Tracking Infectious Diseases

can cause genetic mutations and cancer. Aspergillus will also make gliotoxin that causes host cell destruction and it makes proteases, such as elastase and catalase, which will break down elastin and otherwise protect the fungus from the immune system.

Protozoans can make adhesins and toxins; they also undergo antigenic variation and some can survive inside the cell’s phagocytic vesicles. Giardia lamblia has an adhesive disk that binds to mucosal cells in the intestines. It causes inflammation of the intestinal lining but doesn’t invade the cells directly. Antigenic variation can happen with organisms that cause malaria. The organism Trypanosoma brucei, which causes African sleeping sickness, makes a capsule and undergoes antigenic variation.

Helminths, such as those that cause schistosomiasis, can penetrate intact skin by making elastin and other proteases. Helminths that are very large will evade the immune system and organisms like trichinella and other roundworms will have a cuticle surrounding them that evades the immune system. Still others express glycans on the cell surface that mimic host cells so they evade the immune system. Some will make proteases that degrade antibodies.

TRACKING INFECTIOUS DISEASES

Infectious diseases are tracked by epidemiologists. This field of study looks at both disease transmission and disease etiologies. It studies populations of individuals and the specifics of the population that make it susceptible to a given disease. Individuals that have a disease are said to have morbidity with the morbidity rate being a certain number of diseased persons in a standard set of the population or a percentage of the population.

Two important topics in epidemiology are incidence and prevalence. Prevalence is the number of people with the disease at a particular point in time, while the incidence is the number of new cases of a disease over a specified period of time. The lifetime prevalence is the number of people who get the disease in their lifetime. The prevalence of a chronic disease will always be higher than the incidence because the disease doesn’t go away. Mortality relates to death from a disease compared to a standard number in the population.

Some diseases are called sporadic diseases that aren’t seen often and have no geographic concentration. Tetanus would be considered a sporadic disease. Those diseases that are present within a certain geographic area are called endemic diseases. Malaria is an endemic disease in some areas. Those diseases that peak over a short period of time are called epidemic diseases. Influenza is an epidemic disease. Epidemic diseases are often caused by antigenic drift. Pandemic diseases are those that have worldwide impact.

The first goal of the epidemiologist is to identify the causative agent or etiological agent. Experimentation needs to happen so as to identify if there is a pathogen leading to the disease. It is particularly hard to do when the symptoms are nonspecific. Much of understanding of the causative agent relates to Koch’s postulates.

The CDC or Centers for Disease Control and Prevention is the agency involved in diseases and their causes. There are registers of reportable diseases that must be reported to the CDC by physicians, including things like measles and HIV disease. There is a weekly report made on each of these diseases that is published for doctors and other healthcare providers.

As mentioned in a previous chapter, the first epidemiological study was done in London by John Snow, who studied cholera in 1854. The origin of the disease was mapped to certain water sources and public health changes were made to eliminate the causative agent in the water.

Some diseases have a common source spread of the disease, such as a specific water source. This can be from a point source spread, with a short time period of contamination, a continuous common source spread, such as an infected kitchen with Norovirus, or intermittent common source spread, which does not cause infection all of the time. Propagated spread involves the infectability of each person who gets the disease with no single source of the infection. This usually involves person-to-person spread.

Epidemiological studies come with different study designs. Observational studies are gathered through measuring different findings or doing a questionnaire or survey with no manipulation of the patients. These tend to be ethically easier to do.

There are different approaches to doing an observational study. Descriptive studies look at the patients with the disease and examine medical records. They are often early studies to see if there is a particular pattern to the disease. The analytical epidemiological study looks at certain populations affected by a disease. This narrows down the possible etiologies of the disease. Retrospective studies look at information from the past regarding current cases of a disease, while prospective studies monitor the disease course after the disease has been established. d

Cohort studies look at a particular age of people or a particular group of people and follow them prospectively and retrospectively. Case-control studies look at small groups of people with the disease and study their past. Cross-sectional studies look at all people in a specific point in time.

Experimental studies are different from observational studies because they actually do something with the participants. Certain treatments can be given to see what the outcome of the treatment might be. The proofs made in Koch’s postulates requires experimental studies—some of which might not be ethical to accomplish. A good experimental study is double-blinded so that neither the patient nor the researcher knows what treatment is being given.