Viral Pathogenesis — Questions

The median tissue culture infectious dose (TCID50) is the dilution of a virus preparation that infects half of the inoculated cell cultures, and it gives a quantitative measure of infectious titre. In virulence studies, such as those on highly pathogenic avian influenza H5N1, it allows the replication and infectivity of different strains or conditions to be compared on a common scale, alongside related endpoints such as the median lethal dose (LD50) and median infective dose (ID50) in animals. It measures infectivity rather than disease severity directly, and human virulence cannot be assessed experimentally in this way; it is inferred instead from the severity observed in natural infection.

A cytokine storm is a massive, self-amplifying and dysregulated release of pro-inflammatory cytokines, principally interleukin-6 (IL-6), interleukin-1, tumour necrosis factor, and interferon-induced chemokines, that is neither scaled to the infection nor switched off as it resolves. The result is widespread tissue injury rather than controlled defence: diffuse alveolar damage and the acute respiratory distress syndrome (ARDS), capillary leak, hypotension and multi-organ failure.

In severe COVID-19 it marks the hyperinflammatory phase, with raised IL-6 and ferritin and a tendency to microvascular thrombosis, and the same mechanism drives the lethal pneumonias of highly pathogenic avian influenza and is thought to have contributed to the exceptional mortality of the 1918 influenza pandemic. Because the damage is immune-mediated, treatment targets the response, with corticosteroids and IL-6 blockade in the most inflamed patients.

The cytopathic effect is the visible morphological change a virus produces in the cells it infects, and its pattern is often characteristic enough to suggest the agent.

Cell rounding, detachment, and lysis. The commonest effect: infected cells round up, detach from the monolayer, and lyse. Seen with the enteroviruses and many other lytic viruses.

Syncytium (giant-cell) formation. Viral fusion proteins fuse an infected cell with its neighbours into a multinucleated giant cell. Characteristic of the paramyxoviruses, respiratory syncytial virus, measles virus, and herpes simplex virus.

Inclusion bodies. Aggregates of viral components or altered cell regions, in the nucleus or cytoplasm. Intranuclear inclusions occur in herpesvirus and adenovirus infection (the Cowdry type A inclusion of herpesviruses); cytoplasmic inclusions include the Negri bodies of rabies and the inclusions of the poxviruses.

Cell rounding in grape-like clusters. Adenoviruses round cells into characteristic clusters.

Vacuolation and ballooning. Some viruses produce cytoplasmic vacuoles or swollen, ballooned cells.

Not every virus is cytopathic in culture: rubella virus and many others replicate without an obvious effect and are detected by other means. The severity of the cytopathic effect in culture does not predict the severity of human disease.

Virus-associated haemophagocytic lymphohistiocytosis (HLH) is a hyperinflammatory syndrome in which the immune response, rather than the virus itself, drives the disease.

It is the extreme of the hyperinflammatory axis: a relentless, self-sustaining activation of CD8 cytotoxic T cells and macrophages. Normally the cytotoxic response kills infected cells and then terminates itself through perforin-dependent killing; when that off-switch fails, through an inherited defect of the perforin pathway (familial HLH) or an acquired one, the response cannot be shut down.

The activated lymphocytes and macrophages pour out cytokines (interferon gamma, interleukin-6, tumour necrosis factor and others) in a self-amplifying loop. Activated macrophages then engulf the host’s own blood cells (haemophagocytosis) in the bone marrow, spleen and liver, producing the progressive cytopenias.

Epstein-Barr virus (EBV) is the commonest viral trigger, infecting T or natural killer (NK) cells and driving continuous cytokine production, often in a host with an underlying defect of perforin-dependent killing. Cytomegalovirus is another, particularly in the immunocompromised.

Because the immune response is the disease, immunosuppression is therapeutic (immunomodulation, and etoposide-based protocols when severe) while the triggering infection is treated, a reversal of the usual principle that immunosuppression worsens infection.

Emergence is driven by viral, host, and ecological factors acting together.

Viral factors. Error-prone replication generates a quasispecies from which variants with new properties are selected; reassortment (as in influenza A) and recombination can produce sudden, large genetic change. A virus long adapted to its natural host is often of low virulence there, but the same virus on jumping to a new host species, with which it has no evolutionary history, may cause severe disease. Acquisition of a new receptor usage or a virulence determinant, such as a polybasic haemagglutinin cleavage site, can extend host range or pathogenicity.

Host and ecological factors. Most emerging viruses are zoonotic, crossing from an animal reservoir at points of contact widened by deforestation, agricultural intensification, the wildlife trade, and urbanisation. Expanding vector ranges, human population growth, displacement and travel, and growing numbers of immunocompromised hosts all increase spillover and onward spread. Breakdown of control measures, including falling vaccine coverage, allows re-emergence of agents once suppressed.

Examples. SARS-CoV-2 (a coronavirus spillover causing a pandemic), pandemic and highly pathogenic avian influenza (reassortment and host-range change), Zika and Ebola viruses (expansion from limited foci into large outbreaks), and mpox (spread beyond its previous range). Re-emergence is seen where measles or poliovirus return as vaccination coverage falls. The unifying theme is a virus meeting a new or newly susceptible host population.

A complete answer traces how a virus reaches the central nervous system, how it crosses into it, and how it injures it.

Reaching the central nervous system. Most encephalitic viruses arrive through the blood, after a primary infection and viraemia seed the cerebral vasculature. Two haematogenous routes exist: virus in the vessels of the meninges or choroid plexus crosses into the cerebrospinal fluid and infects the ependyma and underlying brain, or virus crosses the blood-brain barrier directly. It may infect the endothelium, be carried across inside infected leukocytes (as with the human immunodeficiency virus, HIV, and measles), or, as with West Nile virus, exploit a barrier made leaky by inflammatory cytokines. The alternative is the neural route: retrograde axonal transport along peripheral nerves, used by rabies virus and, on reactivation, by herpes simplex and varicella-zoster virus.

Injuring the brain. Disease arises from direct neuronal infection and from the immune response to it. The histological hallmarks are neuronal necrosis, neuronophagia (engulfment of dying neurons), and perivascular cuffing by mononuclear cells, the last reflecting the immune response itself. The balance of direct and immune-mediated injury varies: rabies virus is barely cytopathic yet uniformly lethal through functional neuronal failure, herpes simplex produces necrotising temporal-lobe damage, and in others the immune infiltrate drives much of the harm.

Outcome depends on the virus, the region infected, and the vigour and timing of the immune response, which is also why oedema in the closed cranial compartment is so dangerous.

Herpes simplex virus illustrates the full arc of a latent, neurotropic infection, from a productive primary infection to lifelong carriage with reactivation.

Entry and lytic replication. The virus enters through skin or mucosa, usually at a site of minor abrasion, and replicates productively in epithelial cells, producing the vesicles of primary oral (herpes simplex virus type 1) or genital (type 2) infection.

Neuro-invasion. From the epithelium the virus enters the endings of sensory nerves and its capsids travel by retrograde axonal transport to the neurons of the corresponding sensory ganglion, the trigeminal ganglion after oral infection and the sacral ganglia after genital infection.

Latency. In these long-lived, non-dividing neurons the virus establishes latency: the genome persists as a circular episome, lytic genes are silenced, and only the latency-associated transcripts are expressed. With no viral antigen displayed and little major histocompatibility complex on neurons, the latently infected cell is effectively invisible to cytotoxic T cells and antibody.

Reactivation. Triggers such as stress, ultraviolet light, fever, or immunosuppression provoke reactivation, the resumption of productive replication. New virus travels by anterograde axonal transport back to the periphery, producing recurrent lesions near the original site. Reactivation is the cellular event; the repeat clinical episode is the recurrence. Asymptomatic shedding occurs between episodes and accounts for much transmission.

After a virus enters the blood for the first time (the primary viraemia, often clinically silent), it seeds secondary organs. Replication there releases a much larger, higher-titre wave into the blood, the secondary viraemia, which carries the virus to the target organs responsible for the characteristic disease.

Option A describes the primary, not the secondary, viraemia. Cell-associated virus (B) and lymphatic spread (D) are real features of dissemination but do not define the secondary viraemia, a plasma-phase event. Low-level virus during latency (E) belongs to persistent infection, not the stepwise viraemia of an acute systemic infection.

Herpes simplex virus is carried as a truly latent, non-replicating genome in sensory neurons, so its return to active replication is reactivation, the cellular event that proceeds from genuine latency.

Recrudescence is better reserved for the clinical flare of a persistent, low-level infection that has continued to replicate rather than one emerging from true latency. Recurrence is the repeat clinical episode that results, the cold sore itself, not the underlying event. Reinfection would be a fresh infection acquired anew from outside, not return of the resident virus. Primary infection is the first encounter, which has long since passed.

Susceptibility and permissiveness are independent: a cell is susceptible if it carries the functional receptor for entry and permissive if it can also support the full replication cycle. Here the virus enters (so the cell is susceptible) but cannot complete replication (so it is non-permissive), and the result is an abortive infection, genome in and no progeny out.

The other options misuse the pair. Entry requires susceptibility, so a permissive but not susceptible cell is contradictory; a productive or lytic infection needs both properties; and latency and chronic infection are patterns of persistence, not the description of a single abortive event.

Haemagglutinin must be cleaved by a host protease to become fusion-active. A monobasic cleavage site is recognised only by trypsin-like proteases confined to the respiratory and gastrointestinal tracts, so infection stays local. A polybasic site is cleaved by furin-like proteases present in tissues throughout the body, allowing systemic spread and severe disease.

Option B describes the monobasic, low-pathogenicity site. The polybasic site enables rather than blocks cleavage and fusion (C). Sialic-acid linkage preference is a separate determinant of tropism and transmissibility, not a function of the cleavage site (D), and the cleavage site is not an antibody-escape mechanism (E).

Age at infection is the strongest determinant of whether hepatitis B becomes chronic: persistence follows over 90% of infections acquired in infancy but only 5 to 10% of those acquired in adulthood, with intermediate rates in between. Immune immaturity at first encounter favours persistence, which is why interrupting neonatal transmission is the prevention priority.

Option B reverses the relationship, and age clearly does influence chronicity (A). Most adults clear the virus rather than progressing to chronic disease (C), and infancy infection usually persists rather than clearing in the first year (E).

Poliovirus destroys a narrow set of cells although its receptor is widespread, which shows tropism is multi-factorial. A cell must be both susceptible (carry the receptor) and permissive (supply the intracellular factors the virus needs and lack restriction) before it yields progeny. The mismatch between CD155 distribution and the neurons poliovirus actually kills is the classic demonstration.

Receptor distribution matters but is not sufficient (A). Route of entry shapes the initial distribution without setting target-organ tropism (C). Viruses have defined tropisms rather than infecting every cell (D), and the receptor enables, not blocks, entry (E).

A large prospective cohort found that Epstein-Barr virus infection raised the later risk of multiple sclerosis around thirtyfold, with a marker of nerve injury rising only after EBV seroconversion, and multiple sclerosis is very rare in the EBV-seronegative. EBV is therefore regarded as a necessary cause, though not a sufficient one, since most infected people never develop the disease. The leading mechanism is molecular mimicry, the immune response to Epstein-Barr nuclear antigen 1 (EBNA1) cross-reacting with a central nervous system protein.

The other options reverse or misstate this relationship.

Pathogenicity is the qualitative capacity of a virus to cause disease at all: a virus either is pathogenic or it is not. Virulence is the quantitative degree of disease it causes, the property used to compare a virulent strain against an attenuated one. Option A reverses the two.

The terms are not synonymous (C), and neither describes transmissibility or infectivity, which are separate properties (D). Virulence is not fixed: it is relative to the host and is meaningfully compared only between related viruses (E).

Viral infections are classified on two axes: duration (transient or persistent) and extent (localised to the entry surface or systemic, spreading through the body). Crossing the two gives four patterns.

A transient infection is cleared once the immune response develops, usually leaving immunity. A persistent infection is not cleared, and it subdivides by the nature of the ongoing infection:

Latent, in which the genome lies dormant in a long-lived cell with no infectious virus made between episodes, but with the capacity to reactivate, as in the herpesviruses.

Chronic active, in which the virus continues to replicate, often abundantly, with slowly progressive or delayed disease, as in hepatitis B, hepatitis C, and HIV.

A separate group, the persistent infections with rare late complications, sits within the systemic category: an apparently resolved infection that produces a delayed disease years later, as measles does in subacute sclerosing panencephalitis (SSPE). Overlaid on every pattern is the distinction between asymptomatic (subclinical) and symptomatic infection.

A complete answer follows the ordered stages of pathogenesis and anchors them to one worked example.

The sequence. A virus first crosses a body surface at its portal of entry and undergoes primary replication at or near that site. From there it spreads beneath the epithelium into the lymphatics and reaches the draining lymph nodes. Replication there delivers virus into the blood as a primary viraemia, usually clinically silent, which seeds secondary organs. Replication in those organs releases a much larger secondary viraemia, which carries the virus to the target organ responsible for the characteristic disease. Injury to that organ produces symptoms, and shedding from an accessible surface allows transmission. The incubation period is the time these sequential steps take, and the outcome turns on whether the developing immune response halts the virus before it reaches its target.

Poliovirus as the example. Poliovirus enters by the faecal-oral route and replicates in the lymphoid tissue of the gut, the Peyer patches. It spreads to the blood as a primary viraemia, replicates in secondary sites, and seeds a second viraemia. In a small minority it invades the central nervous system, by crossing the blood-brain barrier or via peripheral nerves, and replicates in and kills the anterior horn motor neurons, causing paralysis. Most infections are asymptomatic because circulating antibody clears the virus before it reaches the central nervous system, which is exactly why the kinetics of the antibody response decide the outcome.