Airborne Microbes of Medical and Agricultural Importance Mcb 403

8/18/2019 Airborne Microbes of Medical and Agricultural Importance Mcb 403

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AIRBORNE MICROBES OF MEDICAL AND AGRICULTURAL IMPORTANCE

Introduction

Aeromicrobiology involves various aspects of intramural (indoor) and extramural (outdoor)

aerobiology as they relate to the airborne transmission of environmentally relevant

microorganisms, including viruses, bacteria, fungi, yeasts, and protozoans.

The aeromicrobiological (AMB) a!"#ay describes: (1) the launching of bioaerosols into the

air (!) the subse"uent transport via diffusion and dispersion of these particles and finally (#)

their deposition. $n example of this path%ay is that of li"uid aerosols containing the influenza

virus launched into the air through a cough, sneeze, or even through tal&ing. These virus'associated aerosols are dispersed by a cough or sneeze, transported through the air, inhaled, and

deposited in the lungs of a nearby person, %here they may begin a ne% infection.

onventionally, the deposition of viable microorganisms and the resultant infection are given the

most attention, but all three processes (launching, transport, and deposition) are of e"ual

importance in understanding the aerobiological path%ay.

E$!ram%ral aeromicrobiology

This is the study of microorganisms associated %ith outdoor environments. In the extramural

environment, the expanse of space and the presence of air turbulence are t%o controlling factors

in the movement of bioaerosols. nvironmental factors such as *+ radiation, temperature, and

relative humidity modify the effects of bioaerosols by limiting the amount of time aerosolized

microorganisms %ill remain viable. In this section, %e %ill be discussing the spread of

agricultural pathogens as an example of extramural aeromicrobiology.

Agriculture

ontamination of crops and animals via bioaerosols has a large global economic impact. ice

and %heat are t%o of the ma-or staple crops that are important to %orld food security. a-or

pathogens of such crops are the %heat rust fungi. These spore'forming fungi cause some of the

most devastating of all diseases of %heat and other grains. In 1//#, one type of %heat rust (leaf

rust) %as responsible for the loss of over 0 million bushels of %heat in 2ansas and 3ebras&a,


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*4$. This epidemic spread of %heat rust and the resulting economic destruction produced are

indicative of the impact that airborne microbial pathogens can have on agriculture.

4pores of %heat rust are capable of spreading hundreds, if not thousands, of &ilometres through

the atmosphere. The airborne spread of rust disease has been sho%n to follo% a predictable trend,

%hich start during the fall %ith the planting of %inter %heat in the southern plains. $ny rust'

infected plant produces thousands of spores, %hich are released into the air by either natural

atmospheric disturbance or mechanical disturbance during the harvesting process. 5nce airborne,

these spores are capable of long'distance dispersal, %hich can cause do%n%ind deposition onto

other susceptible %heat plants. The generation time of ne% spores is measured in %ee&s, after

%hich ne% spores are again released from vegetative fungi into the $6 path%ay.

$s a control measure, chemical treatment such as %heat rust fungicide (a pesticide) can be used

to control the pathogen. 7o%ever, many pesticides have an extremely long half'lives and their

residence in an ecosystem can be extremely harmful, thus ma&ing the option undesirable. $s an

alternative, attempts are being made to breed strains of %heat that are more resistant to the fungi.

$nother method used for controlling phytopathogenic fungi is spore monitoring as a disease

control strategy. In this approach, the life cycle of the fungi, and especially the release of spores,

is monitored, and fungicide application is timed to coincide %ith spore release. This approach

minimizes use of harmful chemicals. Thus, efficient $6 path%ay, sampling, monitoring,

detection, and modeling have the ability to aid in the control of airborne pathogens.

The airborne spread of pathogenic microorganisms is also highly important in the animal

husbandry industry. The occurrence of foot'and mouth disease is an example of the importance

of bioaerosols in the spread of airborne disease. $lso, there is gro%ing evidence that

gastrointestinal pathogens are also important in airborne transmission of disease among animals.

8or example, the transmission of Salmonella typhimurium among calves that are housed

individually in small pens is believed to be through bioaerosols spread because the initialsymptoms resembled those of pneumonia and appeared randomly %ithin these animals, t%o

factors that are not characteristic of oral transmission. 5ral transmission generally occurs

se"uentially from one pen to the next, %hereas aerial transmission can carry organisms past

nearby pens, infecting calves randomly. 8urthermore, it has been discovered that S. typhimurium

could survive for long periods in an airborne state, and calves and mice exposed to aerosolized S.


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typhimurium develops symptoms, proving that gastrointestinal pathogens could be spread via

aerosolization. 8inally, it has also been sho%n that aerosolized Salmonella enteritidis could infect

laying hens. The hens sho%ed clinical symptoms and %ere shedding the test strain of salmonellae

in their feces %ithin a fe% days. Thus, the $6 path%ay can be important even in the spread of

diseases for %hich pathogens are not normally considered airborne.

I&!ram%ral aeromicrobiology

The home and %or&place are environments in %hich airborne microorganisms create ma-or

public health concerns. In comparison %ith extramural environment, intramural environments

have limited circulation of external air and much less *+ radiation exposure. Indoor

environments also have controlled temperature and relative humidity, %hich are generally in the

ranges that permit extended microbial survival. Thus, these conditions are suitable for the

accumulation and survival of microorganisms %ithin many enclosed environments, including

office buildings, hospitals, laboratories, and even spacecraft. In this section, %e %ill be

discussing these three diverse environments as it relates to intramural aeromicrobiology.

Buildings

any factors can influence bioaerosols and therefore ho% 9healthy or ho% 9sic& a building is.

These include the presence and;or efficiency of air filtering devices, the design and operation of

the air circulation systems, the health and hygiene of the occupants, the amount of clean outdoor

air circulated through the building, the type of lighting used, the ambient temperature in the

building, and the relative humidity.

4ome pathogens are uni"uely adapted for survival and transmission in the intramural

environment. 5ne good example of such organism is Legionella pneumophila, the causative

agent of both ontiac fever (4ee description and epidemiology in

previous sections). $side from natural reservoirs %here this organism can be found, there aremany human'made systems %ithin %hich legionellae can find a niche. These include cooling

to%ers, evaporative condensers, plumbing systems, %hirlpools, sho%er heads, and hot'%ater

faucets. In the case of the $merican


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convention, this proliferation led to the subse"uent aerosolization and spread of the organisms

throughout the building.

$s a control measure to arrest the gro%th and spread Legionella spp., the operating temperatures

of hot'%ater plumbing systems should be maintained at ?@A, all potential places %here %ater

can stagnate in %ater pipes should be avoided, ozonization units, dry convective heat exchange

designs should be installed in cooling to%ers %hile any design that could potentially mix the %et

system %ith the supply air should be avoided. 6iocidal agents such as chlorine or copper can also

be effective %hen used regularly at lo% levels.

Public health

>athogens in the $6 path%ay can be a potential source of deadly diseases, but $6 path%ay

also has potential for use in immunization against disease. 8or example, influenza (flu) stri&es

millions every year and &ills about !, of the most vulnerable patients. In-ectable flu vaccines

are currently being used that decrease the severity of the disease, but they are not %idely used

because they re"uire a painful in-ection in the arm.

urrently in development is a flu vaccine that is delivered by nasal spray. $erosol delivery may

actually be a better method for vaccination against respiration'associated diseases because the

vaccine is delivered onto the mucous membranes, %hich are the first line of defense against

respiratory infection. Thus nasal sprays may increase the levels and specificity of the immune

response, especially in these vulnerable areas.

Hospitals and Laboratories

7ospitals and microbiology laboratories are the t%o indoor environments %ith perhaps the

greatest potential for the aerosolization of pathogenic microorganisms. 7ospitals, because they

are centres for the treatment of patients %ith diseases, have a high percentage of individuals,

including patients and staff, %ho are active carriers of infectious, airborne pathogens. 5f

particular concern are neonatal %ards, surgical transplant %ards, and surgical theatres, all critical

areas %here the control of nosocomial infection is imperative.


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The microbiological laboratory is also a breeding ground for pathogenic agents. Bue to this,

handling procedures have been developed and refined to protect laboratory %or&ers. ven under

the strictest of conditions, ho%ever, aerosolization events may occur.

5f great concern in this regard are the laboratories in developing countries that are seldom

e"uipped %ith up'to'date biosafety e"uipment. This poses greater ris&s to %or&ers and students

in such laboratories. 3otorious laboratory colonizers such as Pseudomonas spp, spore formers

such as Cram positive Bacillus and Clostridium as %ell as other airborne pathogens are freely

dispersed in such laboratories in spite of its attendant conse"uences.

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Airborne Microbes of Medical and Agricultural Importance Mcb 403