Viral Pathogenesis: Entry/Dissemination/Spreading Flashcards
define pathogenesis
disease origin
define virulence
the relative measure of pathogenicity of a virus to cause disease in a certain animal species (can only compare between very similar viruses, like two different flaviviruses, but NOT dengue virus and ebola)
define viremia
presence of virions in the blood; either free in blood or in cells associated with white blood cells (ex. feline leukemia)
passive viremia: occurs when virus is introduced into blood (insect bite)
active viremia: the result of viral replication
define infectious dose
the amount of virus required to start a productive infection; the specific number of virions differs for each virus
define localized infection
some viral infections remain localized at a mucosal site and don’t spread from this site of infection
define systemic infection
also called disseminated infection; some viral infections spread beyond the primary site of infection and produce viremia
discuss the different routes of viral exposure and virus shedding
skin: most viruses canNOT enter an animal host with intact skin (outer layer dead), but can enter through iatrogenic needle use or improper biosecurity, or defects in skin; if virus gets in skin will likely stay there and manifest as rash
respiratory tract: most common route; giant surface area, hella amt of replicating cells, breathing in and out constantly mixes air with outside environment and gives more chance for contact; but viruses have to overcome mucus layer and then alveolar macrophages, but can be good shedding via coughing and sneezing
GI tract: eating and drinking also constantly mixes with outside environment, plus peristalsis keeps mixing contents for viruses to find their perfect match cell, but GI tract hella hostile to microbes bc acidity and secretory IgA
eye: constant bathing of conjunctiva with tears and eyelid movement to wash away microbes; some viruses can infect superficial parts of the eye and cause conjunctivitis and keratitis; viral uveitis is as result of systemic viral infection, NOT local replication of virus in cornea or conjunctiva
urogenital tract: STDs of animals; some viruses may enter the host via the urogenital tract and produce localized or systemic infections
describe the different routes of viral shedding
- respiratory tract: mucus, saliva, aerosol
- GI tract: diarrhea
- skin: crusts from lesions, fluid from vesicles, infected epithelial cells
- urogenital tract: infected fluids, aborted fetus, urine (if virus infects kidney epithelial cells), semen (if virus infects testicle or accessory sex glands)
- mammary glands: milk
SHEDDING OFTEN PRECEDES CLINICAL SYMPTOMS
define the acute localized infection-mucosal pattern of infection
- respiratory mucosal viral infections: hit and run viruses that produce local (and sometimes severe) respiratory disease but do not disseminate; produce clinical disease by causing cytopathic inflammation in respiratory tract
- intestinal mucosal viral infections: hit and run viruses that produce local GI disease but do not disseminate; produce cytopathic effect in GI tract and clinically present with diarrhea and/or vomiting, leading to dehydration and metabolic abnormalities;
young, old, malnourished animals most susceptible
BIG PICTURE:
virus starts to replicate; only see disease signs when have high enough levels (shedding may overlap with disease signs), but immune system usually kicks in and clears)
define the acute systemic infection pattern (3)
- infect the host through a particular route and starts infection at that site
- infection is able to spread and disseminate to other parts of the host
- viremia
ex. parvovirus, canine distemper
persistent
virus strikes somehow when immune system is tricked and thinks virus is self, so virus can replicate without detection for a long time before manifesting symptoms
define the acute systemic infection with local persistence infection pattern
how host affects what type of disease you’ll see
ex. neonate puppy with no colostrum gets CDV which can disseminate to CNS and die
older puppy with colostrum: CDV can cause symptoms and may recover but virus might have snuck into CNS and persist for years until one day get old dog encephalitis
immunocompetent dogs: can stop CDV infection before viremia; will just get symptoms
define the persistent viruses that host cannot clear infection pattern
ex. chronic bovine viral diarrhea virus: infected at right time of gestation where fetus thinks BVDV is actually self, and will allow it to replicate but can recombine with a host gene and become cytopathic and kill hella quickly
basically animal thought a virus was itself, lets it replicate, is asymptomatic, sheds, and then can recombine with one of its genes and then replicate a viral recombinant that will kill them
define the latent viruses that periodically flare up infection pattern
ex. herpes viruses infect long-lived cells (like neurons), viral genome sticks around and turns off all but 1-2 genes and sits and waits until host gets stressed (old age, shipping stress, other infections) and causes an acute flare-up where virus is replicating and sheddable; once virus starts replicating though, host can suppress some, but not all viral replication
define the slow viruses that cause disease much later infection pattern
retroviruses with long incubation period between exposure to infectious agent and development of clinical disease (months-years); clinical disease that becomes increasingly severe until results in death; pathology usually localized to one or two organ systems (CNS and/or lung and/or lymphatics)
