Malaria

The association of an unknown virus with the malarial parasite has been considered as the cause. J. Thomson's work in Rhodesia in 1923-24 gave strong evidence in favour of P. falciparum as the causal organism of all local cases and possibly elsewhere. He showed that P. falciparum can almost always be found in the blood ; and that cases had been living in a district heavily in fected by P. falciparum. He found one case of ter five months and another after 4o years' residence; he noted its occurrence only in people who had had attacks of malaria, though possibly un recognized as such at the time, and had taken their quinine in adequately in amount and over an insufficient period of time. A similar observation was made in the cases which developed black water fever on their way home, or after they had left an endemic area even for some time. The finding of the other species of parasite, both of which tend more to chronicity and are readily found in the blood stream, may, when present, be, in Thomson's view, but an indication of a mixed infection.

Diagnosis.

In the diagnosis of malaria in endemic localities the estimate of inhabitants infected is based on the percentage rate of an enlarged spleen or of a positive blood finding amongst a proportion of the population. For the latter, the examinations of a thick and a thin film of blood are made. In the absence of the parasite an estimate of the relative number of large mononuclear (endothelial) cells to other white cells in the blood may be made in certain cases, but the evidence obtained is not conclusive. An increase above the basic figure of 15% endothelial cells of the total white cells was taken by Stevens and Christopher as indi cative of actual or recent malaria in Europeans living in the tropics. Blood cell counts made at some time prior and sub sequent to the finding of the malarial parasite did not show the presence of a constant relative increase of endothelial cells, nor was this a constant factor in latent malaria, even when the clinical diagnosis was obvious and the most reliable factor, an enlarged spleen, present. With regard to the finding of the parasite in the blood in these cases of "latent malaria in England" it must be remembered that but a small drop is taken and that parasites are not easily found except at the period of a relapse. The clinical signs of latent malaria most commonly found are, in order of fre quency, an enlarged spleen, anaemia, functional disorders of the heart and enlargement of the liver. The tendency to chronicity and febrile relapses in malarial infections is very marked.

Distribution.

Malaria is a world-wide disease with endemic foci in all countries. It is most prevalent and extremely common in the tropics with the native population and high infection rate, constant heat, moisture and water for the breeding of the anoph eline mosquito and requisite temperature for the development of the sexual stage of the parasite therein. As the poles are approached, the foci of endemicity gradually become less. For the development of the parasite in the mosquito an adequate temperature is necessary. On an average the three forms, P. malariae, P. vivax and P. falciparum, take 14, I i and six days, respectively, the time varying with the temperature. In sub, tropical countries, malarial outbreaks follow the rainy seasons, when opportunities arise for the breeding of the mosquitoes. Nor way and Sweden are examples of countries wherein Anopheles are present but malaria absent.

In Great Britain malaria was formerly not uncommon, but became rare until the return of many infected troops. Related to the presence of these carriers there were 235 indigenous cases in England in 1917, the number falling to four in 1924, thus corresponding with the cure of the malarial carriers. There exist in England three species of Anophelinae in which the parasites may develop and be transmitted from man to man, but with treatment of infected cases from abroad there is now practically an absence of the human carrier of the parasites, or at least of parasites with sufficiently numerous sexual forms in the blood, a minimum of 12 per cu.mm. of blood being considered necessary for development in the mosquito. Climatic conditions, particular ly temperature and humidity, influence the endemic prevalence of malaria. A mean daily temperature exceeding 6o° F is necessary for its propagation for the full development of the parasite in the mosquito, P. falciparum requiring a very high temperature; P. malariae completing its cycle at a low one, and P. vivax over a wide range of low to high. This explains the seasonal incidence of the three types of malaria, the relatively high incidence of P. falciparum, the most malignant parasite, and the large number of infected inhabitants and high death-rate in the tropics and subtropics, and its rarity in indigenous cases in temperate climes, the presence of the endemic foci of P. rnalariae in the cooler hill country in the tropics and subtropics and the universal distribu tion of P. vivax.