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21.8: Case Study Lyme Conclusion and Chapter Summary

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    22610
  • Case Study Conclusion: What’s Lurking in the Woods

    The bacteria in Figure \(\PageIndex{1}\), labeled with red, yellow, and green in the photomicrograph above, are the tiny culprits responsible for Lyme disease. They are the bacteria Borrelia burgdorferi, which, when transmitted through the bite of an infected tick, can make humans very sick.

    Screen Shot 2019-03-31 at 4.07.55 PM.png
    Figure \(\PageIndex{1}\): (NIAID; CC BY 2.0; https://flic.kr/p/9CjrAh)

    As you learned in the beginning of the chapter, Lauren came down with symptoms of Lyme disease after visiting her grandparents in New Jersey and spending time in the woods there. Lauren’s symptoms included a distinctive bulls-eye rash that is characteristic of Lyme disease (known formally as an erythema migrans rash — shown below) and flu-like symptoms, including fever, chills, fatigue, headache, and body aches. In addition to these symptoms, Lyme disease can cause facial palsy (loss of muscle tone in the face, which can occur on one or both sides), and joint pain and swelling. These symptoms are illustrated below. Lyme disease can also cause swollen lymph nodes; neck stiffness; pain, numbness, or tingling in various parts of the body; heart problems; dizziness; inflammation of the brain and spinal cord; and short-term memory problems. Clearly, Lyme disease can be quite serious, which is why early diagnosis and treatment is so important.

    medical illustration of Erythema migrans, medical illustration of Bell's Palsy, and medical illustration of an arthritic knee
    Figure \(\PageIndex{2}\): Some symptoms of Lyme disease. (Centers for Disease Control and Prevention; public domain; lyme signs symptoms index)

    In a way, Lauren was lucky that her symptoms included the bulls-eye rash because it helped her get diagnosed early. In about 20-30% of cases of Lyme disease, there is no rash. Even if there is a rash, it may not have the “classic” bulls-eye appearance, which can make it hard to identify.

    In addition to noting Lauren’s symptoms and time spent in the woods of the northeastern U.S. where Lyme disease is prevalent, Lauren’s doctor took a blood sample to confirm the diagnosis of the disease. The first blood test for Lyme disease is usually an enzyme immunoassay (EIA), which detects antibodies against the bacteria that cause Lyme disease. Similar to HIV testing, which you learned about in this chapter, it can take time after the initial infection for antibodies against the pathogen to be produced by the body and detectable in the blood. In the case of HIV, this can take up to three months, while in Lyme disease it can take up to two months.

    Lauren’s EIA test results came back as “indeterminate” (neither conclusively positive or negative), possibly because she was recently infected. The Centers for Disease Control and Prevention (CDC) recommends two-step testing for Lyme disease if the initial EIA test is indeterminate or positive, meaning that the second type of test should be performed to confirm the diagnosis. This second test is called an immunoblot, or Western blot test, which is another way of detecting antibodies in the blood.

    Why would a second test be needed if the first test came back positive? This is because the EIA test can give false positives, often due to the presence of other diseases, such as tick-borne relapsing fever, syphilis, and some autoimmune disorders. The Western blot test gives more information and may be able to distinguish between Lyme and other diseases. Even the Western blot can give false positives, so it must be administered correctly (i.e. at the correct time after infection) and the results must be interpreted carefully by an experienced medical professional. The risk of false positives and the need for careful interpretation is similar to the reasons why widespread screening for some types of cancer can be controversial, as you learned earlier in the chapter.

    When Lauren’s doctor did the Western blot test, it confirmed that she did have antibodies against the bacteria that cause Lyme disease. This result, combined with her symptoms and presence in areas where Lyme-disease infected ticks are common, caused her doctor to confirm a diagnosis of Lyme disease. Recall that he started Lauren on medication immediately, because of the high likelihood that she had Lyme disease and the importance of early treatment. Given what you have learned in this chapter, what type of medication do you think he prescribed? If you guessed an antibiotic, you are correct! Because the pathogen is bacterial, antibiotics are generally effective in treating Lyme disease. A two to four week course of oral antibiotics is usually sufficient, although extreme cases may require intravenous antibiotics.

    Within a week of starting the antibiotics, Lauren was beginning to feel better, although she continued to have fatigue and body aches for several weeks afterward, which is common. By two months after treatment, Lauren was back to normal. A small percentage of people with Lyme disease are not so lucky. In those people, Lyme disease symptoms can continue for more than six months after treatment. This is called Post-treatment Lyme Disease Syndrome (PTLDS) and the cause is not yet known. Most medical experts think that PTLDS is due to damage to body tissues and the immune system that occurred during the original infection, and not a continued active infection with B. burgdorferi, although the causes are still under investigation. More research needs to be done to better understand this more chronic version of Lyme disease.

    Lauren is relieved to have recovered, but she wonders how she got infected in the first place. She never saw a tick on her or felt a tick bite, which is not uncommon. The ticks that spread Lyme disease are very small (see below) and their saliva contains a substance that has anesthetic properties, so a person may not feel their bite. They often bite and attach themselves on areas of the body where they are hard to see, such as the scalp, armpit, and groin.

    Relative sized of several ticks at different life stages.
    Figure \(\PageIndex{3}\): The relative sizes of a tick that can transmit Lyme disease, compared to a dime. Most people are infected by the bite of immature ticks called nymphs, which are less than 2 mm in size. Adult ticks can also spread Lyme disease. (Centers for Disease Control and Prevention; public domain; lyme transmission index)

    As you now know, this method of infectious disease transmission is called vector transmission. The disease-causing pathogen is the B. burgdorferi bacteria and the vector is the tick. This is similar to malaria, where the pathogen is transmitted through the bite of a mosquito vector. Like malaria, Lyme disease is endemic to particular geographic regions, based on the presence of the vector organism. Lyme disease risk is high in certain areas of the U.S. (see map below), because of where the tick species that transmit Lyme disease live.

    Lyme Disease Risk Map
    Figure \(\PageIndex{4}\): The relative sizes of a tick that can transmit Lyme disease, compared to a dime. Most people are infected by the bite of immature ticks called nymphs, which are less than 2 mm in size. Adult ticks can also spread Lyme disease. (Centers for Disease Control and Prevention; public domain; Lyme Disease Risk Map.gif)

    In the northeast, mid-Atlantic, and north-central U.S., the tick species that transmits Lyme disease is the blacklegged tick, or deer tick, Ixodes scapularis; and on the Pacific coast, it is transmitted by the western blacklegged tick, Ixodes pacificus. A research study published in 2016 showed that the range of these species is rapidly expanding, and that half of all counties in the U.S. are now home to these tick species. Cases of Lyme disease have tripled in the U.S. in the last 20 years, and it is estimated that 300,000 Americans are infected each year. Lyme disease is the most common vector-borne disease in the United States.

    How can you prevent getting this common infectious disease? You don’t necessarily need to avoid spending time in nature, but you should take preventative measures if you are outside in an area with Lyme disease. These include avoiding walking through thick vegetation where ticks commonly live, using insect repellent, bathing after being outdoors, and checking yourself for ticks daily if you are likely to be exposed. You may want to enlist a friend or family member to check areas you can’t easily see, such as your scalp.

    If you do see a tick attached to your body, it is important to remove it quickly and carefully. Removing a tick within 24 hours of attachment can greatly reduce your chance of getting Lyme disease since it can take 36-48 hours for a tick to transmit the disease-causing bacteria. Remove the tick with tweezers by steadily pulling straight up, as shown in the picture below. Visit the CDC website and consult with your physician for more detailed instructions on proper tick removal.

    clipart style image showing the proper removal of a tick using a pair of tweezers
    Figure \(\PageIndex{5}\): Pull ticks out by grasping them near the head and pulling straight up. (Centers for Disease Control and Prevention; public domain; lyme removal index)

    If you are concerned about a tick bite or think you may have symptoms of Lyme disease, please consult a physician. Many websites or laboratories advertise types of tests for Lyme disease that have not been scientifically proven to be valid. The good news is that when properly diagnosed and treated early, as Lauren was, Lyme disease can usually be cured.

    Chapter Summary

    In this chapter, you learned about the general causes of disease, and details about several specific diseases. Specifically, you learned that:

    • Homeostasis is needed for good health. Homeostasis refers to maintaining internal conditions in a steady state. Homeostats are physiological mechanisms that keep internal variables within normal ranges.
      • The homeostat that controls blood glucose concentration involves pancreatic beta cells, which secrete insulin, and alpha cells, which secrete glucagon. These two hormones control blood glucose concentration in two negative feedback loops, with insulin-lowering values that are too high, and glucagon raising values that are too low.
    • If homeostats fail to perform properly, homeostatic imbalance and disease may result. For example, failure of the homeostat that controls blood glucose concentration causes high blood glucose levels and diabetes. Homeostats also start to fail as people age.
    • There are many underlying causes of homeostatic imbalances that lead to disease. Infectious diseases are caused by pathogens such as bacteria and viruses. Noninfectious diseases are caused by genes or environmental factors other than pathogens, such as toxic exposures or unhealthy habits.
    • Some diseases, such as flu, are acute, or short-term, diseases. Other diseases, such as heart disease, are long-term or even lifelong diseases.
    • At the population level, diseases may occur as sudden outbreaks, called epidemics. If epidemics spread through multiple populations or even worldwide, they are called pandemics. Endemic diseases, in contrast, occur at about the same rate year-round in populations.
    • The science that studies diseases in human populations is epidemiology. The results of epidemiological research form the cornerstone of public health. The father of epidemiology is John Snow, a 19th-century English physician whose investigations pinpointed the cause of cholera outbreaks in London. His work eventually led to significant improvements in public health around the world.
    • All infectious diseases are caused by infections with pathogens, or disease-causing agents, many of which are microorganisms. Types of pathogens and examples of the diseases each type causes include: bacteria (e.g., tuberculosis and strep throat), viruses (e.g., influenza and the common cold), fungi (e.g., ringworm and athlete’s foot), protists (e.g., malaria and giardiasis), helminths (e.g., tapeworm and hookworm), and prions (e.g., CJD and mad-cow disease).
      • In the 19th century, Robert Koch developed four criteria, or postulates, for deciding whether a disease is caused by a particular microorganism. The postulates are now viewed as sufficient but not necessary criteria. They still inform the basic approach to identifying pathogens and historically led to the discovery of many human pathogens.
      • Pathogens cause disease by invading and multiplying in host tissues, causing damage and releasing toxins. Typically, the more pathogens there are in the host, the greater is the severity of the disease. However, pathogens also vary greatly in their virulence.
      • Transmission of pathogens from infected to noninfected human hosts can occur through a variety of different routes: airborne transmission, direct contact, fecal-oral transmission, vector transmission, vertical transmission, and sexual transmission. Prions can be transmitted via eating contaminated nervous tissue from an infected individual.
      • Infectious diseases must be correctly diagnosed so the appropriate treatment can be prescribed. Most infectious diseases can be treated with drugs if not cured. Hygienic habits, especially frequent handwashing, and immunizations are the most effective ways to prevent the spread of infectious diseases. A high level of vaccination in a population provides herd immunity to population members who cannot be vaccinated for medical reasons.
      • Emerging infectious diseases are new infectious diseases that are appearing for the first time in human populations, mainly because of human actions such as encroachment on wild lands. Emerging infectious diseases come about in a number of ways. For example, some pathogens of nonhuman hosts jump to human hosts and start causing disease in them.
    • A sexually transmitted infection (STI) is an infection caused by a pathogen that spreads mainly through sexual contact. This may include vaginal, anal, and/or oral contact.
      • Most STIs are caused by pathogens that can infect the body only via direct contact between mucous membranes. Such pathogens generally cannot spread through nonsexual skin contact, although some can also be transmitted through body fluids such as blood and breast milk.
      • Types of pathogens that are sexually transmitted include parasites, such as crab lice and the protozoa that cause trichomoniasis; bacteria, including those that cause chlamydia, gonorrhea, and syphilis; and viruses, such as those that genital herpes, genital warts, and AIDS.
      • Common symptoms of STIs include genital sores, genital discharge, and painful urination. However, many cases of STIs are asymptomatic.
      • Bacterial STIs can generally be cured with antibiotics. Viral STIs can be treated with anti-viral drugs, but the viruses may not be completely eliminated.
      • If STIs go untreated, some may eventually lead to more serious diseases, especially in females, who may develop the pelvic inflammatory disease (PID) and its sequelae of infertility or ectopic pregnancy. Untreated syphilis is dangerous in both sexes. It typically advances through several stages over the decades to invade internal organs and cause death.
      • A few STIs can be prevented with vaccines. An example is a human papillomavirus (HPV) infection, which sometimes leads to genital warts or cervical cancer. The HPV vaccine is recommended for all girls and boys aged 11-12 years old.
      • For STIs without vaccines, avoiding sexual contact is the only sure way to prevent transmission. Practicing safe sex behaviors — such as proper condom use — can greatly reduce but not totally eliminate the risk of transmission.
    • Human immunodeficiency virus (HIV) is a sexually transmitted virus that infects and destroys helper T cells of the immune system. It is usually transmitted through sexual contact but can also be transmitted through contaminated blood or breast milk. HIV infection is diagnosed on the basis of a blood test for antibodies to the virus.
    • AIDS stands for acquired immunodeficiency syndrome. AIDS is a disease that develops in people with untreated HIV infections, typically several years after their initial infection with the virus. AIDS is diagnosed when the immune system has been weakened to the point that it can no longer fight off opportunistic diseases that do not normally occur in healthy individuals.
      • HIV infection and AIDS are a worldwide pandemic with the highest population rates in sub-Saharan Africa where the virus first emerged. Death and disability due to AIDS have been economically devastating to these populations.
      • The development of new anti-retroviral drugs to treat HIV infection has changed HIV infection from a fatal to a chronic disease. The drugs keep the virus at low levels, reducing the risk of transmission as well as reducing the risk of the infection progressing to AIDS.
      • Until an HIV vaccine is developed, reducing the risk of HIV transmission depends on factors ranging from individual behaviors such as effective condom use to public health policies such as needle-exchange programs.
    • Noninfectious diseases include all diseases that are not caused by pathogens. Noninfectious diseases are generally caused by a combination of genetic and environmental factors other than pathogens. Noninfectious diseases are the leading causes of death globally.
      • Risk factors for noninfectious diseases include age, gender, inherited genes, and environmental factors including exposures such as radon and behaviors such as smoking. Most behavioral risk factors for noninfectious diseases can be avoided, so many noninfectious diseases are considered preventable diseases. Risk factors such as age, gender, and genes cannot be avoided but should be considered in diagnosing, treating, and preventing noninfectious diseases in individuals.
      • Cystic fibrosis is an example of a genetic noninfectious disease. It is inherited as an autosomal recessive trait, caused by a mutation in a gene called CFTR. It results in thick mucus that blocks mucus-secreting organs such as the lungs and intestines, causing recurrent respiratory infections and malabsorption of nutrients. Medical interventions can help people with cystic fibrosis live into middle adulthood.
      • Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. It is a major cause of death in developed countries. Most cancers are noninfectious diseases caused by a combination of genetic and environmental factors. Few are caused mainly by inherited genes.
      • Cardiovascular disease is a class of diseases that involve the heart or blood vessels, such as coronary artery disease and stroke. Cardiovascular disease is the leading cause of death globally. Major precursors of cardiovascular disease include hypertension and atherosclerosis. Obesity and diabetes are additional major risk factors. Most cases of cardiovascular disease could be prevented by modifying risk factors through medications and behavioral changes.
      • Type 2 diabetes accounts for 90 percent of all diabetes cases. It generally develops due to insulin resistance, although the reduction in insulin secretion may exacerbate the problem. Risk factors for type 2 diabetes include obesity, with or without the other indicators of metabolic syndrome, which is called pre-diabetes. Poorly controlled diabetes can lead to heart attacks, blindness, kidney failure, and other serious health problems.
    • Cancer is a group of more than 100 diseases, all of which involve abnormal cell growth with the potential to invade or spread to other parts of the body. The most common type of cancer is the type of skin cancer called basal cell carcinoma, which is usually easy to cure. Less common but more deadly cancers include lung, colorectal, prostate, and breast cancers.
      • Cancer generally occurs when the cell cycle is no longer regulated due to DNA damage to two types of genes: proto-oncogenes, which normally promote division of normal cells; and tumor-suppressor genes, which normally inhibit the division of abnormal cells. Transformation of a normal cell into a cancer cell is a multi-stage process involving accumulated damage to these genes.
      • Once a normal cell transforms into a cancer cell and starts dividing out of control, cancer cells can spread from the original site. Cancer cells can invade nearby tissues, spread through lymph vessels to regional lymph nodes, or spread through the bloodstream to distant sites in the body, which is called metastasis. New cancers that forms at a distant sites are called metastases.
      • There are many possible underlying causes of the DNA damage that leads to cancer, so cancer has many risk factors. DNA damage can be inherited from parents or result spontaneously from environmental exposures to carcinogens. Environmental risk factors include radon, UV light, air pollution, and behaviors such as smoking, unhealthy diet, and lack of exercise.
      • Early diagnosis and treatment are the keys to curing cancer, although not all cancers are curable. Cancers may be detected early through routine screening (e.g., by mammograms) or by patients or health care providers noticing early warning signs, such as unusual bleeding or a nagging cough. A definitive diagnosis of cancer requires a biopsy, in which a tissue sample from the patient is examined microscopically. A biopsy may also reveal the type of cancer (e.g., carcinoma or sarcoma) and its stage (degree of severity, such as whether it has spread).
      • Many types of treatments for cancer exist, including surgery, chemotherapy, radiation therapy, and immunotherapy. The first three types of treatment directly target cancer cells, while the last type of treatment is directed at helping the immune system fight cancer.

    Chapter Summary Review

    1. ____________   feedback loops help maintain homeostasis by keeping variables within a normal range.

    2. Explain what generally happens to homeostatic mechanisms as people age, and how this relates to susceptibility to disease in elderly people.

    3. Give an example of a noninfectious disease that can cause an infectious disease.

    4. True or False. Epidemiologists only study diseases that can be transmitted between people.

    5. True or False. Some cases of cancer are preventable.

    6. Explain how type 1 diabetes can lead to cardiovascular disease. 

    7. For each of the following diseases, state whether the pathogen is a bacterium, virus, fungus, protist, helminth, or prion.

    a.  Creutzfeldt–Jakob disease

    b. Strep throat

    c. Ringworm

    d. Giardiasis

    e. Hepatitis

    f. Pinworm

    8. Which type(s) of pathogens (listed in question 7) are not considered to be living organisms? Explain your answer.

    9. True or False. Transmission of viral pathogens can sometimes be prevented by immunization.

    10. True or False. Human papillomavirus can cause cancer of the penis.

    11. Proper washing can help prevent the spread of:

    A. Sexually transmitted diseases

    B. Infectious respiratory illnesses

    C. Cystic fibrosis

    D. A and B

    12. a. What does “screening” for a disease mean?

          b. Compare and contrast screening for STIs to screening for cancer in terms of potential benefits and drawbacks.

    13. Which is more likely to result in a chronic disease instead of an acute disease – a bacterial STI or a viral STI? Assume the proper treatment is given. Explain your answer and give an example of each type of STI.

    14. Can a couple that does not engage in penile-vaginal intercourse still transmit STIs to each other if they engage in other types of unprotected sexual activity? Why or why not?

    15. What are two ways that STIs can be transmitted that do not involve sexual activity?

    16. What is metabolic syndrome and why is it such a cause for concern?

    17. What is the relationship between HIV and AIDS?

    A. HIV causes AIDS.

    B. AIDS causes HIV.

    C. They are different terms for the same thing.

    D. AIDS can make a person more susceptible to HIV.

    18. Viral load refers to:

    A. The financial impact of a viral disease.

    B. The amount of damage a virus does to an individual.

    C. How widely a virus has spread across a population.

    D. The amount of virus in a sample of an infected person’s blood.

    19. Explain the roles of genetics and the environment in the development of cancer.

    20. If breast cancer metastasized to a patient’s brain, what stage would this cancer most likely be classified as? Explain your answer.

    21. What are two healthy lifestyle choices that you can make that can reduce your risk of disease? Explain your answer, and identify some of the diseases that may be able to be avoided.

    22. What is the most frequently reported bacterial STI in the United States?

    A. HPV

    B. Genital herpes

    C. Chlamydia

    D. PID

    23. What is the difference between a disease vector and a pathogen?

    24. Antiretroviral drugs are used to treat:

    A. HIV

    B. Tetanus

    C. Cholera

    D. Malaria

    25. What is one way that screening for skin cancer is performed