From Alzheimer’s to Zebrafish: Eclectic Science and Regulatory Stories 84
In the past few years, emergent disease outbreaks have increased and nearly all have
involved zoonotic or species-jumping infectious agents. These zoonoses are caused either
by apparently new agents or by previously known agents appearing in places or in species
in which the disease was previously unknown. As an example, closely related strains of
some viruses may mutate into “new viruses” that cause human disease, as evidenced by
the newly reported swine flu pandemic.
Emergence of new zoonotic pathogens is accelerating for several reasons: global
human and livestock populations have continued to grow, bringing increasingly larger
numbers of people and animals into close contact transportation has advanced, making it
possible to circumnavigate the globe in less time than the incubation period of most infec-
tious agents ecological and environmental changes brought about by human activity are
massive and bioterrorism activities, supported by rogue governments as well as orga-
nized amateurs, are increasing, and in most instances the infectious agents of choice seem
to be zoonotic.1
As noted above, there were recent reports of transmissions of two groups of swine
influenza viruses in humans. One group consisted of triple reassortants of viruses from
pigs, humans and birds.2 (Reassortant viruses contain two or more pieces of nucleic
acid from different parents. Such viruses are produced in cells co-infected with different
strains of a given virus.) This virus has circulated in pigs for more than a decade. The
other group is a recent reassortant of the triple-reassortant swine influenza A(H1) viruses
and a Eurasian swine influenza virus, resulting in the swine-origin influenza A(H1N1)
virus.3 The letters H and N refer to hemagglutinin and neuraminidase, two proteins on
the surface of the influenza A virus. (There are 16 known H subtypes and nine known N
subtypes the subtypes differ because of changes in the proteins.) Both groups mentioned
above are H1 viruses that appeared in humans and swine during the deadly 1918 Spanish
flu pandemic. Thus, the virus now circulating in Mexico and other parts of the world is a
derivative of the virus that was responsible for infecting about 97% of the people on earth
and killing at least 50 million of them.4 It was deservedly known as the “mother of all
pandemics.” Before it faded away in 1920, Spanish flu killed more people than any other
outbreak of disease in human history.5
Transmission
Zoonotic diseases are not uncommon. There are at least 39 diseases people catch directly
from animals and about 48 diseases people may contract from the bite of insects that have
bitten an infected animal.6 Just recently, researchers at the University of Edinburgh com-
piled a list of 1,415 microbes known to cause diseases in humans. Of these, 868, or 61%,
are known to be currently transmitted from other animals to humans. (Animals provide
an environment for microbes quite similar to the human body.) The largest number come
from dogs and cats (43%) and from domestic livestock, primarily horses, cattle, sheep,
goats and pigs (39%). Additional microbes that cause human diseases come from rodents
(23%), other primates (13%), birds (10%), marine mammals (5%) and bats (2%). The total
is more than 100% because some microbes, such as the rabies virus, may be transmitted to
humans from more than one animal. Cats, dogs, bats, skunks and raccoons can be reser-
voirs for rabies.7
Arthropods can also transmit zoonotic diseases. Arthropods consist of insects (flies,
fleas and mosquitoes), arachnids (ticks, mites and spiders) and crustaceans (crayfish and
crabs). In a classic example, biting insects that have previously bitten an infected ani-
mal pass along a zoonotic disease. Yersenia pestis, the bacterium that caused the bubonic
plague, was harbored in rodents, but was transmitted to humans by flea bites. The plague
bacterium is ubiquitous it is harbored in more than 200 different mammalian reservoir
hosts. The plague was spread from Asia to Europe by the transit of flea-infested materials.8
In the past few years, emergent disease outbreaks have increased and nearly all have
involved zoonotic or species-jumping infectious agents. These zoonoses are caused either
by apparently new agents or by previously known agents appearing in places or in species
in which the disease was previously unknown. As an example, closely related strains of
some viruses may mutate into “new viruses” that cause human disease, as evidenced by
the newly reported swine flu pandemic.
Emergence of new zoonotic pathogens is accelerating for several reasons: global
human and livestock populations have continued to grow, bringing increasingly larger
numbers of people and animals into close contact transportation has advanced, making it
possible to circumnavigate the globe in less time than the incubation period of most infec-
tious agents ecological and environmental changes brought about by human activity are
massive and bioterrorism activities, supported by rogue governments as well as orga-
nized amateurs, are increasing, and in most instances the infectious agents of choice seem
to be zoonotic.1
As noted above, there were recent reports of transmissions of two groups of swine
influenza viruses in humans. One group consisted of triple reassortants of viruses from
pigs, humans and birds.2 (Reassortant viruses contain two or more pieces of nucleic
acid from different parents. Such viruses are produced in cells co-infected with different
strains of a given virus.) This virus has circulated in pigs for more than a decade. The
other group is a recent reassortant of the triple-reassortant swine influenza A(H1) viruses
and a Eurasian swine influenza virus, resulting in the swine-origin influenza A(H1N1)
virus.3 The letters H and N refer to hemagglutinin and neuraminidase, two proteins on
the surface of the influenza A virus. (There are 16 known H subtypes and nine known N
subtypes the subtypes differ because of changes in the proteins.) Both groups mentioned
above are H1 viruses that appeared in humans and swine during the deadly 1918 Spanish
flu pandemic. Thus, the virus now circulating in Mexico and other parts of the world is a
derivative of the virus that was responsible for infecting about 97% of the people on earth
and killing at least 50 million of them.4 It was deservedly known as the “mother of all
pandemics.” Before it faded away in 1920, Spanish flu killed more people than any other
outbreak of disease in human history.5
Transmission
Zoonotic diseases are not uncommon. There are at least 39 diseases people catch directly
from animals and about 48 diseases people may contract from the bite of insects that have
bitten an infected animal.6 Just recently, researchers at the University of Edinburgh com-
piled a list of 1,415 microbes known to cause diseases in humans. Of these, 868, or 61%,
are known to be currently transmitted from other animals to humans. (Animals provide
an environment for microbes quite similar to the human body.) The largest number come
from dogs and cats (43%) and from domestic livestock, primarily horses, cattle, sheep,
goats and pigs (39%). Additional microbes that cause human diseases come from rodents
(23%), other primates (13%), birds (10%), marine mammals (5%) and bats (2%). The total
is more than 100% because some microbes, such as the rabies virus, may be transmitted to
humans from more than one animal. Cats, dogs, bats, skunks and raccoons can be reser-
voirs for rabies.7
Arthropods can also transmit zoonotic diseases. Arthropods consist of insects (flies,
fleas and mosquitoes), arachnids (ticks, mites and spiders) and crustaceans (crayfish and
crabs). In a classic example, biting insects that have previously bitten an infected ani-
mal pass along a zoonotic disease. Yersenia pestis, the bacterium that caused the bubonic
plague, was harbored in rodents, but was transmitted to humans by flea bites. The plague
bacterium is ubiquitous it is harbored in more than 200 different mammalian reservoir
hosts. The plague was spread from Asia to Europe by the transit of flea-infested materials.8