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Zoonotic pathogens can be transmitted by either direct or indirect contact. Direct
transmission is the immediate transfer of an agent from a reservoir to a susceptible host.
Indirect transmission is the transfer of an infectious agent carried from a reservoir to
a susceptible host. Direct transmission occurs through touch, bite, scratch or aerosol.
Examples include ringworm (touch), rabies (bite), cat scratch disease (scratch) and influ-
enza (aerosol). Indirect transmission occurs when microbes are transferred from animals
to humans by fomites (bedding, feces), an intermediary (sick animal to person to person),
food or water contamination (infective agent on item ingested by human) or an arthropod
such as a mosquito (infective agent transmitted by bite). Examples of indirect transmis-
sion include Escherichia coli 0157:H7 (fecal contaminated meat is the primary source of
infection), Toxoplasma gondii (where the mother contracts the disease from an animal and
transmits it placentally to the fetus), mad cow or bovine spongiform encephalopathy (food
contamination by prions) and West Nile virus (mosquito bite).9
Classification
Zoonoses are classified by their source. There are zoonotic diseases from bacteria, viruses,
fungi, parasites and even prions. Major bacterial zoonoses and their causative organisms
include anthrax (Bacillus anthracis), brucellosis (Brucella melitensis), campylobacteriosis
(Campylobacter jejuni), cat scratch disease (Bartonella henselae), botulism (Clostridium botuli-
num), food poisoning (Clostridium perfringens), erysipeloid (Erysipelothrix rhusiopathiae), E.
coli infections, glanders (Burkholderia mallei), leprosy (Mycobacterium leprae), leptospirosis
(Leptospira species), listeriosis (Listeria monocytogenes), ornithosis (Chlamydophila psittaci),
pasteurellosis (Pasteurella multocida), plague (Yersinia pestis), salmonellosis (Salmonella
typhi), shigellosis (Shigella species), staphylococcal infections (Staphylococcus species),
tuberculosis (Mycobacterium species) and cholera (Vibrio cholerae).10
Important viral diseases from animals and the associated viral family include cold
sores, genital herpes and chicken pox/shingles (Herpesviridae) smallpox (Poxviridae)
AIDS (Retroviridae) yellow, dengue and West Nile fever (Flaviviridae) poliomyeli-
tis (Picornaviridae) influenza (Orthomyxoviridae) rabies (Rhabdoviridae) lassa fever
(Arenaviridae) and Marburg and Ebola hemorrhagic fever (Filoviridae).11
Fungal zoonoses acquired through animal-contaminated soil and the causative agent
include cryptococcosis (Crptococcus neoformans) North American blastomycosis, Gilchrist’s
disease and Chicago disease (Blastomyces dermatitidis) histoplasmosis (Histoplasma capsula-
tum) San Joaquin fever and coccidioidomycosis (Coccidioides immitis) and pneumocystosis
or parasitic pneumonia (Pneumocystis carinii). The latter was a rare disease until the advent
of HIV.12
Parasites are classified into different groups based upon factors such as cell type,
size and locomotion. General categories of medically significant parasites include pro-
tozoa, helminths and arthropods. Protozoan zoonoses and the causative agents include
amebiasis, also called amebic dysentery (Entamoeba histolyltica) cryptosporidiosis
(Cryptosporidium parvum) and giardiasis (Giardia duodenalis). Giardia can be transmitted
from humans to dogs and rodents and vice versa. Mammals such as dogs, cattle, cats, pri-
mates, sheep, pigs and rodents are direct and indirect sources of human infection. Other
protozoan diseases include leishmaniasis (Leishmania spp.) toxoplasmosis (Toxoplasma
gondii) and trypanosomiasis (Typanosoma spp). Helminths are macrocytic worms. The spe-
cies parasitic in animals belong to two phyla: Platyhelminthes (flatworms) and Nemotoda
(roundworms). Diseases caused by helminths include diphyllobothriasis (Diphyllobothrium
spp.) tapeworm (Taenia saginata and T. solium) hookworm (Necator americanus and others)
and trichinosis (Trichinella spiralis).
The phylum Arthropoda contains over 80% of all animal species, making it the largest
group of living organisms. It includes flies, mosquitoes, fleas, mites, lice and bedbugs.
These insects serve as vectors for countless diseases.13
Zoonoses: From Paleolithic Times to the Swine Flu
Zoonotic pathogens can be transmitted by either direct or indirect contact. Direct
transmission is the immediate transfer of an agent from a reservoir to a susceptible host.
Indirect transmission is the transfer of an infectious agent carried from a reservoir to
a susceptible host. Direct transmission occurs through touch, bite, scratch or aerosol.
Examples include ringworm (touch), rabies (bite), cat scratch disease (scratch) and influ-
enza (aerosol). Indirect transmission occurs when microbes are transferred from animals
to humans by fomites (bedding, feces), an intermediary (sick animal to person to person),
food or water contamination (infective agent on item ingested by human) or an arthropod
such as a mosquito (infective agent transmitted by bite). Examples of indirect transmis-
sion include Escherichia coli 0157:H7 (fecal contaminated meat is the primary source of
infection), Toxoplasma gondii (where the mother contracts the disease from an animal and
transmits it placentally to the fetus), mad cow or bovine spongiform encephalopathy (food
contamination by prions) and West Nile virus (mosquito bite).9
Classification
Zoonoses are classified by their source. There are zoonotic diseases from bacteria, viruses,
fungi, parasites and even prions. Major bacterial zoonoses and their causative organisms
include anthrax (Bacillus anthracis), brucellosis (Brucella melitensis), campylobacteriosis
(Campylobacter jejuni), cat scratch disease (Bartonella henselae), botulism (Clostridium botuli-
num), food poisoning (Clostridium perfringens), erysipeloid (Erysipelothrix rhusiopathiae), E.
coli infections, glanders (Burkholderia mallei), leprosy (Mycobacterium leprae), leptospirosis
(Leptospira species), listeriosis (Listeria monocytogenes), ornithosis (Chlamydophila psittaci),
pasteurellosis (Pasteurella multocida), plague (Yersinia pestis), salmonellosis (Salmonella
typhi), shigellosis (Shigella species), staphylococcal infections (Staphylococcus species),
tuberculosis (Mycobacterium species) and cholera (Vibrio cholerae).10
Important viral diseases from animals and the associated viral family include cold
sores, genital herpes and chicken pox/shingles (Herpesviridae) smallpox (Poxviridae)
AIDS (Retroviridae) yellow, dengue and West Nile fever (Flaviviridae) poliomyeli-
tis (Picornaviridae) influenza (Orthomyxoviridae) rabies (Rhabdoviridae) lassa fever
(Arenaviridae) and Marburg and Ebola hemorrhagic fever (Filoviridae).11
Fungal zoonoses acquired through animal-contaminated soil and the causative agent
include cryptococcosis (Crptococcus neoformans) North American blastomycosis, Gilchrist’s
disease and Chicago disease (Blastomyces dermatitidis) histoplasmosis (Histoplasma capsula-
tum) San Joaquin fever and coccidioidomycosis (Coccidioides immitis) and pneumocystosis
or parasitic pneumonia (Pneumocystis carinii). The latter was a rare disease until the advent
of HIV.12
Parasites are classified into different groups based upon factors such as cell type,
size and locomotion. General categories of medically significant parasites include pro-
tozoa, helminths and arthropods. Protozoan zoonoses and the causative agents include
amebiasis, also called amebic dysentery (Entamoeba histolyltica) cryptosporidiosis
(Cryptosporidium parvum) and giardiasis (Giardia duodenalis). Giardia can be transmitted
from humans to dogs and rodents and vice versa. Mammals such as dogs, cattle, cats, pri-
mates, sheep, pigs and rodents are direct and indirect sources of human infection. Other
protozoan diseases include leishmaniasis (Leishmania spp.) toxoplasmosis (Toxoplasma
gondii) and trypanosomiasis (Typanosoma spp). Helminths are macrocytic worms. The spe-
cies parasitic in animals belong to two phyla: Platyhelminthes (flatworms) and Nemotoda
(roundworms). Diseases caused by helminths include diphyllobothriasis (Diphyllobothrium
spp.) tapeworm (Taenia saginata and T. solium) hookworm (Necator americanus and others)
and trichinosis (Trichinella spiralis).
The phylum Arthropoda contains over 80% of all animal species, making it the largest
group of living organisms. It includes flies, mosquitoes, fleas, mites, lice and bedbugs.
These insects serve as vectors for countless diseases.13
Zoonoses: From Paleolithic Times to the Swine Flu