Insects and Human DiseasesInsect Ecology/Biological Control

Fall 2014Buyung A. R. Hadi

Basic terminology

Pathogen

Host Vector

Basic terminology: Pathogen

• Parasite– Facultative vs. Obligate– Ecto vs. Endoparasite– When is a parasite considered a pathogen?

Basic terminology: Vector

• Mostly blood feeding• Types of vectors:

– Mechanical vectors– Intermediate vectors – pathogen propagates asexually/ only 

complete parts of development inside vector (e.g. Borreliaburgdorferi – Tick  Lyme disease)

– Definitive vectors – pathogen propagates sexually inside vectors (e.g. Plasmodium – Anopheles Malaria)

– Why is this important?

Basic terminology: Host

• Infected Host• Infective Host

– Symptomatic host– Carrier host

Basic terminology: Host

• Primary vs. Secondary Host ~ Definitive vs. Intermediate Host/Vector

• Amplifying vs. Dead end Host• Reservoir Host

Insects, Pathogens and Napoleon:The Syrian campaign, 1799, Bubonic plague

Bubonic plague• Pathogen: Yersinia pestis. Gram 

negative bacteria.• Vector: Oriental rat flea 

(Xenopsylla cheopis)– Transmitted by flea bites– Does not infect the flea, but 

caused blockage (biofilm) of flea’s foregut

• Host: Rodents (rats, etc.)

Bubonic Plague is a zoonotic disease

• Zoonotic diseases– Primarily animal disease  spillover to human host

• Other animals serve as the reservoir hosts– e.g. West Nile Virus, sparrow, C. tarsalismosquitoes

• Anthroponotic diseases– Primarily human disease  sometimes spillover to other animals

• Human and sometimes the vectors serve as the reservoir hosts

– e.g. Rickettsia prowazekii , body lice  Typhus

Bubonic plague

Epizootic Form

Enzootic formEpidemic Form

Generalized Epidemiological Forms

Insects, pathogens and Napoleon:The Haitian campaign, 1802, Yellow fever

Yellow Fever• Pathogen: Yellow fever virus, RNA virus 

from the family Flaviviridae• Vector: Aedes aegepty and other 

Aedes mosquitoes• Low rate of transovarial transmission 

on the enzootic  (Haemagogusequinus) and epidemic vector (A. aegepty)

‘Vertical’ transmission of pathogen within vector

• Transstadial – e.g. Lyme disease spirochetes within the larval tick

• Transovarial – e.g. Yellow Fever Virus within Aedesmosquitoes

• Venereal – e.g. La Crosse virus in Aedes triseriatus

‘Horizontal’ Transmission

• Mechanical Transmission– No propagation or development of the pathogen within the vector– e.g. Salmonella contamination of vegetables by house fly

‘Horizontal’ Transmission

• Mechanical Transmission– No propagation or development of the pathogen within the vector– e.g. Salmonella contamination of vegetables by house fly

• Biological Transmission– Propagative/Multiplicative Transmission

• Propagation but no development of the pathogen  usually viral diseases• E.g. Yellow Fever Virus in Aedesmosquitoes

– Developmental Transmission• Development but no propagation of the pathogen  numbers of pathogen 

inoculated is equal to or less than the those acquired• e.g. Microfilaria Wuchereria brancofti (causing agent of elephantiasis) in Culex

mosquitoes

– Cyclopropagative Transmission• The pathogen develop and reproduce asexually within the pathogen  numbers 

of pathogen inoculated is more than the those acquired• E.g. Plasmodium sp. In Anophelesmosquitoes (malaria)

1803: Louisiana Purchase

Insects, pathogens and Napoleon:The Russian campaign, 1812, Typhus

Pediculus humanus humanus/corporisBody Louse

Pediculus humanus capitisHead Louse

“...the lice seemed to seek supremacy, for their number on both officers and privates was in the thousands.”

Private Jacob Walter’s personal papers, 1812

Typhus• Greek Typhos: smoky or hazy 

neuropsychiatric symptoms shared by Typhus and typhoid fever

• Pathogen: Rickettsia prowazekii(epidemic typhus)  obligate endoparasite in both the vertebrate host and the invertebrate vector  The lice usually die of infection

– Anthroponotic disease

• Vector: Pediculus humanuscorporis (human body lice)

• Unlike mosquito‐borne pathogen, louse‐borne typhus has a rather different infection route

Routes of Horizontal Transmission

• Anterior‐station transmission– Pathogen is delivered via the 

mouthparts, usually during vector feeding (e.g. Yellow Fever virus by Aedesmosquitoes) 

• Posterior‐station transmission– Pathogen is delivered via the 

vector’s contaminated faeces(e.g. Rickettsia prowazekii by body lice  Typhus)

Take home message: If you want to conquer the world,

hire an entomologist!

No, seriously. How do we deal with these insect-borne pathogens?

• Disease triangle approach

• Target one of the sides of the triangle, and you terminate the necessary relationships for the disease to continue

Pathogen

Host Vector

Vector Control – StrategiesCase study: Malaria

• Antimalarial drugs  Quinine, before travelling to Malaria prone regions

• Malaria vaccine  not yet available, development has been underway

Pathogen = Plasmodium falciparum

Host=Human Vector=

Anopheles mosquitoes

Vector Control – StrategiesCase study: Malaria

Pathogen = Plasmodium falciparum

Host=Human Vector=

Anopheles mosquitoes

• Transgenic mosquitoes that do not transmit malaria• They’re alive!• They are not out 

and about yet• http://www.plosmed

icine.org/article/info:doi/10.1371/journal.pmed.1000020

Vector Control – StrategiesCase study: Malaria

Pathogen = Plasmodium falciparum

Host=Human Vector=

Anopheles mosquitoes

• Use mosquito repellent or insecticide treated mosquito nets– Bill Gates gets this!! 

(http://www.gatesfoundation.org/topics/Pages/malaria.aspx#) 

• Area‐wide Anopheles eradication using insecticide or other methods