Today’s chapter, is going to be on new and reemerging infectious diseases. Let’s again try to define these terms. Infectious diseases with an incidence in humans that have increased in the past thirty years or threaten to increase in the near future we tend to term new and reemerging infections. Now new infectious diseases that have come about in that, roughly in that time period would include HIV/AIDS, SARS, Lyme disease, Nipah virus, influenza H5N1 or H1N1. Infectious diseases are however often times emerge in new places and they are sometimes called reemerging diseases. This would be diseases such as West Nile virus and monkey pox. Then there are diseases that reemerge in drug resistant forms. A particularly worrisome group which includes malaria, multiple drug resistant tuberculosis, and some bacterial pneumonias and sexually transmitted diseases. Now there are a number of factors that have contributed to the arising of new and reemerging infectious diseases. These might include increased population density, inadequate infrastructures for water and sanitation, movements of people through travel and social disruption, the centralized production of food and it’s distribution, environmental changes, misuse and overuse of antibiotics and other drugs, changes in human behavior and sometimes dysfunctional governments. Let’s look at some of these in greater detail. In terms of the increase in population it was estimated that the population in 2000 was about 6.1 billion people and by 2050 this will come close…become close to doubling to 9.4 or 11.2 billion people. We’re also seeing a tremendous increase in urbanization going from 47% in 2000 to 60% to 65% increasing urbanization of the global population. Now as the population of the world increases of course there are going to be greater contacts between humans and wildlife in habitats that normally we don’t venture into and because of this interaction with both humans and our animals, we may see the rise of new viruses. The coverage of water supply, particularly in least developed countries, is also very problematic.
Particularly in both urban and rural areas in least developed countries the percentage that have access to potable water is anywhere from 50% to 60% Most of the drawing of water in rural areas and in urban areas is work of women. That is, they are the ones who are doing all the work of getting it from wells, from rivers and so on and taking that water back to their home. This is assuming that there is not an indoor water tap. And in this, the bringing of water into urban areas through trucks and so on where people are then forced to come to this area, collect the water, take it back to their home, is another way of distributing but it’s also very problematic as that source could be contaminated as well as the buckets and vessels which carry the water back to the home could also be contaminated. It also limits the amount of water that’s oftentimes available. The disposal of human waste is an increasing problem. The privies are directly linked to a body of water which connects to other parts of the city and that…that fecal disposal is of course quite unsatisfactory. We are increasingly discovering that open defecation may well be linked to under nutrition in children where this is a broad problem. The increase in travel both internationally and locally also provides opportunities for diseases to move about. We can move Ebola from one country into another in a matter of a few hours by air and we have seen this…this happen. No place in the world is unconnected to any other. We are really truly a global village. The production of food on an industrial basis has allowed for increasing availability of food, both grains and meat, but has also intensified the use of antibiotics and the raising of animals and where a central food supply is contaminated that contaminated food then travels far and wide. It’s not the same as getting milk from your local farmer or buying vegetables at the local farmers market. When we buy vegetables and fruits in many of our large cities of the world we have no idea where this comes from. Here’s the production of chickens in an industrialized production where thousands upon thousands of chickens are grown together in oftentimes extremely inhumane circumstances where they have no movement and so on and one can see that where a disease would enter this population it would destroy many, many chickens in a short period of time. This is of course quite different than what takes place oftentimes in local areas where there may be one purveyor of chickens from a few individuals in the community. Where ecology is involved is of course in climate changes, where temperatures have increased in certain areas which have allowed for the introduction of mosquitoes for example that had not been there in the past.
The tires are often traded internationally but when one trades in tires, there’s oftentimes water that is in the bottom of the tire and those…that water will you don’t need much to set up a breeding ground for mosquitoes. Those mosquitoes can then carry the viruses from where they came from to where they are going. And then the misuse of and overuse of antibiotics has led to the development of antibiotic resistance. We have only had antibiotics really for about 70 years. A blink in the eye in terms of evolution and yet we have managed to create a number of organisms that are extremely difficult to treat and far more expensive to treat. For example, multiple drug resistant tuberculosis is much more expensive to treat than is regular tuberculosis. Artemisinin which is the last, the latest drug we have to treat malaria, the last in the line, resistance is developing to that even though we treat these…malaria now with three drugs to try to avoid resistance to artemisinin. Tetracycline for cholera, multiple drug resistant gonorrhea which is a problem and in our hospitals, multiple drug resistant staphylococci which can infect wounds and cause devastating diseases. I would now like to focus on one particular new disease that has taken the world by storm over the last 30 plus years and that’s HIV which leads to AIDS. One might ask, where did HIV come from? Now is it a….. It’s defined as a new disease. It most likely came from somewhere in sub-Saharan Africa. There is a, an infection in some of the great apes, simian immune deficiency virus. It might well have been that someone in the forest or in a market was skinning a, a chimpanzee or a related species and cut themselves with a knife that carried the blood of this animal and therefore infected themselves. And unfortunately this virus adapted itself to humans. It then spread from that person through sex most likely, but it could have been through other means to another person and so on, until there was a critical mass of individuals who were infected and then the epidemic gradually increased. And then of course spread globally over time. We don’t know the initial events but it would seem to be that this was a zoonosis where the organism adapted itself very effectively to human beings. The distribution of HIV even though it came upon the world within probably five or six years it had been introduced everywhere, you can see that the distribution is of quite variable with most cases occurring in sub-Saharan Africa and if one looks at the population to case ratio, it’s even higher in that part of the world. Which raises an interesting question: why though HIV appeared in the world in many parts of the world around the same time, why has the spread been heterogeneous, not only globally, but even with the epidemic in sub-Saharan Africa? And I’d like to explore those ideas for just a moment to look at why this might happen and what does it tell us about prevention of these conditions? Now the spread of HIV in sub-Saharan Africa between 1984 and 1999 was quite interesting because it in 1984 it was fairly evenly distributed. By 1999 this had become a disease much more in the very southern parts of Africa and eastern Africa and West Africa in some parts have remained somewhat unchanged, Senegal, Mauritania, Niger, and so on. The dark red represents the highest prevalence of HIV. Now there are certain risk factors, I’ll call these proximal determinants that are associated with HIV/AIDS, the getting of…the obtaining of this infection. Clearly transfusion with contaminated bloods or contaminated needles and syringes which is like a mini-transfusion. Unprotected sex with a single or particularly multiple partners can increase your risk substantially. Mother to child transmission through delivery and breast feeding and interestingly the non-circumcision of men in certain parts of the world. Let me look at that last issue. Male circumcision interestingly enough is probably one of the oldest forms of surgery going back to at least 2200, 2300 years before the common era, as seen in this hieroglyphic from ancient Egypt. The circumcision rates in different countries in sub-Saharan Africa you can see are quite different with the highest level of…of AIDS in those countries which have the lowest level of male circumcision. Now why…why should male circumcision protect one? Well, it turns out that the inner lining of the foreskin contains cells which have a particular avidity for the HIV virus as well as the human papillomavirus which is associated with cervical cancer. Also when one removes the foreskin, the skin underneath hardens a bit and is more impenetrable probably to the virus itself. So there are biological reasons for why this particular simple operation reduces risks. We should remember that the initiation or the institution of…of male circumcision which by the way has nothing to do with female circumcision, was related to tribal or religious identity. It was not initiated initially to as any type of health measure. Certainly we didn’t know about these issues hundreds of years ago when these practices were introduced. Now distal determinants, that is those factors that affect the proximal determinants would be socioeconomic conditions, domestic violence, the status of women, the degree and type of migrant labor, sexual practices and values as defined by culture, and we have to be very careful that we don’t associate practices and values with any kind of ethical or moral character of the individuals involved. They have their antecedents in other…in other issues. And then concurrent partnerships. This is having many partners over a week as they have…having those many partners serially over the course of a year. Now what strategies then can we use based on these risk factors to reduce the possible risk for HIV? Well, if we delay the age of sexual debut that is when adolescents begin to have sex, we clearly reduce the time when they’re going to be exposed as we would be reducing the number and the exposure to high risk partners. If we reduce the degree of concurrency that would also reduce risk. If we limit alcohol and drug use prior to sex we make sure that the inhibitions to using condoms for example are not taken away. We can increase the level of male circumcision where the rates of HIV are very high and this would be again very much in southern Africa but not in…and East Africa but not in many other parts of the world. We could presumptively treat some sexually transmitted diseases to reduce transmission and certainly test the blood supply and needle exchange and syringes for I.V. drug users. One could also treat individuals who are HIV positive and thereby limit the transmission of this to their partners. This is problematic however and is not as easy to implement. If one looks at this schema of the HIV infection, one will notice that the first few weeks, months is when we have the highest level of viral particles in our blood, viremia. It is during that period of time when one is most likely to transmit the infection. So if one has a number of partners during that time, has sex shortly after one is infected, you are more likely to transmit the infection than when you enter the asymptomatic period which can go on for a number of years. Now we can measure the viral load in the blood and we can also look at other associated factors such as the CD4 count but it’s those first few weeks, months maybe, that we are most likely to transmit the infection because we don’t know we’re infected. There is no way of telling other than to do frequent HIV tests which is somewhat impractical. We can also reduce the risk of infection post exposure. That is, if someone is exposed, we can give nevirapine for the transmission, to reduce the transmission of mother to child. We can treat with ARVs as I noted post exposure or even give it prophylactically especially to high risk groups, people who have multiple partners for example. The use of vaginal microbicides has also been recommended but these are the factors that you can do post exposure. Clearly it’s far more practical if we can prevent the infection to begin with. Well, what I’ve tried to do in the…in this brief chapter is to look at new and reemerging infectious diseases. Why they occur, what are the risk factors in the modern day world. I then focused on HIV/AIDS as an example of a new and reemerging disease.