Lecture 15 Learning Objectives

Explain how yellow fever spread to the Americas

Endemic in Central Africa; introduced into the Americas from the transatlantic slave trade -> forced movement of enslaved West Africans; many Africans had partial immunity due to long-term exposure, but Europeans & Native Americans had it severe; Aedes aegypti mosquitoes transmitted the virus & produced major urban epidemics

Explain how yellow fever impacted historical events such as the Louisiana Purchase and the Spanish American War

Louisiana Purchase: Napoleon sent ~30,000 French troops to suppress Haitian Revolution. Yellow Fever killed ~27k. This weakened France's colonial ambitions & made the sell Louisiana territory to US. Spanish-American War:

Yellow Fever was endemic in Cuba so it threatened US troops; US Army Yellow Fever Commission was formed & sent to Cuba to determine how the disease is transmitted

Explain how the mechanism of yellow fever transmission was discovered

Early discovery: mosquitoes could act as disease vectors.

Later (Walter Reed & Yellow Fever Commission): present in blood; passed thru bacteria-trapping filters, indicating that it was a virus; was not transmitted directly between ppl.

Identify the vector of flavivirus infection (Genus species)

Genus: Aedes;

Species: Aegypti;

Name: Aedes Aegypti

Explain the factors that make a pathogen eradicable

1) Limited host range: preferably only infect humans;

2) Effective intervention: vaccine/vector-control method must reliably interrupt transmission

Explain why yellow fever is not a good candidate for eradication

Yellow fever circulates in the sylvatic cycle in non-human primates; this makes complete eradication more difficult than originally thought.
sylvatic cycle: transmission of yellow fever between forest mosquitoes and non-human primates

Describe the vaccine design and efficacy for yellow fever

virus was attenuated by repeated passages in chick embryo fibroblasts;

this vaccine remains one of the most effective viral vaccines ever developed.

Explain the advantages and disadvantages of using DDT as a control method for yellow fever

Advantage: DDT spraying reduced Aedes aegypti populations and contributed to a decline in urban yellow fever transmission.

Disadvantages: harmful effects on wildlife, especially birds and aquatic life;

Explain the advantages and disadvantages of using SIT to control flaviviruses

SIT: male mosquitoes are sterilized using non-lethal radiation or chemicals, resulting in mutations in the germline; when they mate, offspring die or are non-viable; goal is to reduce mosquito population by preventing reproduction.

Advantage: releases sterile males to out-compete wild males and reduce reproduction.

Disadvantage: requires release of large numbers of males and does not directly remove existing adult females.

Explain the advantages and disadvantages of using RIDL to control flaviviruses

RIDL: release of insects carrying a dominant lethal; mosquitoes are genetically engineered using transposon-like genetic elements containing specific genes; construct includes a lethal gene that disrupts development and a promoter that ensures the gene is only active in female mosquitoes during early development; released males mate with wild females -> no biting females -> reduced transmission.

Advantage: reduces biting female mosquitoes.

Disadvantage: requires genetically engineered mosquitoes and release of large numbers of males.

Explain the state of yellow fever now and what factors allow the problem to persist

Yellow fever remains a major public health concern in Africa, while cases in the Americas are now rare and typically linked to sylvatic transmission. Approximately 200,000 cases occur annually and 30,000 deaths; 90% of cases occur in Africa.

Factors include loss of vector-control programs, DDT restrictions, sylvatic transmission, and barriers to vaccination.

Describe how Wolbachia infected Aedes is used to control flaviviruses

Wolbachia-infected mosquitoes have decreased life span and lower viral replication, making them less likely to spread dengue and yellow fever.

Identify the regions of the world most at risk for yellow fever infection

Africa and South America, especially sub-Saharan Africa and tropical areas of South America.

Describe the factors contributing to the re-emergence of yellow fever in sub-Saharan Africa and South America

Ban of DDT and loss of vector-control programs in South Africa

In South America, outbreaks are linked to human intrusion into forested areas where yellow fever circulates in non-human primates