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Insects Collected from Aircraft Arriving in New Zealand from Abroad

B.—Mechanical Carriers of Organisms Causing Disease in Man

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.

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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.