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Letter
Emerging Human Infectious
Diseases: Anthroponoses, Zoonoses, and Sapronoses
Zdenek Hubálek*
*Academy of Sciences, Brno, Czech Republic
Suggested citation for this article: Hubálek
Z . Emerging human infectious diseases: anthroponoses, zoonoses, and
sapronoses. Emerg Infect Dis [serial online] 2003 Mar [date cited].
Available from: URL: http://www.cdc.gov/ncidod/EID/vol9no3/02-0208.htm
To the Editor: The source of infection has always been regarded
as an utmost factor in epidemiology. Human communicable diseases can be
classified according to the source of infection as anthroponoses (when
the source is an infectious human; interhuman transfer is typical), zoonoses
(the source is an infectious animal; interhuman transfer is uncommon),
and sapronoses (the source is an abiotic substrate, nonliving environment;
interhuman transfer is exceptional). The source of infection is often
the reservoir or, in ecologic terms, the habitat where the etiologic agent
of the disease normally thrives, grows, and replicates. A characteristic
feature of most zoonoses and sapronoses is that once transmitted to humans,
the epidemic chain is usually aborted, but the clinical course might be
sometimes quite severe, even fatal. An ecologic rule specifies that an
obligatory parasite should not kill its host to benefit from the adapted
long-term symbiosis, whereas an occasionally attacked alien host, such
as a human, might be subjected to a severe disease or even killed rapidly
by the parasite because no evolutionary adaptation to that host exists
(1). In this letter, only microbial infections are discussed;
metazoan invasion and infestations have been omitted.
Anthroponoses (Greek “anthrópos” = man, “nosos” = disease) are diseases
transmissible from human to human. Examples include rubella, smallpox,
diphtheria, gonorrhea, ringworm (Trichophyton rubrum), and trichomoniasis.
Zoonoses (Greek “zoon” = animal) are diseases transmissible from living
animals to humans (2). These diseases were formerly called
anthropozoonoses, and the diseases transmissible from humans to animals
were called zooanthroponoses. Unfortunately, many scientists used these
terms in the reverse sense or indiscriminately, and an expert committee
decided to abandon these two terms and recommended “zoonoses” as “diseases
and infections which are naturally transmitted between vertebrate animals
and man” (3). A limited number of zoonotic agents can
cause extensive outbreaks; many zoonoses, however, attract the public’s
attention because of the high death rate associated with the infections.
In addition, zoonoses are sometimes contagious for hospital personnel
(e.g., hemorrhagic fevers). Zoonotic diseases can be classified according
to the ecosystem in which they circulate. The classification is either
synanthropic zoonoses, with an urban (domestic) cycle in which the source
of infection are domestic and synanthropic animals (e.g., urban rabies,
cat scratch disease, and zoonotic ringworm) or exoanthropic zoonoses,
with a sylvatic (feral and wild) cycle in natural foci (4)
outside human habitats (e.g., arboviroses, wildlife rabies, Lyme disease,
and tularemia). However, some zoonoses can circulate in both urban and
natural cycles (e.g., yellow fever and Chagas disease). A number of zoonotic
agents are arthropod-borne (5); others are transmitted
by direct contact, alimentary (foodborne and waterborne), or aerogenic
(airborne) routes; and some are rodent-borne.
Sapronoses (Greek “sapros” = decaying; “sapron” means in ecology a decaying
organic substrate) are human diseases transmissible from abiotic environment
(soil, water, decaying plants, or animal corpses, excreta, and other substrata).
The ability of the agent to grow saprophytically and replicate in these
substrata (i.e., not only to survive or contaminate them secondarily)
are the most important characteristics of a sapronotic microbe. Sapronotic
agents thus carry on two diverse ways of life: saprophytic (in an abiotic
substrate at ambient temperature) and parasitic (pathogenic, at the temperature
of a homeotherm vertebrate host). Typical sapronoses are visceral mycoses
caused by dimorphic fungi (e.g., coccidioidomycosis and histoplasmosis),
“monomorphic” fungi (e.g., aspergillosis and cryptococcosis), certain
superficial mycoses (Microsporum gypseum), some bacterial diseases
(e.g., legionellosis), and protozoan (e.g., primary amebic meningoencephalitis).
Intracellular parasites of animals (viruses, rickettsiae, and chlamydiae)
cannot be sapronotic agents. The term “sapronosis” was introduced in epidemiology
as a useful concept (6-8). For these diseases the expert
committee applied the term “sapro-zoonoses,” defined as “having both a
vertebrate host and a nonanimal developmental site or reservoir (organic
matter, soil, and plants)” (3,9). However, the term sapronoses
is more appropriate because animals are not the source of infection for
humans. While anthroponoses and zoonoses are usually the domains for professional
activities of human and veterinary microbiologists, respectively, sapronoses
may be the domain for environmental microbiologists. The underdiagnosis
rate for sapronoses is probably higher than that for anthroponoses and
zoonoses, and an increase should be expected in both incidence and number
of sapronoses. Legionellosis, Pontiac fever, nontuberculous mycobacterioses,
and primary amebic meningoencephalitis are a few sapronoses that have
emerged in the past decade. In addition, the number of opportunistic infections
in immunosuppressed patients has grown markedly; many of these diseases
and some nosocomial infections are, in fact, also sapronoses.
As with any classification, grouping human diseases in epidemiologic
categories according to the source of infection has certain pitfalls.
Some arthropod-borne diseases (urban yellow fever, dengue, epidemic typhus,
tickborne relapsing fever, epidemic relapsing fever, and malaria) might
be regarded as anthroponoses rather than zoonoses because the donor of
the infectious blood for the vector is an infected human and not a vertebrate
animal. However, the human infection is caused by an (invertebrate) animal
in which the agent replicates, and the term zoonoses is preferred. HIV
is of simian origin with a sylvatic cycling among wild primates and accidental
infection of humans who hunted or ate them; the human disease (AIDS) might
thus have been regarded as a zoonosis in the very first phase but later
has spread in the human population as a typical anthroponosis and caused
the present pandemic. Similarly, pandemic strains of influenza developed
through an antigenic shift from avian influenza A viruses. For some etiologic
agents or their genotypes, both animals and humans are concurrent reservoirs
(hepatitis virus E, Norwalk-like calicivirus, enteropathogenic Escherichia
coli, Pneumocystis, Cryptosporidium, Giardia, and Cyclospora);
these diseases might conditionally be called anthropozoonoses. Other difficulties
can occur with classifying diseases caused by sporulating bacteria (Clostridium
and Bacillus): Their infective spores survive in the soil or in
other substrata for very long periods, though they are usually produced
after a vegetative growth in the abiotic environment, which can include
animal carcasses. These diseases should therefore be called sapronoses.
For some other etiologic agents, both animals and abiotic environment
can be the reservoir (Listeria, Erysipelothrix, Yersinia
pseudotuberculosis, Burkholderia pseudomallei, and Rhodococcus
equi), and the diseases might be, in fact, called saprozoonosis (not
sensu 9 ) in that their source can be either an animal or an abiotic substrate.
For a concise list of anthropo-, zoo-, and sapronoses, see the online
appendix.
References
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