A

• orthomyxovirus

• enveloped virus w/helical capsid

• segmented RNA genome

• 2 types: equine & avian origin (not clinically distinct)

• high transmission in kennel environments

CIV A clinical signs

• mild: low fever, cough, nasal discharge

• severe: high fever, marked coughing, tachypnea/increased effort, danger of pneumonia

Bovine respiratory syncytial viurs

• pneumovirus

• non-segmented RNA genome

• enveloped w/helical capsid

• highest incidence in adult cattle

• high infectivity, medium pathogenicity, low mortality

BRSV clinical signs

• high fever, tachypnea, rhinitis/nasal discharge, cough, bacterial superinfections (pneumonia)

• mild signs often inapparent

BRSV pathogenesis

• aerosol transmission → inhaled virus particles attach to respiratory epithelium → fusion glycoproteins mediate virus entering cytoplasm → replication in cytoplasm → spread to bronchiolar + alveolar epithelium

Respiratory disease complexes

• occur in cows, pigs, & dogs

• multifactorial - 1+ virus ± bacteria

• influenced by host immunity status & environmental factors

Bovine respiratory disease complex

• aka shipping fever or enzootic calf pneumonia

• clinical signs (fever, inappetence, cough, lethargy, nasal discharge) 1-2 weeks after stressful event

• viruses: herpes type 1, parainfluenza type 3, BRSV

• bacteria: pasteurella, mycoplasma, haemophilus

• prevent by reducing stress (weaning, shipping, arrival), improving facility management, & vaccines (2-3 weeks before shipping + 1 day after arrival)

Porcine respiratory disease complex

• viruses: flu, circovirus, coronavirus, haemagglutinating encephalomyelitis

• bacteria: haemophilus, bordetella, mycoplasma, actinobacillus, strep, pasteurella

• caused by environment (temperature changes, humidity, pollutants) & management

Canine respiratory disease complex

• aka kennel cough

  • parainfluenza type 5 + secondary bordetella

  • pneumococcal pneumona: influenza + Strep. pneumoniae colonization from oropharynx to lower lung

Routes of GI infection

• oral: short incubation - infection of epithelium + transcytosis through M cells

• hematogenic

Fluid balance in SI

• villi responsible for maintaining

• balance between absorption by enterocytes vs secretion by crypt cells

Localized GI tract viruses

• coronaviruses (enveloped, icosahedral capsid, RNA genome)

  • high frequency of mutations + RNA recombination

• rotaviruses (more robust that coronaviruses)

• astrovirus (young turkey)

Transmissible gastroenteritis of swine

• major cause of diarrhea in young pigs in winter

  • piglets more susceptible due to lower gastric acid + slower renewal of enterocytes

• short incubation period, fecal-oral route

• causes V+ & profuse watery D+

• <1week old piglets usually die

Rotavirus

• fecal-oral transmission

• <24 hour incubation period

• clinical signs most commonly watery diarrhea in young animals due to villi blunting/fusion (low mortality)

Mechanisms of diarrhea

• enterocyte destruction via virus-encoded toxin → malabsorption

• stimulation of enteric nervous system → increased motility

• milk increases osmotic dysregulation (can’t be digested by enterocytes)

Treatment/prevention of GI viruses

• vaccination of mothers → passive transfer

• supportive therapy via fluids

• facility management (biosecurity)

Parvoviruses

• cause systemic infection including the GI tract

• small, naked capsid, very resistant in the environment

• replicate in nucleus of actively dividing cells

• clinical signs: severe enteritis + leukopenia

  • can see cerebellar hypoplasia in kittens + myocarditis in puppies

Feline panleukopenia virus

• infected via respiratory tract → viremia → lymphoid organs → viremia

• causes leukopenia (lower leukocytes = higher mortality), high fever, V+/bloody D+

• effective modified live vaccine

Canine parvovirus 2

• arose from point mutations in the capsid protein of feline panleukopenia

• oral transmission

• clinical signs: leukopenia, V+/bloody D+

• live-attenuated vaccine (maternal antibodies can interfere

Persistent infections (chronic vs latent)

• virus constantly replicates & is continuously shed

vs

• period of time during the course of infection in which the virus can’t be detected

can be caused by DNA or RNA viruses

Herpesviruses

• causes latent infections

• transported retrogradely to root ganglia for latency, terminated spontaneously or via stressors and transmitted anterogradely back to mucosa

• enveloped DNA virus, highly species-specific

Herpes transmission

• mostly direct mucosal contact

• can occur through droplets & fomites

• persistent infection with continuous or periodic shedding

Gallid herpesvirus 1

• causes infectious laryngotracheitis in chickens

• 1 week incubation, slowly infects flock over weeks

• causes URI symptoms (“pump handle respiration”) & fowl diphtheria (membrane forms at tracheal bifurcation)

Gallid herpesvirus 2

• causes Marek’s disease (T cell lymphoma) in young chickens

• often causes wing or leg paralysis, high mortality in young

• may be subclinical w/shedding in older animals

• can cause cutaneous or ocular signs

• transmitted via feather dust

Control/prevention of gallid herpesviruses

• live-attenuated + vector vaccines in egg

• biosecurity critical

Bovine viral diarrhea virus

• enveloped RNA flavivirus

• oronasal route, primary replication in tonsils → viremia + organ infection

• acute infection usually overcome easily

Hemorrhagic syndrome

• bleeding at eyes & mucosa caused by BVDV

• rare

Transplacental infection of calves with BVDV

• can cause stillbirths in first 90 days (1st trimester) of gestation

• cause persistently infected calves w/acquired immunotolerance in 2nd trimester

  • calves usually end up smaller

  • develop mucosal disease (100% fatal) after a few years - switch from non-cytopathic to cytopathic virus

• calves usually normal with BVD antibodies in 3rd trimester

• can cause torticollis

Avirulent vs virulent BVDV

regulation of viral production via cleavage of NS3 protein

• NS2-3 auto-processing via NS2

vs

• insertion of gene sequences between NS2 & 3 gene, increase protein cleavage

• higher levels of NS3 = faster replication = increased cytotoxicity

Mucosal disease

• develops in a persistently infected cow when BVDV becomes cytopathic

• bloody diarrhea, fever, lesions in mouth & interdigital clefts

• 100% fatal

BVDV control

• ID & remove persistently infected calves via antigen ELISA or PCR

• limit contact of pregnant cows with the rest of herd

• vaccinate

Retroviruses

• Enveloped RNA viruses w/2 copies per virion

• reverse transcriptase transcribes viral RNA genome into DNA copy

• integrase integrates DNA copy into cellular genome

• DNA → mRNA → proteins → new virus particles via budding

• difficult to rid infection

FeLV

• low prevalence in US, kittens most susceptible

• transmitted by saliva/nasal secretions or fomites

• may be abortive (viremia prevented), acute (2-3 weeks shedding, becomes carrier), or progressive (bone marrow infection w/lifelong viremia & immunosuppression)

FIV

• low prevalence

• transmission through bites

• lifelong infection with shedding in saliva, blood. & milk

• depletes CD4 T cells & causes lymphoma/neuro disease

Lentiviruses of sheep/goats

• caprine arthritis encephalitis virus + Maedi-Visna virus

• cause chronic encephalomyelitis, arthritis, & mastitis

  • intersitial pneumonia, lymphoid hyperplasia

• transmitted in bodily fluids

• infected macrophages carry virus to target organs

• local replication & cytokine induction → aberrant tissue growth

Equine infectious anemia virus

• transmission via flies or iatrogenic

• lifelong chronic infection w/periodic clinical relapses (wasting)

• fever, weakness, jaundice, anemia/thrombocytopenia, bloody stools, & petechial hemorrhage + kidney Dz due to immune complexes

Treatment/prevention of retroviruses

• no treatment, no efficacious vaccines

• testing & biocontainment

Transmissible spongiform encephalopathies

• progressive, fatal, neurodegenerative Dz of animals due to prions (abnormal pathogenic transmissible proteinaceous agents, host-encoded)

• accumulations of prions lead to vacuoles in brain + amyloid plaques

• ex: scrapie, CWD, BSE, TME

Scrapie

• in sheep/goats

• detectable in multiple tissues, can verify via third eyelid biopsy

• causes pruritus (& hair loss) + neuro signs

CWD

• in deer, elk, & moose (mostly captive deer)

• causes chronic wasting + PU/PD

• transmitted via direct contact or environmental exposure