Microbiology and Immunology

Zooplankton WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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Zoonoses are relevant for humans because of their

species-jumping ability. Because many of the causative micro-

bial agents are resident in domestic animals and birds, agri-

cultural workers and those in food processing plants are at

risk. From a research standpoint, zoonotic diseases are inter-

esting as they result from organisms that can live in a host

innocuously while producing disease upon entry into a differ-

ent host environment.

Humans can develop zoonotic diseases in different

ways, depending upon the microorganism. Entry through a cut

in the skin can occur with some bacteria. Inhalation of bacte-

ria, viruses, and fungi is also a common method of transmis-

sion. As well, the ingestion of improperly cooked food or

inadequately treated water that has been contaminated with

the fecal material from animals or birds present another route

of disease transmission.

A classic historical example of a zoonotic disease is yel-

low fever. The construction of the Panama Canal took humans

into the previously unexplored regions of the Central

American jungle. Given the opportunity, transmission from

the resident animal species to the newly arrived humans

occurred. This phenomenon continues today. Two examples

are illustrative of this. First, the clearing of the Amazonian rain

forest to provide agricultural land has resulted in the emer-

gence of Mayaro and Oropouche virus infections in the wood-

cutters. Second, in the mid 1990s, fatalities in the

Southwestern United States were traced to the hanta virus that

has been transmitted from rodents to humans.

A number of bacterial zoonotic diseases are known. A

few examples are Tularemia, which is caused by Francisella

tulerensis, Leptospirosis (Leptospiras spp.), Lyme disease

(Borrelia burgdorferi), Chlaydiosis (Chlamydia psittaci),

Salmonellosis (Salmonella spp.), Brucellosis (Brucella

melitensis, suis,and abortus, Q-fever (Coxiella burnetti), and

Campylobacteriosis(Campylobacter jejuni).

Zoonoses produced by fungi, and the organism respon-

sible, include Aspergillosis (Aspergillus fumigatus). Well-

known viral zoonoses include rabiesand encephalitis. The

microorganismscalled Chlamydia cause a pneumonia-like

disease called psittacosis.

Within the past two decades two protozoan zoonoses

have definitely emerged. These are Giardia(also commonly

known as “beaver fever”), which is caused by Giardia lamblia

and Cryptosporidium, which is caused by Cryptosporidium

parvum. These protozoans reside in many vertebrates, partic-

ularly those associated with wilderness areas. The increasing

encroachment of human habitations with wilderness is bring-

ing the animals, and their resident microbial flora, into closer

contact with people.

Similarly, human encroachment is thought to be the

cause for the emergence of devastatingly fatal viral hemor-

rhagic fevers, such as Ebola and Rift Valley fever. While the

origin of these agents is not definitively known, zoonotic

transmission is assumed.

In the present day, outbreaks of hoof and mouth disease

among cattle and sheep in the United Kingdom (the latest

being in 2001) has established an as yet unproven, but com-

pelling, zoonotic link between these animals and humans,

involving the disease causing entities known as prions. While

the story is not fully resolved, the current evidence supports

the transmission of the prion agent of mad cow disease to

humans, where the similar brain degeneration disease is

known as Creutzfeld-Jacob disease.

The increasing incidence of these and other zoonotic

diseases has been linked to the increased ease of global travel.

Microorganisms are more globally portable than ever before.

This, combined with the innate ability of microbes to adapt to

new environments, has created new combinations of microor-

ganism and susceptible human populations.

See alsoAnimal models of infection; Bacteria and bacterial

infection

ZZooplanktonOOPLANKTON

Zooplankton are small animals that occur in the water column

of either marine or freshwater ecosystems. Zooplankton are a

diverse group defined on the basis of their size and function,

rather than on their taxonomic affinities.

Most species in the zooplankton community fall into

three major groups—Crustacea, Rotifers, and Protozoas.

Crustaceans are generally the most abundant, especially those

in the order Cladocera (waterfleas), and the class Copepoda

(the copepods), particularly the orders Calanoida and

Cyclopoida. Cladocerans are typically most abundant in fresh

water, with common genera including Daphnia and Bosmina.

Commonly observed genera of marine calanoid copepods

include Calanus, Pseudocalanus, and Diaptomus, while abun-

dant cyclopoid copepods include Cyclops and Mesocyclops.

Other crustaceans in the zooplankton include species of opos-

sum shrimps (order Mysidacea), amphipods (order

Amphipoda), and fairy shrimp (order Anostraca). Rotifers

(phylum Rotifera) are also found in the zooplankton, as are

protozoans (kingdom Protista). Insects may also be important,

especially in fresh waters close to the shoreline.

Most zooplankton are secondary consumers, that is,

they are herbivores that graze on phytoplankton, or on unicel-

Sheep can act as host for a number of zoonotic disease pathogens.

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