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Diseases

Ochlerotatus notoscriptus

We know that some mosquitoes carry viruses like dengue or parasites like malaria that cause disease in people. But what about diseases agents transmitted by mosquitoes that threaten New Zealand's native wildlife. Did you know that birds and lizards can get malaria too?

This page outlines some of the threats to New Zealand's native wildlife from diseases carried by mosquitoes.

Disease agents vectored by mosquitoes

West Nile virus (WNV)

West Nile virus was first isolated from a human in the West Nile province of Uganda in 1937. Since then it has been recorded in humans in other parts of Africa, Europe in the 1960s, Asia, and, most recently, North America. Before 1994, only a small number of WNV outbreaks were reported, isolated in both space and time, and WNV was considered to have little effect on human health. WNV also had little effect on wildlife. Since then, outbreaks have been more frequent, larger, and the cause of increasing numbers of human, equine, and bird deaths.

WNV was first detected in North America in 1999, and by 2003 had spread west into California, north into Canada, and south into Mexico and the West Indies. WNV has been much more virulent in North America than in Europe, perhaps because it was newly arrived and there was no or little host immunity. By the end of 2003 WNV had caused the death of 566 humans, hundreds of horses, many other mammals and reptiles, and tens of thousands of birds in the USA.

Which mosquitoes carry WNV?

At least 75 species of mosquitoes in 11 genera have been found infected with WNV, including species active during the day, and species active at dawn and dusk. The predominant (primary) vectors are mosquitoes of the genus Culex (subgenus Culex). Culex quinquefasciatus, which has been established in New Zealand for some time, is a vector of WNV in Asia and southern North America. The endemic New Zealand species Culex pervigilans would most likely be a vector for WNV as well.

Two invasive mosquitoes not present in New Zealand, Aedes albopictus and Ochlerotatus japonicus, are often intercepted at New Zealand ports and airports. Both are highly susceptible to WNV infection and can transmit the virus by bite. If either of these artificial container-breeding mosquitoes became established, they would probably disperse throughout New Zealand to provide a nationwide vector.

A Threat to native fauna?

Birds are the principal hosts for WNV, although mammals, reptiles, and amphibians can also be infected.Competent hosts are those that attain high levels of infectious virus in the bloodstream and retain these levels for sufficiently long (1–4 days) to infect biting mosquitoes. Birds can do this, and therefore act as amplifying hosts, whereas mammals (including humans), reptiles, and amphibians usually cannot, and therefore are termed incidental hosts. Thus, the virus cycles principally between birds and bird-biting mosquitoes.

WNV in North America, unlike in Africa, Europe, and Asia, has been characterised by the mortality of thousands of birds from 225 species in 55 families. The highest mortality rate has been in American crows (Corvus brachyrhynchos), which suffered between 40% and 68% mortality. However, most birds that carry the disease survive WNV infection, as indicated by the high number of live birds with antibodies present.

Potentially New Zealand could face a scenario similar to the USA if WNV became established here. Populations of birds suffering high mortality rates could drive some of our endangered native species to extinction. The virus would not be good for humans and other mammals and reptiles either. For more information download the report Risk Assessment for the Establishment of West Nile Virus in New Zealand in PDF format. 

Avian and Lizard Malaria

Malarial parasites (Plasmodium) have a complex life cycle in which they alternate between vertebrate and insect hosts. Each species of Plasmodium infects only one or a few pairs of hosts. About 130 described species of Plasmodium infect birds, mammals, and reptiles over most of the world's tropical and temperate areas. Human malaria is caused by four species of Plasmodium that are carried and transmitted by some species of Anopheles mosquitoes. Anopheles mosquitoes are not present in New Zealand.

Which mosquitoes carry Avian and Lizard Malaria?

Culex quinquefasciatus, whichwas introduced into New Zealand in the 1800s, is usually the mosquito vector for avian malaria. The vectors of Plasmodium species which cause lizard malaria are not mosquitoes but phlebotomine sand flies.

In Hawaii avian malaria has had a significant negative impact upon native birds. Previously free of mosquitoes, Culex quinquefasciatus became established there in 1826. However, the extinction of many bird species in the 1920's and 1930's and the range reduction of other species occurred with the introduction of avian malaria.

Most species of Plasmodium in the world are parasites of lizards. Despite being regarded as a relatively benign infection, several lizard malaria species produce anaemia, tissue damage and mortality in their hosts. The study of a Californian population of the western fence lizard Sceloporus occidentalis found that 25% of adult lizards are infected with Plasmodium mexicanum at any one time. Infection produces a deficit in blood haemoglobin concentration that reduces oxygen transport capacity and stamina. The insect host and vector of P. mexicanum is a sand fly Lutzomyia vexatrix.

Avian malaria has been recorded in New Zealand from yellow-eyed penguins on Stewart Island and Otago Peninsula, and little blue penguin from Codfish Island, but little else is known about the extent of avian malaria in New Zealand. Lizard malaria (Plasmodium lygosomae) was first described from the New Zealand native moko skink Oligosoma moco in 1948.

New Zealand is unlikely to have the type of scenario that occurred in Hawaii, with subclinical avian malaria probably present in penguins much of the time. One study in the early 1990s found 100% of yellow-eyed penguins on Otago Peninsula infected, but without obvious symptoms. Culex quinquefasciatus is not found south of Christchurch, so either another vector such as the endemic Culex pervigilans, is transmitting avian malaria in these locations, or the penguins are being infected elsewhere.

Similarly lizard malaria is probably asymptomatic most of the time in the lizard lifecycle, and is unlikely to be a threat to endemic reptiles in New Zealand.

Avian Pox

There are different strains of Poxvirus that cause disease in many different groups of animals, including seals and birds. The method of virus transmission is variable with mosquitoes and other biting insects often having a role as mechanical vectors. It can be transmitted by the same mosquitoes that transmit avian malaria, and the two diseases are often found together. In New Zealand avian poxviruses have been diagnosed from several domestic and wild bird species and several different strains of avian pox are present.

Which mosquitoes carry Avian Pox?

Culex quinquefasciatus, whichwas introduced into New Zealand in the 1800s, is often the mosquito vector for avian pox, but many other means of transmission are possible.

A Threat to native fauna?

Avian pox has been introduced to parts of the Galápogos Islands and is infecting domestic chickens, as well as some species of Darwin's finches. The mosquito vector Culex quinquefasciatus is present on the islands, although the link from infected domestic birds has not been proved. In North America it would appear that avian pox is a relatively recent arrival and rarely affects waders and shore birds. Climate warming has been predicted to increase the incidence of avian pox because it will promote more rapid development of the Culex vectors.

Murray Valley encephalitis (MVE) virus and Kunjin (KUN) virus

These two Australian endemic viruses cause Australian encephalitis in humans. Symptoms are variable, from mild to severe with permanent impaired neurological functions, which sometimes are fatal. At least two species of heron and two species of egrets circulate virus after infection with low does of MVE and KUN. Other hosts for MVE include grey kangaroos and rabbits (high viraemic response), pigs, dogs and chickens (moderate response), and calves, lambs and wallabies (low response).

Which mosquitoes carry MVE and KUN?

The natural endemic cycle involves birds as the vertebrate hosts and Culex annulirostris in northern Australia. MVE has also been recorded from Aedes rubrithorax, Ochlerotatus vigilax, Aedes vittiger, and Culex quinquefasciatus.

A Threat to native fauna?

Endemic Australian bird viruses would be expected to show little or no symptoms in native birds, as is found with WNV in Africa. However, these viruses could pose a big threat to birds in New Zealand, as WNV did to North American birds on reaching there in 1999. More information about MVE and KUN viruses is available on the University of Sydney web site.

Ross River virus (RR) and Barmah Forest virus (BF)

In Australia the enzootic cycle of RR virus appears to be between mosquitoes and native mammals, most likely kangaroos and wallabies. During periods of intense virus activity there seems to be little doubt that the cycle can also be between humans and mosquitoes. RR virus is usually a non-fatal viral infection. People infected by Ross River virus may suffer pain and tenderness in muscles and joints, fever, chills, sweating, headache and tiredness. A rash may also occur on the trunk and limbs for a time. The symptoms subside eventually and leave few or no after-effects. BF virus, which is similar to RR, causes a wide variety of symptoms from rashes with fevers, to arthritis that can last from months to years.

Which mosquitoes carry RR and BF?

RR virus has been isolated from many mosquito species in Australia, indicating wide susceptibility among mosquitoes. In inland regions, the major vector is Culex annulirostris which breeds in freshwater habitats, especially in irrigated areas. Along coastal regions, saltmarsh mosquitoes represent the major threat, including Ochlerotatus vigilax and Ochlerotatus camptorhynchus in northern and southern coastal regions respectively. In the domestic urban situation, there is evidence to suggest that Ochlerotatus notoscriptus may be a vector, while Culex quinquefasciatus is not. Although Ochlerotatus camptorhynchus became established in New Zealand in 1998, it is actively being eradicated, and is not thought to be carrying RR virus. All cases of RR virus reported in New Zealand have been acquired overseas.

Little is known about the natural hosts of BF virus, but it is carried by the same mosquitoes that carry RR.

A Threat to native fauna?

RR virus has only rarely been isolated from birds. Birds are probably incidental hosts, and playing a negligible role as reservoirs and in virus transmission. Nothing has been reported of RR virus symptoms in Australian amphibians, reptiles or birds. Concern about RR probably has greater relevance to humans and other mammals, than to native wildlife. However, its effects on New Zealand native amphibians, reptiles and birds should it become established here, are unknown.

Western equine encephalitis (WEEV) and St Louis encephalitis (SLEV)

These viruses are endemic to parts of the Americas and are similar in effect to the closely related WN virus, with Culex vectors transmitting the viruses among passerine avian hosts. Humans and horses can be affected by these diseases too, sometimes fatally. Climate warming is predicted to promote more rapid development of these viruses in the vectors, and to increase the winter survival of the Culex vectors themselves.

The likelihood of these vectors and viruses establishing in New Zealand is low, but if they did, their effect could be similar to that of WN virus reaching North America in 1999. Native New Zealand birds could be highly susceptible to these novel diseases and high mortalities likely.