Last of the Mosquitoes

AnnouncementsSpeaking Today???On Deck for Next Tuesday: Sean Allen, Lauren TorbettAbout the Quiz …Read Chapter 16 for next Tuesday, Chapter 17 for next Thursday

Yellow Fever

• Historically second only to malaria but now lags dengue.• Potential for catastrophic outbreaks persists.• Like Dengue, a Flavivirus• Called “Yellow Fever” because of the jaundice that it

often creates• First arbovirus (arthropod borne virus) associated with

human disease.– Carlos Finlay, Cuban physician, first suggested the connection

between mosquitoes and yellow fever– Walter Reed conducted human experiments with US soldier

volunteers to confirm the connection.– Reed’s discovery enabled the US to complete the Panama

Canal where the French could not.

Principal Endemic ZonesNote: Yellow fever has never been reported in Asia

Population here will double between 2009 & 2020

Three cycles• Endemic (Sylvatic)

– Cycles in arboreal monkeys– Mosquitoes are not common ground level– Loggers encounter infected mosquitoes– Transmission: monkey – mosquito – human is one way– Produces sporadic cases

• Intermediate– Monkey – human and back– Mostly African savannah– Most common form of outbreak in Africa

• Epidemic (Urban)– Loggers enter a town with container breeding mosquitoes (esp. Ae.

aegypti)– Mosquitoes acquire the virus– human – mosquito – human cycle established.

Symptoms• Increasing Viremia 3 – 6 days post-infection• Initial flue-like symptoms: fever and chills, severe headache, back pain,

general muscle aches, nausea, fatigue, and weakness• 1-2 day remission• “Toxic phase”- flu-like symptoms return, with abdominal pain (from hepatic

involvement). • Hepatic coagulopathy produces jaundice and numerous hemorrhagic

symptoms:– Hematemesis (black vomit)– Epistaxis (nose bleed)– Gum bleeding– Petechial and purpuric hemorrhages (bruising).

• Late stage (“malignant”) hypotension, shock, metabolic acidosis, acute tubular necrosis, myocardial dysfunction, and cardiac arrhythmia.

• Terminal stage - Confusion, seizures, coma, death from cardiac failure• Survivors of the hepatic phase often have secondary bacterial infections

and kidney failure.• Weakness/fatigue may last months in survivors.

Encephalitis

• Encephalitis – an acute inflammation of the brain– Encephalomyelitis - When both the brain and spinal

cord are involved– Encephalomyelitis often follows viral encephalitis and

so is often referred to as “post-infection encephalitis”

• Separate from but may occur with meningitis• Can be either bacterial (including protozooan) or

viral• Among the viral forms, arboviruses are the most

common (other common forms are HIV secondary, rabies, herpes simplex)

Arboviral Encephalitis

• Most arboviruses can cause encephalitis

• All important mosquito viruses can cause encephalitis

• Only some of these have encephalitis as a primary symptom

• Survivors of the initial encephalitis proceed to encephalomyelitis hence the name for the mosquito viruses.

Mosquito Encephalomyelitis Viruses in North America

• Togoviridae (all Alphaviruses) – Bird viruses– EEE (most pathogenic to humans)– WEE– VEE

• Flaviviridae (JE complex) – Bird viruses– West Nile– SLE

• Bunyaviridae – Chipmunk/Squirrel viruses– LaCrosse

MOSQUITO SURVEILLANCE PROGRAMS

• Used for Anticipated Health Threats

• Mosquito surveillance programs detect:– When each species is present– When each life stage is present– Disease risk

• Conducted by public health agencies

• May be combined with disease surveillance programs

Two kinds of surveillance 

• Mosquito population monitoring – Eggs– Larvae– Adults

• Impact Assessment – Disease risk– Economic impact

• Most economic assessments are veterinary

Population Monitoring: Eggs

Oviposition Traps (see example)For container breeders

Sod/Soil/Debris SamplingRequires a means of separating eggs from sample (see machine on right)

Gravid Traps (see example)Culex/Coquillettidia egg rafts can usually be found

Larval Sampling – Main sampling tool is the larval dipper

Important note: This tool does NOT indicate absolute populations as commonly used but is a relative measure. Typically used for presence/absence sampling.

Different techniques are used with different habitats/species

• Shallow Skim- Anopheles

• Partial submersion- Around emergent vegetation,

• Complete submersion- For quick diving species such as some Aedes and Psorophora,

• Dipper as a background- woodland pools, for early season species.

• "Flow in" method- Shallow water with debris.

• Scraping- Sampling from submerged clumps of vegetation.

• Simple scoop- Most common, especially for Culex.

• Salt marsh- Standard Protocol for potholes with vegetation.

Sampling Adults

• Males – Not interested in them

• Blood questing females – Light/CO2 traps

• Gravid females – Gravid traps

• General – Resting Boxes (Culeseta & Anopheles)

• Non-attraction traps – sample at random

CDC - Light/CO2 Baited Trap• Can use gas, dry ice, or

chemical sachet• Generally overnight• May be used with light• Some differentiate by time

Other Traps for Blood Questers

Mosquito magnet used as a sampling tool Sonic trap mimics the

sound of a dog’s heartbeat

Gravid Traps1. Generally left overnight

2. The type of water affects trap catch

3. Very susceptible to rainfall

4. May also find eggs on/near water

Impact Assessment

• Biting Rate– Human Bait– Animal Bait

• Disease Risk– Prevalence in Mosquito Population– Prevalence in Host Population

Human Landing/Biting Rate1. Fixed time interval2. Sample at same time of day

3. Standardized human

4. Widely restricted

Sentinel Animals

More commonly used as an early warning for disease risk

Landing Rates & Impact

• Human Nuisance – Must know two things:– Number of bites people are willing to tolerate– Bite rate that results in action

• Disease risk – Must know the proportion of bites that result in new infections

Final Thoughts on Mosquito Population Sampling

• All commonly used methods are relative – None give absolute estimates.

• Different species are sampled at different rates for each technique.

• Generally, must pre-define which species in the mosquito complex is the target of a sampling program then select the appropriate technique(s).

Determining Disease Prevalence in Mosquito Populations

Sort into pools

Many diseases have simple strip tests available (tend to be around 70% accurate, but their cheap & easy).

This one costs $60 & does 20 tests

More accurate and sophisticated techniques are available

• Most are based on an amplification procedure.

• Such procedures require facilities with appropriate containment as well as specialized equipment & training.

• Cost prohibitive in many management programs (typically $20 – 25/test).