This book was loaned to me by a friend who is a microbiologist at the UW Medical School. He marked three or four chapters for me to read but it was so interesting I read the whole thing. The subtitle is Animal Infections and the Next Human Pandemic and it serves as interesting background on the current coronavirus. The author is David Quammen. The book was published in 2012, after SARS, about the same time as MERS, but long before the current coronavirus outbreak.
The book starts with an introduction to the phenomenon called zoonosis. This is an animal infection transmissible to humans. Examples are AIDS and flu. Smallpox is not a zoonosis; it only affects humans.
With zoonosis diseases, they often lurk in a host animal while not harming the animal. The disease and the animal have co-evolved so the pathogen lives in the reservoir without harming it. Nearly all zoonotic diseases result from infection by one of six kinds of pathogen: viruses, bacteria, fungi, protists, prions and worms. Viruses are the worst because they evolve quickly and are not affected by antibiotics. Viruses have no means of locomotion. They aren’t even life forms. And viruses are so small, they aren’t caught in filters and can only be seen with an electron microscope.
Zoonosis diseases include Marburg, Lassa, Ebola, HIV, Hendra, avian flu, Nipah, West Nile and SARS. Why so many recently? It could be they were previously undiagnosed. The author feels that it is because human development has put large groups of people near animals. Human activity is causing disintegration of natural ecosystems. Millions of pathogens have evolved for millions of years but have been limited by their geographical range. These native germs are now disbursed to a wider world where there are no natural defenses.
Emerging diseases are defined as an infection whose incidence is increasing following its first introduction into a new host population. That new host often being humans. Spillover is the moment when a pathogen passes from one species to another. Vectors are different from host reservoirs. Mosquitos carry malaria from one person to another via blood. But the insects are not hosts.
SARS started in Southern China and spread rapidly to the rest of China and other parts of the world, carried by humans. After a lot of lab work, SARS was identified as a coronavirus and the first to ever inflict serious illness upon humans.
The wild markets for food in China mix many types of animals together in very tight circumstances, allowing pathogens to spread. The virus was found in palm civets, a type of wild cat. All civets at breeding farms were killed. Then it was discovered that Civets were not the reservoir. After testing all kinds of creatures in Hong Kong, horseshoe bats were found to be a reservoir, although it wasn’t clear they were the only reservoir.
With SARS, symptoms appear before the person is contagious, which is good. With the Flu of 1918, people were contagious before showing symptoms, which is much worse. When the next big one comes, if it is contagious before showing symptoms and it has a high mortality rate, it will be devasting. (The current coronavirus is contagious before symptoms appear.)
Viruses were never viewed until the 20th Century because they are so small. There is no consensus as to whether viruses are alive; they are sub-cellular. Viruses vary from 14 nanometers to 300 nanometers. They have four basic challenges:
- Getting from one host to another.
- Penetrating a cell.
- Commandeering the cells equipment and resources in order to reproduce itself.
- Getting back out of the cell.
Viruses are ‘a piece of bad news wrapped in a protein,’ that protein wrap known as a capsid. A virus outside a cell is called a virion. A virion is not a cell. Inside the capsid is genetic material. That material can be either DNA or RNA. DNA is double-stranded and generally repairs mistakes (so the two strands match up). RNA has no such corrective mechanism as they are single-stranded. The result is that RNA viruses mutate profligately.
Mutation provides genetic variation, which is the raw material that natural selection operates on. Most mutations are harmful and an evolutionary dead end. Some are helpful and live on. RNA viruses evolve faster than any other organism on Earth. Viruses can live benignly within a host or even be helpful. But once spillover occurs, the rules change.
Viral survival depends on transmissibility, which includes replication and transmission. Replication occurs within cells of a host. Transmission can be via sneeze, blood or other means. Blood transmission usually requires a vector, such as a mosquito. Blood-borne viruses can also spread by way of shared needles or transfusions. Sexual transmission is good for viruses that can’t survive exposure to light or outside air.
Pathogenicity is the capacity of the microbe to cause disease. HIV and Rabies have very high death rates. They depend on spreading before the host dies. The evolutionary success of a bug is directly related to its rate of transmission through the host population and inversely but intricately related to its lethality, the rate of recovery from it, and the normal death rate from all other causes.
RNA viruses are unstable. They mutate a lot and can’t grow very large. With so many limitations, they tend to jump species a lot; spillover. They jump from a reservoir host to another animal. Those that jump to humans come from many sources:
- Hantavirus from rodents.
- Yellow fever from monkeys.
- Monkeypox from squirrels.
- Herpes from macaques.
- Influenzas from wild birds to domestic birds and sometimes pigs.
- HIV from chimps.
But the scariest new viruses are transmitted by bats. Bats are mammals. In fact, bats make up 25 percent of all mammal species. And, or course, they can fly, so they can have a wide range.
(The current coronavirus seems to have originated in the wet markets of China. It has again, as in SARS, been found in civets. And, again like SARS, the suspicion is that civets aren’t the real problem. The Chinese horseshoe bat is thought to be the reservoir and the source of spillover into humans. Bats can and do carry a lot of viruses without getting sick themselves. Bats are the reservoir for Marburg, Nipah, Hendra, and probably Ebola.)
These outbreaks of flu and other zoonotic diseases are a result of changes humans have made. There are over seven billion people on Earth and we’re headed towards nine billion before things level off. We live at high densities. We penetrate the great forests and have disrupted the ecosystems of the planet. Ecological circumstance provides opportunity for spillover. We are inseparable from the natural world.
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