Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY RNA tumor viruses

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The Centers for Disease Controlreport about 600–1,000 cases

occurring annually, but this number may be underestimated

due to underreporting. Rickettsia rickettsiiare carried and

transmitted by four species of the hard-shelled tick, all of

which feed on humans, wild and domestic animals, and small

rodents. When a tick feeds on an infected animal, the bacteria

are transmitted to the tick, which can in turn infect other ani-

mals with its bite. Human-to-human transmission of R. rick-

ettsiidoes not occur. Once inside the human bloodstream, the

bacteria invade cells that line the small blood vessels.

The symptoms of Rocky Mountain spotted fever reflect

the presence of bacteria inside blood vessel cells. Within two

to 12 days of being bitten by an infected tick, the infected per-

son experiences a severe headache, fever, and malaise. After

about two to four days, a rash develops, first on the extremi-

ties, then the trunk. A characteristic sign of this disease is that

the rash involves the soles of the feet and palms of the hands.

If the disease is not treated with antibiotics, the infected blood

vessel cells lyse, causing internal hemorrhage, blockage of the

blood vessels, and eventual death of the cells. Shock, kidney

failure, heart failure, and stroke may then occur. Rocky

Mountain spotted fever is often fatal if not treated.

A similar but milder disease is rickettsial pox, caused by

R. akari.These bacteria are transmitted by mites which live

preferentially on the common house mouse, only occasionally

biting humans. Rickettsial pox is characterized by a rash that

does not affect the palms or soles of the feet. The rash includes

a lesion called an eschar-a sore that marks the spot of the

infected mite bite. The mild course of this disease and the

presence of the rash sometimes leads to its misdiagnosis as

chicken pox, but the eschar clearly distinguishes rickettsial

pox from chicken pox.

Outside of the United States, spotted fevers such as

North Asian tick typhus, Queensland tick typhus, and bouton-

neuse fever are caused by other rickettsia species. As their

names suggest, these diseases are found in Asia, Mongolia,

and the Siberian region of Russia; in Australia; and in the

Mediterranean region, Africa, and India, respectively.

Symptoms of these spotted fevers resemble those of rickettsial

pox. Although these spotted fevers share some of the symp-

toms of Rocky Mountain spotted fever, they are milder dis-

eases and are usually not fatal.

Three forms of typhus are also caused by rickettsia.

Epidemic typhus is caused by R. prowazekii,a bacterium that

is transmitted by the human body louse. Consequently,

episodes of this disease occur when humans are brought into

close contact with each other under unsanitary conditions.

Endemic typhus and scrub typhus are caused by R. typhiand

R. tsutsugamushi, respectively. Transmitted by rat fleas,

endemic typhus is a mild disease of fever, headache, and rash.

Scrub typhus, named for its predilection for scrub habitats

(although it has since been found to occur in rain forests,

savannas, beaches, and deserts as well) is transmitted by chig-

gers. Unlike endemic typhus, scrub typhus is a serious disease

that is fatal if not treated.

Not all rickettsia cause disease. Some species, such as R.

parkeriand R. montana,normally live inside certain species of

ticks and are harmless to the insect. These rickettsia are non-

pathogenic (they do not cause disease) to humans as well.

With the exception of epidemic typhus, no vaccine

exists to prevent rickettsial infection. Prevention of these dis-

eases should focus on the elimination of insect carriers with

insecticides and wearing heavy clothing when going into areas

in which rickettsial carriers dwell. For instance, appropriate

clothing for a forest expedition should include boots, long-

sleeved shirts, and long pants. Treating the skin with insect

repellentsis also recommended to prevent insect bites.

RNA RNA tumor viruses TUMOR VIRUSES

RNA tumor virusescontain ribonucleic acidas their genetic

material. The virusesderive their designation from their asso-

ciation with tumors.

RNA tumor viruses are retrovirusesthat possess the

reverse transcriptase enzyme that manufactures deoxyribonu-

cleic acid(DNA) from the RNA template. Indeed, retrois the

Latin word for backwards.

The history of RNA tumor viruses extends back to the

first decade of the twentieth century. In 1908, it was demon-

strated that fluid from a chicken that has leukemia could cause

cells to be cancerous, even after the fluid had been filtered to

remove all bacteria-sized organisms. Three years later, Peyton

Rousidentified one such factor, now named the Rous Sarcoma

Virus. By the 1950s, many oncogenic viruses had been dis-

covered and the RNA nature of these viruses had been estab-

lished. In the 1960s, the reverse transcriptase enzyme was

discovered. Finally, in 1981, the first human retrovirus was

discovered, this being the Human T-cell Leukemia Virus

(HTLV-1). The latter virus is a well-known tumor viruses.

There are two groups of RNA tumor viruses, the

Oncovirinae and the Lentivirinae. Examples of the first group

include the Rous Sarcoma Virus, HTLV-1, and HTLV-2 (which

is also known as hairy cell leukemia virus). A prominent exam-

ple of the second group is the Human Immunodeficiency Virus

(HIV). A characteristic of HIV and other members of the second

group is the long period of latency before symptoms of infec-

tion appear.

As for many viruses, the investigation of RNA tumor

viruses involves growing the virus in a cultureof whatever

eukaryotic cell the virus is able to replicate inside. Then, the

virus is purified. Subsequently, the virus can be studied using

a variety of molecular and genetic techniques, and the electron

microscopeto assess the shape of the virus particles.

Some RNA tumor viruses never exist outside of the host

cell, and lack an envelope around their genetic material.

Viruses such as the mouse mammary tumor virus have an

envelope that has spikes protruding from the surface. Other

RNA tumor viruses contain spikes that are less prominent.

Lentiviruses are an example of the latter shape.

The envelope of RNA tumor viruses comes from the

membrane surrounding the host cell. The virus acquires this

envelope as it emerges from the host cell. Within this enve-

lope are distinctive proteins, which are coded for by the enve-

lope, or env, gene. Another characteristic component of RNA

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