Identifying the Causative Agent
Reading: Introduction to Seeking the Cause
Have students read the introduction to the module. The purpose of this reading is to introduce students to the science of epidemiology and inform them that they will be learning about the three major ideas in epidemiology – identifying the first cases of an infectious disease, determining the causative agent, and tracking how it spread.
The purpose of this activity is to have students model the second big idea of epidemiology, identifying the cause. Working in teams, students analyze epidemiological, medical, and laboratory data and use information about various causes of respiratory distress to narrow down the candidates and identify the causative agent.
For each team of 4 students:
- One copy of each report (epidemiological data, medical data, and laboratory data)
- One copy of Respiratory Distress Table
- Poster board and markers (optional)
- Divide the class into teams of 4. Distribute one copy of each report and the Respiratory Distress table to each team
- Have students read the introduction to the activity in the student materials
- Inform the students that in order to determine the cause of the disease, they must use each report and the table. You may allow access to the Internet (except for Googling the answer. If they do, they are still required to develop a reasonable and logical explanation of why this solution is the most likely one. Just knowing the answer is not sufficient.)
- Remind students that they must keep track of why they eliminated some information and deemed other information important. They need to record the rationale behind each decision and be responsible for justifying every decision. Their conclusions must be based on analysis of the data in relation to symptoms, method of detection, immune response, characteristics of the causative agents of lung disease, treatment, and target population.
You may want to have students prepare a poster and present their findings to the class. The poster should contain the following information:
- The nature of the epidemic; is it an infectious or noninfectious disease?
- The probable causative agent of the epidemic, if infectious.
- The process by which you came to the decision about the causative agent.
- The rationale for each decision.
- Recommendations for preventing further spread of the disease.
The following is an example of reasoning students might use based on the information in the data reports. Each tab represents the type data on which conclusions are based. The combination of these reports should lead to the conclusion that the hantavirus is the causative agent of the epidemic.
Conclusions based on Epidemiological Data
(using Report 1 and Table):
- Relationships between individuals and their living and working conditions suggest that direct contact is not necessarily a requirement for the transmission of the causative agent.
- Cases occurred in rural areas.
- Environmental conditions indicate an increase in the rodent population.
- Disease candidates would include plague, hantavirus, or chemical toxin.
- These data must be connected to medical and laboratory data.
Conclusions based on Medical Data
(using Report 2 and Table):
- Victims show respiratory symptoms.
- The disease is not antibiotic-sensitive or responsive to anti-protozoal drugs.
- The disease, therefore, could be a virus, a chemical toxin, or antibiotic-resistant bacteria.
- Data eliminates Toxoplasma, pneumocystis; age range eliminates
- Remaining candidates include Legionnaires’ disease, viruses, and
- These data must be connected to epidemiological data or laboratory data.
Conclusions based on Laboratory Data
(using Report 3 and Table):
- The disease agent cannot be visualized by light microscopy; unlikely to be parasite or the larger bacteria. Probably viral in origin since it cannot be visualized.
- Infectious agent unable to be grown in culture.
- Immunological assay shows presence of white blood cells, indicative of intracellular pathogen.
- Antibody detection suggests Ebola or hantavirus.
- These data must be connected to medical data or epidemiological data.
Hantavirus was first identified in Korea as a disease of the renal system. The virus was transmitted in the feces of rodents. Infected individuals suffered fatigue, fever, weakness, and kidney failure. Initial identification of the agent of infection in the Four Corners epidemic as hantavirus was met with skepticism by some scientists, as respiratory distress was the characteristic symptom.
All strains of hantavirus known then caused only kidney problems not respiratory distress. However, analysis by molecular biological techniques such as polymerase chain reaction (PCR) and DNA sequencing revealed that the Four Corners virus was a new strain of hantavirus. This virus had mutated and the change resulted in new tissue specificity for the virus; this particular strain now targeted and grew in cells of the respiratory system.
The Four Corners virus was officially named Muerto Canyon, referring to the valley inside the Navajo Nation—Muerto Canyon, meaning Valley of Death.
Report One: Epidemiological Data
Data about infected individuals (victims)
- Geographical distribution—lived in New Mexico and other parts of the Southwest, South, and Northwest United States
- Habits—worked in a variety of jobs; some held positions in maintenance or cleaning; no unusual hobbies
- Living Conditions—lived in trailers, small homes; generally in rural areas
- Relationships—in some cases victims were related; one instance of engaged couple as victims; primarily isolated cases of unrelated, uninvolved individuals
- Travel—no pattern of foreign travel or association with anyone who has traveled
Data about the environment
Recent rains and good growing conditions for seeds, nuts, berries, and insects had resulted in increase in rodent population.
Report Two: Medical Data
- Symptoms—respiratory distress, flu-like symptoms, fever, muscle aches, coughing, difficulty in obtaining oxygen
- Victims—range of ages; many young adults
- Treatment—antibiotics ineffective; no response to anti-protozoal drugs (drugs which eliminate parasitic, single-celled organisms or protozoans)
- death by suffocation
- rapid occurrence of death after onset of symptoms
- air sacs in lungs filled with fluid, presumably from blood plasma leaking from pulmonary (lung) veins
Report Three: Laboratory Data
- Mass spectral analysis—negative for chemical toxins and heavy metals
- Examination of lung tissue and blood smears by direct stain and light microscopic analysis—negative for visible pathogens
- Growth in culture from blood and lung tissue—no growth demonstrated
- large increase in number of white blood cells
- results of antibody detection assays:
- negative for Toxoplasma gondii, Streptococcus pneumoniae, Mycoplasma, influenza virus
- inconclusive for Ebola virus, hantavirus, pneumocystis, Legionella, Pasteurella pestis
This discussion is designed to help students reflect on their understandings of how the origins of diseases are tracked and their spread determined.
What kinds of information would help an epidemiologist determine that a disease was being caused by an infectious agent rather than being a noninfectious disease?
Students should recognize that the pattern of disease distribution can distinguish infectious diseases from heritable diseases or diseases caused by lifestyle (heart disease for example) but may be less clear-cut when it comes to diseases caused by the environment. The rates of the spread of the disease and the numbers of individuals involved should also indicate that the disease is infectious. The epidemiologist would also want laboratory data such as information on antibodies and cultures for microbial growth.
What kinds of information about the origin of a disease can be gained from examining the symptoms of the disease?
Students should recognize that many different diseases exhibit similar symptoms.
How would understanding the origin of the disease and how it is spread help in stopping the hantavirus outbreak?
Students should realize that by knowing the viral nature of the cause and that the vector that carries the virus is a mouse, treatment for victims can be administered and plans for eliminating the vector can halt the spread of the disease. You may want to share the following with your students:
Many cultures have established rituals, taboos, and customs which, in part, were developed to avoid illnesses observed to be associated with certain hygienic or food practices. Do you know of any and, if so, is there a basis for the practice in science?
Students may mention that certain religions forbid eating pork, a taboo that may be the result of the concern about contracting trichinosis from undercooked pork. Some cultures forbid pregnant women from drinking alcohol, which can cause developmental problems in the fetus.