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Submodule 6: Microbes and Fungi -- the Pathogens
Introduction and General Characteristics
Invasive non-native pathogens have had a profound impact on the health of humans, plants, and animals in the U.S as well as around the world. The movement of disease-causing microbes around the world has taken its toll on many different organisms, and continues to intensify with increased global transport of cargo, people, animals, and plants. The efforts of many different organizations and governmental agencies from all countries are needed to monitor this problem and curtail pathogen introductions. In addition, open information sharing among scientists is critical to insure that the best methods for treatment and control are available and in effect. Public health efforts need to address human health threats from exotic and emerging diseases.
Fungi
Protozoa (photo courtesy USGS)
Bacteria (photo courtesy NSF)
Virus (photo courtesy NIH)
Fungi: Fungi are primarily multicellular eukaryotic organisms. Invasive fungi are often introduced into the U.S. on imported lumber or nursery plants, or on human travelers. Many can also be carried by soil. In addition, some fungi reproduce by releasing large quantities of wind-borne spores that can travel long distances before landing on a suitable medium and germinating. Careful inspections often fail to detect infections because, except for at certain stages of its life cycle, a fungus may be undetectable. In addition, it is difficult for inspectors to determine if a newly identified fungus poses any danger of becoming an invasive organism in this country. Even after many years of research, it is estimated that only 10% of all fungi have been classified and described, and few have been studied in depth. Because of this, mycologists, or those who study fungi, have little information about the characteristics of invasive, pathogenic fungi. Another confounding factor is that a particular fungus can inhabit many different organisms, and may be pathogenic to some of these hosts but not to others.
Protozoa: Protozoa are single-celled eukaryotic organisms. Most nonindigenous protozoa that become detrimental in new environments are found in aquatic environments. These organisms are commonly found in ballast water and cause diseases of marine and freshwater fish. Treatment of ballast water (such as heat or filtration) can easily kill protozoa before they can cause harm.
Bacteria: Bacteria are single-celled, prokaryotic organisms. Bacterial pathogens are most commonly introduced into the U.S. within infected organisms, in soil, or in ballast water, although there are numerous other pathways. Live organisms imported into the U.S. are periodically inspected for evidence of bacterial diseases. Failure to detect harmful bacteria can occur due to lack of obvious symptoms or because inspectors are unfamiliar with the symptoms. Some states are looking into the possibility of treating pathways such as ballast water to kill potentially invasive bacteria before they can spread; this could stop invasion of dangerous bacteria such as Vibrio cholerae, which causes cholera, and has been detected in ballast water in ships from Bangladesh. The ARS (Agricultural Research Service) has been working on a program to characterize and rapidly identify such potentially invasive bacteria using molecular techniques.
Viruses: Virus particles are organisms, smaller than most bacteria, which require a host to metabolize and replicate. Invasive viruses are usually brought into this country within a host organism that they have infected. A problem for inspectors is that viruses can be present is seemingly healthy organisms, which makes potential introductions easy to miss. Many virus particles can also live outside their host for extended periods in environments such as in ballast water and on equipment.
Particular plant and animal species are subject to blanket rejections by inspectors when entering the U.S. due to the high probability they will be infected with known pathogens. However, not all invasive pathogens are a problem in all areas of the country. Environmental conditions play a major role in determining areas where pathogens may become a problem.
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