Dengue I: epidemiologyBill Indge

Dengue

Mosquito

HumanMonkey

The cycle of infection

Dengue

Egg

Larva

Pupa

Adult

• Female mosquitoes need a blood meal for their eggs to develop.• Male mosquitoes feed mainly on nectar. They do not feed on

blood.

Eggs are laid in water containers and tree holes, close to or in houses. The eggs can survive out of water for periods of 6 months or more.

Larvae feed on small aquatic organisms

The main vector — Aedes aegypti

Dengue

• Humans are the main hosts of the virus but it can also infect monkeys.

• When a female mosquito takes a blood meal from a person infected with dengue fever, the cells lining her gut become infected with the virus. The virus then spreads to other tissues in the mosquito including the salivary glands. When the mosquito bites another person, saliva containing the virus is introduced into their blood.

• There is a brief incubation period in a person infected with the virus of 4–7 days before the symptoms become apparent.

• Many people infected with dengue virus show only mild symptoms such as an uncomplicated fever. Others are severely ill, and in a small proportion dengue is life-threatening.

The virus

Dengue

Aedes aegypti Aedes albopictus

Closely associated with human habitation. Often found indoors.

Associated with thickets and wooded areas. Mainly a garden mosquito

Feeds mainly on humans and to a lesser extent, domestic animals.

Feeds on humans and a wide range of other vertebrates

Usually bites more than one person to complete blood meal

Usually completes blood meal from one person

Needs several blood meals to complete its cycle of egg-development

Needs only one blood meal to complete its cycle of egg development

The vectors of dengue

Dengue

A. aegypti and A. albopictus are two important vectors of dengue. Which of these two species do you think is the more important vector? Use the information in the table to explain your answer. (4 marks)

The key to answering a question like this is to note the instructions:

• ‘Explain’ means that a reason is required, so you will not get credit for simply copying information from the table.

• ‘Use information in the table’ means that you must do this if you want to gain marks. It would be a good idea to refer specifically to the table in your answer.

Before writing anything, think about the mark allocation. 4 marks means you need to make four valid A-level points, so:

• you are not likely to score anything for making a 50:50 decision as to which species is the more important vector.

• there are 4 marks available and four comparisons are made in the table. It would seem logical to try to offer an explanation based on each of these comparisons.

Dengue

• The evidence suggests that Aedes aegypti is the more important vector.

• Closely associated with people/occurs more often in houses so more likely to come into contact with humans.

• Feeds mainly on humans so more likely to be carrying dengue virus/more likely to have bitten infected person.

• Bites more than one person per blood meal/bites several people per cycle of egg development. This increases the number of people to which it can transmit the virus.

• Bites more than one person per blood meal/bites several people per cycle of egg development. This increases the probability of biting an infected person.

A. aegypti and A. albopictus are two important vectors of dengue. Which of these two species do you think is the more important vector? Use the information in the table to explain your answer. (4 marks)

Dengue

1950 1970 1990 2010Year

1.0

0.8

0.6

0.4

0.2

0

1.0

0.8

0.6

0.4

0.2

0

A newspaper article concluded that the increase in reported cases of dengue was caused by global warming. Use information in the graph to evaluate this conclusion. (3 marks)

Dengue

In simple terms, to evaluate something means to judge its worth, so examiners have to be fair to you. If they want you to evaluate something, there has to be more than one side to the argument.

In this case we ought to be looking for some evidence to support the argument and some evidence that fails to support it. Try this approach:

• Start by looking for evidence to support the conclusion. It is usually easy to find supporting evidence so you might just as well start with something straightforward.

• Now write the word ‘but’ and look for ways in which the data fail to support the claim.

A newspaper article concluded that the increase in reported cases of dengue was caused by global warming. Use information in the graph to evaluate this conclusion. (3 marks)

Dengue

• There is a positive correlation between the mean number of cases of dengue fever and the mean increase in global temperature/as the mean number of cases of dengue fever increases, so does the mean increase in global temperature.

(but)

• Correlation does not mean causation/just because two things are correlated, it does not mean that one causes the other.

• There may be other factors involved/example of another factor such as

more deforestation

increased urbanisation

increased travel

A newspaper article concluded that the increase in reported cases of dengue was caused by global warming. Use information in the graph to evaluate this conclusion. (3 marks)

Dengue

15 20 25 30 35 40

Temperature/°C

70

60

50

40

30

20

10

0

Time/ days

Time taken from egg hatching to mosquito becoming adult

Time taken from feeding until dengue virus appears in saliva of mosquito

Dengue

15 20 25 30 35 40Temperature/°C

100

80

60

40

20

0

Percentage of hatched larvae surviving to become adults

Percentage of adults completing a blood meal within 30 minutes

Dengue

It has been suggested that a mean increase in global temperature of 2°C would lead to an increase in the number of cases of dengue fever. This is because it would:

• shorten the time taken for the dengue virus to appear in the saliva of a mosquito after feeding

• shorten the mosquito life cycle, resulting in greater numbers of mosquitoes

• lead to mosquitoes biting more frequently because they would be dehydrated

Dengue

• The data show the effect of specific temperatures covering a range from 5 to 40°C. A rise of 2°C will have different effects at different starting temperatures.

• In addition, the data refer to specific temperatures maintained constant in the laboratory. In the field, temperatures vary on a daily and seasonal basis.

• Temperatures will also vary from one part of a mosquito’s range to another.

1 The data in the graphs were obtained from laboratory experiments. Care needs to be taken in using these data to support the suggestion above. Explain why. (2 marks)

Dengue

• The shorter the time spent feeding, the greater the probability of the mosquito surviving to transmit the virus/and biting another person.

2 (a) Explain how an increase in the percentage of adult mosquitoes completing a blood meal within 30 minutes could lead to an increase in the number of cases of dengue. (1 mark)

Dengue

• Between (approximately) 28 and 33°C, the time taken from feeding until dengue virus appears in saliva of mosquito shortens. This increases the probability that a mosquito will be able to transmit dengue virus.

• Between (approximately) 15 and 20°C, time taken from egg hatching to mosquito becoming adult decreases. This will allow more generations in a given period of time. Above (approximately) 20°C, however, the percentage of hatched larvae surviving to become adults decreases.

• There is no evidence here to support the suggestion that a rise in temperature would lead to mosquitoes biting more frequently because they would be dehydrated.

2 (b) To what extent do the other data in the graphs support the suggestion? Explain your answer. (3 marks)

This resource is part of Biological Sciences Review, a magazine written for A-level students by subject experts. To subscribe to the full magazine go to www.hoddereducation.co.uk/biologicalsciencesreview