Shoo Fly, Don’t Infect Me!
Mechanical Vectors of Bacterial GI
Diseases
By: Liz Melton
March 9th, 2012
TABLE OF CONTENTS
Common Mechanical Vectors:
Cockroaches ……………………………………………………………………………………..............…… 2
Musca Domestica …………………………………………………………………….……………………… 2-3
Musca Sorbens ……………………………………………………………………………………………….. 3
Musca Chrysoma ……………………………………………………………………………………..……… 3-4
Fly Life Stages ……………………………………………………………………………….…………………4-5
Mechanical Transmission Process…………………….……………...………………………………. 5-6
Mechanically Transmitted GI Diseases:
Salmonella ……………………………………………………………………………………………………... 6-7
Typhoid …………………………………………………………………………………………………………. 7
Bacillary Dysentery (Shigellosis)……………….…………………………………………………..… 7-8
Summarized Scenarios:
Role of Cockroaches and Flies as Mechanical Vectors in Egypt …………………………. 8
Role of Cockroaches as Mechanical Vectors in Thailand ………………………………...… 8
Role of House Flies in Cholera Transmission in India ………………………………………. 9
Recent Research:
The housefly (Musca domestica) as a carrier of pathogenic microorganisms in a
hospital environment ……………………………………………………………………………………… 9
Reduction of transmission of shigellosis by control of houseflies …………………………. 10
Political Ties and Wartime Relevance:
PATH & WaterAid America …………………………………………………………………………….. 10
Armed Forces Pest Management Board TECHNICAL GUIDE NO. 30 Filth Flies
Significance, Surveillance and Control in Contingency Operations ……………………. 11
AFPMB: Disease Vector Ecology Profile for Somalia..…………………………………….11-12
References:
Reference List ..……………………………………………………………………………………………..13-14
INTRODUCTION
Civil strife, diminished immune status, food shortages, antibiotic resistance,
and utterly unsanitary living conditions taint all settlements undergoing
humanitarian crises. Such catastrophes yield irreparable harm to innocent
bystanders, reducing everyday routine to disarray and disorganization. Refugee
camps are characterized by chaos. Internationally displaced persons are forced to
inhabit incredibly close quarters; in many cases, this equates to: defecating sans any
form of privacy, consuming meals and water of the poorest quality, and failure to
bathe in overcrowded, filthy locations. Most unsettling then, is looming potential for
rapid and virulent spread of disease via mechanical transmission: “a form of indirect
transmission in which the infectious agent does not undergo physiologic changes
inside the vector”1.
This background delineates modes of bacterial gastrointestinal disease
transmission in humanitarian crisis communities. To combat mechanical disease
transmission, this paper first distinguishes features of mechanical vectors that
supply distinct connection to areas of humanitarian crisis. Owing to feeding and
reproductive behaviors, several mechanical vectors such as cockroaches, houseflies,
filth flies, and blow flies become exceedingly prevalent in disaster zones.
Publications hailing from Egypt, Thailand, India, and Somalia provide concrete
examples of fly and cockroach infectivity in humantarian crises-esque scenarios.
Documented spread of pathogens by way of cockroaches and flies near latrines,
open-air markets, military bases, village animal pens and trash dumps, and hospitals
are also examined within this piece. Finally, a discussion of political influence and
wartime repercussions may instigate further research on intervention strategies for
control of mechanical vectors of GI bacterial disease.
Common Mechanical Vectors
Common Cockroach
Common cockroaches are typified by their 5-73mm length and dual wing
structure2. All cockroaches have a flattened appearance, and are yellowish brown in
color. What differentiates cockroaches from other beetles or land-bound insects is
their incredible agility. Moreover, they exhibit endophagic behavior, feeding on
garbage and feces indoors3. These arthropods are most active at night, scurrying in
and out of latrines and other warm, man-made structures. Often, they breed
outdoors, and opt to reside in colony form4.
Due to the fact that refugee camps have little to no shelter denying cockroach
entry into makeshift tents, and the propensity for organic waste and feces to be
proximal to sleeping individuals, it is clear how mechanical transmission is not only
possible, but probable5. Unfortunately, cockroaches are responsible for
transmitting numerous bacterial GI diseases including: many forms of diarrheal
diseases, typhoid fever, and dysentery along with a host of other viruses and
diseases2.
Musca domestica (Common Housefly)
Distributed worldwide, Musca domestica, the common housefly, is marked by
its 6-7 mm length, grey color, and four dark longitudinal stripes along its thorax2. It
has been proven that high density of flies is correlated with crowded human
populations in unsanitary conditions. This habitat is a near-exact portrayal of
humanitarian crisis sites. What makes the housefly so intrinsically linked to
mechanical transmission of bacterial GI diseases in humanitarian crises is the
dependency of its life cycle on unsanitary settings4. Although house flies do not
physically lay eggs directly in latrines, landing and feeding on uncovered human and
animal excrement as well as domestic organic waste is essential to house fly
survival. What is more, houseflies are associated with the spread of infantile
diarrhea, shigellosis diarrhea, cholera, other forms of dysentery, and typhoid
fever2,6.
Musca sorbens (Filth fly)
Similar to Musca domestica, Musca sorbens, common name filth fly, is
characterized by a grey color, and usually grows to a size of 6 mm in length2. Unlike
houseflies, however, filth flies are detected by two dark stripes on the thorax, and
normally reside in tropical locations. Musca sorbens flies possess a wing-span of
around 15 mm, and, like most other flies, filth flies breed and engorge in human
feces around settlements7. Filth flies are unique in that they also feed on eye
secretions, especially those of children2. As a result, Musca sorbens is implicated in
the transmission of the eye disease trachoma. In the context of humanitarian crises,
diarrheal diseases are the most commonly transmitted pathogens from filth flies.
Chrysomya (Blowfly)
A compact body 10mm in length, and a shiny blue or green color set
Chrysomya, otherwise known as blow flies, apart from other flies2. Blow flies are
distributed worldwide, and also possess a strong preference for breeding in open
latrines, decomposing meat or fish, garbage, and animal excrement. Recent research
demonstrates that blow flies are most active in refugee camps, simple pit latrines,
market places, defecation holes sans lids7. Luckily, blow flies display contrasting
behavior from cockroaches, and do not enter houses. Nonetheless, Chrysomya
density rises as the number of simple pit latrines in a camp increases7—an
imminent threat to regions in the vicinity of humanitarian crises. These flies are
linked to intestinal worm eggs, diarrheal disease, and dysentery transmission.
Fly Life Stages:
Four discrete phases can be identified in fly genesis. Eggs give way to
maggots/larvae, which eventually pupate, emerging as adults within weeks3. Per
the World Health Organization, female flies seldom lay greater than 150 eggs in one
sitting, which takes place only up to five times within her lifetime8. In general, eggs
are deposited on an assortment of spots, namely: decayed, fermenting, rotting, moist
organic material of animal or vegetable origin, and heaps of animal or human feces2.
Choice of egg-laying venue is dependent on its moisture, texture, and freshness.
Following about an 8-48 hour incubation period, eggs hatch into maggot form3.
Conveniently, larvae thrive on organic matter and animal/human excrement.
Larvae are recognized by their slender, white, legless, maggot-like exterior, and
grow after burrowing far into breeding material. Reaching pupa maturity takes
anywhere from three days to as long as several weeks3. Maggot development is
contingent on temperature, fly species, and the type, quantity, and quality of food8
(high in fecal and waste-contaminated areas of humanitarian crisis). Two to ten
days subsequent to formation of a puparium, the capsule-like case containing the
fly-to-be, the adult fly emerges2. Spreading its wings, and pushing to the surface, the
body of the adult fly dries and hardens.
Adult flies can subsist for one to two months prior to death, conditional on
the species and living conditions. On account of their mouthpart design, flies only
consume food in a liquid, or readily soluble state. Solid food is wetted with saliva,
and sucked up along with water, a vital component to a fly’s livelihood. Flies cannot
exist over 48 hours without drinking water3. A fly’s diet habitually consists of: milk,
sugar, syrup, blood, meat broth, and any materials found in human settlements
ingested at least two to three times a day7.
Mechanical Transmission Process:
Fly breeding mechanisms and filthy feeding habits of cockroaches mentioned
above are precisely what make flies and cockroaches efficient mechanical vectors
and transmitters of bacterial GI diseases. Three hundred fifty fly species in twentynine families have been potentially associated with the spread of food-borne
diseases4. According to Clinical Microbiology Reviews, over fifty species of
synanthropic flies have been reported to be associated with unsanitary conditions,
and are involved in dissemination of human pathogens in the environment, and
twenty-one of these species have been involved in transmission of human
gastrointestinal diseases. A fly’s configuration and flying patterns are inherently
conducive to mechanical transmission. Individual flies have been known to migrate
up to twenty miles, with unsanitary sites as targets. Sucking mouthparts, sticky
footpads, sticky substances on appendages, body and leg hair, and electrostatic
charge on exoskeletons effectively render flies “super transmitters” of disease2,4.
In particular, districts subjected to humanitarian crises are prime breeding
and feeding grounds for these infective arthropods. Waste incurred via food
preparation, handling, vending, and serving waste in public places is considered
hazardous2,7. More disturbingly, feces enhance transmission of infectious agents
much greater than that of any other substrate or medium, a major concern in
humanitarian crises. Ample animal manure and human excrement, garbage, animal
bedding, and decaying organic matter can be found scattered all over refugee camps,
and pose pending danger of mechanical disease transmission7.
Mechanically Transmitted GI Diseases
Salmonellosis:
Salmonellosis is attributed to infection with the bacteria Salmonella.
Salmonella infects the GI tract, and is frequently spread by mechanical vectors
affiliated with humanitarian crisis settings. Infecteds will present with diarrhea,
fever, and abdominal cramps 12 to 72 hours after infection. A sustained fever of
approximately 40° C, and feelings of weakness, headache, loss of appetite, or
materialization of a rash of flat, rose-colored spots are other symptoms of
Salmonellosis. Antibiotics are the suggested treatment for Salmonellosis9.
Salmonellosis persists only 4 to 7 days, though, in some individuals, severity
of diarrhea may prompt patient hospitalization. Salmonella bacteria is capable of
spreading from the GI tract to the portal system, triggering infection of other body
sites, and leading to eventual death without immediate antibiotic treatment.
Furthermore, some patients complain of joint and urination pain, as well as eye
irritation—what is deemed Reiter's syndrome. This form of the disease may
continue for months or years, bringing about chronic arthritis. Infants, the elderly,
and immuno-compromised persons are more liable to this degree of illness. All
three age/status groups are regularly observed in refugee populations, making
Salmonellosis outbreaks grave concerns in humanitarian crises7.
Typhoid fever:
Typhoid fever, caused by the bacterium Salmonella typhi is classified as a lifethreatening illness. In the developing world, this disease is fairly common, about
21.5 million persons are afflicted with typhoid fever each year10. Salmonella Typhi
lives only in humans. Infected individuals harbor bacteria in their GI tract and
bloodstream, and both silent carriers and ill persons shed bacterial residue in stool.
Fecal-oral transmission is a method of contracting typhoid fever, a logical
explanation for enormous prevalence of typhoid where handwashing and clean
water is limited7,11. Additionally, houseflies and cockroaches are known to be
wonderful mechanical vectors of typhoid fever2, and, coincidentally reside in areas
at high risk of endemicity. Typhoid fever is treated and prevented by oral
antibiotics10, which are expensive, and rarely imported in the bulk required by
humanitarian crises settings.
Bacillary Dysentery (Shigella):
As with most bacterial GI diseases, bacillary dysentery invokes bloody
diarrhea, fever, and stomach cramps a day or two after exposure to the Shigella
bacterium. Shigellosis symptoms remain for 5 to 7 days on average; in spite of this,
children younger than two years of age may experience violent seizures12. To make
matters worse, infection by the strain Shigella flexneri, causes post-infectious, and
possibly chronic, arthritis. As aforementioned, chrysomya, houseflies, and
cockroaches all mechanically transmit Shigella bacteria2. Antibiotic treatment
destroys Shigella bacteria, so ampicillin, trimethoprim/sulfamethoxazole,
ceftriaxone, or, among adults, ciprofloxacin will alleviate symptoms12. Regrettably,
Shigella bacteria have become resistant to antibiotics, requiring multiple treatments
at once—a near impossibility to administer in humanitarian crises7.
Summarized Scenarios
The role of cockroaches and flies in mechanical transmission of medical important parasites:
METHODS:
1) Surveyed residential areas in Khaldyia Village, Egypt 2009 – 2010
 45 cockroaches collected in control, 178 total collected
 Flies from human feces also observed
2) Parasites isolated/identified from defecation areas in and around houses
RESULTS:
-Cockroaches trapped in the toilets of:
 houses with pit latrines: 98 parasites/ml
 houses with water system: 31 parasites/ml
-Pathogens isolated from external and internal surface of 98% of test cockroaches
(Adapted from: Gehad T. El-Sherbini, Eman T. El-Sherbini. The role of cockroaches
and flies in mechanical transmission of medical important parasites. Journal of
Entomology and Nematology. 2011;3(7):98-104.)4
The majority of cockroaches from the samutprakarn province of Thailand are carriers of
parasitic organisms:
METHODS:

920 cockroaches were obtained Samutprakarn province of Thailand
o 18 open-air shopping markets in 5 districts
RESULTS:
 1 subdistrict of this province
o captured during feeding time in natural habitat
 wet smear and modified acid-fast bacilli staining identified
parasites from the external surface or cuticle of the
cockroaches
-498/920 (54.1%) of the cockroaches harbored parasitic organisms
**Results elucidate that markets giving refuge to cockroaches cultivate colonies that
carry several pathogens activating transmission of illness such as diarrhea or bowel
disorder.
(Adapted from: Pennapa Chamavit, Panupong Sahaisook, Nunthawadee Niamnuy.
The majority of cockroaches from the samutprakarn province of Thailand are
carriers of parasitic organisms. EXCLI Journal. 2011;(10):218-222.)5
Vector potential of houseflies (Musca domestica) in the transmission of Vibrio cholerae in
India:
METHODS:
1) Small village 15 km away from Delhi city outbreak of diarrhea
2) 150 houseflies captured with sterilized nets from animal pens, yards or houses
3) Collect stool samples from 50 patients within 24 hrs of illness, before antimicrobial
o all subjects of lower socioeconomic status
o all exhibiting signs of gastroenteritis
4) Samples were transferred to tubes of high pH (8.4) of alkaline peptone water
suppressing commensal intestinal bacterial growth to permit uninhibited
multiplication of V. cholerae
RESULTS:
-60% fly pools were positive for V. cholerae
**This study bestowed convincing evidence that houseflies acted as mechanical
vectors, and aided in dissemination of V. cholerae biotype El Tor.
(Adapated from: R. Fotedar. Vector potential of houseflies (Musca domestica) in the
transmission of Vibrio cholerae in India. Acta Tropica. 2001;(78):31–34.)6
Recent Research:
The housefly (Musca domestica) as a carrier of pathogenic microorganisms in a hospital
environment:
METHODS:
1) Collect Musca domestica from:
 surgical ward (All India Institute of Medical Sciences Hospital)
 remote residential area 5 km away as a control
2) A total of 113 flies were collected:


65 from a surgical ward (test)
48 from a residential area (comparison)
RESULTS:
-10 genera of bacteria isolated from the test group, 9 from control group
-However, the load of bacteria carried by the test group of flies was significantly
more.
**Evidence for houseflies acting as vectors of potentially pathogenic bacteria in a
hospital environment is a frightening, and very real prospect.
(Adapted from: R. Fotedar, U. Banjeree S. Singh Shriniwas, A Verma. The housefly
(Musca domestica) as a carrier of pathogenic microorganisms in a hospital
environment. Journal of Hospital Infection. 1992;(20):209-215.)13
Reduction of transmission of shigellosis by control of houseflies (Musca domestica):
Prospective Crossover Intervention Study
METHODS:
1) 2 military field bases several km apart in early summer, 1988
2) Implement and track progress of intensive fly control measures at one base, keep
another as control
RESULTS:
-After 11 weeks, fly counts were 64% lower on the bases exposed to fly control
-Clinic visits for diarrheal diseases dropped by 42%:
 85% for shigellosis (76% for antibodies to Shigella)
 57% for antibodies to enterotoxigenic Escherichia coli
**Leads one to believe that control of mechanical vectors is on the horizon, and is an
achievement necessary for reducing the number of GI diseased individuals in
humanitarian crises.
(Adapted from: Dani Cohen, Manfred Green, Colin Block, Raphael Slepon, Ruhama
Ambar, Steven Wasserman, Myron Levine. Reduction of transmission of shigellosis
by control of houseflies (Musca domestica). The Lancet. 1991;(337).)14
Political Ties and Wartime Relevance
PATH & WaterAid America:
Two leading NGOs, WaterAid America and PATH (Program for Appropriate
Technology in Health) have joined forces in order to “defeat a global killer” that
entertains little attention on our side of the continent: diarrheal disease. Both
philanthropic organizations aspire to ameliorate international living conditions
which currently foster the transmission of diarrheal diseases. With a focus
primarily on children, PATH and WaterAid will be conducting extensive research in
endemic communities on the upshot of clean water and sanitation solutions and
dispensation of adequate resources. Thus, surveillance carried out by NGOs akin to
WaterAid and PATH may be an alternative route for testing exploratory mechanical
vector control strategies in humanitarian crisis locations.
(Adapted from: (May, 2009). Defeating a Global Killer: The Need for Action on
Diarrheal Disease. www.who.int/entity/pmnch/events/2009/InvitationD
Devent.pdf. [Flyer]. February 13, 2012.)15
Armed Forces Pest Management Board TECHNICAL GUIDE NO. 30 Filth
Flies: Significance, Surveillance and Control in Contingency Operations:
Historically, arthropods have plagued military operations whether it be as a
result of physical distress induced by spread of infection, or simply by sustained,
non-stop pestering base-side. Evidenced by journals dating back to World War I,
Colonel J.C.G. Ledingham of the Royal Army Medical Corps, Mesopotamian
Expeditionary Force found a strong correlation between fly density and the
incidence of dysentery. World War II battles, especially that of El Alamein in North
Africa, bore extreme troops loss to to dysentery presumably from flies as well.
Farther down the historical line, flies were proven a “monumental nuisance”
throughout the Vietnam War, with testimonies regarding one mess hall in which fly
infestation was so heavy it was difficult to eat without ingesting one or two.
The Persian Gulf War of 1991 posed parallel problems, and fly infestation
finally began to be established as a chief issue in humanitarian relief operations.
Somalian relief forces encountered brutally insufficient sanitation, and detailed vast
fly infestations within military bases, and refugee shelters. Realizing that a blend of
deficient sanitation, and growing livestock populations (inevitably initiating a
greater garbage and excrement load), was a recipe for fly and cockroach influx, the
Armed Forces Pest Management launched the creation of Disease Vector Ecology
Profiles, and technical guides to narrow the risk of mechanical transmission of
disease in wartime and humanitarian crises.
(Adapted from: Armed Forces Pest Management Board TECHNICAL GUIDE NO. 30
Filth Flies: Significance, Surveillance and Control in Contingency Operations
Published and Distributed by Armed Forces Pest Management Board Information
Services Division Forest Glen Section/Walter Reed Army Medical Center,
Washington, DC 20307-5001, Office of the Deputy Under Secretary of Defense
(Installations and Environment))7
Armed Forces Pest Management Board: Disease Vector Ecology Profile for
Somalia:
Disease Vector Ecology Profiles (DVEPs) single out relevant vector-borne and
other militarily significant diseases in countries with an emphasis on: “essential
epidemiology, vector bionomics, behavior, and pesticide resistance”. A Somalian
example on “Fly-Associated Diseases: ENTERIC DISEASES” highlights agents of
disease, vector transmission, and vector bionomics. Generally, profiles are
constructed as compilations of contemporary research on both vector
characteristics and control measures. DVEPs are constantly evolving owing to swift
Third World urban development, civil strife, ecological change, and invariable
growth in the number of countries tormented by humanitarian crisis.
(Adapted from: (15, September, 1993). Disclosure. Armed Forces Pest Management
Board: Disease Vector Ecology Profile for Somalia, 4th Ed. Armed Forces Pest
Management Board. Retrieved from: file:///C|/Inetpub/wwwroot/AFPMB/pubs
/dveps/somalia.htm.)11
Reference List
1. Centers for Disease Control and Prevention: Public Health Foundation. Glossary of
Epidemiology Terms. Glossary. http://www.cdc.gov/excite/library/glossary.htm#T.
April 25, 2007. February 13, 2012.
2. Keiding J. The housefly—biology and control. Training and information guide
(advanced level). Geneva, World Health Organization, 1986 (unpublished document
WHO/VBC/ 86.937; available on request from Division of Control of Tropical
Diseases, World Health Organization, 1211 Geneva 27, Switzerland).
AND Ch. 2, Adapted from: Birley, 1991; Thomson, 1995; Sabatinelli, 1996; Chavasse
and Yap, 1997; Rozendaal, 1997
3. Thaddeus K. Graczyk, Ronald Knight, Leena Tamang. Mechanical Transmission of
Human Protozoan Parasites by Insects. Clin. Microbiol. Rev. 2005;18(1):128.
4. Gehad T. El-Sherbini, Eman T. El-Sherbini. The role of cockroaches and flies in
mechanical transmission of medical important parasites. Journal of Entomology and
Nematology. 2011;3(7):98-104.
5. Pennapa Chamavit, Panupong Sahaisook, Nunthawadee Niamnuy. The majority of
cockroaches from the samutprakarn province of Thailand are carriers of parasitic
organisms. EXCLI Journal. 2011;(10):218-222.
6. R. Fotedar. Vector potential of houseflies (Musca domestica) in the transmission
of Vibrio cholerae in India. Acta Tropica. 2001;(78):31–34.
7. Armed Forces Pest Management Board TECHNICAL GUIDE NO. 30 Filth Flies
Significance, Surveillance and Control in Contingency Operations
Published and Distributed by Armed Forces Pest Management Board Information
Services Division Forest Glen Section/Walter Reed Army Medical Center,
Washington, DC 20307-5001, Office of the Deputy Under Secretary of Defense
(Installations and Environment)
8. Orin S. Levine, Myron M. Levine. Houseflies (Musca domestica) as Mechanical
Vectors of Shigellosis Reviews of Infectious Diseases. 1991;13(4):688-696.
9. Centers for Disease Control and Prevention. Salmonella. What is Salmonellosis?
September 27, 2010. http://www.cdc.gov/salmonella/general/. February 13, 2012.
10. Centers for Disease Control and Prevention. National Center for Emerging and
Zoonotic Infectious Diseases. Typhoid Fever. October 5, 2010. http://www.cdc.gov/
nczved/divisions/dfbmd/diseases/typhoid_fever/. February 13, 2012.
11. (15, September, 1993). Disclosure. Armed Forces Pest Management Board:
Disease Vector Ecology Profile for Somalia, 4th Ed. Armed Forces Pest Management
Board. Retreived from: file:///C|/Inetpub/wwwroot/AFPMB/pubs/dveps/
somalia.htm.
12. Centers for Disease Control and Prevention. National Center for Emerging and
Zoonotic Infectious Diseases. Shigellosis. November 16, 2009. http://www.cdc.
gov/nczved/divisions/dfbmd/diseases/shigellosis/. February 13, 2012.
13. R. Fotedar, U. Banjeree S. Singh Shriniwas, A Verma. The housefly (Musca
domestica) as a carrier of pathogenic microorganisms in a hospital environment.
Journal of Hospital Infection. 1992;(20):209-215.
14. Dani Cohen, Manfred Green, Colin Block, Raphael Slepon, Ruhama Ambar, Steven
Wasserman, Myron Levine. Reduction of transmission of shigellosis by control of
houseflies (Musca domestica). The Lancet. 1991;(337).
15. (May, 2009). Defeating a Global Killer: The Need for Action on Diarrheal Disease.
www.who.int/entity/pmnch/events/2009/InvitationDDevent.pdf. [Flyer]. February
13, 2012.