The Plague: It was the Gerbils

Aug. 7, 2015

By Medical Discovery News

In the past 800 years, many things have been blamed for the plague that swept through Europe in the Middle Ages: the alignment of the planets, bad air, lack of proper hygiene, black rats, and their fleas. Now scientists have data that suggests the climate in Central Asia at that time influenced the size of the great gerbil population, which triggered cycles of plague in Europe. These furry little rodents carried the plague bacterium, as did the fleas that fed on them. When the gerbil population shrank, the fleas found alternate hosts like horses, humans, and eventually rats, which then made their way to Europe and triggered the plague pandemics.

The plague was caused by the bacterium Yersinia pestis. It is transmitted to humans through the bite of a flea that has fed on an infected rodent. Plague outbreaks have afflicted humans for thousands of years and changed the course of history. The first recorded plague pandemic began in 541 and was named the Justinian Plague after the 6th century Byzantine emperor. Frequent outbreaks for the next 200 years are likely to have killed over 25 million people. The second pandemic, called the Great Plague or the Black Death, began in China and spread westward along trade routes to Constantinople and into Europe. About 60 percent of Europeans died, eliminating entire towns.

The third pandemic, or Modern Plague, also began in China and spread to Hong Kong by 1894. Rats hitching rides on steamships spread the plague to port cities around the world for the next 20 years, killing about 10 million people. By then scientists were able to identify the bacterium responsible and how it spread. Efforts to control the rat population eventually ended the pandemic. It continued to infect people (although in much smaller numbers than before) during the 20th century, such as in Vietnam during the war. The bacterium is still in the reservoir of wild rodents, and today most cases of plague are in sub-Saharan Africa and Madagascar. The plague can be effectively treated with common antibiotics, but if left untreated it has a high mortality rate.

Since there are still lots of rats in Europe, some wonder, why is there no plague? Researchers proposed that each time, the plague actually started in Asia. To test their theory, they examined climate records using the rings of trees. The incidence of plague did not correlate with climate changes in Europe, but it did with changes in Asia. It was already known that the Asian great gerbil carries Yersinia pestis, and when the weather in Asia was good, gerbils thrived, but when it turned bad, their population would crash. Then their fleas would seek another host such as human traders and their pack animals, who spread the plague to other parts of the world. They found no evidence that rodents in Europe carried Yersinia pestis, so that would explain why cases of the plague disappeared between pandemics.

So don’t worry about the little gerbils in the pet store – they are not carrying the plague.

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An Unwelcome Gift from Gorillas

July 3, 2015

By Medical Discovery News


You probably know that Acquired Immunodeficiency Syndrome (AIDS), which has affected 79 million people and killed 39 million since 1981, is the result of Human Immunodeficiency Virus (HIV). What you may not know is that there are several different types of this virus and they did not all come from the same source, making the search for HIV’s origins lengthy and complicated.

There are four groups of HIV-1: M, N, O, and P. Each of them was transmitted between African primates as simian immunodeficiency viruses (SIVs) before infecting humans, and each crossed species to humans independently. More than 40 African primates carry SIVs, which emerged up to 6 million years ago. It is likely that transmission to humans has occurred many times when hunters where exposed to the blood and tissues of infected animals. However the isolation of humans in Africa limited the spread of SIVs that crossed into humans until the last century.

It was not until modern travel allowed infected humans to move from the bush to cities and from there to other cities and countries that an HIV strain such as M took hold among humans, leading to a global pandemic. Group M causes more than 90 percent of AIDS cases and currently affects 40 million people worldwide. We already know that it came from chimpanzees in southern Cameroon. Group N also came from chimpanzees, but has infected less than 20 people.

Group O has infected about 100,000 people in Cameroon, Chad, Gabon, Niger, Nigeria, Senegal, and Togo. Although anti-retroviral drug combinations have made HIV infections survivable, many in Africa and the developing world do not have access to these treatments. Group P has only been isolated from two people. The origins of groups O and P were previously unknown, but now their source has been definitely confirmed: gorillas.

Scientists gathered fecal samples from western lowland, eastern lowland, and mountain gorillas, screening them for SIV antibodies and genetic information. Despite testing many wild troops of gorillas throughout Cameroon, Gabon, the Democratic Republic of Congo, and Uganda, the virus was identified at only four sites. Two strains of SIVs from southwestern Cameroon resembled HIV Group P and one from central Cameroon resembled Group O.

Not only does this data prove that gorillas were the immediate source of groups O and P, but the genetic information revealed that the viruses originated through a cross-species transmission from chimpanzees to gorillas. These are the same chimpanzees that infected humans, leading to groups M and N. Chimpanzees and gorillas share the same habitat, so the virus could have infected a gorilla if it bit a chimpanzee with SIV or was exposed to its blood or tissues.

Understanding the origins of HIV in humans is crucial if we want to prepare for additional viruses, especially SIV variants, entering the human population in the future, which will remain a risk as long as humans continue to hunt and eat primates.

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The Birth of Ebola

May 1, 2015

By Medical Discovery News

Colorized micrograph of Ebola by Dr. F.A. Murphy

For most Americans, the Ebola scare seems to have come and gone, but that doesn’t mean the outbreak is over in Africa or that we’ve seen the last of the virus, especially considering its history. Scientists believed that Ebola is relatively new as far as viruses go – only 10,000 years old. However, ancient animal bones show that Ebola appeared between 16 and 23 million years ago, perhaps even earlier.

The Ebola virus was discovered in 1976 during two outbreaks in what was then called Northern Zaire (now the Democratic Republic of the Congo) and Southern Sudan. The outbreaks were actually caused by two different strains of the Ebola virus named Zaire and Sudan, with 90 and 50 percent mortality rates respectively. Since then, three other strains have been identified: Tai Forest, Bundibugyo, and Reston, which is the only one that doesn’t affect people. Overall, there have been 27 outbreaks, but the current outbreak that started in March 2014 is by far the worst, infecting almost 25,000 people and killing over 10,000, thereby making it the world’s first Ebola epidemic.

Ebola is a member of the filovirus family, which also includes the Marburg virus discovered in 1967. Filoviruses are zoonotic, meaning they replicate in other animals, their natural reservoirs, before transmitting to humans. The Ebola virus’s natural reservoir is African fruit bats, so it can transfer to humans who come into contact with an infected bat or another species that has been infected, such as chimpanzees, antelope, and porcupine. Then the virus can spread from person to person.

New research into the origins of filoviruses shows that they have evolutionary ties that go back millions of years. Scientists tracked the viruses’ origins by looking for pieces of their genetic information in fossilized animal bones. While using the bones to study the genomes of ancient voles and hamsters, they found the same pieces of the viruses’ genetic material in the same locations in both rodent species. This suggests that the viruses have existed at least as long as the two species have.

Given the billions of bases each animal has in its genome, it is highly unlikely that these fragments of viral genetic information would have been inserted in exactly the same locations during different infections. Scientists therefore concluded that the virus had infected a common ancestor of these two rodents sometime before the Miocene Epoch, 5-23 million years ago, around the time the great apes arose. Furthermore, the viral genetic elements more closely resemble Ebola than Marburg, meaning the two viruses had already diverged from each other. Sometime before then, the two viruses shared a common ancestor that has not yet been identified.

This means that these viruses have been coevolving with mammals for millions and millions of years, much longer than previously believed. An understanding of the origins and evolution of filoviruses could help us better prevent outbreaks of them and hopefully even create a vaccine that would be effective against all of them.

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Viruses Up to Bat

April 11, 2014

By Medical Discovery News


In tales, bats are feared because they could be blood-sucking vampires in disguise. Obviously, Dracula isn’t real, but science has recently uncovered a dark secret that bats have been keeping: viral reservoirs.

Reservoirs are bodies of collected water. Viral reservoirs are a collection of viruses carried by one species. Bats are an important source for a variety of viruses that can infect other animals and humans, such as deadly viruses SARS, Ebola, and MERS.

Bats are among the most abundant and diverse vertebrates on earth and are found on every continent except Antarctica. Their ability to maintain viruses may date back to ancient times. Viruses can cause persistent infections in bats or they can lay dormant. Since bats also have relatively long lifespans – up to 25 years – if they have a persistent virus they have a good chance of infecting others with it, especially since they can fly and travel long distances. Bats also live in close-knit communities, so they are likely to pass infections to other bats, thereby maintaining viruses in the population. Some viruses spread by direct contact, while others such as rabies can be spread by droplets of saliva, mucus, urine, or feces. 

While scientists have known for a while that bats are a source of the rabies virus, they have recently isolated almost 70 other viruses from bats. Most of these only infect fellow bats, not other animals or people, but they do carry some dangerous human pathogens like Japanese Encephalitis, Chikungunya, Rift Valley Fever, Nipah viruses, and Hendra viruses. 

Henipaviruses were first discovered as the cause of an outbreak of an acute respiratory illness in two humans and 22 horses in Hendra, a suburb of Brisbane, Australia, in 1994. The virus kills 75 percent of horses who are infected and 60 percent of people. That’s an especially deadly virus. Since then, there have been 39 Hendra outbreaks in horses, two of which spread to people. That virus was later found to be genetically related to another virus called the Nipah virus, which emerged in Malaysia in 1999. There have been nine more outbreaks of the Nipah virus since then, killing almost half of the people infected. Bats were probably responsible for many of these outbreaks.

In the latest study, 42 percent of the 2,000 Straw-coloured Fruit Bats from 12 African countries harbored Henipaviruses. About one-third of the bats also carried a rabies-like virus called Lagos bat virus. Since Henipaviruses can be easily transmitted, people living near bat populations could be at risk of infection.

Before mass bat hunts begin, it’s important to know that bats play essential roles in the ecosystem and cannot be eliminated without drastic consequences. Therefore people need to be cautious and vigilant about potential exposure to bat viruses. Ongoing research will hopefully create new antiviral vaccines that protect people. As humans continue to invade wildlife areas, so will the possibilities of contracting new viral infections. 

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