Eukaryotes (kingdom Protoctista; subkingdom Protozoa)
eg: Plasmodium relictum
Malarial parasitism has been a major cause of decline in the Hawaiian avifauna since the introduction of foreign birds and mosquitoes gathered momentum during the past century, especially in the duration of WWII. In the Southern hemisphere, Penguins have also been found to be particularly susceptible to the parasite, but no severe outbreaks have occurred hitherto. Yet the irreversible effect of Plasmodium relictum on the vulnerable endemic birds in one of the most ecologically important Pacific island groups have been pronounced, and the raid is still under way.
Plasmodium is a protozoan that parasitizes red blood cells of the infected hosts, causing acute illness and death in species that are prone to such attacks. Mosquitoes, mainly of the species Culex quinquefasciatus, are the principal transmitting agent of the disease as they spread the parasite into the blood stream of the native birds when they secrete their saliva into the birds from which they are drawing blood. Dispersal is rapid throughout the infected organism's body and in the local bird communities, especially Passerines (small songbirds) that constitutes more than 90% of the surviving endemic bird species in Hawaii. Endemic birds that have evolved in the entire absence of the protozoan is especially vulnerable to the disease, which causes a high mortality rate compared to the introduced bird species that have become resistant to a large degree to the parasite. The range of native species is restricted by the presence of Mosquitoes in the lowland areas, and indeed many Hawaiian honeycreepers have retreated to the upland swamps and forests that are relatively free of Mosquitoes. However, both the vector and host have large ranges and infected birds may bring the parasite into relatively unaffected communities into which pretty much all the relict communities of native birds are retreating. This is likely to jeopardize the remaining bird populations and cause their extinction.
Plasmodium has a complex life cycle consisting of stages both in the Mosquito and the host bird. The infectious-stage organisms that are transmitted from the Mosquito to the host species are called Sporozoites. After entry into the host body the parasite begins to attack the red blood cells of the host. The nutrients obtained from the erythrocytes are used in development into the Gametocytes and Merozoites. The former reproduce sexually after being ingested by Mosquitoes during blood feeding and the latter, with its associated toxins, cause the symptoms of high fever apparent in host birds.
Attempts to control malaria have been thwarted due to the difficulty to eradicate the Mosquito transmitters. Captive populations of native birds are receiving medical treatment but this is not a long-term solution, and the future of surviving birds is uncertain.
Prokaryotes (kingdom Monera)
While most people do not realize it is so, our lives are intimately tied with that of bacteria, whether the connotations are positive or negative. As introduced organisms many bacteria have had negative impacts on wildlife and humans, especially with disease-containing pathogens. Bacteria that cause diseases in humans are widely studied; those causing widespread impact among the wildlife receive less coverage. However, epidemic-causing bacteria can lead to huge economic losses in ways as portrayed by the example below.
eg: Pasteurella multocida
This bacterial species is now found all over the world and is the causative organism of a disease that has affected many domestic and wild animals throughout its range in the form of outbreaks. An array of diseases caused by this pathogen includes a common poultry epidemic known as fowl cholera or avian cholera, is reputed to kill the infected animal within a day of the first contact.
Pasteurella multocida inhabit the upper respiratory tract of many large vertebrates, notably that of poultry, cattle, swine, even domestic cats and dogs. Rodents can also act as vectors and as Rats and Mice have successfully invaded a new wide range over much of the world, the Avian Cholera bacteria is also spread to where suitable hosts reside. Strains within this species group have caused outbreaks of avian cholera in chickens, turkeys, geese and ducks; other respiratory diseases and haemorrhagic septicemia in ruminants, which include cattle and sheep, atrophic rhinitis in pigs, septicemia in rodents and lagomorphs, and even rare infections in humans, are also attributable to this bacterium. Intensive animal farming practices that constrict the space available to individual livestock animals have resulted in high population densities that are auspicious for the transmission of the bacterium among the herds. The bacterium is adept at modifying their characteristics according to the abundant host present around the locality, with a high mutation rate. A number of different lineages have been identified that are specifically suited to life in different hosts. Though not especially motile and dies under sunlight exposure, P. multocida can thrive in various estuarine and riparian environments, living in the respiratory systems and digestive tracts of many animals, mainly avian hosts.
The ecological impact of this bacterium can be serious in places where farms have allowed the bacterial population to proliferate, affecting wild birds such as wild waterfowl and birds of prey that feed on waterfowl corpses. Infected birds show acutely manifesting symptoms as convulsive fits, disorientation, panting, and muscle stiffness; chronic signs of infection include lethargy and shivering, with mucous discharges from the respiratory tract accumulating around the nostrils and beak. Egested materials are pasty and yellow, containing the bacterium, which can be spread further, and sometimes with bloodstains from intestinal ulceration; this is followed by gradual loss of weight, fever, diarrhoea, and inflammated joints. Transmission is quick from bird carcasses into the surrounds, and an outbreak can be initiated very easily; in the extreme, in places where wetlands occur with large congregations of waterfowl and waders, avian cholera caused by the bacterium can kill thousands of birds at a time. Epidemics have been reported from the wetlands sites, consequently, near farms in the Mid-west and the Southern states. Released bacteria enter wild birds, some of which are resistant to the bacteria and simply acts as a vector, such as gulls. Some susceptible individuals contain the bacterium but show no symptoms of infection.
No cure has been found for the diseases caused by Pasteurella multocida, and most control methods tend to focus on the preventive side, and measures implemented target at minimizing any chance of dispersal. Infected carcasses are removed to protect the predatory birds and mammals that are principally scavengers. Pools contaminated are drained and flushed or sterilized when possible. Bacterium carried by dead birds can remain in waters around for a long time, and in the first place the unhygienic conditions of poultry farms must be rectified so that the accumulation of contaminated faeces and overcrowding of birds do not occur to grant chances for the survival of the bacterium; at any rate, however, it is very difficult to eliminate this introduced species and periodic outbreaks still occur in areas where the presence of P. multocida is noted.
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Pathogenic Viruses (kingdom Sub-organisms)
Pathogenic Viruses are endoparasites that affect a wide range of organisms and there are species that base their life cycles on all sorts of other organisms from animals, plants to bacteria. There are still debates as to whether viruses are organisms, as they do not show all the normal characteristics of organisms. However in terms of biotic interactions they are effectively self-propelled invaders especially when introduced to foreign lands where the other organisms have not been evolving under the influence of the virus and are susceptible to parasitism.
eg: Flavivirus sp. --- West Nile Virus (WNV)
First identified in a female patient suffering from an unknown illness in back in 1937, the West Nile Virus obtains its name from a district in Uganda, where this case had occurred. The endoparasite relies on a two-stage host cycle, living alternately in the arthropod and avian vector, similar to the protozoan responsible for avian malaria. The Virus thrives in scattered sites around Asia Minor and Middle East to Eastern Europe and South to Africa, and outbreaks were rare but entailed high fatality rate among the victims, up to 10% in some outbreaks. The outbreak in 1974 in South Africa affected around 3000 people. From 1999 onwards, the WNV has been found also in the New World, and most States in USA have seen instances of infection. In USA in 2003, more than 200 people died from the viral infection, out of over 9000 documented clinical cases. The same strain of virus that is present in USA has been identified from dead waterfowl in Middle East, but the means of introduction is yet to be known.
Despite the fact that only about a fifth of the infected people develop the West Nile Fever, the disease associated with WNV, the virus has been found in many domestic fowl and wild mammals such as Bears, Raccoons, Chipmunks and Horses, the latest being especially prone to WNV attack and often acts as a carrier. Over 138 species of birds in the US has also been found to carry the WNV, but the main vectors are, yet again, Mosquitoes. House Mosquitoes ( Culex pipiens ) and others, such as Aedes and Anopheles, have been tested to find WNV hitching a ride in the insect, usually in the salivary glands of the Mosquitoes. Mammals are usually the cul-de-sacs of the WNV life cycle, as they tend to develop illnesses from the viral infection and die as a result.
West Nile fever surfaces with symptoms such as headache and aching body muscles, skin rash, and fever within a fortnight of infection. Japanese Encephalitis and meningitis can also develop from the viral attack, and coma could result, followed by death. And yet no cure has been found for diseases with WNV as the causative pathogen.
Preventive measures may be useful with the knowledge that most long-range carriers are likely to be imported poultry and other animals originating from the native range of the WNV. Quarantine for the virus is carried out to decrease the chance of its spread. Vaccination of Horses, which are prone to WNV infection as shown by the fact hat over 10000 have been shown positive in the WNV test in America alone in as recent as 2002, are practiced to hamper the dispersal of West Nile Virus. At the frontiers, Mosquito control remains the most urgent method to adopt against the Virus; it is hoped that with the use of larvicide in pools where Mosquitoes breed, and the eradication of possible container breeding grounds, the level of Mosquito-transmitted pathogens can be reduced.