Biology Reference
In-Depth Information
Alien Human Diseases and Their Evolution
Vectors and agents of human disease are also subject to rapid evolution in
response to many agents of selection. Vectors of disease, ranging from
insects to rodents, have evolved resistance to chemical pesticides. Resur-
gence of human malaria, for example, is due in part to the evolved resist-
ance of
Anopheles
mosquitoes to the insecticide DDT in the 1960s and to
pyrethroid insecticides more recently (Hemingway et al. 2002). Disease
agents themselves evolve to escape control by modern antibiotics and
other drugs. Resistance to the drug chloroquine by three of the four
human malaria parasites (
Plasmodium
spp.) has contributed to the resur-
gence of human malaria (Wellems 2002).The invasion of new geograph-
ical areas by strains of many of these disease agents and their vectors is cre-
ating opportunities for rapid evolution of new forms of disease.
The influenza virus provides an example of complex, continuing evo-
lution of a disease agent of humans, other mammals, and birds.The orig-
inal natural reservoir of these viruses is thought to be waterfowl, shore-
birds, and gulls. The virus, with an RNA core consisting of ten genes,
shows a number of major subtypes that differ in the structure of two sur-
face proteins, hemagglutinin and neuraminidase. Various subtypes based
on differences in these proteins have apparently arisen by rapid evolution
following the transmission of the virus to other animals, including swine,
horses, poultry, and humans.
The influenza virus apparently evolves in two ways.Within each sub-
type, mutations occur in the hemagglutinin gene, a process known as anti-
genic drift. These slightly altered forms of the virus enable it to escape
strong control by antibodies to former strains of the virus. This requires
the creation of new vaccines annually to protect humans. Occasionally,
new subtypes that differ to a greater degree in hemagglutinin structure,
originating by recombination between different strains in a nonhuman
animal host, enter the human population, an event known as antigenic
shift.The major human flu pandemics of 1918, 1957, 1968, and 1977 were
due to such shifts.The recent year-to-year changes that require new vac-
cines are the result of antigenic drift.
In 1997, an unusual outbreak of influenza occurred in Hong Kong,
killing 6 of the 18 people infected.This virus represented a new subtype
that entered the human population from chickens. It possessed a mutant
form of a different protein, labeled PB2, which differed in amino acid
substitutions at one or two positions (Hatta et al. 2001). This protein is
thought to influence the process of virus replication in cells of the host.
