The Transfer of Disease by Zoonoses
What are zoonoses?
A zoonosis is an infectious disease naturally transmissible between animals and man. The pathogen causing this transfer of disease is called ‘a zoonotic pathogen’. This can occur by either direct contact with the infection, for example the contamination of food, water and handling of animals (as seen with Salmonella, Leptospira and Escherichia coli), or the disease can also be transferred through the use of an intermediate species – a vector. The word zoonosis originates from the late 19th century, with ‘zoo’ meaning animals and ‘nosos’ meaning disease.
Three quarters of all current human diseases, and the majority of the most devastating diseases have all been caused by a zoonotic pathogen.
An increase in the transfer of disease by zoonoses is caused by heavy rainfall, as this can cause run-off from urban, suburban and rural landscapes which consequently can carry wastewater and protozoa into untreated water such as that used for agriculture, recreational activities and drinking. The protozoa can also enter the coastal waters and can therefore contaminate a vast range of shellfish and also infect many marine vertebrae.
Examples of zoonotic transfer within the marine environment:
Marine mammals can carry microbes (bacteria, fungi and viruses) that all can cause illness in humans. Many marine mammals may appear healthy, however, they can still be carrying organisms that are dangerous to humans. They have been found to carry many different pathogens associated with food poisoning for example (Escherichia coli, Salmonella, Listeria). Much like other wildlife species out of the marine environment, seals and sea lions can defecate Giardia (a protozoan) within their faeces. The Giardia protozoan can cause diarrhoeal symptoms along with other serious illness in humans. Much less common than the transfer of this pathogen is the transfer of the rabies virus and Mycobacterium tuberculosis, both of which can infect marine mammals and consequently pass to humans through the food chain.
Sea Lion sp.
A mycoplasma that is found in seals has been known to cause “seal finger” in humans; if a person has been bitten or has been handling a contaminated seal. The symptoms of this include acute swelling, discharge and in some cases there can also be some joint involvement. Another zoonotic transfer that occurs is that of seal pox. Seal pox is a zoonotic disease affecting pinnipeds (seals and sea lions), which is mainly seen within rehabilitation centres or in convalescing animals. It is transferred when broken skin comes into contact with the virus; for example being bitten by a seal. The virus can cause pox sores in humans, which can persist for up to a year. Seals and sea lions are also able to transmit leptospirosis.
Both crabs and crayfish are able to transmit Paragonimiasis a disease, which causes chronic inflammation of the lungs. The disease itself takes 23-35 days to reach the lungs after the pathogenic worm has been ingested. The disease is passed on to humans through the consumption of raw crabs and crayfish. By ensuring that the meat is thoroughly cooked you are able to eliminate the pathogen and prevent its spread to humans.
Birds are associated with the dissemination of the causative agents of a large number of diseases that can affect the health of humans. Feral pigeons, the birds we so commonly associate with our towns and cities, are descended from the Rock Dove; a bird of coastal cliffs. More than 110 pathogens (8 viruses, 55 fungi, 41 bacteria and 6 protozoa) have been reported to affect pigeons. With both gulls and humans often living in the same environments, and there being an increase in the number of gulls occupying more urbanized areas, the chances of zoonotic transfer of disease has been increased. The diseases are most commonly transferred by faecal contamination; for example within water supplies. The pathogens most often carried by gulls originate from sites such as waste transfer stations, landfills and those handling untreated sewage. As the gulls feed at these locations and then visit drinking water reservoirs, grazing pastures and other sites which have a significance in the human food chain, they have the potential to readily transmit diseases. Visiting gull nesting sites, many of which now occur in urban environments, may, in addition to diseases associated with faecal matter, also expose humans to pathogens such as ticks, lice and fungal spores.
Electron micrograph of the Influenza A virus
The migratory and behavioral patterns of some birds, provide further mechanisms for the occurrence of a greater range of zoonotic diseases and for them to have an increased potential for global spread. Avian influenza is a zoonotic disease commonly seen within ducks and shore birds. The mutation that infects the birds is able to mutate in order to infect humans. The Influenza A virus (H5N1) is seen within both the Mediterranean gull and within waterfowl. The virus has been shown to be able to survive within the environment for a long period of time and is able to be spread just through touching surfaces contaminated with the disease. Once a bird has been contaminated with Avian influenza they are able to release the pathogenic organisms in order to transfer the disease, for example within their faeces and saliva, for as much as ten days. The symptoms of influenza within a human are the typical flu like symptoms however it can and has caused fatalities.
The British Pest Management Manual (2007)
International Journal of Environmental Health Research. Volume 6, Issue 1, 1996. Jeremy J. Hatch