Episode three: malaria, tuberculosis, and HIV/AIDS

I’m getting my lectures and other materials together, so I thought that it might be nice to tell you a little about the focus of the course I will be teaching, which is called “The Biology and Public Health of Malaria, Tuberculosis, and HIV/AIDS.”

Why those three infectious diseases?  I chose them because they remain significant causes of mortality despite the fact that every single one is preventable, and two of them can be cured.  I also chose them because social categories such as gender, race, and socioeconomic status have strong effects on both the risk of contracting one of these illnesses and also the risk of dying as a result. Additionally I’m a microbiologist and so I chose them for teaching purposes, because one of them is caused by a bacterium (Mycobacterium tuberculosis), another by a virus (Human Immunodeficiency Virus), and the third by a eukaryotic parasite (Plasmodium species).

It is also very interesting that despite the importance of these infections and concentrated research effort, there is no immunization series that can effectively protect children or adults from any of these agents.  (Some studies indicate that an immunization called BCG can protect young children from some forms of severe tuberculosis early in their lives, but this protection does not protect immunized people later in life and can complicate diagnosis of tuberculosis, so it is not used in low-prevalence countries such as the U.S.)  Unfortunately, these three infectious agents have taught us how little we understand the human immune system, which has more connections among cells than are found in the whole nervous system!  Even if we had effective immunizations, we might not have the political will to build the human and physical infrastructure needed to distribute and administer the immunizations where they are most needed.  Certainly the infrastructure to store and administer affordable antimicrobials is not in place, even though those antimicrobials exist.

I also plan to use these diseases to teach about evolution, because all three infectious agents have evolved in response to our attempts to treat them, and in the case of malaria and tuberculosis, we humans have evolved in response to their attempts to infect us.  

The World Health Organization likes to explain the causes of mortality by scaling everyone who died in a given year to 1,000 people.  For the most recent available data (2004), the top ten causes of mortality in low income countries were:

1.     Lower respiratory disease (112/1000)

2.     Coronary heart disease (94/1000)

3.     Diarrhoeal disease (69/1000)

4.     HIV/AIDS (57/1000)

5.     Stroke (56/1000)

6.     Chronic obstructive pulmonary disease (36/1000)

7.     Tuberculosis (35/1000)

8.     Neonatal infections (34/1000)

9.     Malaria (33/1000)

10. Prematurity and low birth weight (32/1000)

Numbers 1, 3, 4, 7, 8, and 9 are directly caused by infectious disease, while infectious disease in a mother can contribute substantially to number 10, too.

For the world considered as a whole, the list looks like this:

1.     Coronary heart disease  (122/1000)

2.     Stroke (97/1000)

3.     Lower respiratory infections (71/1000)

4.     Chronic obstructive pulmonary disease (51/1000)

5.     Diarrhoeal diseases (37/1000)

6.     HIV/AIDS (35/1000)

7.     Tuberculosis (25/1000)

8.     Trachea, bronchus, lung cancers (23/1000)

9.     Road traffic accidents (22/1000)

10. Prematurity and low birth weight (20/1000)

Even though malaria isn’t listed, it is a significant contributor to #10.   Malaria is also a major downer for economic prosperity – people so sick they can’t get out of bed don’t generate as much income or wealth as healthy people.

In Botswana, which by some measures is a middle-income country, HIV/AIDS is the #1 cause of over-all mortality, while tuberculosis is #4.  For children under 5 in Botswana, HIV/AIDS  is the #2 cause of death, while malaria is #4.  Tuberculosis is the most common “opportunistic infection” affecting HIV+ people worldwide, so there is a connection between deaths from TB and HIV/AIDS, with some difficulty in separating the two in some cases.  It is exciting that the over-all crude death rate in Botswana has fallen substantially from about 32/1000 in 2003 to about 8.5/1000, very close to the U.S. death rate.  This is due in no small part to Botswana’s enviable medical infrastructure and the availability of anti-HIV medications (antiretrovirals, or ARVs) and antibiotics to treat tuberculosis.  Check out this animation showing the data regarding the rise, fall, and rise of health in Botswana.  Public health measures WORK!!

Infectious disease remains an important cause of death worldwide, and is likely both caused by social inequality and a contributor to the perpetuation of that inequality.   People interested in the impacts of infectious disease on development might want to check out gapminder.org and look at under-5 child mortality as one of the variables.  There is also a great visual summary of HIV infection data on that same site.

Lest I end with all gloom and doom, it is worth pointing out that this has been a year with lots of good news on the infectious disease front, too, so stay tuned.  And cheers to all the microbiologists out there devoting their lives to making the world a safer place.