7F Ch3 Cheat Sheet: Mosquito-borne Diseases

🎯 Top 5 Traps

1

Most important Michigan mosquito-borne diseases are caused by VIRUSES — not bacteria, not protozoans. WNV, SLE, EEE, LAC, JCV are all viral. Malaria (eradicated) is the only protozoan-caused mosquito-borne disease. Heartworm is a filarial worm — affects dogs, not the answer to "diseases of humans."

2

LAC = CHILDREN's disease. JCV = ADULTS more affected. Both are California encephalitis (CE) group viruses with similar symptoms, but the demographics flip. Easy exam trap — students who learn one assume the other behaves the same way.

3

EEE primary vector = Culiseta melanura (bird-only, the natural cycle). EEE Michigan BRIDGE vector = Coquillettidia perturbans. Cs. melanura keeps the virus circulating among swamp birds; Cq. perturbans is what feeds on infected birds AND on humans/horses. Don't confuse the two.

4

EEE fatality: 30 to 50 percent in humans, 90 to 95 percent in horses. Survivors often left with permanent neurological damage (mental retardation, behavioral changes, convulsions, paralysis). Children especially severe. The numbers are heavily exam-tested.

5

"Dead-end host" means the virus can't get back out — NOT that the host dies. A dead-end host is outside the natural transmission cycle: insufficient viremia to re-infect a mosquito. Humans, horses, and some bird species are dead-end hosts for WNV, SLE, EEE, and LAC. Most dead-end hosts survive infection.

🔢 Numbers You Must Know

Number

What It Represents

1937

Year WN virus was first isolated (West Nile province, Uganda)

2002 — 644 / 51

Michigan WNV epidemic year: 644 human cases, 51 deaths

1975 — 93 / 3

Michigan SLE outbreak year: 93 human cases, 3 deaths

30 to 50 percent

EEE fatality rate in humans

90 to 95 percent

EEE fatality rate in horses

4 to 20 percent

WNV/SLE historical fatality rate range — most deaths in age 50+

80 to 800 to 1

Silent-to-symptomatic infection ratio for WNV/SLE — most infections are asymptomatic

1 to 2 weeks

Mosquito extrinsic incubation period (EIP) at summer temperatures (longer when cooler)

3 to 4 days

Vertebrate viremia window during which mosquitoes can pick up the virus

1968

First reported human LAC case in Michigan (4-month-old boy)

late May to October

Seasonal range of LAC activity, coinciding with peak mosquito populations

about 25 percent

Share of Michigan mosquito species capable of transmitting dog heartworm

5 to 6 months

Time for heartworm to develop to adult stage in the dog (delays detection)

2 to 2.5 weeks

Heartworm larval development time inside the mosquito at Michigan summer temperatures

5 years / 2 years

Adult heartworm lifespan in canine / microfilariae lifespan

late August

Typical Michigan peak for Culex pipiens populations — coincides with WN/SLE human outbreaks

🔀 Easily Confused

Pair / Group

Distinguishing Feature

WNV vs SLE

Same Culex vectors (Cx. pipiens primary in MI), same wild-bird reservoir, similar urban/suburban ecology. WNV: arrived MI 2001, ongoing — corvids highly susceptible. SLE: rare in MI since 1975 outbreak (93 cases, 3 deaths).

LAC vs JCV

Both California encephalitis (CE) group viruses; both transovarially transmitted (from female mosquito to her eggs). LAC: Aedes triseriatus vector, small-mammal reservoir, CHILDREN affected. JCV: Aedes stimulans & Ae. communis (Midwest), deer (ungulate) amplifying host, ADULTS more affected.

EEE primary vs bridge vector

Primary (natural cycle): Culiseta melanura — feeds almost exclusively on birds, keeps virus circulating in swamp birds. MI bridge vector: Coquillettidia perturbans — feeds on infected birds AND on humans/horses, links the natural cycle to mammalian hosts.

Natural cycle vs Dead-end host

Natural cycle: virus circulates between reservoir vertebrate (usually birds or small mammals) and mosquito vector. Dead-end host: vertebrate that doesn't develop sufficient viremia to infect mosquitoes — outside the cycle. Humans, horses, some birds are dead-end for these diseases.

Viral vs Parasitic mosquito diseases

Viral: WNV, SLE, EEE, LAC, JCV — current Michigan threats. Parasitic: malaria (protozoan, eradicated in U.S.) and dog heartworm (filarial worm, animal disease primarily). Different agent classes, different control implications.

Urban (WN/SLE) vs Rural (EEE) ecology

Urban/suburban: WN/SLE — Cx. pipiens breeds in artificial containers, catch basins, sewage, gutters; vectors thrive near humans. Rural: EEE — disease centered near freshwater swamps where Culiseta melanura and Coquillettidia perturbans breed.

Aedes triseriatus vs Aedes japonicus

Both container/tree-hole mosquitoes. Ae. triseriatus: native, wooded sites, primary LAC vector. Ae. japonicus: exotic, mixed sites — possible WNV/SLE bridge vector. Avoid confusion with the Aedes that lays eggs on damp soil (vexans, trivittatus).

🦠 Mosquito-Borne Disease Quick Reference

Disease

Agent / Primary MI Vector

Reservoir & Key Notes

West Nile (WNV)

Virus / Culex pipiens

Wild birds — corvids highly susceptible. Most significant arboviral disease in U.S. MI 2002: 644 cases, 51 deaths. Symptoms severe in age 50+. Fatality 4-20%.

St. Louis (SLE)

Virus / Culex pipiens, Cx. quinquefasciatus

Wild birds (sparrows, starlings near humans). Urban/suburban. Sporadic in MI; 1975 outbreak: 93 cases, 3 deaths.

Eastern Equine (EEE)

Virus / Culiseta melanura (natural cycle); Coquillettidia perturbans (MI bridge)

Wild swamp birds (blue jays, cardinals, catbirds, wood thrushes, robins). Rural — freshwater swamps. 30-50% human fatality, 90-95% horse. Severe in children. Pheasants & emus also affected.

La Crosse (LAC)

Virus (CE group) / Aedes triseriatus

Small mammals (squirrels, chipmunks, field mice, rabbits). Late May to October. CHILDREN's disease. Transovarial transmission. Rarely fatal but possible behavioral after-effects.

Jamestown Canyon (JCV)

Virus (CE group) / Aedes stimulans, Ae. communis (Midwest); Culiseta inornata (West)

Likely deer (ungulates) as amplifying host — NOT rabbits or squirrels. ADULT disease (vs LAC). Anopheles punctipennis & A. quadrimaculatus = potential late-season vectors.

Dog heartworm

Filarial worm (Dirofilaria immitis) / about 25% of MI species — Aedes vexans, Ae. trivittatus, An. punctipennis, An. quadrimaculatus, Cx. pipiens

Dogs (and foxes, wolves); cats lesser. Adult worms in right heart chamber/pulmonary arteries. 5-6 mo dog development; 2 to 2.5 wk in mosquito. Detect annually; prophylaxis available.

🔄 Virus Transmission Cycle Reference

Concept

Specification

Mosquito EIP (extrinsic incubation period)

1 to 2 weeks at summer temps for virus to reach salivary glands. Longer at cooler temps. After EIP, mosquito is infectious for the rest of its life.

Vertebrate viremia window

Reaches infective level within less than a week post-bite; lasts 3 to 4 days. A mosquito must feed during this window to pick up the virus.

Dead-end host

Vertebrate whose viremia doesn't reach the level needed to infect feeding mosquitoes — outside the natural cycle. Humans/horses/some birds for WNV, SLE, EEE, LAC.

Asymptomatic-to-symptomatic ratio

WNV/SLE: 80 to 800 silent infections per 1 case showing symptoms. Most infected people never know.

Demographic risk

WNV/SLE: severe symptoms concentrated in age 50+. EEE: severe in children especially. LAC: children. JCV: adults.

Transovarial transmission

Female mosquito passes virus to her eggs — overwintering mechanism for LAC and JCV in Aedes egg stage. Helps maintain virus across years in endemic areas.

Overwintering reservoirs

Cx. pipiens overwinters as mated adult females in protected sites (outbuildings, attics, rodent burrows) — possibly carries WNV/SLE through. Aedes (LAC, JCV) overwinter in egg stage. Anopheles: probably eggs and possibly hibernating females.

💡 Memory Hooks

LAC vs JCV ages: "Children get LAC (Little Ankle-biters Catch it). Adults get JCV (Jamestown — Just Came of age)." Both California encephalitis group, opposite demographics.

EEE fatality numbers: "Thirty-to-fifty for the rider, ninety-to-ninety-five for the horse." Human 30 to 50 percent, equine 90 to 95 percent.

EEE vector pair: "Melanura among the birds, Perturbans among us." Culiseta melanura keeps EEE in the bird cycle; Coquillettidia perturbans bridges to humans and horses in Michigan.

Heartworm reach: "A quarter of mosquitoes can carry it." About 25 percent of Michigan species are biologically capable. Owners should treat dogs prophylactically every season.

Dead-end host: "Off-ramp, not death." A dead-end host is outside the cycle — not killed by it. Humans usually survive WNV, SLE, LAC; the virus just can't get back out.