Coronavirus
The virus specifically known as SARS-CoV-2, or COVID-19, caused the 2019 global pandemic.
SARS stands for Severe Acute Respiratory Syndrome.
The pandemic is believed to have started following a laboratory leak in Wuhan, China, although this has been a source of political debate, where initial explanations asserted that there was a natural, zoonotic origin for the virus (i.e., originating from animals, e.g., bats).
Has characteristic spike proteins that create a crown-like appearance on electron microscopy.
The spike proteins bind to ACE2 receptor, allowing for viral fusion with host respiratory epithelium.
Presentation can range from mild respiratory symptoms similar to the common cold (rhinovirus) all the way to severe respiratory disease with multi-organ failure.
Many different vaccine types exist – i.e., mRNA (Moderna; delivers mRNA coding for the spike protein), viral vector (AstraZeneca; delivers mRNA in a harmless viral capsid), and killed (Sinovac; delivers inactivated, killed virus).
Both live viral infection as well as vaccination are known to cause rare adverse effects, such as Bell’s palsy and myocarditis, although these effects are not unique to coronavirus and can rarely happen with many viral infections and vaccines.
Vaccination mandates and their political implications were (and still are) a source of contentious debate.
Prior to the 2019 pandemic, coronavirus was known to cause SARS in China in 2002 and Middle Eastern Respiratory Syndrome (MERS) in Saudi Arabia in 2012.
Vaccination schedule for children now recommends IM vaccine starting at 6 months; 2-3-doses.
Hantavirus
Causes pulmonary syndrome and hemorrhagic fever.
Spread by rodents/mice.
This virus is known to get rarely asked on Step 1, where they apparently mention hantavirus in the vignette in a patient with a fatal hemorrhagic pulmonary syndrome, and then the answer is just “mice” for how it’s acquired.
Influenza
Causes respiratory distress, fever, and myalgias (muscle pain). For USMLE purposes, the myalgias are exceedingly HY as a vignette finding that usually suggests the flu over other diagnoses.
Has 8 segments, two of which are hemagglutinin and neuraminidase.
Hemagglutinin mediates viral attachment to the cell by enabling its binding at sialic acid receptors.
If a question asks about the molecule most flu vaccines are targeted against, the answer is hemagglutinin.
Neuraminidase allows for newly synthesized viral particles to leave the host cell. This enzyme cleaves sialic acid residues, which normally bind the new viral particles within the cell. Once these residues are cleaved, the viral particles can leave the cell.
Drugs such as oseltamivir and zanamivir are sialic acid analogues that function as neuraminidase competitive inhibitors. In other words, they prevent the virus from leaving the cell. If the USMLE asks which drug prevents viral spread within a community, or they tell you a drug is given and now host cells are “packed with virions” (because they can’t leave the cell), the answer is one of the -mivirs.
Antigenic drift is point mutations in hemagglutinin and/or neuraminidase, where the virus has changed slightly. It leads to seasonal epidemics.
Antigenic shift is due to two influenza viruses entering a cell, one of human origin, the other of animal origin (such as bird or swine), where they engage in reassortment of viral segments, leading to a completely novel influenza virus. It leads to generational pandemics.
If a patient gets a bacterial lobar pneumonia following recent convalescence from influenza infection, USMLE likes S. aureus as a HY cause. The USMLE will not play trivia where they list S. aureus alongside S. pneumo and you’re forced to choose. What they’ll do is say something about how a guy recently recovered from a viral illness in which he had high fever and myalgias, and now he has a pneumonia caused by a gram-positive coccus in clusters –> answer = S. aureus. In contrast, S. pneumo is gram-positive diplococci.
Vaccine is given fall or winter every year. USMLE really cares about this. So much so, they will sometimes say “April” in a vignette, where giving flu vaccine is wrong, or they’ll say “January” or “October,” where giving it is correct.
IM killed vaccine: start age 6 months, then give yearly throughout life; safe to give during pregnancy.
Intranasal live-attenuated vaccine: ages 2-45; immunocompetent, non-pregnant persons only.
Parainfluenza virus
Aka paramyxovirus.
Causes laryngotracheobronchitis (croup).
Presents as hoarse, barking, or seal-like cough in school-age kid. The Q can say the cough gets better when his dad brings him out into the cold air.
Neck x-ray shows “steeple sign,” which is sub-glottic narrowing.
Don’t confuse the steeple sign of croup with the thumbprint sign of epiglottitis caused by H. influenzae type B.
Sometimes the Q can give you easy vignette of croup, but then the answer is just “larynx” (literally inflammation of the larynx, trachea, and the bronchi). “Sub-glottic” means below the area of the vocal cords. The larynx is the area encompassing the vocal cords.
Tx is supportive. If they force you to choose an actual Tx, however, nebulized racemic epinephrine is the answer.
Respiratory syncytial virus (RSV)
Answer on USMLE for a kid <18 months old who has low-grade fever and bilateral wheezes.
Tx is supportive care on USMLE. Don’t choose answers like ribavirin or palivizumab.
Mumps
Causes POM –> Parotitis, Orchitis, Meningitis.
Doesn’t typically cause rash.
Vaccine is MMR, which is live-attenuated.
Measles
Aka rubeola; causes a head-to-toe macular popular rash.
The notion of “cough, coryza, conjunctivitis” as = measles is absolute garbage and a flaming joke. No idea why there have been resources over the years that have perpetuated this trash. These symptoms are non-specific for viral infections in general (e.g., rhinovirus, RSV, etc.). I frequently see students get easy NBME Qs wrong where they think measles is the diagnosis in the setting of these symptoms.
Can cause Koplik spots (pathognomonic whiteish lesions on buccal mucosa).
MMR vaccine is live-attenuated; contraindicated in pregnancy; not contraindicated in HIV.
Immigrant Hx on USMLE sometimes implies unvaccinated status.
Can rarely cause subacute sclerosing panencephalitis (reactivation of latent infection in the CNS in teenagers).
Rubella also causes a head-to-toe macular popular rash, but rather than Koplik spots, suboccipital and post-auricular lymphadenopathy (tenderness on back of head and behind ears) is characteristic.
Rabies
Causes encephalomyelitis that is nearly always fatal.
Spread by the bite or scratch of infected animals, such as bats, skunks, raccoons, and wild dogs.
Travels up peripheral nerves to the CNS. Has very long incubation period of >1-3 months before symptoms appear.
Presents as flu-like illness that progresses to neurologic features, as well as pathognomonic findings such as hydrophobia (fear of water), aerophobia (fear of drafts of air), and hyper-salivation.
Negri bodies are characteristic inclusions seen on electron microscopy of infected neurons.
Ebola
Causes hemorrhagic disease, where patient initially has flu-like illness, followed by development of internal and external bleeding, leading to death nearly always.
Bats are the most likely reservoir, with human-human spread occurring via contact with bodily fluids (i.e., blood, vomitus, feces, sweat).
As of 2019, a recombinant vaccine was created (rVSV-ZEBOV) that has been shown to be 98% effective.
