Researching a Disease: Heart Disease, Tobacco Mosaic Virus and Wheat Rust

Every disease is a story about tissues

A heart attack is a story about cardiac muscle starved of oxygen. A mottled tomato plant may be a story about chloroplasts wrecked by a virus. An orange-streaked wheat field is a story about a fungus punching through the epidermis of leaves and stems. In this lesson you will investigate diseases the way a scientist does: ask a sharp question, find trustworthy sources, keep careful records, and explain the disease in terms of the cells, tissues, organs and systems it affects.

Step 1: a good research question

A weak question: "What is heart disease?" (too broad; the answer is a list of facts.)

A strong question: "How does atherosclerosis damage the heart's tissues, and how well do lifestyle changes reduce the risk?" It is focused, links to tissues and organs, and needs evidence to answer.

Steps 2 and 3: find and evaluate sources

Use a mix of print (textbooks, library books), electronic (government health agencies, university and hospital sites, scientific journals, databases from your school library) and human sources (an interview with a nurse, a farmer, a pharmacist or a plant pathologist).

Evaluate every source with questions like these:

• Currency: when was it published or updated? Is that recent enough for this topic?
• Authority: who wrote it? Are they experts? Is it peer-reviewed?
• Accuracy: is the evidence shown? Do other reliable sources agree?
• Purpose and bias: is it trying to inform, persuade or sell? Who benefits?
• Relevance: does it actually answer your question at the right level?

Step 4: record and cite

Keep notes in your own words with the source beside each note. Record a full citation for every source. Many Ontario science classes use APA style. For a web page the pattern is:

Author or Organization. (Year, Month Day). Title of page. Site Name. URL

and in your text you cite briefly, such as (Author, Year).

Case 1: Heart disease (coronary artery disease)

What happens (tissues and organs): the heart muscle gets its own blood from the coronary arteries. In atherosclerosis, fatty material called plaque builds up inside the artery wall, under the thin epithelial lining. The artery becomes narrower and stiffer, so less blood reaches the cardiac muscle. During exercise, the muscle may not get enough oxygen, causing chest pain (angina). If a plaque cracks, a blood clot can form and block the artery completely: the cardiac muscle beyond it is starved of oxygen and begins to die. That is a heart attack (myocardial infarction). Because cardiac muscle cells rarely divide in adults, damaged muscle is replaced by scar tissue that can't contract.

System effects: a weakened heart pumps less blood, so every system gets less oxygen and nutrients.

Risk factors: smoking, high blood pressure, high LDL cholesterol, diabetes, physical inactivity, unhealthy diet, family history and age.

Prevention and treatment: not smoking, being active, healthy eating, and controlling blood pressure, cholesterol and diabetes; medicines; angioplasty (a balloon opens the artery, often leaving a mesh tube called a stent) and bypass surgery (a vessel from elsewhere is used to route blood around the blockage).

Case 2: Tobacco mosaic virus (TMV)

The organism: TMV infects tobacco, tomatoes, peppers and many other plants. In the 1890s it became the first virus ever discovered, when scientists found that something smaller than any bacterium could pass on the disease.

What happens (cells, tissues, organs): the virus enters plant cells through tiny wounds, takes over the cells to make copies of itself, and spreads from cell to cell and through the phloem to the rest of the plant. It damages chloroplasts in the leaf mesophyll, producing a mosaic of light and dark green patches. Leaves may curl or pucker. With less photosynthesis, the plant is stunted and produces less fruit.

Spread and control: TMV spreads by contact: hands, tools, clothing and infected plant material. The virus is very stable and can survive a long time in plant debris and dried leaves. There is no cure for an infected plant, so control relies on removing infected plants, washing hands and tools, and growing resistant varieties.

Case 3: Wheat rust

The organism: rusts are fungi. Wheat can be attacked by stem rust, leaf rust and stripe rust.

What happens (tissues and organs): a rust spore lands on a wheat leaf or stem, grows into the plant through the stomata, and spreads its threads between the cells, stealing water and nutrients. It then forms pustules that burst through the epidermis, releasing millions of rust-coloured spores. Damaged dermal tissue lets the plant lose more water; damaged mesophyll means less photosynthesis; stem rust can weaken the stem so plants fall over. The result is fewer, smaller grains.

Spread and control: spores travel in the wind, sometimes hundreds of kilometres. Canadian plant scientists, including researchers at Agriculture and Agri-Food Canada, monitor rust each season and breed rust-resistant wheat varieties. But rusts evolve: a strain of stem rust first detected in Uganda in 1998 and named Ug99 in 1999 can overcome several resistance genes, so breeders keep searching for new ones. Farmers also use fungicides and crop rotation.