Basic concepts in virology

1 What structural components are used to categorize viruses?

Viruses can be classified according to the following:

Nucleic acid

Ribonucleic acid (RNA) vs. deoxyribonucleic acid (DNA)

Single-stranded vs. double-stranded

Segmented vs. nonsegmented

Capsid symmetry (icosahedral vs. helical)

Size

Presence or absence of an envelope

There are many RNA viruses; all contain single-stranded RNA (ssRNA) except rotavirus, which contains a double-stranded DNA (dsDNA) genome. They are most easily categorized by their capsid symmetry and nucleic acid polarity. The nucleic acid polarity is either + or − sense; viruses with + sense polarity have RNA strands that function directly as messenger RNA (mRNA). Viruses that contain − sense genomes (like influenza and parainfluenza) rely on RNA-dependent RNA polymerases to make a + sense template for transcription.

All DNA viruses except the Parvoviridae family contain dsDNA. All are enveloped except Parvoviridae, Adenoviridae, and Papovaviridae. If you can memorize the viruses that are all medically relevant DNA viruses, then you know by default any other virus must contain RNA (Figs. 22-1 and 22-2).

Figure 22-1 Classification of major viral families based on genome structure and virion morphology. A, DNA viruses. C, circular genome; L, linear genome. B, RNA viruses. S, segmented genome. + or − refers to nucleic acid polarity (see text).

(From Rosenthal K, Tan J: Rapid Review Microbiology and Immunology. Philadelphia, Mosby, 2007.)

Figure 22-2 Virion structures.

(From Rosenthal K, Tan J: Rapid Review Microbiology and Immunology. Philadelphia, Mosby, 2007.)

Step 1 Secret

Although it is of little clinical relevance, you are expected to know how to classify viruses according to their genomes (DNA vs. RNA, single- vs. double-stranded, linear vs. circular), capsids (helical or icosadehedral), and presence or absence of a membrane. For instance, you may be given a clinical vignette with a question that asks, “The causative agent of this patient’s disease is _____.” Instead of naming viruses, you might see answer choices that reflect the composition of the virus (e.g., “a dsDNA virus with a circular genome”). Remember, the USMLE testmakers love to ask second- and third-order questions!

Unfortunately, the only way to learn this information is simply to memorize it. Tricks and mnemonics are of great use here.

Of the DNA viruses, any virus core name ending with an “a” (papilloma, polyoma, hepadna) has a circular genome, and the rest are linear (adeno, parvo, pox, herpes). All of these viruses have double-stranded genomes except parvovirus, because it is the smallest DNA virus (“parvo” is derived from the Latin word for small). All of the DNA viruses replicate in the nucleus except poxvirus, which is large enough to carry its own DNA-dependent RNA polymerase. Enveloped viruses include hepadnavirus, herpesviruses and poxvirus; the rest are naked. You can remember this because poxvirus is large, so it has an envelope, and the other two enveloped DNA viruses both begin with “H.”

2 Name the disease associated with each DNA virus listed in Table 22-1.

Table 22-1 DNA Viruses

Family	Member Virus(es)	Disease(s)
Parvoviridae	Parvovirus B19	Fifth disease, aplastic anemia, arthritis, hydrops fetalis in utero
Herpesviridae	Herpes simplex virus types 1 and 2 (HSV1 and HSV2)	Oral and genital lesions Temporal lobe encephalitis
	Varicella-zoster virus (VZV)	Chickenpox (varicella), herpes zoster
	Epstein-Barr virus (EBV)	Infectious mononucleosis
	Cytomegalovirus (CMV)	CMV retinitis, congenital deafness
	Human herpesvirus types 6 and 7 (HHV6 and HHV7)	Roseola infantum (exanthema subitum)
	HHV8	Kaposi sarcoma in HIV infection
Poxviridae	Variola	Smallpox
Poxviridae	Molluscum contagiosum virus	Umbilicated papules (spread among wrestlers and sexual partners)
Hepadnaviridae	Hepatitis B	Hepatitis
Adenoviridae	Adenovirus	Conjunctivitis, pneumonia, gastroenteritis and pharyngitis
Papovaviridae	Human papillomavirus (HPV)	Serotypes 16, 18 cause cervical dysplasia
Papovaviridae	JC virus	Progressive multifocal leukoencephalopathy

HIV, human immunodeficiency virus.

3 Cover the right-hand column in Table 22-2, and using the clinical description given, name the most likely virus.

Table 22-2 Classic Clinical Manifestations of Infection with Different Viruses

Description	Virus(es)
Rash with “slapped cheek” appearance	Parvovirus B19
Descending maculopapular rash, Koplik’s spots	Measles (rubeola) virus
Typically causes gastroenteritis but may cause paralysis by destruction of anterior horn cells	Poliovirus
Cervical cancer in sexually active smoker	HPV (serotypes 16,18)
Parotitis, orchitis, and possible sterility in males	Mumps virus
Cataracts leading to blindness in newborns	Rubella virus
Painful vesicular lesions in dermatomal pattern; virus remains dormant in dorsal root ganglion	Varicella-zoster virus
Acute retinitis in patient with AIDS	CMV
Genital warts	HPV (serotypes 6, 11)
Painful genital vesicular lesions	HSV2 (occasionally HSV1)
Hepatitis in pregnant women, with high mortality rate	Hepatitis E virus
Fatigue, splenomegaly, and atypical lymphocytosis in a teenager; positive heterophile antibody test result	Epstein-Barr virus (EBV)
Gastroenteritis on cruise ship	Norwalk virus
Common cause of gastroenteritis in children	Rotavirus
Common cold viruses	Coronaviruses, rhinoviruses
Most common cause of bronchiolitis in children	RSV
Segmented genome can undergo reassortment, causing epidemic shift pneumonia	Influenza A virus
Severe encephalitis after an animal bite; intracytoplasmic Negri bodies in neurons	Rabies virus
Neonatal encephalitis	HSV or CMV

AIDS, acquired immunodeficiency syndrome; CMV, cytomegalovirus; HPV, human papillomavirus; HSV, herpes simplex virus; RSV, respiratory syncytial virus.

Basic concepts in parasitology

1 What are protozoa?

Parasites can be classified as protozoa or metazoa. Protozoa are single-celled eukaryotic organisms. The medically important protozoa and their associated diseases are listed in Table 22-3.

Table 22-3 Protozoa Commonly Causing Disease in Humans

Protozoan	Associated Disease
Entamoeba histolytica	Amebic dysentery (may cause sterile, “flask-shaped” liver abscess)
Giardia lamblia	Giardiasis (foul-smelling diarrhea after drinking contaminated water)
Cryptosporidium spp.	Diarrhea in immunocompromised person
Trichomonas vaginalis	Trichomoniasis (vaginitis with green discharge)
Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale, Plasmodium malariae	Malaria; spread by Anopheles mosquito (only P. vivax and P. ovale cause recurrent infection due to liver hypnozoite stage) Treat with primaquine to eradicate; adverse reaction in patients with G6PD deficiency is hemolytic anemia
Toxoplasma gondii	Toxoplasmosis (from cat feces) Can cause intracerebral infection in HIV infection
Leishmania spp.	Leismaniasis (cutaneous and visceral)
Trypanosoma brucei	African sleeping sickness (African trypanosomiasis) Transmitted by tsetse fly
Trypanosoma cruzi	Chagas disease (American trypanosomiasis), reduviid kissing bug Dilated cardiomyopathy, dementia, and megacolon

G6PD, glucose-6-phosphate dehydrogenase; HIV, human immunodeficiency virus.

2 What is the difference between cestodes, nematodes, and trematodes?

They are all helminths (worms). Cestodes are flatworms (tapeworms), nematodes are roundworms, and trematodes are flukes.

3 Cover the left column in Table 22-4, and from the description of the infection at the right, name the helminth that causes it.

Table 22-4 Helminths Commonly Causing Disease in Humans

Helminth	Manifestation(s)/Mechanism(s) of Infection
Cestodes (Flatworms)
Taenia solium (pork tapeworm)	Infection from eating pork leads to intestinal worm; infection from egg ingestion causes cysts to encrust in brain
Taenia saginatum (beef tapeworm)	Transmitted by undercooked beef (mostly asymptomatic)
Diphyllobothrium latum (fish tapeworm)	Extremely long intestinal tapeworm that causes vitamin B₁₂ deficiency and anemia; can be acquired by eating raw fish
Echinococcus granulosus	Ingestion of eggs in dog feces, causing cysts in liver, lungs, and brain Rupture of cysts causes allergic reaction
Nematodes (Roundworms)
Enterobius vermicularis (pinworm)	Anal itching with white worms visible in perianal region; positive result on Scotch tape test
Ascaris lumbricoides (giant roundworm)	Intestinal infection, but worms pass from intestine to lungs Marked eosinophilia Eggs have rough, bumpy surface
Ancylostoma duodenale or Necator americanus (hookworms)	Larvae directly penetrate the skin and attach to intestinal mucosa, causing chronic blood loss and anemia
Trematodes (Flukes)
Schistosoma hematobium	Hematuria after swimming in the Nile Egg has small terminal spine Increased risk of bladder squamous cell cancer
Schistosoma mansoni or Schistosoma japonicum	Free-swimming cercariae released from snails infect the human host Eggs are antigenic and induce granuloma formation Pipestem fibrosis of liver
Clonorchis sinensis	Biliary obstruction in patient from southeast Asia
Paragonimus westermani	Transmitted by eating raw crab meat, resulting in gastrointestinal and pulmonary disease