PU535 Unit 3 Transmission Pathway Analysis Example

Fifteen Days From One Neighbor to the Next: A Transmission Pathway Analysis of Urban Dengue That Prints a Duration on Every Arrow, From a Viremic Resident Through a House-Bound Mosquito to the Susceptible Household Next Door

[Student Name]

Purdue University Global

PU535: Public Health Biology - A Global Perspective

Unit 3 Transmission Pathway Analysis

[Instructor Name]

[Date]

The city, its water schedule and its case counts are composite; the durations, the vector behavior and the research cited are real.

The Chain in Five Boxes

1. A resident with virus in the blood, infectious to mosquitoes for about five days.

2. A female Aedes aegypti takes a blood meal from that resident and acquires the virus.

3. The virus multiplies inside the mosquito and reaches the salivary glands after about eight to twelve days.

4. The same mosquito bites a susceptible neighbor and delivers virus in its saliva.

5. That neighbor incubates the virus for four to ten days before fever, if fever comes at all.

Each link below is written in the same three parts: what happens, how long it takes, and what the surroundings contribute. Keeping one pattern makes the links comparable, which matters because control works on whichever link is weakest.

Link 1: The Person Who Is Infectious

A person infected with dengue carries virus in the blood for roughly five days, starting a day or so before fever and ending as the fever breaks. Any mosquito biting during that window can pick the virus up. The brief point that decides the shape of an urban outbreak is who counts as that person. Most infections never produce illness severe enough to reach a clinic, and people with no symptoms at all still carry enough virus to infect mosquitoes feeding on them (Duong et al., 2015). The composite city's 1,180 reported cases therefore stand in for a far larger number of infectious person-days, most of them in people who never stopped working.

Link 2: The Blood Meal

Only the female mosquito feeds on blood, and she needs it to make eggs. Aedes aegypti feeds in daylight, often indoors, and often takes several small meals from several people to fill one batch of eggs. That habit matters more than any single property of the insect: a mosquito that bites four people in a household instead of one turns a single infectious resident into four chances of acquiring the virus, and later into four chances of passing it on. The surroundings contribute here through screens, bed nets used in daytime and repellent, all of which lower the number of bites rather than the number of mosquitoes.

Link 3: Inside the Mosquito

Once swallowed, the virus has to cross the mosquito's gut, multiply, and reach the salivary glands before any later bite can transmit it. That interval, the extrinsic incubation period, averages about nine days and falls between roughly five and fifteen depending on temperature (Chan & Johansson, 2012). Heat shortens it; a cool week stretches it. The mosquito stays infective for the rest of her life afterward, so the critical question is whether she survives the waiting period at all. A female that lives two weeks may transmit several times; one that lives six days transmits nothing, however many times she has bitten. This is why the classic vector models treat daily survival as the term that dominates transmission (Smith et al., 2012). A mosquito that dies on day seven of a nine-day incubation has fed, acquired the virus and infected nobody.

Link 4: The Bite That Puts the Virus Back

Transmission happens through saliva injected while the mosquito probes for a vessel, which is why a bite that is interrupted can still be infective. The surroundings decide who is available to be bitten. Because Aedes aegypti seldom travels far from the house where it emerged, usually staying within the same block or the next one, the people bitten are the people who live and work beside the water that produced the mosquito (Harrington et al., 2005). Dengue in this city therefore spreads house to house and street to street rather than across town, except where people carry the virus themselves on a bus or a motorbike.

Link 5: The Susceptible Neighbor

After an infective bite, four to ten days pass before any fever, with about six days typical (Chan & Johansson, 2012). Susceptibility is specific: four dengue serotypes circulate, infection with one gives lasting protection against that one only, and a later infection with a different serotype carries a higher risk of severe illness. In a city where three serotypes have circulated in a decade, the pool of people susceptible to the serotype arriving this season is large even among those who have had dengue before.

Where the Mosquitoes Come From

The habitat belongs inside the vector link, not in a separate heading, because container water is what keeps the chain turning between rains. Piped water arriving three days a week means that nearly every household stores water, in drums, buckets and cement tanks beside the door. These containers hold clean, shaded standing water within meters of sleeping people, which is what Aedes aegypti needs; a survey of containers in such neighborhoods usually finds a small minority of container types producing most of the adult mosquitoes. Rain adds breeding sites, but domestic storage supplies them all year, which is why cases in the composite city never fall to zero in the dry months.

Adding the Durations Up

Printing a number on each arrow lets the analysis do arithmetic the boxes alone cannot. One turn of the cycle runs from a person becoming infectious, through about nine days inside the mosquito, to a neighbor's fever six days after the infective bite: roughly fifteen to twenty days from case to case. That figure explains the city's reported pattern, where surveillance plots show case peaks two to three weeks apart rather than a smooth rise. It also sets the deadline for action. A household visit made three weeks after a reported case arrives after the generation it was meant to interrupt, which is why vector teams in dengue programs are dispatched within days of a case report rather than weekly.

Which Link Control Should Break

Each link invites a different intervention, and the durations say which is worth the money. Removing or covering domestic containers cuts the number of adult mosquitoes and acts on every link at once, but only if it reaches most households, since mosquitoes from one uncovered drum serve the whole block. Screens and daytime repellent reduce bites in one house and protect that house alone. Treating patients does nothing to the chain, since no treatment shortens human viremia. Measures that kill adult females, including indoor spraying during an outbreak, act on the term the models say matters most, daily survival, but their effect lasts only as long as the insecticide does. The analysis ends by recommending container reduction as the standing program, with adult control reserved for the weeks when case reports cluster, and with each recommendation tied to the arrow it shortens. Dengue's global expansion gives that argument urgency: the number of infections each year is now estimated in the hundreds of millions, most of them in cities of this kind (Bhatt et al., 2013).

References

Bhatt, S., Gething, P. W., Brady, O. J., Messina, J. P., Farlow, A. W., Moyes, C. L., Drake, J. M., Brownstein, J. S., Hoen, A. G., Sankoh, O., Myers, M. F., George, D. B., Jaenisch, T., Wint, G. R. W., Simmons, C. P., Scott, T. W., Farrar, J. J., & Hay, S. I. (2013). The global distribution and burden of dengue. Nature, 496(7446), 504-507. https://doi.org/10.1038/nature12060

Chan, M., & Johansson, M. A. (2012). The incubation periods of dengue viruses. PLoS ONE, 7(11), e50972. https://doi.org/10.1371/journal.pone.0050972

Duong, V., Lambrechts, L., Paul, R. E., Ly, S., Lay, R. S., Long, K. C., Huy, R., Tarantola, A., Scott, T. W., Sakuntabhai, A., & Buchy, P. (2015). Asymptomatic humans transmit dengue virus to mosquitoes. Proceedings of the National Academy of Sciences, 112(47), 14688-14693. https://doi.org/10.1073/pnas.1508114112

Harrington, L. C., Scott, T. W., Lerdthusnee, K., Coleman, R. C., Costero, A., Clark, G. G., Jones, J. J., Kitthawee, S., Kittayapong, P., Sithiprasasna, R., & Edman, J. D. (2005). Dispersal of the dengue vector Aedes aegypti within and between rural communities. The American Journal of Tropical Medicine and Hygiene, 72(2), 209-220. https://doi.org/10.4269/ajtmh.2005.72.209

Smith, D. L., Battle, K. E., Hay, S. I., Barker, C. M., Scott, T. W., & McKenzie, F. E. (2012). Ross, Macdonald, and a theory for the dynamics and control of mosquito-transmitted pathogens. PLoS Pathogens, 8(4), e1002588. https://doi.org/10.1371/journal.ppat.1002588