Related to Horton Hospital Malaria Therapy Unit
Description:

‘Benign Tertian Malaria.- For the purpose of obtaining supplies of mosquitoes infected with Plasmodium vivax, eighty-four batches of Anopheles maculipennis were fed on suitable cases and incubated until sporozoites were present in the salivary glands. The number represents approximately one batch per month during the period of the work... [478-479]

In addition, for various reasons, we have infected a considerable number of patients with P. vivax by direct blood inoculation instead of by mosquito bites.

Quartan Malaria.- In 1928 we obtained from Dr. Kirschbaum, of the Friedrichsberg Hospital, Hamburg, a strain of quartan malaria which we have used for the treatment of forty-five patients, all of them being infected by direct blood inoculation.

Malignant Tertian Malaria.- We have used a tropical strain of P. falciparum for the treatment of twenty patients, all being infected by direct blood inoculation, an Italian strain for the treatment of three patients, of whom two were infected by the bites of infected maculipennis fed upon crescent cases in hospital in Rome, the third by direct blood inoculation, and a Sardinian strain for the treatment of three patients, the infection of one of them being by the bites of maculipennis caught in nature in a house at Posada in that country, and brought by hand to England, and in the others by inoculation of blood from that case.’ (479)

 

‘As long ago as 1901 it was found that, even under what seemed to be the most favourable conditions, certain individual mosquitoes in a successful batch either failed altogether to become infected or became infected very lightly. In 1908, Darling again called attention to the point, and in 1921, Wenyon laid stress on it in recording the results of his feeding experiments in Macedonia. In our first laboratory report we were inclined to attribute the differences to technical circumstances. Yorke and Macfie, in their paper read at a meeting [486-488] of this Society in 1924, attributed the differences to a state of the patient's blood rather than to varying receptivity of different individual mosquitoes of the batch. From the results of many experiments in which we were careful to separate out for subsequent dissection, a number of mosquitoes which had all sucked (as nearly as could be ascertained) the same amount of blood, we are now of opinion that the variations are too great to be attributed to any other reason than that individuals of the same species of anopheles vary greatly in their receptivity to infection. The variations are as marked between individual mosquitoes which have been fed several times on a suitable case as they are in single feeding experiments... Possibly the reason for the wide differences may be that the lining of the stomach wall of some individuals in a batch of mosquitoes is not so permeable to vermicules as is the lining of others. It is suggestive in this connection that in heavily infected insects the zygotes are distributed almost equally over the whole stomach, while in lightly infected insects they are present towards the posterior end only, the infected and the uninfected parts of the wall being often sharply separated by a ring of zygotes. The distribution on some infected stomachs suggests that the lining (cuticula as Grassi called it) is very thin over only a small area of the internal layer of epithelial cells, or is actually ruptured or absent at that spot; for the distribution indicates that the vermicules had found and streamed through that single place of entry to distribute themselves in fan-like fashion or in a cluster among the epithelial cells. (See the plate facing this page). This peculiar distribution suggests a histological rather than a biochemical explanation. If the explanation were to lie in a histological character of this kind, it would give support to the observations of Huff and others, who claim that, by breeding out from particular individual female Culex pipiens which have failed to become infected when fed on the avian parasite, they have succeeded in obtaining a race of which all the members are refractory... But perhaps there is more in the latter aspect of the subject than meets the eye, for we were inclined to think that our own maculipennis of England had become developed into a race refractory to P. falciparum, until, by taking a batch to Italy, we found that 50 per cent. of individuals readily became infected when a "good infector" was available.’ (486-488)

 

'In view of the importance of "domiciliary infection" in the spread of indigenous malaria in England, we have made some experiments to ascertain the period required for maculipennis to become infective (sporozoites in the glands) when, after a single feed on a suitable case of benign tertian malaria, they are kept in the bedroom of a house during the months of July to September. For one of these experiments the readings of the dry and wet-bulb thermometer twice daily are recorded on the accompanying graph. For purposes of comparison similar batches of maculipennis fed on the same patient at the same [488-489] time were kept at a known constant temperature in an incubator. The periods required at bedroom temperature in different experiments were fifteen days in two experiments, and twenty-three days in a third. The time in which the control batches became infective in the incubator at a temperature which did not vary between 76° and 80°F. dry bulb and 74° and 78°F. wet bulb was nine days.’ (488-489)

 

'Theoretically the species of anopheles which suck the largest volume of blood at a meal should be those most often found to be infected and to carry the greatest numbers of zygotes. But in the field this is not the usual finding, and even in experimental work it seldom or never happens that in single feeding experiments the number of zygotes found can be correlated at all with the amount of blood ingested, or that the increased infections resulting from second [489-490] and third feeds bear any obvious relation to the amounts of blood taken in at those feeds. The subject is of interest in connection with endeavours to forecast the probability and amount of infection in mosquitoes fed upon carriers whose blood contains a known number of ripe gametocytes (or exflagellating male forms) per cubic millimetre. The late Dr. Darling worked systematically at this problem... He assumed that half the gametes would be females capable of developing into zygotes in the mosquito's tissues, and therefore that in the above example he ought to be able to find 544 zygotes in mosquitoes fed once, 1,088 in those fed twice, and 1,632 in those fed thrice. In actual experiments, however, the numbers of zygotes found never approached those figures, and from the fact that only fifty zygotes were found in a mosquito when 1,632 were predicted by the formula, he concluded that 97 per cent. of all the female gametes taken into the stomach of a malaria-carrying mosquito fail to develop into zygotes. If this were the general rule, it would indicate not only that anopheles are hardly more successful malaria-carriers than culicines, but that any differences of carrying-power between different species of anopheles would be so small as to be inappreciable.

... We have worked out the predicted number of zygotes in relation to the actual numbers found by dissection in a number of experiments, but... the numbers of zygotes found indifferent individuals of a batch of mosquitoes fed at the same time on a case vary so greatly that a figure representing the usual or average loss would be of little or no service for comparing the relative carrying-power of different species. This is one of the reasons why great caution is necessary in concluding, from the results of comparative feeding experiments, that a particular species of anopheles is a "better malaria carrier " than another.’ (489-490)

 

'Batch 2 was infective from 30th January to 9th February, sporozoites being found in the salivary glands of all mosquitoes dissected during that period. With the mosquitoes of the batch there were nine successes and three failures, two of the failures being with mosquitoes which, by dissection immediately after they had bitten the patients, were proved to [492-493] have sporozoites in their salivary glands at the time of biting. This finding is usually considered to be unequivocal evidence that sporozoites were injected by the mosquito when it bit the patient, and therefore as proving that failure, when it occurs, is due to the patient's insusceptibility to malaria. This was the decision made at first in the two cases mentioned, but we were obliged to reverse it a month later when both patients were successfully infected by mosquitoes of the next batch.

Batch 33 was also one in which, after 13th February, when the batch became infective, all the mosquitoes dissected were found to have sporozoites in their salivary glands. All cases bitten on the 14th and 15th February were successful. On the 16th the batch was taken to Torquay and from there on the 17th to Exeter and Exminster; thence, on the 20th, back to London. The results during this period and later were as follows:-

...[table of results (shows travel of mosquitoes from Torquay-Exter-Exminster-Horton-Harpenden-London)]...

In this group cases 158 to 162 are those about which it has to be decided whether sporozoites were injected or not. None of those patients could be considered "insusceptible" because all were successfully infected a short time after the first failure. It must be concluded that the mosquitoes were responsible for the failures, but the dissection results indicated that all of them had sporozoites in the salivary glands at the time of biting.’ (492-493)

 

‘we are obliged to admit that in our experience the test of finding sporozoites in the salivary glands of a mosquito after biting, is not good evidence that sporozoites were injected by that mosquito when it bit the patient. This observation, unfortunately, adds to the difficulties of experimental work on malaria. Taking it into account, it is obvious that great caution is necessary in coming to a decision that failure is due to the patient being "insusceptible " or "immune." It is preferable, in our opinion, to meet the difficulty of explaining failures such as are recorded in the above [493-494] examples by assuming that a requirement for successful infection is that sporozoites, in addition to being present in the cells of the salivary gland, must be lying free in the common salivary duct of the mosquito at the time of biting. On this assumption it is easy to account for a sequence of failures following asequence of successes, and vice versa, as well as for single failures which appear rather frequently in a scattered manner among the successes in our records.

We are fortified in making this assumption by recalling to mind the complex anatomical structures and interrelated mechanism by which a biting mosquito injects a drop of saliva into the wound before commencing to suck blood, and the difficulty, during die microscopic examination of a dissected salivary gland, of expressing sporozoites from the substance of gland cells into the common salivary duct and salivary cup, and of driving them thence with the saliva into the salivary channel along the hypopharynx. It is easy to imagine that, in the natural act of biting, the suction force of the pump-like mechanism of the salivary cup may not always be sufficient to draw sporozoites, as well as saliva, into the cup.

...

In an earlier paper I mentioned some "failures " among anaemic patients whose state of general health was very poor. This is perhaps a point worthy of study in tropical regions where ankylostomiasis is prevalent and severe. In those regions it may not always be correct to assume that all native children with few or no parasites have acquired an "immunity."’ (493-494)

 

'Patients who have passed through a long attack of benign tertian malaria ending in freedom from fever without being treated by quinine (so-called "spontaneous recovery ") still continue to have parasites in their blood. These are patients whom I have already mentioned as being those in whom the infection has been allowed "to smoulder." Parasites are sufficiently numerous to be found even in thin films for three or four weeks, but later their continued presence can only be ascertained when the patient is subject to some change of habit, such as being allowed to be up from bed and to walk about. At such times parasites can be found again, and often there is a recrudescence of fever. The blood picture and parasite findings in these patients are not different from what is found in many native children in hyperendemic areas.

... That the parasites are still present for many months is proved by the occurrence (usually between the eighth and tenth month after the date of primary infection) of a definite recurrence of fever and reappearance of parasites in the peripheral blood...

... after there has been a recurrence and "spontaneous recovery," endeavours to bring on a fresh attack by reinfection with the same parasite fail.’ (495)

 

‘The question is often asked whether these results assist to explain the immunity problem as it presents itself in untreated native communities in the tropics and, if so, in what manner. The question is not easy to answer, because, so far as I am aware, no one has yet studied immunity among native races except as a mass problem. It is known that among those races the parasite rate percent., the spleen rate per cent., and the average number of parasites per cubic millimetre of blood decrease with advancing age periods, but from these data we can only guess very roughly at what is actually happening to individual members of the community. The next step is not for those who are studying induced malaria in Europe, but for those who are studying hyperendemic malaria in the tropics. Instead of parasite and spleen rates per cent. of populations of different ages, what seems to be needed is an intensive study of individual infants and children in a localised hyperendemic area from the point of view of the kind of results just recorded. Such an enquiry should start with the newly born, and each individual should be under continuous observation until the usual age of naturally acquired immunity is reached.’ (497)

 

‘The graph on page 508 has been prepared to indicate the significance of some of our observations in explaining the peculiar seasonal incidence of benign tertian malaria in Northern Europe and some other countries. It is based on what actually happened in the cases of 107 patients who were under our observation at Horton continuously for more than six months, most of them much longer. For the purposes of the graph, the events have all been brought to the [508-509] same starting point by assuming that the date of primary infection in all cases was the 1st September.

It will be seen that the seasonal clinical incidence is about the same as that of natural benign tertian malaria in Northern Europe, and that the "Spring rise," about which so much has been written, is due to recurrences in persons who had their primary attack in September, together with primary attacks in persons whose infections in September remained latent through the winter... in natural circumstances the occurrence of these cases in the spring is not due to any special climatic or other environmental condition peculiar to that season, but results from the fact that April and May are between thirty and forty weeks after September, which was the month in which infection occurred. If the month of infection had been April instead of September, there would have been a "winter rise" in November and December.’ (508-509)

 

‘Under the heading prophylaxis I mentioned that the results of giving quinine in cases infected by direct blood inocuIation are quite the reverse of those obtained in cases infected by the bites of mosquitoes. For this reason, useful conclusions on treatment cannot be drawn from results obtained in blood inoculated cases. The explanation of the difference may be that by blood inoculation only the forms of the parasite which live in red blood corpuscles are introduced, whilst by the natural method of infection a form of the parasite is introduced which has always lived, not in red blood corpuscles, but in tissue cells. In the mosquito it has lived in the celIs of the stomach wall and of the salivary glands, and it has not been dependent on haemoglobin for nutriment. Evidently quinine has not the same action on a tissue parasite of that kind, as it has on the ordinary red cell forms. Clinically a striking difference between the two classes of case is that true (long interval) relapses, and recurrences are [515-516] not observed (so far as we can ascertain) in inoculated cases, while they occur in 50 per cent. of mosquito-infected cases...

The fact that blood-inoculated cases of malaria are cured permanently by a single short course of quinine has had the unfortunate result of making many believe that induced malaria, even when caused by the bites of mosquitoes, is not comparable with the naturally acquired disease as it occurs in the tropics.’ (515-516)