West Nile Virus

This activity uses the pre-made Stella model. To download, you must hold down the "ctrl" key and click. Then select the "download link option". We will investigate four assumptions: (you may now open your models)

• Temperature - The average daily temperature.
• VirulenceinB - The virulence of the virus in birds when birds don't need to cluster around a few sources of water. VirulenceinB = 1 means that birds always get sick, 0 means they never get sick.
• InitialSickBirds - The number of birds infected at the beginning.
• BdaytoRecover - For the birds that recover, the number of days that they can infect mosquitoes that bite them.

The investigation strategy is to first look at the effects of different assumptions about how virulent West Nile Virus is in birds. We then look at how a change in temperature interacts with the virulence. Finally, we look at the effect of having more sick birds at the beginning of summer.

Base Case: Open the model. If you need to clear the graph, use the Dynamite icon at the bottom of the graph. Set the sliders at:

• Temperature = 75
• VirulenceinB = 0.1
• InitialSickBirds = 0
• BdaytoRecover = 3

Run the model.

Bdead is the number of dead birds each day. How many birds are dead initially? Why does it stay zero for the first two weeks (days 1 to 14)? (Hints: How high is the curve the first day? You used one of the sliders to tell the model how many sick birds there were the first day. The model has assumptions about how quickly the birds get sick and die)

MvirusIncubate is the number of mosquitoes that have the virus but are not yet infectious. At this point, we say the virus is incubating. How many incubating mosquitoes are there in the model initially?

Why is the number of mosquitoes that are incubating the virus decreasing in the first two weeks? (Hints: Why do mosquitoes start incubating the virus? Why do mosquitoes quit incubating the virus?)

Why does the number of mosquitoes incubating the virus start to increase in the third and fourth weeks?

Minfectious is the number of infectious mosquitoes. How many infectious mosquitoes are there at the beginning? Why does the number of infectious mosquitoes increase for the first two weeks? Why does it fall rapidly during the third week?

Binfectious is the number of birds that can infect mosquitoes that bite them. Why does the number of birds that can infect mosquitoes increase in the first two weeks? The number of infected birds goes down in the third week and then back up. Why does this happen?

How many infectious birds are there at the end of the summer (day 120)? Is the number of infectious birds increasing or decreasing? Why?

Moderate virulence: Use the Dynamite icon at the bottom of the "Population Changes in Current Run" graph to clear it. Set the sliders at:

• Temperature = 75
• VirulenceinB = 0.2
• InitialSickBirds = 0
• BdaytoRecover = 3

Run the model.

How have the curves changed? Why do the curves look different in the last three months of the graph (days 30-120)?

In general, during the first month, you see the effects of the 40 infected mosquitoes that were there at the beginning. After about day 50, how the disease spreads or fails to spread is due to how different assumptions make the disease more or less infectious.

Extra credit: Can you find the "tipping point" for VirulenceinB, where the curves go from decreasing in the last three months to increasing there? One way to do this is to try all the values between 0.1 and 0.2. Another way is to set the slider half way between 0.1 and 0.2, run the model, and see how the curves go. If the curves still decrease, move the slider halfway between 0.15 and 0.2 and run the model again. (The tipping point is about 0.15.)

High virulence: Use the Dynamite icon at the bottom of the "Population Changes in Current Run" graph to clear it. Set the sliders at:

• Temperature = 75
• VirulenceinB = 1.0
• InitialSickBirds = 0
• BdaytoRecover = 10 (This is higher than what has been observed in real birds so far, but there might be a new version of the virus.)

Run the model.

What does having VirulenceinB = 1 mean?

How have the curves changed now? Why?

Note: It is hard to tell from the graph what happens during the first month. You could export the values to Excel to investigate this further.

Temperature Effects (1): Use the Dynamite icon at the bottom of the "Population Changes in Current Run" graph to clear it. Set the sliders at:

• Temperature = 90
• VirulenceinB = 0.1
• InitialSickBirds = 0
• BdaytoRecover = 3

Run the model.

How have the curves changed from the Base Case now? Why?

Temperature Effects (2): Use the Dynamite icon at the bottom of the "Population Changes in Current Run" graph to clear it. Set the sliders at:

• Temperature = 90
• VirulenceinB = 1.0
• InitialSickBirds = 0
• BdaytoRecover = 10 (This is higher than what has been observed in real birds so far, but there might be a new version of the virus.)

Run the model.

How have the curves changed from the High Virulence Case now? Why?

Migration of Sick Birds: Use the Dynamite icon at the bottom of the "Population Changes in Current Run" graph to clear it. Set the sliders at:

• Temperature = 75
• VirulenceinB = 0.1
• InitialSickBirds = 10
• BdaytoRecover = 3

Run the model.

Does changing the initial number of sick birds or mosquitoes change whether the disease is increasing or decreasing at the end of the summer? Why or why not?

Extra Credit: You can go into the model and change a number of other assumptions.

• StandingWater - The amount of water in ponds, old tires, flowerpots, and so on, as a percentage of normal. So 100 means that all the ponds are full, 80 means that ponds are drying up, and 120 means that there is flooding.
• BmortalityRate - The fraction of the birds that die instead of recovering.
• BdaystoDie - The number of days that birds who eventually die are sick.
• Populations of susceptible and infected birds and mosquitoes.

Can you use the table of information about West Nile Virus in different kinds of birds to calculate values for mortality rates and days to die for a population that is 1/3 American crows, 1/3 robins, and 1/3 blue jays? (Answer: Take averages from the numbers in the table. So the days infectious would be 1/3* (3.8 + 4.5 + 4) = 4.1)

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