Transport of an Infectious Agent

Transmission involves the transport of an infectious agent from the reservoir to the host. It is the most important link in the chain of infection. Pathogens can be transmitted from the reservoir to a susceptible host by various routes (Sobsey and Olson, 1983).

a. Person-to-Person Transmission

The most common route of transmission of infectious agents is from person to person. The best examples of direct contact transmission are the sexually transmitted diseases such as syphilis, gonorrhea, herpes, or acquired immunodeficiency syndrome (AIDS).

Coughing and sneezing discharge very small droplets containing pathogens within a few feet of the host (droplet infection). Transmission by these infectious droplets is sometimes considered as an example of direct contact transmission.

b. Waterborne Transmission

The waterborne transmission of cholera was established in 1854 by John Snow, an English physician who noted a relationship between a cholera epidemics and consumption of water from the Broad Street well in London. The waterborne route is not, however, as important as the person-to-person contact route for the transmission of fecally transmitted diseases. The World Health Organization (WHO) reported that diarrhoeal diseases contracted worldwide mainly by contaminated water or food, killed 3.1 million people, most of them children (WHO, 1996). In the United States, waterborne disease outbreaks are reported to the U.S.

 Environmental Protection Agency (U.S. EPA) and the Centers for Disease Control and Prevention (CDC) by local epidemiologists and health authorities; the system was started in the 1920s (Craun, 1986a, b; 1988). During the period 1971– 1985, 502 waterborne outbreaks and 111,228 cases were reported. Three-quarters of the outbreaks were due to untreated or inadequately treated groundwater and surface waters. Gastrointestinal illnesses of unidentified etiology and giardiasis are the most common waterborne diseases for groundwater and surface water systems. The outbreak rate (expressed as the number of outbreaks/1000 water systems) and the illness rate (expressed as numbers of cases/million-person year) decrease as the raw water is filtered and disinfected (Craun, 1988).

c. Foodborne Transmission

Food may serve as a vehicle for the transmission of numerous infectious diseases caused by bacteria, viruses, protozoa, and helminth parasites. The World Health Organization estimates that accidental food poisoning kills up to 1.5 million people per year. In the United States, it is estimated that foodborne illnesses affect some 6 to 80 million persons / year, leading to approximately 9000 death (Altekruse et al., 1997). Food contamination results from unsanitary practices during production or preparation. Several pathogens and parasites have been detected in risky foodstuffs such as shellfish, vegetables, raw milk, runny eggs or pink chicken, turkey, ground beef and ground pork, alfalfa sprouts, and unpasteurized apple juice/cider. Their presence is of public health significance, particularly for foods that are eaten raw (e.g., shellfish, fresh produce). There is also an increased risk among the elderly and immunocompromised people (HIV and leukemia patients, and those taking immunosuppressive drugs such as steroids, cyclosporine, and radiation therapy).

Vegetables contaminated with wastewater effluents are also responsible for disease outbreaks (e.g., typhoid fever, salmonellosis, amebiasis, ascariasis, viral hepatitis, gastroenteritis). Raw vegetables and fruit become contaminated as a result of being handled by an infected person during processing, storage, distribution or final preparation, or following irrigation with fecally contaminated water (Seymour and Appleton, 2001). Vomitus (estimation of 20 to 30 million virus particles released during vomiting) from infected food handlers can also contaminate exposed food and surfaces via production of bioaerosols.

In England and Wales, viruses accounted for 4.3 percent of all foodborne outbreaks for the period 1992–1999, with Norwalk-like viruses (NLVs) being the most commonly found agents (O’Brien et al., 2000). Outbreaks of hepatitis were associated with fresh produce (e.g., salads, iceberg lettuce, diced tomatoes, frozen raspberries).

Shellfish (e.g., oysters, clams, mussels) are significant vectors of human diseases of bacterial, viral, and protozoan origin. Several surveys have been carried out worldwide to show the presence of pathogens in shellfish samples. The use of molecular techniques (RT-PCR) has helped in the detection of enteric viruses (hepatitis A virus, Norwalk-like virus, enterovirus, rotavirus, and astrovirus) in oyster and mussel samples in France (Le Guyader et al., 2000). In Switzerland, 8 of 87 imported oyster samples were positive for Norwalk-like viruses (Beuret et al., 2003). Moreover, infectious oocysts of Cryptosporidium were detected in mussels and cockles in Spain (Gomez-Bautista et al., 2000).

Enteric viruses (enteroviruses, Norwalk-like viruses, and adenoviruses) were detected in 50–60 percent of mussel samples in two sites in the west coast of Sweden (Hernroth et al., 2002). Of 36 mussel samples from the Adriatic Sea, 13 were contaminated with hepatitis A virus and 5 samples with enteroviruses (Croci et al., 2000). In several surveys, E. coli was not found to be a good indicator of virus presence in mussels.