Case Studies and Project Ideas: Tuberculosis

Disease Background:

Tuberculosis has been one of the most prevalent infectious diseases in the world. It is very contagious and for that reason can result in huge epidemics. Some of the earliest human skeletons found have had hunchbacked spines--one of the results of tuberculosis--and ancient Egyptian mummies have often been unearthed and diagnosed as having suffered from some form of tuberculosis infection. Tuberculosis is also commonly known as consumption, because it usually results in the body literally shriveling up and wasting away.

In 1882, Robert Koch demonstrated that consumption was actually caused by a tiny microorganism known as tubercle bacillus. It had been known before this time that consumption could be transmitted by swallowing or inhaling discharges from infected patients. With Koch's discovery, the science of studying the contagious effects of tuberculosis could begin, but there were still many things that needed to be known before any epidemics could be controlled.

In the nineteenth century, tuberculosis was responsible for a huge number of deaths. The disease was widespread in both Europe and North America and came to be known as the "great white plague". Even clothing reflected the prevalence of the disease. The high collars on men's shirts are thought to be a means to hide the sores that commonly develop on the necks of tuberculosis sufferers. People of this period who suffered from tuberculosis include John Keats, Henry David Thoreau, Robert Louis Stevenson, and Ralph Waldo Emerson. As late as 1911, tuberculosis was the main cause of death in the United States.

Luckily, this is no longer the case. Control of the tuberculosis virus today can be thanked to the development of a vaccination called BCG. After receiving a shot of this vaccination a person is much less likely to be infected by tuberculosis. BCG typically reduces the chance of contracting the disease by a fifth. If tuberculosis has actually developed in a patient, a chemotherapy treatment called isoniazid must usually be carried out with varying degrees of success.

Epidemiology:

Modeling tuberculosis is very challenging, because there are so many different populations to consider: those unvaccinated, those vaccinated, those unvaccinated but exposed to the disease, those vaccinated but exposed to the disease, and those people who eventually contract the complete disease. In addition, one needs to consider the number of people receiving chemotherapy treatment both after becoming infected and also after developing the actual disease. Despite the high number of population groups, modeling tuberculosis has been tackled by many scientists and mathematicians.

Models of a disease like tuberculosis are very important because they can help governments in determining their health care strategies. For example, in one case a group of mathematicians experimented with different ways of stopping an epidemic for the same population. The first slowly reduced the number of cases for 12 years and then completely erased the epidemic after 20. The second quickly cut into the number of cases in the first six years and then slowly got rid of the rest after 20. The first was cheap, but meant that many people would have to endure tuberculosis. The second was much more expensive, but resulted in less human suffering. Slightly adjusting the model so that it could yield an intermediate solution might result in the best policy for a government to use in order to fight an epidemic.

Sudan: A Case Study

Sudan, a country in eastern Africa, presents an interesting study for tuberculosis epidemics. It is a developing country located primarily between Egypt, Chad, Ethiopia and the Red Sea. The capital of the nation is Khartoum with a population of around half a million. In the 1960s, Sudan overall had approximately 10 million people, but the country now has over 20 million. This can be attributed largely to the fact that during the 1950s, Sudan had a extremely high yearly birth rate of about 50 births per 1000 people as well as a large refugee population entering from other countries. The population has exploded despite Sudan having a quite high yearly death rate of 20 deaths per 1000 people. Much of the country is remote and medical facilities are in scant supply. Thus dealing with epidemics such as tuberculosis is a very difficult challenge to the leaders of this country. Diseases in the area typically sweep from east to west, from Chad to the Red Sea, along a corridor known as the "epidemic belt" of the Sudan.

One of the great epidemics of tuberculosis occurred during the 1960s. At this time, the number of Sudanese testing positive for the disease reached a high point. In addition, the number of deaths due to tuberculosis also peaked during this period--50 people died for every 100,000 in population during the year 1960. Although tuberculosis is not often fatal, it can be extremely crippling. Consequently, the number of BCG vaccinations was appropriately increased until the late 1960s. Fear of another smallpox epidemic diverted resources away from tuberculosis vaccinations.

Tuberculosis Testing and Vaccination in Sudan 1961-1965

Year	Tested	Positive	%	Vaccinated
1961	114,663	40,023	34.9	53,929
1962	228,656	59,928	26.2	112,942
1963	273,904	63,801	23.3	154,891
1964	181,708	39,780	21.9	103,635
1965	98,719	24,815	25.3	49,191
Total	897,650	228,347	26.3	484,588

The table above shows the data collected during a tuberculosis testing and vaccination campaign during a peak of tuberculosis cases in the 1960s. From the results, it is easy to see that tuberculosis had infected a large portion of the population--around 25%. This gives some idea of the rate at which the population was being infected. Vaccinations during this period were also high--about 100,000 vaccinations per year.

Analysis:

STELLA makes creating the tuberculosis model much easier. Although it needs to be a big model to take care of the vaccinated and unvaccinated populations, it can still be made using the same simple stock and flow tools. Try modeling just an unvaccinated group first. Remember that the tuberculosis model requires an original population, an intermediate stage where the suspect has been exposed to the virus but has not yet contracted it, and a final stage where the disease has become active. Next, double the size of your model by adding a vaccinated group. See how this changes your results. Then add birth and death rates. Finally, see how treating the actual tuberculosis patients and the patients only exposed to the virus with chemotherapy would effect the spread of the disease.

Developed by
Graphic of Shodor Logo