Biomedical Engineering Reference
In-Depth Information
rickettsiae have a theoretical ID
50
of one organism or
infective particle when introduced parenterally. Agents
causing tuberculosis, coccidioidomycosis, and histoplas-
mosis have an ID
50
of the order of 10 or fewer organisms or
infective particles when exposure is via the respiratory
route. Oral infective doses for bacterial enteric pathogens
may vary from 10
1
organisms for Shigella to 10
8
organisms
for Vibrio comma.
The risks of infection following exposure to agents in
the environment are usually secondary to those associated
with more direct manipulations of infectious materials. The
capability of an agent to survive in the environment directly
influences procedures used for decontaminating work
surfaces as well as the containment practices, equipment,
and facilities used for laboratory and research animal
activities.
Virulence
The capability of a microorganism to produce infection and
disease in a host may significantly influence the occupa-
tional risk assessment. Fresh field isolates should be
considered fully virulent within the limits of the agent.
Isolates attenuated by passage on artificial media, tissue
culture, or laboratory animals may pose lower infection
risks than unmodified agents.
Attenuated strains of yellow fever, vaccinia, and
polioviruses are safe and effective vaccines for use in
humans. These strains maintain infectivity and elicit an
antibody response, but characteristically do not produce
a generalized disease in healthy human adults. Intentional
or accidental passage of these vaccine strains in human or
animal hosts may, however, significantly increase the
virulence and result
Activity Conducted
The manner in which procedures are performed may
significantly influence the infection risk for personnel.
Laboratory manipulations of clinical specimens and
cultures of Legionella pneumophila are commonly
handled on the open bench using good laboratory prac-
tices. Such activities have not resulted in reported disease.
Animal aerosol challenge studies using concentrated
liquid cultures may produce infections
in exposed
personnel
typical of
the presumed natural mode of
transmission.
The quantity and concentration of materials handled
may also be associated with increased infection risks. Large
volumes of cultures associated with the propagation of
infectious agents may represent an inherently greater
infection hazard than activities typical of isolation and
identification of pathogens.
Persons having the most intimate and direct contact
with experimentally or naturally infected animals, such as
veterinarians, animal caretakers, and researchers, are
obviously at greater infection risk than those with indirect
or no contact.
in human-to-human or animal-to-
animal transmission.
Strains attenuated for one species may retain virulence
for other related or nonrelated species. Live rabies vaccines
attenuated for dogs may produce clinical disease in cats,
foxes, and skunks. In the past century, the only two docu-
mented cases of rabies in laboratory workers, one in
a research facility and the other in a vaccine production
facility, resulted from exposure to an attenuated dog
vaccine strain (ERA) and a strain fixed by mouse passage
(Street Alabama Dufferin), respectively.
Other Considerations
A number of other considerations influence the assessment
of risk in activities involving infectious agents. The avail-
ability and use of safe and efficacious vaccines may
significantly reduce individual infection risks (e.g. hepatitis
B virus, Venezuelan equine encephalitis virus, and
smallpox virus vaccines) and also reduce the level of
primary containment required for laboratory and animal
studies. However, vaccination must never be used to
eliminate primary containment or personal protective
equipment.
The availability of and access to prompt and informed
medical staff and the use of specific and effective therapy
also reduce individual
Survival in the Environment
The physical environment of laboratories and animal
facilities is typically hostile to the growth stages of many
primary pathogens. Drying, exposure to ultraviolet light,
ambient temperatures, lack of nutrients, and residual
chemicals and cleaning products on surfaces reduce the
survival of most fastidious pathogens outside of the host or
selected growth media.
Coxiella burnetii, M. tuberculosis, many systemic and
dermatophytic fungi, hepatitis B virus, and spore-forming
bacteria are exceptions; they remain viable on surfaces or
as droplet nuclei for extended periods. Occupational
infections may occur as a result of inhaling infectious
droplet nuclei (tuberculosis, Q fever), contact of intact skin
(Microsporum canis), or exposure of mucous membranes
(hepatitis A virus, leptospirosis) with splashes, sprays, or
contaminated fluids.
infection risks following overt
exposures.
The consequences of infection are unfortunately
often confused with the risk of infection. The grave
consequences of infection with rabies, herpes B,
Creutzfeldt
e
Jakob agent, and human immunodeficiency
viruses (HIV) are not indicative of infection risks of
persons regularly working with these agents. Ordinary
