The Mosquito-Borne Diseases

Ross River (RR) & Barmah Forest (BF)

Both diseases are caused by viruses which are transmitted to humans through the bite of a mozzie. Serological studies and laboratory investigations have indicated that native mammals, most likely kangaroos and wallabies are natural hosts for Ross River (RR) virus but little is known about the hosts of Barmah Forest (BF) virus. Both viruses have been isolated from many mozzie species, including those that breed in freshwater habitats especially in irrigated areas. Along coastal regions, saltmarsh mozzies represent the major threat whereas floodwater mozzies are the main transmitters in inland regions following heavy rains or floods.

A wide variety of symptoms may occur from a transient rash and mild illness with fever to polyarthritis affecting chiefly the ankles, fingers, knees and wrists but other joints may be infected. The disease is not fatal but can last from months to years. There is no known specific treatment.

RR has become of serious concern in Australia. For most of Australia the disease peaks through summer and autumn months particularly January to March. In southwestern Australia and eastern Victoria, RR starts early in spring and peaks in early summer. RR peak outbreaks occur in NSW in January through to May.

Areas under intensive irrigation and localities close to saltmarshes are most productive for mozzie populations. RR fever is also present in urbanised areas, including major capitals such as Sydney, although rural residents are at greater risk alone. During 1991 -1997 the number of cases averaged 5,000 per annum in Australia. In 1996 there were 4,721 recorded cases of Ross River Fever in Queensland

Australian Murray Valley Encephalitis (MVE) and Kunjin (KUN) virus

Australian Murray Valley Encephalitis (MVE) and Kunjin (KUN) virus are different viruses with slightly different clinical symptoms which are variable, from mild to severe with permanent impaired neurological functions, to sometimes fatal. That is, cases vary from sudden onset of fever. Anorexia and headaches are common, while vomiting, nausea, diarrhea and dizziness may also be experienced. Brain dysfunction may be experienced after a few days with lethargy, irritability, drowsiness, confusion, convulsions and fits; neck stiffness can be expected and both coma and death may ensue. It is rare for recovery from the encephalitic syndrome to occur without some residual mental or functional disability. There are no specific therapies to treat the disease or control the virus.

It's acknowledged that both MVE and KUN viruses have a natural endemic cycle, which involves water birds as the vertebrate host and the mozzie as the major insect that transmits the disease. Its uncertain whether the virus/es are introduced prior to outbreaks, or whether they are endemic there at undetectable levels and only become evident with periods of intense bird and mozzie breeding following intensive flooding. Previous outbreaks were consistent with extraordinary rainfalls and widespread flooding in the eastern Australian watershed and the Murray-Darling basin in particular.

Japanese Encephalitis (JE)

Japanese Encephalitis (JE) has spread throughout eastern Asia including India, Japan, China and southeast Asia. It appeared in Australia in 1995. Serological evidence is that transmission occurred in pigs in 1996 and into humans in 1998 and that it had been in PNG for eight or more years. Its natural cycle in Asia involves water birds and mozzies with pigs being also involved as an amplifying host and providing a link to humans through their proximity to housing. Symptoms of infection are similar to MVE virus

Dengue Fever (DF)

Dengue Fever (DF) is the most important viral disease transmitted by mozzies and affecting humans on a world scale. Symptoms range from mild fevers to a severe and potentially life threatening haemorrhagic disease. It's a debilitating infection of comparatively short duration with a high attack rate but a low fatality rate. Its classic symptoms affect older children and adults with fever, violent headache and severe pains in the muscles and joints following an incubation period of 5-8 days and lasts about 4-7 days. Recovery is usually complete although convalescence may be long. A more sever form, Dengue Haemorrhagic Fever (DHF) involves internal bleeding and is sometimes associated with severe shock and occurs most frequently in infants and young children. No specific treatment exists.

It commonly occurs in most tropical and sub tropical countries and so is present in the northerly parts of Australia. North Queensland is especially susceptible and Medical Authorities regularly warn about DF, particularly in the popular tourist areas such as Cairns. It has also been recorded north of Sydney.

Malaria

Malaria occurs when humans are infected with a blood parasite that is transmitted by a mozzie when she has a blood meal. Its one of the most widespread of all human parasitic diseases with the annual world estimated incidence in 1998 at 110 million cases with about 2.5 million deaths per year. The parasite invades liver cells and red blood cells which they disrupt causing fever. Thus malaria is characterized clinically by fever - usually periodic - varying degrees of anaemia and splenic enlargement and a range of other syndromes involving organs such as the brain, liver and the kidneys.

In Australia, Malaria has been endemic but malaria was declared eradicated from the country in 1981, though 700-800 cases occur here each year in travelers infected elsewhere and the region of northern Australia above 19*S latitude is the receptive zone for malaria transmission.

In the last few years there has been evidence of mozzies entering Australia via commercial airlines and subsequently transmitting malaria which is known as 'airport malaria'.

West Nile Virus

West Nile virus (WNV) is a virus of the family Flaviviridae. Part of the Japanese encephalitis (JE) antigenic complex of viruses, it is found in both tropical and temperate regions. It mainly infects birds, but is known to infect humans, horses, dogs, cats, bats, chipmunks, skunks, squirrels, and domestic rabbits. The main route of human infection is through the bite of an infected mosquito. West Nile virus has been described in Africa, Europe, the Middle East, west and central Asia, Oceania (subtype Kunjin), and most recently, North America. WNV has three different effects on humans.

The first is an asymptomatic infection. The second is a mild febrile stage (termed West Nile Fever) has an incubation period of 2 to 8 days followed by fever, headache, chills, excessive sweating, weakness, swollen lymph nodes, drowsiness, pain in the joints and symptoms like those of influenza or the flu. Occasionally there is a short-lived truncal rash and some patients experience gastrointestinal symptoms including nausea, vomiting, loss of appetite, or diarrhea. All symptoms are resolved within 7 to 10 days, although fatigue can last for some weeks and swollen lymph nodes can take up to two months to resolve. The third is a neuroinvasive disease termed West Nile meningitis or encephalitis. In many infected individuals the ratio between the three states is roughly 110:30:1. The more dangerous encephalitis is characterized by similar early symptoms but also a decreased level of consciousness, sometimes approaching near-coma. Deep tendon reflexes are hyperactive at first, later diminished.

The virus is transmitted through mosquito vectors, which bite and infect birds. The birds are amplifying hosts, developing sufficient viral levels to transmit the infection to other biting mosquitoes which go on to infect other birds and also humans. The infected mosquito species vary according to geographical area; in the US Culex pipiens (Eastern US), Culex tarsalis (Midwest and West), and Culex quinquefasciatus (Southeast) are the main sources.