Scrub Typhus

We Know Its Cause But Not Its Cure—Yet

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THIS article, reprinted from Health Horizon /’ is by Kenneth Mellanby, QBE, MA, ScD, Reader in Medical Entomology at the University of T nnrinr) Vnh’nnl ni Trnryirnl MorlirinP tie served with the typhus Research Laboratory, SEAC. He was recently appointed Principal of the University College which is to be established in Nigeria This article on scrub typhus will be of particular interest to residents in Papua, New Guinea, and the Solomons, where the cause and high mortality rate of the disease were well known in the 30’s. In a region where fevers of various kinds and intensities are almost the norm, and men land women) live great isolation, often far from medical aid, it is probable that there were even more cases of “Japanese River Fever” was it rZL 9 d neraUV Call6d th6n) tHan Residents 'ot these areas are still Residents of these areas are stm contracting the disease. Secondary growth which has sprung up around plantations, abandoned during the war, appears to be a happy huntingaround for the mite or bush moka grouna jor me mue, or ousn moiza.

BEFORE 1940, not one person in a thousand in Britain had ever heard of scmb typhus, or disease” as it should more properly be called. Even in medical circles, most

people were rather hazy about it, and few would consider that it could ever present an important problem of practical hygiene.

Scrub typhus was first described by Japanese medical authorities as occur ring in Japan and Formosa. Later it was found in Malaya, Indo-China, the Netherlands’ East Indies, New Guinea and the adjacent islands, and in the north of Queensland, in Australia. Although it was known to be widespread, and although the Japanese had stated that a case mortality as high at 50 per cent, of those developing the disease might be expected, the total number of actual cases had been small, particularly among Europeans.

Everyone had anticipated that malaria would play a major part in any tropical campaign in the war, and these expecta tions were fully justified. Both in New Guinea and Burma during the first year after Pearl Harbour, malaria claimed more victims than the Japanese, or than all the other diseases put together.

But after that a remarkable thing happened. As a result of researches carried out in many countries it was found possible to control malaria, and in New Guinea, for instance, where the in cidence had been 4,000 admissions to hospital per 1,000 troops a year (i.e., on an average every man had four attacks each year), the rate of admission to hos pital for malaria dropped to 50 cases per 1.000 (i.e., to one-eightieth of the previous level).

Malaria had ceased to be a major medical problem, and scrub typhus, which had previously been overshadowed, now revealed its importance. Not only had malaria decreased but (particularly in New Guinea) improved treatment almost com pletely eliminated deaths from that disease; whereas the number of deaths from scrub typhus remained high.

During the period when almost every one had malaria, and when our forces

were retreating and living under terrible conditions, many cases of scrub typhus must have passed unrecognised. In the same way, among the natives of these countries where malaria is endemic, any fever is usually ascribed to that cause, and scrub typhus is overlooked.

To the average soldier, there was some thing mysterious and rather sinister about scrub typhus. He knew about malaria, how it was caused and how pre vented. He realised that by applying the measures he had been taught he could do a great deal to keep himself free from infection, and he knew that even if he be came infected, his chances of a complete recovery were good.

But about scrub typhus he had heard very little, and his officers were equally ignorant. There seemed to be little that a man could do himself. If anyone did develop the disease it was known that he was in for an extremely serious illness which might well prove fatal.

ALL this was bad for military morale.

The military authorities decided that something drastic must be done.

Much basic knowledge about scrub typhus was available, and active research had been in progress at the outbreak of war, particularly in Malaya. But there were many gaps in our knowledge. It was not possible for our scientists to put for ward practical recommendations in detail which they could guarantee would control the disease.

For this reason, American. Australian and British research teams were sent to work in the main centres of incidence, and further fundamental investigation was carried out in the laboratories in Britain It may be as well to explain just what this disease—scrub typhus—is, from the victim’s point of view.

Infection is due to the bite of an in fected mite, a tiny creature closely re

lated to the harvest mite of Britain. The bite usually passes unnoticed, particularly in a country where one is attacked by mosquitoes and other noxious pests at all hours of the day and night.

After an incubation periocj of about ten days, symptoms appear. Often the site where the bite took place becomes a characteristic little ulcer. The patient has a high temperature, a severe headache and often a generalised rash.

In the second week various complica tions of a nervous or pulmonary nature are common, and the patient is critically ill. Deaths may be as high as 50 per cent., though the overall figure for our forces was in the region of*lo per cent.

So far no satisfactory curative drug has been found, though several substances have given promising results. The best treatment is good nursing, and for that reason nursing sisters were sent to the furthest forward medical units in Burma, so that those with scrub typhus did not have to be subjected l to the very arduous journeys to base hospitals.

THE GERM which causes scrub typhus is a “rickettsia,” a form of life barely visible under the microscope and in some ways intermediate between the filter-passing viruses and ordinary bacteria. Other different kinds of rickettsiae cause louse-borne typhus, tick- and flea-typhus. Rickettsiae cannot be cultured in the laboratory except as parasites on living material, which means that making vaccine is difficult. These difficulties have been overcome in the case of several rickettsial diseases, and it is even possible that a scrub typhus vaccine which has been produced may prove effective; It has been mentioned that what we call the vector, that is to say, the living creature which conveys the infection to

man, is a mite. These creatures live in the soil, and most of their life history is quite independent of man. The adult mite is only a couple of millimetres long and the immature stages are even smaller.

The eggs are laid in the soil, and from them hatch the six-legged larvae, and these, so far as we are concerned, form the important phase. These larvae attack warm-blooded hosts, mammals or birds.

The types which transmit scrub typhus seem normally to prey on rats, and man is generally attacked more by chance than by choice. The larvae becomes firmly at tached to its victim, the rat, and it feeds on the tissue fluids, not on the blood.

Often large orange patches due to hun dreds of tiny attached mites may be found in the ears of rats where the disease is common.

The whole process of feeding takes about, three days, after which the en gorged larva detaches itself and burrows into the earth to transform into a further stage called the nymph, from which the adult insect is eventually produced.

It is important to note that the mite attacks a warm-blooded host only once in the whole life cycle; this means that the rickettsiae of scrub typhus must be pre sent in the eggs of the mite even before they are laid; a case of hereditary trans mission!

Rats are the usual hosts of these mites, and these animals seem to give little skin reaction to the bites, unlike man who usually develops severe irritation when he is attacked. The rats mav also serve as a reservoir of the disease, and most wild species of rats seem well able to tolerate the rickettsia so that they can infect further mites without themselves suffer ing the symptoms of the disease. But as hereditary transmission in the mite occurs, the disease can continue without any need to pass through the blood of the rat. Certainly, the disease can exist

without the presence of man, who seems to be an accidental victim rather than an essential link in the chain of trans mission.

Our troops in the East often got the impression that scrub typhus was a Jap anese “secret weapon” which had been de liberately left behind by the retreating enemy.

This was not the case, though the Jap anese army did something indirectly to increase the danger of the disease. The commonness of these mites largely de pends on the number of rats. Now, the hygiene of the Japanese army left much to be desired, and rats became very abundant around their dirty camps. As the rat population increased so the mites became more abundant, and where in fection with scrub typhus existed, the risk of its spread increased, too.

THE facts about the disease and its transmission had been suspected, but lacked full confirmation, and the first task of the research teams was to establish or refute these conclusions.

The next task was to prevent infection.

In other diseases, such as malaria, pro tection is obtained by a whole series of measures of various importance. And even when an infected mosquito has suc cessfully bitten, drugs prevent the parasite from having its effect.

We had to try to find similar weak links in the chain of scrub typhus infection.

A study of the life of the mites taught us a great deal about their habits, and something about the type of terrain most likely to be infested, but we learned little which would enable us to get rid of the creatures over a wide area.

It is possible to clear considerable tracts of country of mosquitoes by spraying the insecticide DDT from aeroplanes, but in areas where mites also existed, this treat ment had no marked effect upon them;

they live in the soil most of the time and are thus difficult to reach.

Anyhow, mites are much less sus ceptible to DDT than mosquitoes.

Over restricted areas such as camps and hospital sites drastic treatment such as “bulldozing” the ground was effective, but such measures have a very limited appli cation. It is always worthwhile trying to keep down rats near a camp, but to clear large areas of rats is an impossible task in the tropics.

CERTAIN repellant substances have been discovered to keep off mos quitoes: they add to comfort in tropical and temperate countries, and are of some importance in preventing malaria.

Similar substances played a most import ant part in the campaign against scrub typhus but their action was rather to poison the mite than to make the human victim unattractive.

The attacking mite usually walks over the boot, through the sock on to the skin.

If a man is lying on the ground the in sect finds its way in by some other route, but it nearly always walks over the cloth ing. Various chemicals can be applied in very small quantities so that the wearer cannot distinguish treated from untreated clothes. A mite which walks over the substance is killed. One great advantage of these preparations is that they are not readily washed out from the clothes, for in the tropics frequent laundering is a necessity.

Research work taught us that these poisons kill the mites and help to pre vent scrub typhus, but it was not so easy to put the idea into practice. Both in New Guinea and Burma there were no facilities for impregnating large quantities 9f garments with the new insecticide and it was felt that we must relv on the in dividual efforts of the men themselves. A special drill was worked out, and the troops were carefully instructed.

IT is difficult to produce vaccines against all rickettsial diseases, and the parasite of scrub typhus seems to be about the trickiest of all. After many attempts in various countries it looked as if success had been obtained: these organisms grow well in the lungs of the American cotton rat, and it was found that in this way a vaccine could be prepared. on these lines was given highest priority Rats were flown across the Atlantic, special laboratories were built, and the vaccine produced. It was. however, not given a full trial during the war, as bulk supplies only reached SE Asia after the Japanese had surrendered. Further trials will have to be completed before a definite assessment of the vaccine can be made. * n Postwar world, scrub typhus will still be an important medical pro blem over wide areas of tropical and sub tropical countries. Certain kinds of agriculture which encourage the breeding of rats may make the situation more dangerous. If these lands are to be de veloped, as they must be to realise their potentialities and to raise the standard of living, more and more people will risk this trying, infective disease with the unpronounceable Japanese name.