Evolutionary Medicine

Background Evolutionary Medicine

Immunity

Immunity is the ability to resist infectious disease. There are two ways in which a person can become immune to a disease. First, there is innate immunity, which is the natural way the human body protects itself. The human body has anatomical barriers, such as the skin and mucus membranes. These are the first lines of defense to keep an infectious disease microbe out of the human body. The mucus membranes are found in any cavity that leads to the outside of the body. The cells that make up the mucus membranes produce mucus, which traps foreign substances that enter the body. Another form of innate immunity is endocytosis. When a foreign invader enters the body, a specialized human cell will surround the invader, ingest it and then dissolve it. If a foreign substance penetrates the human body, an immune response ensues. Defenders rush to the area of penetration. The influx of defenders results in inflammation, which is an attempt to destroy the invader. The last type of innate defense is antimicrobial protein. Macrophages or cells roam throughout our body looking for foreign protein. When such an invader is found, the macrophage transfers it to a helper T cell. The T cell in turn finds the white blood cells that can make the protein, or antibody, that attaches to the specific foreign protein, or antigen. The bonded antibody and antigen are on the surface of an invader such as bacteria. This process not only impairs the invader, but also marks it for attack. ⁴

Acquired immunity results when an infectious disease enters the body and the human body produces antibodies. These antibodies can last for decades or forever. Over time the body builds up specialized B cells that fight infectious disease. If an infectious disease that has previously invaded the body invades again, the B cells that have been created and stored will immediately respond and attack. This process is called immune memory.

Population Size

The size of a host population is important in relation to the spread of an infectious disease. For some common Old World diseases such as, influenza, typhoid, measles, and smallpox, a certain population density was necessary for the disease to spread and be sustained. For example, if an infectious disease spreads and over time, everyone becomes infected, some people will die while the rest of the population will recover and develop antibodies to the disease. The disease will die out when no one is left alive who is susceptible.

Disease load directly correlates to population size. There are three stages: hunter-gatherer, agricultural, and urban. With each stage humans become more vulnerable to a group of diseases, but the disease load is cumulative. For example, in the agricultural society the population would be susceptible to the diseases of the hunter-gatherer stage in addition to the diseases of the agricultural stage. Hunter-gatherer populations can get infectious diseases, but due to their nomadic lifestyle, hunter-gatherer cultures face a more limited disease load. In this kind of society, the group lives in one area for a short period of time, so they are not living in dirty conditions and amongst waste. However, they are still vulnerable and microbes that live on animals and in the soil will infect the people of this most primitive lifestyle at various times. ⁵

As people move into agrarian societies and are no longer roaming the Earth foraging for food, the population expands. "Agriculture sustains much higher human population densities than does the hunting-gathering lifestyle on the average, 10 to 100 times higher." ⁶ Agricultural societies find themselves not only vulnerable to the diseases of hunter-gatherer societies, but also to a gamut of diseases that are associated with domesticated animals and a sedentary lifestyle. Farming communities become surrounded by disease carrying rodents seeking food stores and also by their own waste. The fecal matter can get into the water supply and is often used as fertilizer being spread by farmhands.

The disease load once again increases as people develop cities. The sanitation conditions can once again deteriorate as crowds of people live together. Infectious disease spreads easily when large numbers of people are living together under dirty conditions. People living in urban areas are vulnerable to the microbes that are found in soil and on animals in addition to the infectious diseases that become prevalent and are easily spread in urban environments.

Invasive and Native Species

Another basic principle of biology, which relates to evolutionary medicine is the dynamic that exists between invasive and native species. Native species are organisms that are indigenous to an area. Invasive species, as the name suggests, are organisms that invade the geographical area of a native. Invasive species can grow and spread rapidly. Over time, invasive species may take over the native species. Today, most of our weeds and hay grasses originated in Eurasia. Many of these species were indigenous to Eurasia. The greatest danger of invasive species is the fact that they have no known predators and they disrupt ecosystems. They also can carry many diseases that native species have never encountered.

The plight of the American chestnut tree is a great example of an invasive species introducing a new disease. This tree was not only an important natural resource used by Native Americans, but also a valuable resource for other cultures when making furniture, fence posts and train rails. The Chinese chestnut species was introduced in New York. Along with this tree came a fungus hidden in its bark. The first American chestnut tree died in 1904. The fungus spread its spores with the help of wind and woodpeckers over one thousand miles to the Southern Appalachians and by 1940 the American chestnut was almost wiped out completely. ⁷ The immigration and migration of living species that has been going on for years is stirring the pot and spreading microbes around the world. This movement of species is creating a more homogenous global environment with less and less biodiversity. ⁸

Typhoid Fever

Typhoid fever is a disease of humans caused by the bacteria Salmonella typhi and is spread through the contamination of food or water. S typhi only lives in humans in the intestinal tract or in the bloodstream. The first symptoms of typhoid fever include severe headache, body aches, weakness and constipation. Later, a fever and some red splotches will appear usually around the abdomen. The stomach becomes tender and the spleen may swell. Hair loss and bronchitis are other common symptoms. The bacteria get into the bowels and are excreted with bowel movements. In about 25% of the people the bacteria also appears in their urine. ⁹

In some cases, the bacteria may be able to reside in the gallbladder of a host, enabling such a person to be an asymptomatic carrier of the disease. ¹⁰ This was the case with Mary Mallon, who became known as "Typhoid Mary". In the early 1900's, Mary Mallon cooked for several families in New York City. She was a carrier and was determined to be the source of the outbreaks. Typhoid Mary was quarantined for the rest of her life since she refused to stop cooking and repeatedly ended up killing her employers and members of their families.

Today, the industrialized parts of the world rarely encounter typhoid fever. The modern methods of treating drinking water and waste have greatly reduced the contamination cycle. Unfortunately, third world and developing countries continue to face this disease.

Smallpox

Variola virus causes the human disease smallpox, and it has affected human populations for thousands of years. Humans are the only natural host for smallpox. Fortunately, smallpox was eradicated from the United States in 1949, and the last naturally occurring case in the world appeared in 1977 in Somalia. ¹¹

Smallpox has a higher mortality rate and spreads more easily in large population densities. But, smallpox is also sustainable in small, densely populated rural populations. Smallpox can be contagious when the infected person begins to exhibit symptoms. However, it is most contagious when the rash is present. The contagious stage lasts until the last scab falls off, which could be three weeks. There are three ways the virus is transmitted: 1) through infected droplets in the respiratory tract 2) from the pus in a sore or 3) from dried scabs. The scabs and sores can contaminate clothing and bedding. Touching these items can make the virus airborne, but the virus cannot travel long distances without a host. ¹²

The most obvious symptom of smallpox is the skin rash with lesions that hold pus. The virus enters the body through the respiratory system, but quickly travels to the lymph nodes and then to the blood. From the blood it has access to the internal organs. The virus then invades the blood for a second time and the infected person develops a high fever, a headache, stomach and muscle pains and vomits.

Measles

The strain of the measles virus that affects humans is generally mild. However, when a population has no prior exposure to the disease measles it is much more dangerous. This reveals that historically the virus used to be more serious to humans. A virus called rubeola causes measles. This disease begins in the upper respiratory system and its symptoms include a fever, runny nose and cough. Eventually, the immune response to this virus causes a skin rash. ¹³

Diphtheria

Diphtheria has been around at least since the time of Hippocrates, who described the disease in 4 ^th century B.C. This disease threatened the colonies at various times. From 1735-1740 an epidemic spread throughout New England. It mostly affected children. This infectious disease is caused by a bacterium and affects the upper respiratory system. Symptoms include a sore throat, low fever, and an extra membrane forms over lesions that develop in the upper respiratory tract. The growth of the extra membrane may cause swallowing problems and/or suffocation. The lesions produce a toxin that spreads through the lymph system and the blood to other organs. ¹⁴ Once the toxin spreads throughout the body, it may cause paralysis and heart failure.

Figure 1 ¹⁵

Yellow Fever

Yellow Fever is an arbovirus, or a type of virus carried by biting arthropods, which infects both non-human and human primates. Several types of mosquitoes carry

yellow fever and there are three types of transmission: sylvatic, intermediate, and urban. The sylvatic transmission cycle occurs when humans encroach into the jungle. In this type of transmission mosquitoes usually carry yellow fever from infected monkeys to humans. Intermediate transmission usually occurs when humans live or work near the jungle. In this cycle, yellow fever is transmitted via mosquitoes to humans either from monkeys or from humans. In urban transmission, the virus is spread from human to human through a mosquito vector. ¹⁶ The Aedes aegypti mosquitoes are best able to live in villages, therefore transmitting the virus from one infected person to another. ¹⁷

The virus originated in West Africa, but during the slave trade the virus made it across the ocean to the New World. A mosquito carrying the virus could not survive the entire journey to the Americas, but given a food supply and water supply, the mosquito could survive long enough to lay eggs in the water containers onboard the ship. When these mosquitoes hatched they too carried yellow fever. This cycle enabled yellow fever to cross the Atlantic Ocean to the Caribbean Islands. The virus established itself in the warm climate of the islands. Later, it eventually reached port cities in North America. Since the seasons change in North America, the mosquitoes that transmit the disease could only survive during the warmer months. This meant that an outbreak of yellow fever could only strike during the warmer months and could only last until the first frost killed off the mosquitoes.