3 Viruses Go "Viral"

Reading 2

A Tale of Two Epidemics

In this reading students explore two epidemics – Severe Acute Respiratory Syndrome (SARS), spread by direct contact and the recent epidemic of Zika virus that is transmitted by a mosquito vector and implicated in microcephaly in newborns and in paralysis in adults. Students consider how the mode of transmission and other factors affect the emergence of an infectious disease and its escalation from a localized outbreak to an epidemic.

Students compare the two epidemics and use information about the biology and ecology of the viruses to propose ways the epidemics could be halted. At the end of the reading students revisit the idea that diseases that are transmitted by direct or indirect contact with infected individuals are considered contagious and are introduced to the mathematical construct, R0, that enables an estimation of the degree to which viruses or bacteria are contagious. Students read more about R0 in Reading 3 and work with it in the remainder of the module.

The following tales of two viral epidemics demonstrate how localized outbreaks can escalate to epidemics by very different modes of transmission — one involving direct or indirect contact with infected individuals and the other carried by a mosquito vector. By understanding both the mode of transmission and the ecology of the virus, interventions can be developed that could stop the spread of the disease. As you read keep track in a chart or table of the characteristics Zika and SARS viruses have in common and how they differ.

The Story of SARS

The story of the 2003 epidemic of Severe Acute Respiratory Syndrome (SARS) that spread to 29 countries from a small village in China provides insight into how a disease that should have remained a local outbreak can suddenly and rapidly escalate into a deadly epidemic. Spread through the air in respiratory droplets from coughing or sneezing of infected individuals, the virus is highly contagious. In a five month period the epidemic resulted in 8400 cases and more than 800 deaths, including many healthy young adults. Quarantining thousands of people in their homes averted a greater worldwide pandemic.

Where did this epidemic begin? Using the tools of epidemiology (see Module 2 –Seeking the Cause) scientists tracked the origins of the disease to a food market in Guangdong province in China where food vendors keep small animals in close quarters prior to being sold and slaughtered. Evidence indicated that a palm civet (see Figure 1) acquired the virus from bats, the virus’ reservoir animal (an animal in which the virus can live and reproduce, often without injury to the animal).

palm tree civet
Figure 1: A palm civet. From tnresources.org

In turn, a waitress working in a restaurant specializing in a palm civet dish picked up the virus during preparation or serving of the meal. The waitress, now Patient Zero, infected several patrons of the restaurant who infected the doctor who treated them. The doctor then traveled from China to Hong Kong where, through frequent coughing and expulsion of infected respiratory droplets, he spread the virus around the hotel where he was staying, infecting many individuals. These individuals then boarded airplanes, and infected fellow travelers and people at their destinations. From bat to palm civet to waitress to doctor to individuals around the world, an epidemic was born (see Figure 2).

chain of contagion
Figure 2: The chain of contagion that created the SARS epidemic. Click to enlarge.

The SARS epidemic is an example of a virus moving from one species to another - in this case from bat to palm civet to humans. Mutations in the genetic material of organisms occur continually in the lifetime of the organism. Most of the mutations are silent; that is, they cause no observable change in the traits of the organism. But certain changes in the genomic sequence of an organism may cause a change in protein structure that can alter a trait. Changes in the sequence of the genetic material of a virus may enable the virus to infect a new species.

What was remarkable about the SARS virus is the speed at which it spread globally once it entered its new host, humans. Prior to airplanes, SARS might have remained local within the Guangdong province but air travel has enabled infected individuals to move around the world quickly, spreading the disease as they go.

The Story of Zika

In 2015 Brazil saw a dramatic increase in the number of babies born with microcephaly, a condition in which a baby’s head is much smaller than normal due to abnormal brain development while in the womb, often resulting in developmental delays later in life. Normally microcephaly is a relatively rare occurrence usually caused by exposure of a pregnant woman to drugs, alcohol or environmental toxins, but nearly 3,000 cases were reported in Brazil, about 20 times the number of cases reported in the prior year 2014.

Initial epidemiological investigations implicated an infection during pregnancy with the Zika virus as the cause. Prior to the microcephaly epidemic, Zika virus was a relatively unknown pathogen. Transmitted through the bite of an infected female mosquito Aedes aegypti, (see Figure 3) Zika virus normally causes few or no symptoms in children and adults, although in rare instances it can result in an autoimmune disease, Guillain-Barre syndrome, in which the body’s immune system attacks part of the nervous system leading to muscle weakness and paralysis.

the mosquito known as aedes aegypti
Figure 3: Aedes aegypti taking a blood meal

The past several decades have seen an alarming increase in numbers of Aedes aegypti, which also carries the viruses for yellow fever, dengue, and chikungunya, debilitating diseases affecting millions of people worldwide. This enormous increase in numbers of mosquitoes most likely reflects the explosion of human created trash that litters the world. Needing only small amounts of water that puddle in abandon tires, non-biodegradable containers, and plastic bags to lay their eggs, the mosquito has evolved to live in close quarters with humans, especially in poorer regions of the world where trash removal, mosquito netting, air conditioning, and insect control are limited or non-existent.

Zika virus has been observed by virus hunters making slow progress from the Zika Forest in Uganda to Indonesia to Micronesia and the Polynesian Islands mirroring the movement of populations across the Pacific (see Figure 4). Until 2013, Zika virus seemed mostly innocuous, and not worth worrying about. Then in 2015 it was clear that Zika virus was becoming a very serious concern.

The virus is believed to have arrived in the blood of infected travelers attending the 2014 World Soccer Cup. Because most Zika infections show few or no symptoms, these travelers may not even have realized they were infected. The combination of many individuals carrying the virus in their blood coming in contact with and being bitten by the mosquito vector present in great numbers resulted in an unprecedented rise in Zika infections and the terrible consequences for unborn babies. It is not been determined whether Zika has a non-human animal reservoir.

zika global spread
The global spread of the Zika virus. Click to enlarge.

The rapid spread of Zika throughout Central and South America (see Figure 5) may also be the result of a change in the virus itself. Mutations may have altered the virus enabling it to infect humans more easily or increasing the number of Zika virus particles in human blood. Either of these changes could increase the rate of Zika spread by increasing the chances of a mosquito picking up the virus from an infected individual and transmitting it to another person.

zika infection areas
Areas of major Zika infections as of February 2016. Click to enlarge. Source: Centers for Disease Control
Factors Effecting the Emergence of Infectious Disease

SARS and Zika are examples of infectious diseases that emerged seemingly from nowhere to become major debilitating epidemics. What factors determine whether a virus will enter the human population to cause an emerging disease or will stay obscure and unknown?

Many emerging diseases arise when a virus moves from an animal reservoir to humans. This movement from reservoir to human can occur through direct contact with the animal, as seen with Ebola and SARS viruses, or indirectly through an insect vector, as observed with Zika virus. Situations such as changes in agricultural practices that bring humans in closer contact with animal reservoirs and insect vectors increase the chances that a new disease will emerge.

Changes in a virus’ ability to infect humans can also initiate outbreaks and epidemics of new diseases (emerging diseases) and diseases once considered to be eliminated (re-emerging diseases) such as antibiotic-resistant tuberculosis.

Factors Affecting the Spread of Infectious Disease

Below are three factors that affect the spread of epidemics.

Mode of transmission is an important factor in determining whether a disease will spread. Diseases that are considered contagious, requiring direct or indirect contact with an infected individual, will spread when people live in close contact with each other, when they have practices that bring them in contact with bodily fluids or respiratory droplets of an infected individual, and when rapid travel around the world is common place, enabling infected individuals to come in contact with many people, often before the individual displays symptoms of the disease.

Diseases that are spread by insect vectors such as mosquitoes or ticks may be harder to “catch” but can spread readily if the vectors are abundant and are found in close proximity to humans.

Practices resulting in increases in the density of vectors, such as careless disposal of tires and plastic as seen with Zika, can bring new diseases to a population. Poor sanitation and sloppy food preparation can initiate an epidemic as seen with cholera and outbreaks of salmonella food poisoning.

Predicting Epidemics

Is it possible to predict whether an epidemic will occur and how bad it will be? In many cases mathematical models can help give an idea about whether an epidemic of the disease will occur. For contagious diseases, a mathematical construct called R0 (written R-nought and pronounced R-not) can provide some indication of how extensively an infectious disease will spread through a population under certain conditions. You will be exploring R0 further in Activity 2.