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).

B.—Mechanical Carriers of Organisms Causing Disease in Man

Five important mechanical carriers of organisms causing disease in man are listed in Table 6. Two of these are cockroaches, the cosmopolitan German Cockroach (Blattella germanica), which is already an abundant pest in New Zealand, and the American Cockroach (Periplaneta americana). The latter insect, although very common throughout most of the Pacific and sometimes found in New Zealand wharf sheds, does not seem to have gained a firm foothold in this country. Nine examples of the former species, two living and seven dead, were collected from the baggage compartments and kitchens of five aircraft from Australia, Norfolk Island, and Fiji, while five examples of P. americana, all living, were found in the baggage compartment of two DC4s from Fiji, where this insect is abundant. The medical significance of cockroaches is primarily concerned with food contamination.

page 19

It has been claimed that the causal organisms of tuberculosis, leprosy, amoebic and bacillary dysentery, and cholera may pass unharmed through the digestive tracts of cockroaches, while these insects may also act as the intermediate hosts of certain parasitic worms.

Stomoxys calcitrans, the Stable Fly, which now has an almost world-wide distribution and is common in New Zealand and other parts of the Pacific, can mechanically transmit the causal organisms of anthrax by its bite. The larvae of this species may also cause accidental myiasis in man (Smart, 1943). A living female of S. calcitrans was taken from the cockpit of a DC4 from Sydney on 20th October, 1950, while a dead and dried-up example was found in the astrodome of a DC3 from Fiji.

The cosmopolitan housefly, Musca domestica, is abundant throughout New Zealand. Living and dead specimens definitely referable to this species were collected on 32 occasions in aircraft from Australia, Norfolk Island, Samoa, and Fiji. The number of individuals totalled 134, and no doubt many of the fragmentary muscid flies collected also belonged to this species. M. domestica and some of its close relatives are among the most important of all food contaminators. Among the pathogenic organisms causing disease in man which they have been shown to be able to carry on their mouthparts, their body hairs, and the pads of their feet, are those of typhoid fever, cholera, bacillary and amoebic dysentery, tuberculosis, yaws, conjunctivitis, trachoma, and septicaemia. These flies may also carry the eggs of such parasitic worms as whipworm and hookworm. Chandler (1949) states that houseflies have been found to be able to harbour the causal agent of poliomyelitis, and that the epidemiology of this disease is suggestive of their implication, although as yet there is no evidence that they play a part in its transmission. The chief health interest attaching to the lesser housefly, Fannia canicularis (dead examples of which were three times collected on board aircraft from Australia), is that larvae of this species have from time to time been reported from cases of intestinal and urino-genital myiasis in man (Smart, 1943). M. domestica has also been named in this connection.

A blowfly, Lucilia sericata, which is discussed in more detail in the following section of this account, has been shown by Chang (1943) to be a selective feeder on the cysts of dysentery amoebae in China. Passing unharmed through the alimentary tract of the fly, these cysts may be deposited on food intended for human consumption, for L. sericata has the habit of defecating while feeding. The cosmopolitan European Bluebottle, Calliphora crythrocephala, which some authorities consider to be at least of contributory importance as a food contaminator, was once collected alive from the astrodome of a DC4 from North America.

At first sight, fresh importations of the cosmopolitan species referred to above would seem to be without much significance. When, however, it is considered page 20 with what rapidity importations may be made by means of aircraft from areas where such diseases as amoebic and bacillary dysentery are rife, it will be realized that introductions of already infected insects of these species might well lead to the establishment of epidemic foci in New Zealand.

C.—Biting and Stinging Insects

Species of mosquitoes and wasps not found in New Zealand, and of potential pest significance, were collected during this project. A living female of Mansonia crassipes, a swamp-breeding mosquito common in parts of Fiji, was found in the astrodome of a DC6 from Nandi on 26th November, 1950. During 1946 a dead female of Aëdes albolineatus, a jungle-breeding species occurring in the Indo-Malayan region and several of the Pacific islands, was collected from under a seat in the passenger compartment of a DC3 from Singapore; and a dead female of Culex sitiens, a widely distributed mosquito which breeds in brackish water, was taken from the astrodome of a DC3 on its arrival from Fiji and Norfolk Island on 15th March, 1950. All three of these purely pest mosquitoes would be most unwelcome additions to the New Zealand fauna. Supposing these insects to be capable of establishing themselves here, there are already precedents enough to serve as warnings of the importance which they might assume. There is but scant chance that the tropical Aëdes albolineatus could find breeding places suited to its requirements in this country, but brackish swamps outwardly suitable as larval habitats for C. sitiens, a vicious biter known to have a long flight range in Fiji (Amos, 1947), exist within a mile of the tarmac at Whenuapai. Similarly, swampy farmlands which might provide larval habitats for M. crassipes, are adjacent to Whenuapai Airport, within easy reach of this species (which Amos states to have a flight range of at least one mile) from the tarmac.

Polistes olivaceus, a wasp common in Fiji (Williams, 1947), was taken alive on board aircraft from Nandi and Nausori on two occasions, while the closely related P. macaensis was collected (dead) in the passenger compartment of a DC4 from North America. Bonnet (1948), declaring that the sting of the latter species produces more or less severe local reactions, lists P. macaensis among the medically important insects of Hawaii. A third vespid wasp, Ropalidia socialistica, was recorded (two dead specimens) from the astrodome of a DC3 from Malaya, via Brisbane. This species is stated by Tillyard (1926) to be common in Queensland. Salius bicolor is an Australian spider-hunting wasp, a single dead example of which was also found in the astrodome of a DC3 from Brisbane. The vespid records are of more than ordinary interest in view of the recent establishment of the German Wasp, Vespa germanica L., in the North Island of New Zealand. First reported from the Waikato district in 1945, this species is now abundant in many of the northern districts of the North Island. As is so often the case when page 21 insects become established in a new habitat from which the natural controlling factors of their native environment are absent, V. germanica has become relatively more troublesome here than it is in its European home. Despite recent reports that this wasp is showing some beneficial qualities in that it has been seen destroying flies and moths, its fondness for sweetstuffs has made daytime jam-making rather a hazardous business for housewives living in the invaded districts, and it has now taken to robbing beehives and destroying hive nuclei in the autumn (Paterson, 1949). V. germanica has spread to Whenuapai, where its nests are continually being located and destroyed within airport bounds by the RNZAF insect-control unit, and it has recently been reported as entering aircraft there. This introduces the danger that, unless constant vigilance is exercised, V. germanica may be transported by means of aircraft both to the South Island of New Zealand and to other parts of the Pacific area. The natural preoccupation with the risks of insect introductions attendant upon international air transportation has tended to overshadow the parallel risks from internal air transportation. Although the latter risks may all too easily be regarded merely as an acceleration of the inevitable, due regard should nevertheless be paid to them.