Mosquito Borne Disease and the War in the Pacific

M. Laird, Zoology Department, V.U.C.

Mosquitoes of various genera act as vectors for the organisms which cause malaria, filariasis, dengue fever, and yellow fever. In military campaigns, particularly those in tropical areas, these diseases may offer at least as great a hazard to the combatants as do the operations of the enemy. The truth of this statement had been sufficiently demonstrated long before the Second World War. In the Spanish-American war the U.S.A. suffered more casualties from malaria and yellow fever than from actual combat. The Macedonian campaign in the First World War was disrupted by the effects of malaria, which caused the hospitalization of very large numbers of troops on both the Entente and Allied sides. Coming to the early stages of the recent Pacific campaign, the fall of Bataan in the Philippines was hastened by the facts that about 35,000 men of the U.S. garrison were suffering from malaria and that suppressive drugs for the treatment of this disease were in short supply.

Occurring over the greater part of the land masses of the world, malaria is the most important of the endemic diseases affecting mankind. Records clearly referring to this disease have come down to us from very ancient times, although it was not until towards the end of last century that the true nature and means of transmission of malaria became known. The Grecian legend of the slaying of the Hydra by Hercules is thought to be an allegorical reference to the reclamation of fever-ridden marsh lands. Almost 2,500 years ago Hippocrates gave a clear account of the distinguishing features of the different types of page 11 malaria. The downfall of Ancient Greece is closely associated with the increasing prevalence of malaria in that country, and it is thought likely that other civilizations including that of the Mayas owed their decay at least in part to this disease. When Baghdad was at the height of its power in the eighth to thirteenth centuries A.D., the Arabic physicians were only too well acquainted with the effects of malaria. Throughout the centuries epidemics of the disease have caused the depopulation of once thriving areas and changed the course of history. Even today, 100,000,000 people in India alone are infected with the parasites of malaria; and in that country more than 1,000,000 die annually as a consequence of the disease or its direct economic effects.

Malaria, caused by protozoon parasites of the genus Plasmodium which attack the red blood corpuscles, is spread by the bite of infected Anopheles mosquitoes. These insects develop as aquatic larvae, usually in stagnant pools of various kinds, and the adults are most active at twilight. The disease is of high incidence over many of the South-west Pacific island-groups which became familiar to New Zealand servicemen during the war—the New Hebrides, Solomon Islands, and the Bismarck Archipelago. It does not occur in New Caledonia or the Polynesian islands east of 170 deg. E. The effects of malaria become apparent about ten days after the bite of an an infected Anopheles, the symptoms consisting of alternate chills and feverish spells with either one or two fever-free days between attacks depending on the species of Plasmodium concerned. In the absence of treatment with quinine or certain synthetic drugs these attacks may continue over a long period. Relapses, less severe than the original attacks, may continue throughout life, depending on the resistance of the individual concerned. Tropical or malignant tertian malaria, caused by Plasmodium falciparum, may have a fatal termination. The cumulative effects of long-continued attacks of the less severe forms of malaria may also play an important part in bringing about death.

In the Pacific, human filariasis is caused by nematode worms of the genus Wuchereria. The chief vectors are the cosmopolitan Culex fatigans, a night-biting mosquito, and Aedes scutellaris, a day-biting species. Both insects breed in water held by such natural containers as coconut husks, or such artificial ones as water tanks. Filariasis is particularly prevalent in Fiji and Samoa. The effects of the disease, unlike those of malaria, do not become manifest until after many separate infections over a long period of time. This entails long residence in close proximity to a heavily-infected native population, together with a high incidence of the mosquito vector. The disease never attained prominence among troops during the Pacific war, and few cases were reported among New Zealand personnel. The symptoms include gross swellings in the vicinity of the lymph nodes, which become blocked by the calcified remains of great numbers of the developmental stage of the page 12 parasite. These swellings lead to the conditions described under the name of “elephantiasis”. The progress of filariasis may be arrested, if noticed at an early stage, by sending the patient back to a temperate area.

Aedes aegypti and Aedes albopictus transmit the virus which causes dengue fever. The breeding habits of these day-biting species are similar to those of the vectors of filariasis. Dengue, and various forms of dengue-like-fevers, occur throughout the tropics. The effects become apparent in from seven to nine days after the bite of an infected insect. There are two main crises of fever, and very severe pains in the joints and muscles—hence the name of “breakbone fever” sometimes applied to this disease. Recovery takes place in about a week, there being none of the recurrent ill-effects associated with malaria. The disease very rarely has a fatal termination. However dengue fever is of great importance from the military standpoint, as it is liable to occur in “pandemics”—explosive epidemics which may temporarily incapacitate an entire unit of men.

Yellow fever, a virus disease spread by Aedes aegypti, does not occur in the area under discussion and need not concern us here.

Under peacetime conditions permanent military establishments in malarious areas are maintained in a good state of sanitation. Adequate drainage within camp bounds reduces the number of ground pools suited to the breeding requirements of Anopheles. Control measures are taken against both larval and adult mosquitoes, and personnel generally sleep under mosquito nets or in mosquito-proofed barracks. Thus the chances of servicemen contracting malaria under these conditions are relatively slight.

Combat conditions, however, immeasurably increase the hazard of insect-borne disease. Troops may live for many weeks in Anopheles-infested areas before there is any opportunity to drain permanent pools or carry out any other anti-mosquito measures. Furthermore, innumerable fresh anopheline breeding places are formed as water accumulates in shell-holes, bomb-craters, and the wheel-ruts made by supply and combat vehicles. As personnel are living for the most part in hurriedly improvised shelters, it is very difficult, if not impossible, for them to avoid being bitten by mosquitoes. If the supply of anti-malarial drugs is cut off, malaria soon becomes widespread.

Allied malaria casualties were severe during the Japanese southward drive, largely because of a shortage of suppressive drugs. The Japanese gained by this drive virtually the whole of the world's quinine-producing areas. Thus the Allies were forced to concentrate on the production of synthetic antimalarial compounds, and America began to manufacture large quantities of atebrin for the use of her forces. This drug acts with great energy on the asexual forms of the malaria parasite in the bloodstream, and prevents the development of symptoms of malaria as long as it continues to be taken.

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During the heavy fighting in the Solomons at the beginning of the Allied counter-attack, although anti-malarial drugs were available to the U.S. forces engaged, battle conditions rendered the enforcement of malaria discipline very difficult. The initial malaria casualty rate was thus very high. Before it became possible to undertake control measures against Aedes aegypti, severe epidemics of dengue fever also occurred.

As the northward drive continued, however, and the various island bases were consolidated, an intensive anti-mosquito campaign was developed. Special units were set up to carry out this campaign, and surveys of mosquito breeding places were undertaken in each military area. Squads of hygiene and sanitation personnel and native labourers commenced to drain swampy areas and large permanent pools. Any other pools in the area which for some reason or another could not be drained, but which were actual or potential anopheline breeding places, were sprayed once a week with such larvicides as D.D.T. in diesel oil. In the early stages of the operations in the islands, transport drivers had shown a tendency to avoid rutted sections of roads through coconut plantations by simply following new tracks through the lanes of trees. This caused the needless formation of fresh wheel-ruts and consequently of great numbers of potential mosquito breeding places, complicating subsequent control work. Traffic was now restricted to as few roads as possible, in order to minimize this danger.

In addition to these anti-anopheline measures, malaria surveys of local native populations were made. It was found that the disease was all but universal among the natives, who acted as reservoirs of malarial infection for newly-emerged Anopheles. Thus wherever possible, military camps were built beyond anopheline flight-range of villages, otherwise the natives themselves were given a supervised daily dosage of atebrin.

Control measures were also directed against the vectors of the causal agents of filariasis and dengue fever which, as has already been mentioned, are natural- and artificial-container breeders. The dumping of metal cans, disused rubber tyres, portions of wrecked trucks and aircraft, and other refuse capable of holding water, had brought about a sharp increase in the populations of these mosquitoes. Such disused material was now gathered together into dumps and earthed over. It was also made compulsory for small cans to be flattened and for holes to be pierced in larger metal objects before dumping, in order to prevent their holding water.

At the same time malaria discipline was enforced among troops on the island bases. A daily issue of atebrin was made, and the use of bednets made compulsory. Personnel were required to wear long trousers and long-sleeved shirts from dusk onwards in order to guard against anopheline bites. An added precaution against such bites came with the development and issue of a satisfactory mosquito-repellant based on page 14 dimethyl-phthalate. On application to the skin this substance gives protection from mosquito bites for periods of up to two hours. Besides being of use against mosquito vectors of disease, it gives welcome protection from the attentions of day-biting pest mosquitoes in the jungle.

All these anti-mosquito measures combined to reduce the incidence of insect-borne diseases among Allied troops to a very low level. During the early operations on Guadalcanal Island in the Solomons over a thoustand attacks of malaria had been recorded for each thousand invasion troops; after the institution of full-scale malaria control here the primary malaria rate fell so sharply that in the last two years of the war only a few isolated cases of the disease were recorded from service personnel on the island. The majority of these cases were traceable to infections outside the military area. An even more striking illustration of the efficiency of the control programme is seen in the case of Espiritu Santo Island in the New Hebrides. Here, in what was previously one of the most heavily malarious islands of the Pacific, no primary cases of the disease were recorded from troops over a two-year period.

It has been said that the only ways of eliminating human malaria from the world would be to wipe out either all the anopheline mosquitoes or all the human beings—of these alternatives the second would be decidedly the easier to put into effect! In heavily malarious localities still in more or less virgin condition, the problems of anti-malarial work are so complex that it is quite impossible to do more than keep relatively small areas temporarily free from the disease. Once mosquito control operations in such areas are discontinued, the Anopheles population is soon restored to its former level by invasion from outside the old control limits. Provided that a reservoir of infection is still available to these insects, malaria once more becomes a problem.

However, wartime entomological control experience served to show that, as long as the requisite equipment and organization are available, it is possible to safeguard people entering the tropical islands of the South-west Pacific from mosquito-borne disease.

Suggested reading: Svensson, R. 1940. A Handbook of Malaria Control. Published by the Shell group of oil companies.