Insect-Borne Disease and Australia

Insect-Borne Disease and Australia

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Insect-Borne Disease and Australia


Insect borne diseases loom as a potential threat in many countries including Australia. Australia has been relatively free of insect-borne diseases compared to other southern land masses such as South America and Africa. Australia has several characteristics that influence the prevalence of insect-borne disease. Some these features include low altitudes, tropical forests, Southern Oscillation, and the fact that Australia is an island continent. Some of the most common insect-borne diseases of concern in Australia include Murray Valley encephalitis, Malaria, Dengue fever, Ross Valley virus and Yellow fever. The most common vector in Australia is the mosquito, in particular Aedes aegypti, which is the Dengue vector. Dengue fever is a flavivirus and is the greatest insect-borne disease threat in Australia. Two presentations of Dengue fever are known; classic Dengue fever and Dengue hemorrhagic fever. Classic Dengue fever is fairly common, usually in urban areas, and usually not life threatening. Dengue hemorrhagic fever is a very serious illness. Dengue is usually isolated to North Queensland, which is brought in by infected international travelers and maintained by the large Aedes aegypti population in Queensland. Australia and north Queensland have implemented management plans to control and eradicate Dengue fever and other potential insect-borne viruses. Keys to this plan are disease surveillance, mosquito control and surveillance, and education.

Introduction

Insect-borne diseases are a common threat to travelers, especially when traveling to tropical countries. Insect-borne diseases are usually transmitted by mosquitoes and are caused by several types of microorganisms. Vaccinations, mosquito control, and education are common methods utilized in minimizing the effects of insect-borne disease.

In this paper I will attempt to give an overview of insect-borne disease in Australia. In doing this, I plan on describing the most common insect-borne diseases in Australia, the vectors behind these diseases, and common methods used in prevention of insect-borne disease in Australia.

Australian Geographical Features

Compared to other southern land masses, such as South America and Africa, Australia has been and is relatively free of insect-borne disease (Kettle 1993).

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Related Searches

Several unique features of Australia affect the frequency of insect-borne diseases. The island feature of Australia separates Australia from other land masses and makes mosquito immigration relatively rare (Kettle 1993). The island feature and Australia’s proximity to the equator result in low elevations and tropical climates. Low altitudes increase the survivability of mosquito larvae, which is a result of the high oxygen levels needed for larvae survival (Savage et al. 1990). Tropical and subtropical areas of Australia include the northern and northeastern edges of Queensland, the northern edge of the Northern Territory, and the far northern edge of Western Australia. These areas are the most common breeding areas of mosquitoes, and as a result are also the site of most instances of insect-borne diseases in Australia (Dengue Fever Management Plan 2000).

The Southern Oscillation, an important mode of climatic fluctuations over the Indian and Pacific Oceans, is closely related to eastern and northern Australian rainfall. The Southern Oscillation has been shown to be related to mosquito population numbers and also closely linked to outbreaks of Murray Valley encephalitis (Nicholls 1986).

Mosquito Background

Mosquitoes are blood-sucking insects that serve as the most formidable transmitters of disease in the animal kingdom. Mosquito-borne diseases are caused by human parasites that have a stage in their life-cycle with the capability to enter the blood stream. The parasites use the mosquitoes to complete their life-cycle, replicate, or change in form. The female mosquitoes transmit the parasites by picking them up from the blood of one host and then transmitting them to another host. Although mosquitoes can transmit many diseases to humans, they cannot transmit HIV. HIV is treated as food, and digested, when taken up by a mosquito. The volume of HIV particles circulating in the blood is also extremely low and in order to be transmitted by mosquitoes a disease must be present at high levels in the blood stream (Crans 1998).

Within Australia there are over 300 species of mosquitoes, with only a small number of these posing a threat (Department of Medical Entomology 2003). Only a small number of these feed on humans and an even smaller number has the ability to transmit disease (Department of Medical Entomology 2003). Mosquitoes belong to the family of flies called Culicidae. The genus Aedes contains some of the worst pests and has the ability to transmit Dengue fever (Crans 1998 and Kettle 1993). The genus Anopheles has the ability to transmit Malaria (Crans 1998).

Mosquitoes have six legs and two scale-covered wings. The head of the mosquito consists of a projecting proboscis, which conceals and protects the mosquito’s sucking mouthparts (Department of Medical Entomology 2003). The mosquito life-cycle consists of four main stages; egg, larvae, pupa, and adult (Figure 1). Mosquitoes in the first three stages are aquatic, while the adult stage is terrestrial. Adult females briefly return to the water habitat to lay each batch of eggs. Mosquitoes vary in their flight range, with most dispersing less than two kilometers from their breeding areas. Some mosquitoes will remain within a few meters from their breeding area, while others can fly as far as 50 kilometers downwind from their larval habitats (Department of Medical Entomology 2003.

The average a female mosquito will live is 2-3 weeks, but the male’s lifespan is usually shorter (Department of Medical Entomology 2003). Mosquitoes’ lives are usually cut short due to the effects of weather or because they serve as food for other insects and birds. Mosquitoes are cold-blooded and are incapable in regulating their body temperature. Therefore, mosquitoes function best at 27 degrees Celsius, are lethargic at15 degrees Celsius, and cannot function below 10 degrees Celsius. Mosquitoes can therefore remain active year-round in tropical climates. In temperate climates, mosquitoes are forced into hibernation in the fall and remain in hibernation until spring. Mosquitoes hibernate in various life stages. Eggs can be laid before the onset of cold weather and remain frozen in ice until the onset of spring. Larvae can burrow in muddy freshwater swamps and hibernate until spring when they once again begin to feed and eventually emerge as adults. Adult mosquitoes can also hibernate by seeking shelter in hollow logs, animal burrows, or basements where they enter a stage of torpor until spring arrives (Crans 1998).

Mosquitoes can either be nocturnal, diurnal, or crepuscular feeding. Crepuscular feeding refers to feeding primarily at dawn and dusk. Mosquitoes commonly feed on plant nectar and other plant juices. The female mosquito requires blood to provide protein to her eggs during development, so female mosquitoes “bite” while males do not “bite” (Crans 1998). Female mosquitoes are attracted to potential hosts based primarily on carbon dioxide exhalation, heat, and odor (Department of Medical Entomology 2003). Perfumes, deodorants, and hair sprays have been shown to both mask the chemical cues to repel the mosquitoes and to enhance the attractiveness of the host to the mosquito. Dark colors absorb heat and therefore attract more mosquitoes, while light colors repel heat and act to make the host less attractive to mosquitoes. Once the mosquito has located the host, she locates a capillary and injects saliva, which makes penetration easier and prevents the host’s blood from clotting in her food canal (Crans 1998). The transmission of saliva is commonly the pathway for disease transmission (Department of Medical Entomology 2003). The welt that appears shortly after the “bite” is an allergic reaction caused by the anti-clotting saliva deposited by the mosquito (Crans 1998).

After her blood meal, the female mosquito rests while her eggs develop and then flies to an aquatic habitat to deposit her eggs. Females lay a batch of 50-200 eggs every 48-96 hours, variation in egg numbers is due to temperature and variations in species, the higher the temperature, the shorter the developmental time (Crans 1998). After hatching, the larvae feed continuously and enter the pupa stage after four molt substages (Department of Medical Entomology 2003). Larvae development usually lasts two weeks and relies heavily upon food availability, water conditions, and most importantly, temperature. The pupa stage lasts approximately two days. After two days, the adult emerges and immediately begins to feed and mate (Crans 1998).

Dengue Fever

The common vector of Dengue fever is the vector Aedes aegypti (Dengue Fever Management Plan 2000). Aedes aegypti originated in Africa and has been dispersed throughout the world on ships (Kettle 1993). The mosquito is unusual in that it does not breed in swamps or drains and usually does not bite at night. Aedes aegypti is commonly found in wardrobes and under beds and is referred to as the “domesticated” mosquito or the “cockroach” mosquito (Dengue Fever Management Plan 2000). In the 1970s, Australia was thought to have eliminated the Aedes aegypti population, but recently the mosquitoes have staged a comeback (Kettle 1993). In Australia, Aedes aegypti is found only in Queensland (Dengue Fever Management Plan 2000). In areas like Townsville and Charters Tower, the climate is semi-arid, and Aedes aegypti survives year round (Canyon 2001).

Maximum survival rates of Aedes aegypti has been found to be between 20 and 30 degrees Celsius (Tun-Lin et al. 2000). Low populations of this mosquito are common during dry and cool seasons. Populations increase when temperatures increase and the wet season commences (Canyon 2001). In previous experiments, the sex ratio (female: male) was 1:1 at all temperatures, except for at 30 degrees Celsius, where it was found to be 4:3. Under field conditions at Townsville, Charters Towers, and north Queensland, the duration of immature development varied according to the container position (i.e. shaded or exposed) and the availability of food resources, as well as inversely with temperature. Experiments have indicated that containers with an abundance of organic matter or those under trees tended to produce the largest adult Aedes aegypti, which had faster development and better immature offspring survival rates. Larger Aedes aegypti have been linked to a greater risk of Dengue transmission, so it would seem important to focus on limiting such containers (Tun-Lin et al. 2000).

The Aedes aegypti mosquito becomes infected with Dengue when she bites a human that is viraemic with the Dengue virus (Dengue Fever Management Plan 2000). Viraemic refers to the host having enough of the Dengue virus circulating in their bloodstream to infect the mosquito (Dengue Fever Management Plan 2000). A person can transmit the virus to mosquitoes within three days from contracting the virus (Dengue Fever Management Plan 2000). The mosquito can then transmit the virus the other people after 8-10 days (Dengue Fever Management Plan 2000). The complete cycle takes roughly 14 days (Figure 2).

Dengue is a flavivirus in the family Flaviviridae (Endy et al. 2002). Other Flaviviridae diseases include Yellow fever, Japanese encephalitis and Murray Valley encephalitis. In Australia, Dengue fever is the most significant of all mosquito-borne diseases in terms of mortality, morbidity, and economic cost (Dengue Fever Management Plan 2000). Four distinct serotypes of Dengue have been identified. The four serotypes are DEN-1, DEN-2, DEN-3, and DEN-4. All serotypes produce severe Dengue illness. DEN-3 has shown to be produce more severe Dengue cases while DEN-1 has been found to be most common in Aedes aegypti (Endy et al. 2002 and Chung and Pang 2002). A person in a given location could hypothetically have as many as four Dengue infections, one of each serotype (Dengue Fever Management Plan 2000).

Each year 50 to 100 million cases of Dengue are reported worldwide. Several hundred thousand cases of Dengue result in Dengue hemorrhagic fever, which is usually confined to children under the age of 15. Dengue is currently present in approximately 75 countries worldwide, with 2.5 billion people at risk of infection. Dengue is most commonly found in the urban areas of the tropics. The frequency of Dengue is increasing in recent years and this can be attributed to increased transmission through air travel and the increase of potential breeding sites. The increase in potential breeding sites includes an increase in the number of unused car tires and an increase in consumer containers ideal for mosquito breeding (Dengue Fever Management Plan 2000).

Dengue fever has been contained to only northern Queensland of Australia. Queensland has a history of 13 Dengue outbreaks since 1885 and dating back to 1879 (Figure 3). Dengue fever is not naturally occurring in northern Queensland, but because the mosquito vector, Aedes aegypti, is common to northern Queensland, outbreaks occur whenever Dengue is introduced by travelers (Dengue Fever Management Plan 2000).

Two presentations of Dengue are known; classic Dengue fever and dengue hemorrhagic fever. Classic Dengue fever is relatively common in non-immune worldwide travelers and is mild enough to be frequently unrecognized or to be misdiagnosed as Malaria (Shirtcliffe et al. 1998). Symptoms of classic Dengue fever include (Dengue Fever Management Plan 2000 and Shirtcliffe et al. 1998):

· Sudden onset of fever, which lasts three to seven days
· Intense headache, especially behind the eyes
· Muscle and joint pain
· Loss of appetite
· Vomiting and diarrhea
· Skin rash
· Minor bleeding, generally the gums and nose
· Extreme fatigue

The illness commonly runs a biphasic course with a reoccurrence of fever and rashes, but recovery is usually uncomplicated. Diagnosis is usually obtained using a blood test and an antibody (Shirtcliffe et al. 1998). There is no cure or vaccinations available for Dengue fever at this time (Kettle 1993).

Dengue hemorrhagic fever (DHF) is a serious illness that usually occurs in individuals that are partially immune. DHF is most common in children under the age of 15 (Shirtcliffe et al. 1998). The average fatality rate of individuals with DHF is approximately five percent, but with timely treatment this can be reduced to less than one percent (Dengue Fever Management Plan 2000). Symptoms of hemorrhagic fever include hypotension, increased vascular permeability, thrombocytopenia and hemorrhagic manifestations. Thrombocytopenia is rare blood disorder that affects the platelets of the blood. Characteristics include low platelet count and low platelet survival time. Symptoms include a tendency to bleed excessively into mucous membranes, especially during menstruation (Shirtcliffe et al. 1998).

Malaria

Malaria is a member of the Sporozoa group and is caused by four species of Plasmodium: P.falciparum, P.malariae, P.vivax, and P.ovale. Malaria is transmitted by the mosquito vector Anopheles farauti. Ninety percent of all death due to Malaria has been in Africa. Australia has a unique Malaria feature in that Anopheles farauti has a southern dispersal limit along the latitude of Townsville. The last Malaria outbreak in Australia occurred in Cairns in 1942 and the last returning traveler from Australia diagnosed with Malaria was found in 1962. Australia was officially proclaimed free of Malaria by the W.H.O. in 1981 (Kettle 1993). Although eradicated, 1000 cases are imported annually and occasional cases of local transmission still occur (Russell 1998).

Malaria attacks liver cells and then attaches to erythrocytes and disperses throughout the body. Symptoms of Malaria include severe fever, chills, and sweating. The disease enters a remission, which lasts from a couple weeks to many months, and then returns with another set of fever and chills. Diagnosis is made using a dye stain of the host’s erythrocyte cells. Treatments include the use of chloroquine, amodiaquine, and mefloquine. All these drugs are effective at attacking the Malaria infested erythrocytes (Department of Medical Entomology 2003).

Murray Valley Encephalitis (Australian encephalitis)

The Japanese encephalitis (MVE) serocomplex of flaviviruses compromises 10 members, 9 of which: Alfuy, Koutango, Kokobera, Kunjin, Murray Valley encephalitis, JE, Stratford, Usutu, and West Nile have been isolated form Africa, Australia, Asia, southern Europe, and the Middle East. The tenth member, St. Louis encephalitis, is confined to North, Central, and South America (Poidinger et al. 1996). MVE, like most other encephalitis viruses, is maintained in wild animals and then transmitted to other animals and humans via a mosquito vector. The vector of MVE is the freshwater mosquito, Culex annulirostris (Nicholls 1986).

In the 20 year span from 1968 through 1998 MVE cases occurred in Northern Australia in all but four years (Russell 1998). MVE has a high subclinical rate and researchers have estimated that only 1 in 500 patients with MVE become noticeably ill. Symptoms vary from very mild to severe and commonly include fever, headache, and anorexia. Vomiting, diarrhea, dizziness, and nausea are also experienced at times. Brain dysfunction may be experienced after days of lethargy, confusion, convulsions, and neck stiffness. Coma and death can be caused by this condition referred to as encephalitic syndrome. Diagnosis is obtained through a blood-test using MVE antibodies. There is currently no vaccination for MVE and treatment consists of mainly respiratory treatment (Department of Medical Entomology 2003).

Ross River Virus

Ross River virus is transmitted by the mosquitoes Culex annulirostris and Aedes vigilax (Russell 1994). Ross River virus is endemic and annually active in Australia. Rural townships are most affected by the virus, but urban areas have recently reported cases of the virus (Russell 1994). Ross River virus is also known as epidemic polyarthritis and the symptoms include those of severe arthritis along with fevers and rashes (Department of Medical Entomology 2003 and Russell 1994). Ross River virus is not lethal, but the problems associated with epidemic poly-arthritis are of top economic concern in Australia (Russell 1994). No vaccinations are currently available for Ross River virus and treatments are aimed at reducing the patient’s pain rather than eliminating the virus (Department of Medical Entomology 2003). The frequency of the virus has been increasing in recent years and appears likely to continue as a severe economic and health concern for rural Australia in years to come (Russell 1994).

Yellow Fever

Yellow fever is a member of the flavivirus group. Yellow fever is transmitted among human populations via the Aedes aegypti mosquito, which is also the vector for Dengue fever. Since 1788 only five ships have been recorded as arriving in Australia with a history of Yellow fever, with the last arriving in 1925 (Kettle 1993). The disease possesses two phases, the acute phase and the toxic phase. Symptoms of the acute phase include fever, nausea, muscle pain, and loss of appetite. After three to four days, symptoms disappear and patients usually improve. Fifteen percent of patients with Yellow fever enter the toxic phase within 24 hours. The patient in the toxic phase rapidly redevelops a fever and becomes jaundice, hence the name Yellow fever. Symptoms of the toxic phase include bleeding from the nose, mouth, stomach, and the ears. Kidney function rapidly deteriorates in some cases and half the patients in the toxic phase die within 10-14 days. The remainder recovers from Yellow fever without any significant organ damage (WHO 2003). Fortunately, there is a vaccination that provides 100 percent protection for potential travelers (Kettle 1993).

Disease Prevention and Control

Most methods of disease prevention and control in Australia aim at the control of Dengue and the Dengue vector, Aedes aegypti. This is because Aedes aegypti was originally not native to Australia and is also the transmission vector for Dengue fever and Yellow fever (Kettle 1993). Queensland has implemented two Dengue Fever Management Plans, one in 1994 and one in 2000, which remains in effect until 2005. The 2000-2005 plan consists of three core components: disease surveillance, mosquito surveillance and control, and education (Dengue Fever Management Plan 2000).

Surveillance is the first method of defense against Dengue. The shift has gone from Dengue surveillance to a surveillance of imported Dengue cases. This is because an infected traveler could quickly initiate an outbreak of Dengue to northern Queensland (Dengue Fever Management Plan 2000).

Mosquito surveillance and control has a preventative role during non-outbreak times and is especially critical whenever Dengue is reintroduced by a traveler. Dengue has been said to “have the ability to spread through an area like wildfire and that Aedes aegypti is the fuel needed by the fire.” Control involves yard inspections, removal of potential breeding areas, mosquito traps, insecticides, and biological agents (Dengue Fever Management Plan 2000). Mosquito traps include everything from small-scale zappers to large-scale nets and mammal mimicry vacuum devices, which attract and trap mosquitoes by emitting carbon dioxide (Canyon 2001). Current insecticides include S-methoprene, cypermethrin, and imiprothrin (Dengue Fever Management Plan 2000). Studies have shown that in semiarid towns such as Charters Towers, the practice of treating a relatively small number of key subterranean water habitats with insecticides in the winter results in lower Aedes aegypti populations during the summer, the period of greatest risk for Dengue (Kay et al. 2002).

There are also eradication methods aimed at eliminating mosquito eggs and larva. Methods used to eliminate larva include the elimination of breeding containers, insecticides, and biological control agents (Dengue Fever Management Plan 2000 and Canyon 2001). Most larvae insecticides are aimed at preventing larva development and metamorphosis (Canyon 2001). In some cases, biological control agents are preferred over insecticides due to the harmful side effects on the environment caused by insecticides. Some biological agents used include Copepods and Bacillus thuringiensis (Dengue Fever Management Plan 2000 and Canyon 2001). Copepods are tiny crustaceans that feed on mosquito larvae and occur naturally in ponds and lakes (Dengue Fever Management Plan 2000). Bacillus thuringiensis is a spore forming Bacilli that forms a layer on stagnant water. When digested by mosquito larvae, the bacteria release a toxin that disrupts internal membranes and results in larvae death. Bacillus thuringiensis is particularly useful in that it is safe for the environment and has a very low toxicity to other animals (Canyon 2001). Current cost projections of an eradication program would be tens of millions of dollars and still probably unsuccessful if used (Dengue Fever Management Plan 2000).

Australian control workers cannot eliminate mosquito populations in all homes and areas without the help of the public. For this reason, education is needed and is used to demonstrate effective methods to help eliminate the mosquito population. Education programs include media relations, brochures, billboards, and community training sessions. Education and awareness plans are always in place, but they intensify significantly during disease outbreaks (Dengue Fever Management Plan 2000).

Traveler Precautions

Travelers always have the risk of disease and when traveling to tropical and subtropical areas there is a higher risk of contracting an insect-borne disease. The following precautions will lower that risk (WHO 2003 and Canyon 2001):

· Before traveling, consult a physician and receive all available immunizations

· When mosquitoes are common, wear full-coverage clothing and apply repellants
containing DEET.

· Use bed netting containing permethrin

· Upon returning from abroad, consult a physician if any problems exist

These precautions will not totally eliminate the risk of insect-borne disease when traveling, but when used with education and awareness can significantly lower a traveler’s risk of insect-borne disease (WHO 2003).

Conclusions

Although Australia is relatively free of insect-borne disease in comparison to other southern continents, the threat of insect-borne disease still looms. Australia, particularly north Queensland, is constantly battling to eliminate the population of the Dengue vector, Aedes aegypti. Dengue fever is thought of as the biggest insect-borne disease threat, while Yellow fever, Ross River virus, Murray Valley encephalitis, and Malaria all pose small risks. Management plans aimed at education and mosquito control and disease control have decreased mosquito numbers, but are thought to never be able to totally eradicate Australia from these vectors. To reduce the risk of disease travelers should consult their physician before and after traveling and reduce the risk while in tropical or subtropical areas by using protective clothing and repellants.

References

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