Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Radiation mutagenesis

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In humans, furious rabies patients typically show

bizarre behavior, ranging from episodes of severe agitation to

periods of depression. Confusion becomes extreme as the dis-

ease progresses, and the patient may become aggressive.

Hydrophobia is always seen with this type of disease, until the

patient becomes comatose while showing intermittently

uncontrollable inspiratory spasms. This type of rabies is also

characterized by hypersalivation, from 1–1.6 qt (1–1.5 L) of

saliva in 24 hours, and excessive sweating.

The paralytic form of rabies in humans is often indis-

tinguishable from that of most viral encephalitis, except for

the fact that a patient suffering from rabies remains con-

scious during the course of the disease. Paralysis usually

begins at the extremity exposed to the bite and gradually

involves other extremities finally affecting the pharyngeal

and respiratory muscles.

The dog is a most important animal as a disseminator of

rabies virus, not only to man but also to other animals. Wild

carnivora may be infected and transmit the disease. In the

United States, foxes, raccoons and skunks are the most com-

monly involved. These animals are sometimes responsible for

infecting domestic farm animals.

The disease in wildlife (especially skunks, foxes, rac-

coons, and bats) has become more prevalent in recent years,

accounting for approximately 85% of all reported cases of ani-

mal rabies every year since 1976. Wildlife now constitutes the

most important potential source of infection for both human

and domestic animals in the United States. Rabies among ani-

mals is present throughout the United States with the excep-

tion of Hawaii, which has remained consistently rabies-free.

The likelihood of different animals contracting rabies varies

from one place to the next. Dogs are a good example. In areas

where public healthefforts to control rabies have been aggres-

sive, dogs make up less than 5% of rabies cases in animals.

These areas include the United States, most European coun-

tries, and Canada.

However, dogs are the most common source of rabies in

many countries. They make up at least 90% of reported cases

of rabies in most developing countries of Africa and Asia and

many parts of Latin America. In these countries, public health

efforts to control rabies have not been as aggressive. Other key

carriers of rabies include the fox in Europe and Canada, the

jackal in Africa, and the vampire bat in Latin America.

In the United States, 60% of all rabies cases were

reported in raccoons. The high number of cases in raccoons

reflects an animal epidemic, or, more properly, an epizootic.

The epizootic began when diseased raccoons were carried

from further south to Virginia and West Virginia. Since then,

rabies in raccoons has spread up the eastern seaboard of the

United States. Concentrations of animals with rabies include

coyotes in southern Texas, skunks in California and in south

and north central states, and gray foxes in southeastern

Arizona. Bats throughout the United States also develop

rabies. When rabies first enters a species, large numbers of

animals die. When it has been around for a long time, the

species adapts, and smaller numbers of animals die.

There are few deaths from rabies in the United States.

Between 1980 and the middle of 1994, a total of 19 people in

the United States died of rabies, far fewer than the 200

Americans killed by lightning, for example. Eight of these

cases were acquired outside the United States. Eight of the 11

cases contracted in the United States stemmed from bat-trans-

mitted strains of rabies.

Internationally, more than 33,000 people die annually

from rabies, according to the World Health Association. A

great majority of cases internationally stem from dog bites.

Different countries employ different strategies in the fight

against rabies. The United States depends primarily on vacci-

nation of domestic animals and on immunization following

exposure to possibly rabid animals. Great Britain, in which

rabies has never been established, employs a strict quarantine

for all domestic animals entering the country.

Continental Europe, which has a long history of rabies,

developed an aggressive program in the 1990s of airdropping

a new vaccine for wild animals. The vaccine is mixed with

pellets of food for red foxes, the primary carrier there. Public

health officials have announced that fox rabies may be elimi-

nated from Western Europe by the end of the decade. The

World Health Organizationis also planning to use the vaccine

in parts of Africa.

Though the United States have been largely successful in

controlling rabies in humans, the disease remains present in the

animal population, a constant reminder of the serious threat

rabies could become without successful prevention efforts.

See alsoViruses and responses to viral infection

RRadiation mutagenesisADIATION MUTAGENESIS

Mutationsare caused by DNAdamage and genetic alterations

that may occur spontaneously at a very low rate. The fre-

quency of these mutations can be increased by using special

agents called mutagens. Ionizing radiation was the first muta-

gen that efficiently and reproducibly induced mutations in a

multicellular organism. Direct damage to the cell nucleusis

believed to be responsible for both mutations and other radia-

tion mediated genotoxic effects like chromosomal aberrations

and lethality. Free radicals generated by irradiation of the cyto-

plasmare also believed to induce genemutations even in the

non-irradiated nucleus.

There are many kinds of radiations that can increase

mutations. Radiation is often classified as ionizing or non-ion-

izing depending on whether ions are emitted in the penetrated

tissues or not. X rays, gamma rays (γ), beta particle radiation

(β), and alpha particle (α) radiation (also known as alpha rays)

are ionizing form of radiation. On the other hand, UV radia-

tion, like that in sunlight, is non-ionizing. Biologically, the dif-

ferences between types of radiation effects fundamentally

involve the way energy is distributed in irradiated cell popula-

tions and tissues. With alpha radiation, ionizations lead to an

intense but more superficial and localized deposition of

energy. Primary ionization in x rays or gamma radiation trav-

erses deeper into tissues. This penetration leads to a more even

distribution of energy as opposed to the more concentrated or

localized alpha rays.

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