What is the theory of coexistence in ecology

Viruses and Evolution

The announcement in the St. Galler Tagblatt (April 26, 2004) about a massive carp window sore in Lake Constance only concerned the media briefly. The longer the question of whether the individual Sars cases reported from Beijing could herald the beginning of a global epidemic. The rise in the number of cases of newly reported HIV diagnoses has also made politicians more aware of the disease that has not been defeated. The individual deaths this spring among people in Southeast Asia infected with the bird flu virus do not appear to have been repeated. Nevertheless, such reports are unsettling. We ask ourselves whether we are at the beginning of a new era of recurring epidemics or whether these are long-established phenomena. Or maybe both?


Reports of new infectious diseases trigger existential fears. Security gives us the idea that we deal with thousands upon thousands of microorganisms every day without any problems. Our skin and mucous membranes, our intestines, but also our air and the environment are covered with it. We know from experience in dealing with antibiotics that the intestinal flora is important for our health. It is possible - at least that the other microorganisms in our environment also have an important function. Seen in this way, we can also view infectious diseases as smaller "accidents" in a system in which the coexistence of different micro- and macro-organisms is the rule.


In fact, over the millions of years of our development, we have optimally adapted to the pathogens in the environment. Herpes viruses have existed for ages. They are so well established in our organism in a latent form that we do not feel any of them. A fever blister is a trivial matter that probably only serves the HSV itself for transmission. The Epstein-Barr virus is so well adapted that - if we are infected as a small child - we never feel any of this infection.


These examples are no exception. We now know that 5% of our genetic material (DNA) comes from retroviruses. This is a testimony to viruses that got into the host DNA of our stem cells during our evolution through the transcription and integration of virus RNA.


In addition to the well-adapted coexistence with microorganisms, accidents happen now and then. The emergence of HIV is a prominent example of our time. HIV evolved from a closely related virus, SIV, found in chimpanzees. The rural exodus, contact with and consumption of monkey meat, truck transports across Africa and the associated prostitution were prerequisites for the development of the epidemic. In a few thousand years, this virus will hardly cause any more problems.


Many viruses have the property that they can only multiply efficiently in one species. The adaptation to the new host, which requires a species jump, is often too great for the virus to reproduce efficiently. As a rule, the avian influenza virus cannot cause an epidemic in humans. The pig is needed as an intermediate step. The individual cases of “bird flu” in Southeast Asia last winter were an exception. The virus could be transmitted directly from birds to humans, but overall human-to-human transmission was still relatively inefficient.


Let us summarize: Viruses adapt well to their host over the centuries and can only rarely, under special conditions (or is it a coincidence?) Be transferred to another host, where they can then - as the example of HIV reminds us - cause severe damage . So-called “emerging infections” are therefore never new inventions of nature, but rather the transmission of pathogens from animals to humans. There are numerous other examples of this: Hemorrhagic viruses (Ebola, Dengue, etc.), West Nile virus, Sars, cowpox, etc. Animals are, so to speak, the “reservoir” for new infections in humans.


But not only humans are affected by such microbiological accidents. In 1999 something strange happened in the North Sea. Sick seals were washed ashore en masse. A viral infection was suspected. After a few months it could be proven that the animals were infected with influenza B. Influenza B is a virus that is only known in humans. The phylogenetic analysis showed that the seals must have been infected by virus strains from humans. 50% of the seals in the North Sea had died within a short time of a disease that was new to this species, but hardly ever caused problems for humans.


What promotes the transfer of viral infections from one species to another? In the case of the influenza B epidemic of North Sea seals, an increase in seal populations in the North Sea was suspected. More modern theories assume that population density alone is responsible for triggering an epidemic. According to this theory, the species-specific infectious agents play an important role in the regulation of populations. If a species has a great survival advantage, its individuals live together in ever smaller spaces. The chance that a pathogen can be transmitted increases. This makes the species more susceptible to infection. Proponents of this theory also assume that the world population has only increased so rapidly over the past 400 years through the influence of hygiene measures and vaccinations. According to this theory, it was only the prevention of autoregulation by infectious agents that could throw the world population, which has been stable for thousands of years, out of balance. At least plausible if you look at the development of the world population.


Back to the carp window sill this spring. In the daily newspaper of April 26th, 2004 it is stated that the animals are sick with a virus infection. We read a little further: "Last summer, the carp in particular multiplied rapidly and developed in a previously unknown manner." Perhaps it's just a completely normal regulatory phenomenon in nature. Can humanity still escape this microbial population regulation? Presumably, new infectious diseases will occupy us frequently in the next few years and decades!



PD Dr. med. Pietro Vernazza, Head of Department, Infectious Diseases / Hospital Hygiene Department, St. Gallen Cantonal Hospital.