LESSON 3.2 WORKBOOK - Dashboard - Confluence2.2+Lesson+3.2+Workbook.pdf · Workbook Lesson 3.2 ... Endemic — in the case of an ... Figure 4: The life cycle of the deer tick that

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W o r k b o o kLesson 3.2

DEFINITIONS OF TERMS

Reservoir — a reservoir typically harbors the infectious agent

without injury to itself, and serves as a source from which others

can be infected.

Asymptomatic carrier — is a person or other organism that has contracted an infectious disease,

but has no symptoms.

For a complete list of defined terms, see the Glossary.

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Routes of transmission

Most of the pathogens that we have seen so far can be passed directly from one person to another. However, this is not the only way that diseases can be transmitted. In previous units, we discussed cholera and how it can be contracted through drinking water that is contaminated with feces containing cholera bacteria. In that case, the cholera bacterium lives in the water in between human hosts. In the case of malaria, the parasite lives inside mosquitoes in between human hosts. As we will see, many pathogens have several stages in their life cycles and these stages can include multiple hosts.

Reservoirs

When a pathogen does not use humans to complete its entire life cycle, it must have another host, or a reservoir. As discussed in the last lesson, reservoirs are carriers of a microbe that are usually asymptomatic. Reservoirs can act as sources of pathogens in the environment and a single microbe may use multiple reservoirs. In the case of Ebola, researchers discovered that fruit bats acted as a reservoir

LESSON 3.2 WORKBOOKReservoirs and vectors — Lyme diseases and Malaria

In the last lesson, we discussed Ebola and its natural host, the fruit bat. In this lesson, we will explore different methods that pathogens employ to infect their hosts. Including the use of more than one host species, and the use of vectors. We will also explore the idea that microbes can evolve to live in their hosts without causing disease, and that a microbe deadly to one host can result in an asymptomatic carrier in another.

http://sites.tufts.edu/greatdiseases/student/glossary/

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Vector — an organism that transmits a pathogen from

reservoir to host.

Eastern equine encephalitis virus (EEEV) — a virus that

spreads through mosquitoes. It can cause encephalitis.

Spirochete — spiral shaped bacteria.

CDC — Centers for Disease

Control and Prevention is a federal US public health agency.


1. Which of the following about vectors is true?

.a it’s an organism that transmits a pathogen from a reservoir to the host

.b it’s a potential means for controlling the spread of a disease

.e it’s a transmission route for pathogens

.d a and c

.e all of the above __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

for the virus without becoming ill. This means that Ebola can thrive in the bat population for an extended period of time increasing the likelihood of human exposure which can be fatal to the human host. Another example is the plague bacterium, which can be carried by prairie dogs. We will see more examples of reservoirs in this lesson.

Vectors

Many microbes use different hosts for different parts of their life cycle, but how does a microbe get from one host to another? Many microbes use an animal or an insect vector to move from host to host. Vectors are living transmitters of disease and are a crucial part of the life cycle for many pathogens. Most vectors are insects and the spread of a disease is often limited by the spread of its vectors. With global warming and the transport of insects by humans, numerous vector born diseases are spreading to new places. Perhaps the most famous vector is the mosquito, which transmits numerous diseases includ-ing malaria. Warmer weather allows mosquitoes to survive farther north, so mosquito-borne infections such as malaria and Eastern equine encephalitis virus (EEEV) are spreading. In this case, eradicating the vectors is a potential means to control the spread of the disease.

Lyme disease has a complex life cycle with multiple reservoirs en route to human infection

Lyme disease is currently one of the Center for Disease Control’s ‘Top 10’ emerging infectious diseases worldwide. It can be caused by at least 3 different Borrelia bacteria, which are Gram-negative spirochetes (spirilla). Lyme disease is named after the village of Lyme, Connecticut, where a cluster of cases was diagnosed in 1975. Intensive investigation soon established that Lyme disease is carried by ticks; in fact it is the most common tick-borne disease in the Northern Hemisphere.

Lyme disease has been causing focal epidemics as it spreads in the northeastern and upper Midwestern US. They are thought to be a result of the large increase in deer in suburban and rural areas. This is because the deer tick needs the white-tailed deer to reproduce successfully. So reducing the numbers of deer hosts may help break the deer tick reproductive cycles and their ability to flourish. In the US it has

Figure 2: Borrelia burgdorferi (white spirals), the major cause of Lyme disease in the US.

Figure 1: Prairie dogs are asymptomatic reservoirs for the plague.


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2. Lyme disease is characterized by all of the following EXCEPT:

.a it’s one of the Top 10 emerging infectious diseases worldwide, according to the CDC

.b it can be caused by at least three different Borrelia bacteria, a Gram-positive spirochetes (spirilla)

.c it’s named after the village of Lyme, Connecticut

.d it’s carried by ticks

.e it’s the most common tick-borne disease in the Northern Hemisphere

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been suggested that reducing the deer population in areas with the highest Lyme disease rates from current levels of 60 or more per square mile to 8 to 10 per square mile could reduce tick numbers down to levels too low to spread Lyme and other tick-borne diseases.

Borrelia burgdorferi is transmitted to humans and deer from its natural reservoir in rodents by deer ticks that feed on both humans and deer. Deer are the most common hosts for adult stage ticks; humans are “accidental” hosts because the bacteria from infected people are not transmitted to other humans or deer. Lyme disease bacteria are transmitted from nymph ticks to rodents like the white-footed mouse, and then back from the mouse to the larval ticks.

It works like this: the nymph tick typically feeds on the mouse for several days. If the tick is infected with B. burgdorferi the mouse becomes chronically infected, particularly in the skin where the tick has fed. Then when uninfected ticks feed on the rodent the bacteria passes back to the next generation of ticks. The back and forth transmission is essential because the female tick does not transfer bacteria directly to her offspring. The mice are therefore an essential bridge that allows bacteria to be transmitted from one generation of deer tick to the next.

The vector

The nymph tick is also the major vector that transmits the bacteria to humans. When the tick bites a human two important events occur: First, the bacteria in the tick is exposed to warm mammalian blood. This allows it to change a surface protein it produces to a form that helps it transmit to humans. If the feeding ticks are removed before this changeover of protein is complete, i.e., within 24–36 hours, the bacteria will not transmit successfully to humans. This is why human infection is quite rare — only about 2% of tick bites result in Lyme disease. The second event is that the bacteria is injected into the human along with tick saliva. The saliva contains substances that interfere with the immune response at the site of the bite. This prevents the human host from feeling an itch or pain from the bite, and provides a protective environment that allows the bacteria to survive.

Figure 3: Black legged deer ticks maintain the reservoir of B. burgdorferi in nature. The Nymph form is most respon-sible for human infection because it is so small it often goes unnoticed.

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Endemic — in the case of an infectious disease, it means that an infection remains constantly present in a certain population without having to be imported.


3. Which statement about Lyme disease is true?

.a it is caused by the increase in deer population.

.b it is caused by the Borrelia bacteria which is carried by ticks that reproduce in white-tailed deers.

.c it is caused by white-tailed deers that produces 3 different Borrelia bacteria.

.d none of the above

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The life-cycle of Lyme disease

The 2-year cycle of deer ticks has four stages: eggs, larvae, nymphs, and adults. Larvae hatch in the summer. They are uninfected, but soon acquire B. burgdorferi by feeding on infected mice. The infected larvae molt into nymphs in the fall and are dormant through the winter. Infected nymphs feed on mice in the late spring and early summer. This sets up a natural reservoir of B. burgdorferi. The chronically infected mice then transmit B. burgdorferi to the next generation of uninfected tick larvae. Infected nymph ticks also feed on humans in the late spring and summer causing a peak of human Lyme disease. The nymph ticks then molt into adults.

The natural cycle of B. burgdorferi is important in how Lyme disease occurs. First, Lyme disease is endemic only in regions were all the players in its life cycle coexist. In the US, this is the northeast and mid-Atlantic coastal states, and the Great Lakes regions. Second, most human infections occur in the spring and summer months, when outdoor activities bring people into contact with feeding nymph ticks. Since the ticks are extremely small (approximately 1 mm in length) they are often not discovered for several days, allowing the B. burgdorferi to be activated and transmitted.

Figure 4: The life cycle of the deer tick that is the vector for Lyme disease.


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Syphilis — a sexually trans-mitted disease caused by the

Treponema spirochete.

Encephalitis — inflammation of the membranes covering the

brain.

Cerebrospinal fluid (CSF) — the fluid that bathes the brain and

the spinal cord.

Lumbar puncture — also called spinal tap, is a procedure used to collect CSF from the spine with a

needle.


4. Which is NOT a sign nor symptom of Lyme disease?

.a neurological problems like facial paralysis and encephalitis leading to memory loss

.b circular “bull’s-eye” rash at the site of the bite caused by an inflammatory response to the bacteria

.c fever, headache, muscle soreness, fatigue

.d chronic inflammation of the joints, the heart, the central nervous system, and the skin

.e none of the above __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Lyme disease signs and symptoms

Like syphilis, which is also caused by similar bacteria, Lyme disease progresses through several stages and affects many organs. The incubation period from the infection to the onset of symptoms is usually one to two weeks, but can be much shorter (days), or much longer (months to years).

The classic sign of Stage 1 infection is a circular ‘bull’s-eye’ rash at the site of the bite caused by an inflammatory response to the bacteria. It looks like a bull's-eye because the tick saliva inhibits the inflammation in the area closest to the bite. Only 80% of patients display a ‘bull’s-eye’ rash. During Stage 2, the bacteria enter the bloodstream and spread to the joints, heart, nervous system, and distant skin sites where they cause a variety of symptoms such as fever, headache, muscle soreness, and fatigue. Some patients also suffer from neurologi-

cal problems like facial paralysis and encephalitis leading to memory loss. If the infection is untreated, Stage 2 symptoms may progress to Stage 3 with chronic inflammation of the joints, the heart, the central nervous system, and the skin.

Diagnosing Lyme disease

Lyme disease is usually first diagnosed by the ‘bull's-eye’ rash, facial paralysis or arthritis together with a history of possible exposure to infected ticks. Many people may not recall a tick bite. Borrelia bacteria are very hard to grow in the laboratory, so diagnostic tests measure the amount of antibodies in the blood or cerebrospinal fluid (CSF) via lumbar puncture. Unfortunately, the blood screens only detect 64% of all infections at early stages of the disease! Although this rises to 100% in people with advanced symptoms, such as arthritis, the disease is much harder to treat at this point.

Figure 5: Stages of Lyme disease: Lyme disease can persist for many years and infect a large number of organs. It can be extremely difficult to eradicate once the bacteria have entered the brain, joints and heart.

Figure 6: Lyme disease: up to 80% of patients display a ‘bull’s-eye’ rash.


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Autoimmune reaction — an attack on the host by the

immune system.

WHO — World HealthOrganization is an agency of the United Nations which oversees

international public health issues.

DDT — dichlorodiphenyltrichlo-roethane, a chemical used to kill

insects.


5. To reduce the spread of Lyme and other tick-borne diseases we should

.a design better diagnostics.

.b reduce the deer population.

.c design a better vaccine for humans.

.d All of the above.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Preventing and treating Lyme disease

Removing ticks within 36 hours can reduce transmission of Borrelia to nearly zero. Checking your body for ticks after outdoors activities is a good habit, especially in areas where Lyme disease is prevalent. However, it is important to remove the entire tick, not leaving the head embedded in the skin; leaving the head of the tick behind may allow for transmission of the bacteria even after the body of the tick is removed! The use of protective outdoor clothing such as hats, long-sleeved shirts, and trousers tucked into socks or boots, as well as the use of insect repellents can also minimize exposure to ticks. Outdoor pets can also bring ticks into the house, so medications that prevent ticks form biting

pets can reduce the transmission of Lyme disease to pets and humans.

Antibiotics are the primary treatment for Lyme disease but they are not always effective. Up to one third of Lyme disease patients who have completed a course of antibiotic treatment, continue to have symptoms. Some doctors and scientists believe that the symptoms persist because the Borrelia infection has not been eliminated or because the patient has been co-infected with other tick-borne infections. Other doctors believe that the initial infection may cause an autoimmune reaction that continues to cause serious symptoms even after the bacteria have been eliminated.

Malaria: The most important of all parasitic diseases

Malaria, the most important of all parasitic diseases, occurs in many tropical and semitropical regions. According to the WHO’s 2014 World Malaria Report, in 2013, around 198 million cases of malaria occurred globally, killing around 584,000 people. Ninety percent of malaria-related deaths occur in Sub-Saharan Africa, and 78% of these deaths account to children under the age of 5. Of all infectious diseases, malaria is considered to have caused the greatest harm to the greatest number of people. Malaria was endemic in some areas of the US in the 1800s but has since been eradicated with DDT. Currently, in the US there are about 1,500 cases diagnosed per year and they arise from travelers who

have been in an endemic area, or, rarely from infected mosquitoes that have arrived in planes. But this may not necessarily continue. If climate change continues along its current trajectory, mosquitoes are expected to regain a foothold in many areas (see Fig. 9) currently considered too temperate to support them.

Figure 8: Antibiotics are the primary treatment for Lyme disease but they are not always effective.

Figure 7: After spending time outdoors, it's time for a tick check!


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Protozoa — a group of single-celled eukaryotes.

Sporozoite — the form of the malaria plasmodium that repli-

cates in the liver.


6. Which is true about the transmission of malaria to humans?

.a It occurs through the bite of infected female anopheles mosquitoes.

.b The parasite is injected from the mosquito’s salivary gland into humans when it takes a blood meal.

.c The Anopheles mosquito is the vector while infected humans are the reservoir.

.d All of the above ______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

The malaria parasite and its vector

Malaria in humans is caused by 4 species of Plasmodium protozoa parasites — P. falciparum, P. vivax, P. ovale, and P. malariae. The four species vary in their ability to cause disease because they prefer red blood cells of different ages. Plasmodium falciparum is the most deadly because it invades red blood cells of all ages. Transmission of the parasite to humans occurs through the bite of infected female anopheles mosquitoes. The parasite is injected from the mosquito salivary gland into humans when it takes a blood meal. The parasite then travels rapidly to the liver, where it takes up residence in order to mature. The mature Plasmodium leaves the liver and enters red blood cells, where it divides multiple times, and as a result the red blood cells become full of parasites, and burst.

The natural cycle of malaria

The Anopheles mosquito is the vector, transmitting the malaria parasite to humans. Infected humans are the only actual reservoir.

The life cycle of the malaria parasite is rich in fascinating detail (see Fig. 11 on next page).

1. Let’s start at the beginning of the cycle with an infected female Anopheles mosquito, which has the parasite in her salivary glands.

2. When she feeds on a human, the parasite is injected into the bloodstream and immediately travels to the liver. It can enter liver cells within 30 minutes.

3. Over the next week or two the parasite, in the form of plasmodium sporozoites, multiplies in the liver cells generating large numbers. At the same time, as the sporozoites are dividing they are also maturing to a form that is capable of infecting red blood cells called merozoites.

4. When the merozoite is mature, it is released into the bloodstream, where it enters red blood cells. The merozoite grows and divides inside the red blood cells.

Figure 9: Climate change and malaria — the dark red areas show where malaria may expand to; current spread is in yellow tint.

Figure 10: A female anopheles mosquito whose abdomen is dis-tended with blood.


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Merozoite — the form of the malaria plasmodium that infects

red blood cells.

Anemia — a decrease in the red blood cells count or hemoglobin

levels in the blood.


7. The following can be used to diagnose acute malaria EXCEPT

.a a thick smear of blood is taken under a microscope slide and dyed to color the Plasmodium merozoites blue.

.b an accurate yet expensive PCR analysis.

.c serological testing.

.d none of the above ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

5. After another 2–3 days the infected red blood cells are so full of parasite, that they burst, releasing yet more parasites into the blood stream to infect more red blood cells.

6. While this is happening, a small number of merozoites develop further into male and female forms. If a mosquito feeds again from the human reservoir it can take in the male and female forms. They then develop in the mosquito into the sporozoites and the cycle repeats itself.

Malaria signs and symptoms

The symptoms of malaria are intense and generally show up within a month of infection as very high fever, chills and, later on, anemia (low numbers of red blood cells). The symptoms coincide with the release of large numbers of merozoites from the red blood cells, and occurs because the immune system has

recognized the presence of a foreign invader and has put its defense mechanisms on high alert. After the first parasites replicate in the red blood cells, the infection becomes synchro-nized, so that all infected red blood cells lyse at the same time. Other symptoms of malaria are also caused by the immune system response and resemble influenza (fever, muscle aches, and generally feeling ill). Stomach pain can also occur because of liver damage. Patients with symptoms of malaria are often misdiagnosed, especially if they don’t live where malaria is endemic and if the physician does not ask about travel.

Figure 11: The life cycle of Plasmodium in humans.

Figure 12: Plasmodium merozoites bursting out of red blood cells


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PCR analysis — a method for detection of DNA or RNA at low

levels via PCR amplification.

Serological testing — testing blood serum for the presence of antibodies against a foreign

agent.


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Diagnosing malaria

Malaria is diagnosed in the laboratory by taking a thick smear of blood onto a microscope slide and then using a dye to color the Plasmodium merozoites blue. Different types of plasmodia give different patterns of staining. PCR analysis to detect plasmodium is more accurate than blood tests but is expensive and requires expertise to be performed, and to interpret the results correctly. It isn’t useful in developing coun-tries. Unlike Lyme disease, serological testing for malaria antibodies is of little use for someone with acute infection because antibodies to the parasite do not develop for 3 – 5 weeks, but treatment must begin within 1 – 2 days to stop the infection from spreading to the liver.

Treatments and prevention

The malaria parasite has evolved several strategies to evade the immune system. As a result, people who live where malaria is endemic will still get infected on a regular basis. However, their infections are usually less severe, suggesting that the immune system can control the infection even if it cannot eradicate it.

Drugs are based on quinine, which accounts for the popularity of gin and tonic in the tropics — tonic contains quinine! Quinine stops the parasite from degrading the hemoglobin in red blood cells, thereby blocking a major source for its nutrition. The major antimalarial drug used to be chloroquine. Unfortunately, the parasite has developed resistance to it. Chloroquine-resistant malaria is now widespread in most of Southeast Asia, South America and Africa, and a cocktail of other antimalarials must be used. In addition, alternative measures can be used, including bug nets for windows and beds. They are simple but extremely effective because infected female mosquitoes feed at night.

Figure 14: A mosquito bed net soaked in insecticide can give reasonable protection for under $5, if used properly.

Figure 13: Detecting P. falciparum merozoites in blood samples. Red blood cells (light pink cells) containing the ring form of the malaria parasite (blue rings).


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Carcinogen — an agent that is able to cause cancer.


8. All of the statements below about DDT are true EXCEPT:

.a it is an insecticide.

.b it is used to eradicate the mosquitoes that spread malaria.

.c its use is controversial because it is toxic.

.d it is currently banned for use in the fight against malaria.

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Malaria Control — DDT — For and Against

Malaria remains a major public health challenge in many parts of the world. One way to eliminate infection is to eradicate the mosquito vector. Spraying with pesticide, especially DDT, has been effective but controversial. DDT (dichlorodiphenyltri-chloroethane) was first used to control malaria in the second half of World War II. In response to its success WHO started an anti-malaria campaign in the 1950s and 1960s that relied heavily on DDT. The results were promising. For example, in Sri Lanka, the program reduced cases from about 3 million per year before spraying to just 29 in 1964. Then the program was halted to save money, and malaria rebounded to 600,000 cases in 1968–1969. About the same time public awareness about problems with DDT led many governments to restrict or curtail its use. Animal studies have shown that DDT is a

carcinogen. There is also evidence that in humans DDT can cause liver damage as well as liver cancer, and that it has a negative impact on the reproductive system.

Current recommendations: the double edged sward

Currently DDT remains on the WHO’s list of recommended insecticides and its policy has shifted from recommending spraying only in areas of seasonal or episodic transmission of malaria, to also advocating it in areas of continuous, intense transmission, so DDT use is expected to rise. The major reason is the failure to curtail malaria through drug treatment to kill the parasite, because of the increasing prevalence of drug-resistant forms.

For spraying to be effective, at least 80% of homes and barns must be sprayed, so if enough residents refuse spraying, the whole program can be jeopardized. But there is no doubt that when used properly DDT can significantly curb malaria; however DDT resistance in mosquitoes, which is largely due to its overuse in agriculture, has greatly reduced its effectiveness in many parts of the world. Some countries that still use DDT have shown that alternative insecticides are less effective — in South Africa malaria incidence increased dramatically when the insecticide was switched, but returning to DDT and introduc-ing new drugs brought malaria back under control. Similarly malaria cases increased in South America after DDT use was stopped. Between 1993 and 1995 Ecuador increased its use of DDT and saw a 61% reduction in malaria rates, while other countries that gradually decreased DDT use at the same time saw large increases in malaria.

Figure 15: After DDT was a success against malaria and typhoid in World War II, the WHO instigated malaria eradication programs to spray mosquitoes.


STUDENT RESPONSES

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Remember to identify your sources

What are the public health issues involved with the diagnosis, treatment, and prevention of malaria?

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Why is it important to study the relationship of a pathogen with its environment beyond the human host? What opportunities

for treatment or prevention could be missed if you did not consider the pathogen beyond the human host?

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TERMS

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TERM DEFINITION

Anemia A decrease in the red blood cells count or hemoglobin levels in the blood.

Asymptomatic carrier A person or other organism that has contracted an infectious disease, but has no symptoms.

Autoimmune reaction An attack of the immune system of an organism against its own organs.

Carcinogen An agent that is able to cause cancer.

CDC Centers for Disease Control and Prevention is a federal US public health agency.

Cerebrospinal fluid (CSF) The fluid that bathes the brain and the spinal cord.

DDT Dichlorodiphenyltrichloroethane, a chemical used to kill insects.

Eastern equine encepha-litis virus (EEE)

A virus that spreads through mosquitoes. It can cause encephalitis.

Encephalitis Inflammation of the membranes covering the brain.

Endemic In the case of an infectious disease, it means that an infection remains constantly present in a certain population without having to be imported.

Lumbar puncture Also called spinal tap, is a procedure used to collect CSF from the spine with a needle.

Merozoite The form of the malaria plasmodium that infects red blood cells.

PCR analysis A method for detection of low levels of DNA or RNA with amplification by PCR (polymerase chain reaction). It is used for the detection of many infectious agents.

Protozoa A group of single-celled eukaryotes.

Reservoir A reservoir typically harbors the infectious agent without injury to itself, and serves as a source from which other individuals can be infected.

Serological testing Testing blood serum for the presence of antibodies against a foreign agent often timesan infectious agent.

TERMS

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TERM DEFINITION

Spirochete Spiral shaped bacterium.

Sporozoite The form of the malaria plasmodium that replicates in the liver.

Syphilis A sexually transmitted disease caused by the Treponema spirochete.

Vector An organism that transmits a pathogen from reservoir to host.

WHO World Health Organization is an agency of the United Nations which oversees international public health issues.

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LESSON 3.2 WORKBOOK - Dashboard - Confluence2.2+Lesson+3.2+Workbook.pdf · Workbook Lesson 3.2 ... Endemic — in the case of an ... Figure 4: The life cycle of the deer tick that