I looked, and there before me was a pale horse! Its rider was named Death, and Hades was following close behind him. They were given power over a fourth of the earth to kill by sword, famine and plague, and by the wild beasts of the earth. Revelations 6:7
The Prevalence of Disease Infectious disease – the fourth horseman of the apocalypse – is the largest source of premature deaths and suffering. In 2002, warfare accounted for 0.3% of all deaths worldwide; communicable diseases were responsible for 26%. All of the wars of the 20th Century killed an estimated 111 million combatants and civilians, an average of 1.1 million annually while communicable diseases now kill approximately 14 times as many people annually (Pirages 2005). The 1918 influenza pandemic killed 20-40 million. Clearly, understanding and combating the spread of disease is among the most serious challenges we face today. Moreover, due in part to the adaptability of pathogens, old maladies including tuberculosis, malaria and cholera persist, and previously unrecognized diseases such as Ebola, Hepatitis C and HIV have emerged as new threats.
The relationships between people and microbes have a long history that can be envisioned in distinct stages. Domestication of animals some 10,000 years ago brought people and animals in closer contact. Then greater contact among centers of civilization, beginning about 2500 years ago, facilitated the swapping of diseases and acceleration of their spread. Transoceanic exploration after the 14th and 15th Centuries led to greater exchange – the bubonic plague reaches Europe from Asia, and Europeans brought small pox, measles and influenza to the New World. The origin of syphilis is unknown, but it may have been carried to the New World by Christopher Columbus’ crew, and a less popular theory has its origin in the New World, and brought back to Europe after 1492. Modern transportation and globalization represent an advanced form of this process, in which disease outbreaks potentially can circle the globe in a matter of days, carried by infected passengers or by other organisms on jet planes.
Many factors influence the roles that disease plays in our lives and societies. Infectious diseases strike the underfed, and chronic diseases the overfed. In Africa, infectious and parasitic diseases are responsible for about half of all deaths, vs. 2% in Europe. Moreover, little investment is made in the development of drugs for diseases that largely strike poor people in developing countries. Neither governments nor the private sector ate doing well in this regard, although this void is being partly filled by NGOs and private donors. At present, more than 2.3 million people die each year, primarily in poor countries, from eight diseases that could easily be prevented by vaccination (Pirages 2005). The top killers include respiratory infections, HIV/AIDS, Diarrhea, Tuberculosis and Malaria. Non-communicable, chronic diseases cause more deaths worldwide than communicable diseases, but the latter are far more avoidable.
The Ecology of Infectious Diseases What are infectious diseases, and what do they have to do with ecology? Quite a lot, although we should first note that in this lecture we will focus particularly on diseases that are linked to ecology and biodiversity, as this emphasis ties in with the general theme of our course. Infectious diseases are illnesses caused by a disease agent that can be transmitted from organism to organism. Diseases can take on many forms but are generally characterized by the weakening of the infected organism, the degree of which depends on the organism’s resistance to the infection and the virulence of the disease agent causing it.
The agents of disease fall into two major types. Microparasites include viruses, bacterial pathogens and parasitic protozoans, and cause what we refer to as pathogenic diseases. Examples include ebola hemorrhagic fever (a virus), Lyme disease (a bacterium), and malaria (a protozoan). Macroparasites include flatworms (Platyhelminthes), roundworms (Nematoda) and arthropods; are generally larger and have longer generation times than microparasites; and we refer to these as parasitic diseases. An example of a parasitic disease is schistosomiasis, caused by aquatic trematodes. Regardless, infectious diseases are caused by living organisms – a biotic interaction.
There are two modes of disease transmission. Direct, physical contact between hosts allows disease agents to transfer from an infected host to an uninfected one (STDs, flu). Indirect transmission is facilitated by an intermediate host or a vector (malaria, Lyme, West Nile Virus, dengue, schistosomiasis). Vectors transfer a microparasite from one host to another: mosquitoes transmit malaria, dengue, WNV; ticks transmit Lyme disease and spotted fevers; the Kissing bug transmits Chagas disease.
The role of ecology is already partly apparent, since ecological factors could allow vectors to become abundant, or suppress them, influencing disease dynamics. Ecology also can be important when we consider the host’s role. A host is simply an organism capable of becoming infected with a disease. However, it is useful to recognize three categories of hosts. A host can be a member of the same species, as when you get the flu from a friend. A host can be a member of a different species: for example, West Nile Virus (WNV) infects wild birds and horses as well as humans. Zoonotic diseases are diseases of humans in which the disease agent (pathogen) resides predominantly in a non-human animal host (reservoir) and may be transmitted among hosts (including humans) via the bite of a vector. An abundant animal reservoir can increase the likelihood that humans become infected by increasing the overall availability of infected hosts from which vectors may take a blood meal. Finally, there are parasitic diseases that require more than one host (a secondary host) to complete their life cycle. Schistosomiasis requires a snail host for one stage of its life cycle: control the snails and one controls the disease. Emerging Infectious Diseases EIDs are diseases that are, or have recently, increased in incidence, impact, pathogenicity, and geographical or host range. Some affect wildlife and are a conservation concern. Some affect humans, and are a source of considerable present alarm.
Global declines in amphibian populations along with the
appearance of bizarre deformities in frogs are perhaps one of the most
pressing and enigmatic environmental problems of the late 20th century.
While some declines are attributed to habitat destruction, others are not
associated with obvious environmental factors, and so the global amphibian
decline has been a mystery. Recently, infectious diseases have been
implicated as an agent of amphibian decline. For example, the golden toad
of Costa Rica has declined to perilously low numbers in very recent years,
and was last observed in 1989. Yet it dwelt in well-protected cloud forests in national parks. Several
diseases have been implicated, of which chytridiomycosis (“chytrid”, a
disease found in amphibians in the pet
Atelopus Varius - critically endangered (Robert Puschendorf, IUCN)
The ability of diseases to “jump” from one species to another, typically due to crowding of animals in the pet trade and in domestic rearing, and to the consumption of bushmeat, is a major cause of new, emerging diseases. Monkeypox, SARS, and HIV are examples.
Trade in exotic (and not-so-exotic) animals may expose humans to novel pathogens. The Monkeypox outbreak in United States in June 2003 was caused by pet prairie dogs, a native North American rodent, which were housed in pet stores near to African rodents that serve as natural reservoirs for the Monkeypox virus. The U.S. Government has since banned the import of African rodents by the pet trade.
In the tropical forests of central and West Africa (the Congo Basin rainforests), the greatest threat to vertebrate species is over-hunting for subsistence and commerce – the bushmeat trade. For people living in these areas, up to 90% of total animal protein may be derived from wild animals. And this carries risks. There is evidence to suggest that bushmeat consumption may be responsible for the introduction of many infectious diseases into the human population, including SARS, HIV and Ebola.
SARS is a viral respiratory illness caused by a coronavirus. First reported in Asia in February 2003, within a few months it spread to a few dozen countries in the Americas and Europe. The wild reservoir is unknown, but SARS has been detected in civets (rather like a raccoon/cat) for sale at Chinese meat markets. In response, the Chinese government instituted an eradication campaign of civets. Yet, because it is possible that rodents are the actual reservoir, this destruction of one of their natural predators is potentially counter-productive. The SARS outbreak has been contained and is not currently in the news, although it may re-appear, perhaps after laying dormant in wild populations. How it crossed over to humans in the first place is unknown, though bushmeat consumption is a likely mechanism.
Human Immunodeficiency Virus (HIV) is most closely related to Simian Immundeficiency Virus (SIV), which is widespread in primates. Genetic analyses indicate that HIV-1 originated from at least three zoonotic transmissions of SIV from chimps to humans. Almost certainly this is due to eating chimps – butchering and then consuming bushmeat. First identified from blood samples collected in Kinshasa, Congo, in 1959, the actual origin of HIV-1 in humans probably was much earlier.
So what has caused the recent emergence of HIV? Three things: increased exposure risk, increased viral transmission, and viral adaptation. Increased exposure risk likely is the consequence of the conscription of local people for work as porters and forced labor by colonial authorities. Poorly fed conscripts were no longer able to produce food through agriculture and may have increased their reliance on bushmeat for sustenance. Increased viral transmission likely was the consequence of a massive influx of rural peoples into major cities, as well as disruption of traditional social practices and networks that governed sexual behavior. Viral adaptation is thought to be associated with an inept attempt to control infectious diseases using massive, poorly-funded inoculation campaigns in which the same syringes were used to treat masses of people. By ensuring the rapid passage of a virus through a series of hosts over a relatively short period of time, it is possible to select for more virulent pathogen strains. While this explanation remains speculative, it represents current best understanding, and raises worrisome questions about future emerging diseases.
The cost in human lives, misery, and prosperity are enormous. AIDS was first identified in 1980s, and has caused more than 20 million fatalities to date. The number infected with AIDS is expected to approach 100 million by 2010. Due to its long incubation period, it is years before an infected person displays symptoms, thus increasing the likelihood of further spread. (Read that sentence again.) The SARS outbreak cost in excess of $100 billion in lost trade and tourism.
Biodiversity and Disease Dynamics
Till now, we have emphasized how infectious diseases can threaten wildlife and humans via complex and not yet well understood ecological disruptions. Next we explore how loss of biodiversity can increase the risk of disease. Under some circumstances, loss of biodiversity may increase disease incidence. This occurs when reservoir competence varies among host species and the most competent reservoir hosts tend to become community dominants with biodiversity loss. In contrast, vertebrate communities with high species diversity will contain a greater proportion of incompetent reservoir hosts that deflect blood meals away from the most competent reservoirs, thus diluting the incidence of disease and the risk of its transmission to humans. Lyme disease illustrates this pattern.
Lyme disease is a tick-borne bacterial disease prevalent in North America. The principal reservoir, the white-footed mouse, increases dramatically in abundance in fragmented habitats that contain low vertebrate diversity (due to loss of predators and competitors). Lyme prevalence increases dramatically as a result. However, in more intact community assemblages with greater biodiversity and less pronounced dominance by mice, many of the vertebrates are poor reservoirs, meaning that they are less likely to infect ticks. Thus, as disease prevalence decreases, fewer ticks are infected, and disease risk to humans is reduced. In effect, high biodiversity, when accompanied by a range of host competencies, dilutes infection risk. Unfortunately, the white-footed mouse thrives in small forest fragments created by exurban sprawl, and so the common desire to live in a wooded setting (which typically consists of small fragments of forest) creates the conditions most likely to increase Lyme disease risk.
What Lies Ahead Unfortunately, there is much to worry about. Most pathogens have short generation times and appear capable of high mutation rates and rapid evolution, which represent important factors in disease emergence. Infectious diseases are likely to pose an increasingly large threat to both wild and human populations. In a chart of human mortality over the 20th century one can barely make out WWI, WWII, Korea, Viet Nam – but the 1918 influenza pandemic stands out dramatically. That influenza outbreak caused about 5% mortality, while Ebola causes about 90% mortality. Moreover, modern air travel ensures that an outbreak today would travel extremely rapidly. We have never seen a widespread outbreak of a disease causing such high mortality, and the prospect is truly frightening. The current concern about the spread of highly virulent strains of the bird flu is justified.
Suggested Readings and Links Pirages, Dennis. 2005. Containing infectious diseases. Pp 42-59 In State of the World 2005 – Worldwatch Institute, WRI.
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trade) is believed most likely to
be responsible. Possibly it was introduced accidentally, by the release
of a pet amphibian or fish, or carried by an invading species. Termed
“pathogen pollution”, the introduction and spread of pathogens to novel
habitats appears to be an increasing yet poorly recognized form of
anthropogenic environmental change that can have severe consequences for
ecosystems, similar to those caused by introduction of other exotic
organisms.