A.—Vectors of Organisms Causing Disease in Man

The almost cosmopolitan domestic mosquito Aëdes aegypti, although widespread in Australia and throughout the larger islands of the Pacific, does not occur in New Zealand. This species is the classical vector of the causal organisms of yellow fever, which does not occur in the Pacific, also of those of dengue, which is rife in this area (again excepting New Zealand). A. aegypti was reported once only, a single living female being discovered in the passenger compartment of a DC3 which reached Whenuapai from Nausori, Fiji, on 1st April, 1950.

Culex annulirostris, a most annoying evening-biting pest mosquito, is able to harbour the causal organisms of filariasis (Brug, 1938). This species is not found in New Zealand, although it is very abundant in the Pacific Islands to the northwards and in Australia. The only previous record of C. annulirostris from this country is by Graham (1939), who in 1929 discovered larvae in the hold of a ship page 15 which had arrived at Auckland from Suva. Subsequently, more larvae were found in a barrel on the nearby waterfront, but the destruction of this brood was successful in preventing the establishment of the species. C. annulirostris was recorded from aircraft at Whenuapai on five occasions, one live and five dead females and two dead males being collected. The living female was found in the forward luggage compartment of a DC4 which arrived from Sydney on 20th April, 1950. One of the dead females was in the passenger compartment of a DC6 from Nandi, Fiji, while the remaining specimens were collected from the basal rim of astrodomes of DC3s from Norfolk Island and Fiji.

During 1946, two dead females of Aëdes tongae were collected from the basal rim of the astrodome of a DC3 which had visited Tonga in the course of its previous flight. Farner and Bohart (1945) assume that this mosquito is the Tongan vector of Wuchereria bancrofti, the causal organism of filariasis in the South-west Pacific.

Culex fatigans, an important vector of W. bancrofti in many other parts of the world, is not considered so in the South Pacific and Indonesia. This species has long been established in New Zealand, and is particularly common in the vicinity of Auckland city and Whangarei, a fact supporting the hypothesis of ship-borne introductions having at one time been made (Graham, 1939). Three living females of C. fatigans were recovered from the passenger compartments of two DC6 aircraft from Sydney (3rd March and 3rd April, 1950), while three more, also a living male, were collected in aircraft from Nandi (from the passenger compartment of a DC6, 2nd April, 1950, and the astrodome of a DC3, 6th May, 1950). Two dead males were also discovered in the basal run of the astrodome of a further DC3 from Nandi.

There are several records of the human flea (Pulex irritans) from aircraft. Whitfield (1939) and Mendonça and Cerqueira (1947), who examined very large numbers of insects from aircraft, each recorded but a single flea. Pemberton (1944) mentioned finding several living examples of P. irritans, and suggested that insecticidal sprays in aircraft do not normally reach these insects. Both the fleas found in the present instance were taken alive from a seat in the passenger compartment of a DC4 from North America during 1948. P. irritans is able to transmit the bacillus of bubonic plague under laboratory conditions, but most authorities agree that it is of little if any significance as a vector of this bacillus in nature.

Of the three vectors of human disease organisms recorded from aircraft at Whenuapai but not established in New Zealand, two would probably have but little difficulty in gaining a foothold here. Aëdes aegypti and Culex annulirostris are both widespread species able to adjust themselves to a considerable range of climatic conditions, and there is no reason to suppose that they Would not thrive in the more northerly parts of the North Island. However, there are grounds page 16 for doubting the ability of a tropical mosquito of such restricted distribution as Aëdes tongae to establish itself here, although field experiments would have to be conducted before this point could be finally settled.

Filariasis is a primarily tropical disease, and is endemic through the warmer and more humid regions of the world between the latitudes of 30° N. and 32° S. It is not considered that this disease could become established in New Zealand, and for that matter an efficient vector of the causal agent in its periodic form (Culex fatigans) already occurs here. Dengue, though, has a wider distribution. occurring from 36° N to 35° S. latitude in the Americas, and to a trifle north of 35° S. in Australia (Lumley and Taylor, 1943). One of the last-named authors has reported Aëdes aegypti from the southerly parts of New South Wales, at a latitude equivalent to that of the most northerly parts of North Auckland. There is thus reason to believe that both dengue and the classical vector of its causal virus might be able to become established in at least this portion of New Zealand. Of course, the introduction of A. aegypti already infective for dengue is much more unlikely than that of uninfected mosquitoes. Following its establishment here the species might thus perhaps have only pest significance, until such time as infective mosquitoes, or dengue sufferers in the infective stage, slipped past the quarantine net.

By far the greatest danger to the health of man arising out of the development of air transportation in the South-west Pacific is that of the introduction of anopheline vectors of the plasmodia of human malaria into malaria-free lands. Anopheles is absent from New Zealand, Norfolk Island, New Caledonia, and the Pacific Islands east of 170° E. longitude, although both this genus and malaria are endemic in parts of Australia, in the New Hebrides and Solomon Islands, and throughout the Indo- Malayan region. The reasons for the absence of anopheline mosquitoes from such a broad area of the Pacific are incompletely understood. They may possibly lie in simple geographical isolation, or they may be bound up with physiological and/or ecological barriers which operate naturally to keep Anopheles outside this area. Until more is understood regarding this most important and fundamental problem, we must act as if geographical isolation is the only factor involved, and assume that species of this genus, once introduced into the malaria-free area, are capable of establishing themselves within it. Some support is undoubtedly lent this hypothesis by Lambert's (1941) information that Anopheles and malaria, once introduced into isolated Rennell Island (south of the Solomons) in the late 1930a, rapidly became established there.

Judging from chemical and climatic factors and macroscopic ecological features, there arc no apparent differences between ground pools in the New Hebrides and Solomon Islands utilized by Anopheles farauti Lav., a most efficient vector of the malarial organisms, and pools of similar type in such malaria-free localities as Fiji. page 17 Other invertebrates present often belong to the same genera and even to the same species—Culex annulirostris, for example, often shares larval habitats with A. farauti in the malarious Pacific Islands, and the former mosquito occurs abundantly in Fiji in ground pools which, superficially at least, appear suited to the requirements of this anopheline. Similarly, mosquito larval habitats in the northern parts of New Zealand, although possibly not suited to the tropical A farauti, outwardly appear suited to the requirements of more temperate anophelines, the Australian A. annulipes Walk., for example. The last-named species ranges through the whole of both coastal and inland Australia, according to Lee and Woodhill (1944), and Lee (1948) has recently confirmed the occurrence of this species in Tasmania at a latitude equivalent to that of the northerly portion of the South Island of New Zealand. Although A. annulipes does not normally transmit the malarial organisms in Australia, Roberts (1943) has shown that under experimental conditions this species is just as hospitable to the parasites of human malaria as is A. farauti. Primary cases of malaria in Australia presumably transmitted by A. annulipes, have been reported from as far south as 37° S. latitude (Tebbutt. 1943), that is to say, about the latitude of Auckland. It is important to notice in this connection that, although the world distribution of malaria is more restricted than that of anopheline mosquitoes, and various ecological and physiological considerations combine to prevent these insects acting as vectors in particular localities, a species of Anopheles capable of transmitting malarial parasites but not normally doing so in one area may be an important vector in another. Thus Chandler (1949) mentions that in the U.S.A. Anopheles pseudopunctipennis Theo. is an apparently harmless mosquito, although this same species (possibly a different race) is the chief vector of malarial parasites in mountainous regions from Mexico to Argentina.

One thing is certain: if species of Anopheles capable of transmitting malarial parasites under the conditions prevailing in New Zealand and other parts of the malaria-free area of the Pacific ever become established here, a reservoir of infection will await them in the form of ex-servicemen and others who have contracted malaria abroad and still carry gametocytes in their blood. In Fiji in particular, where there is a large non-indigenous Indian population (many of whom are gametocyte carriers) as well as large numbers of Fijians who contracted malaria while serving in the Solomons during World War II, it is to be expected that the establishment of anophelines would be very quickly followed by primary malaria epidemics. There is an excellent historical precedent for such an eventuality. Hoops (1935), declaring that malaria was not known in Mauritius until the second half of the nineteenth century, stated that anophelines became established in that island following their introduction by sea from Madagascar. These mosquitoes, presumably becoming infective after biting Indian or African coolies who were gametocyte carriers, initiated malaria epidemics which were responsible for a total mortality of 32,00 during the first years 1867–68 alone. page 18 Today, of course, such epidemics could be quickly brought under control by means of modern insecticidal and therapeutic techniques, but not before considerable havoc had been wrought among the non-immune indigenous population. The introduction of Anopheles and malaria into the malaria-free areas of the Pacific, particularly into the less developed islands, would still be nothing short of a medical and economic disaster.

Although no anophelines have yet been collected from aircraft arriving in New Zealand from abroad, abundant instances of the carriage of these mosquitoes by aircraft are on record from other parts of the world. Galvāo (1948), summarizing the results of earlier investigators, listed 534 anophelines representative of 19 species as having been found in aircraft over a sixteen-year period. Furthermore, Anopheles gambiae Giles, one of the most deadly of the anophelines, has been shown by Sicé et al. (1939) to be capable of the normal life processes following an air journey to a new locality. Circumstantial though this evidence may be, it indicates that a definite hazard does exist. Even if the chances of viable anophelines surviving air journeys are as low as Miller et al. (1947) suggest (between 10 and 20 of these mosquitoes per 30,00 aircraft), an appreciation of the havoc which might follow such introductions and of the consequent vast expenditure of man-hours and money which the necessary control measures would entail (cf. the A. gambiae eradication campaign in Brazil—Soper and Wilson, 1943) affords justification enough for the enforcement of preventive measures. During World War II, the gravity of this problem in the South-west Pacific was fully appreciated. In the majority of cases at least, military aircraft and ships travelling from malarious to non-malarious areas were subjected to thorough insecticidal treatment. By the end of the war, there had not been a single instance of the establishment of anophelines on any island known to be non-malarious in this area (Perry, 1947).