Efficient Mosquito Hosts.—It must be remembered that only certain genera and species of Anophelinae are known malaria transmitters; thus Stephens and Christophers, in dissecting 496 mosquitoes of the species M. rossi, did not find a single gland infected with sporozoites.
With M. culicifacies, however, 12 in 259 showed infection. A mosquito which is capable of carrying out the complete sporogonous cycle is an efficient host and in the case of malaria the mosquito is the definitive host (sexual life of parasite).
Malarial Index.—Mosquito dissection is one method of determining the endemicity of malaria or the malarial index. There are two other methods: 1. by noting the prevalence of enlarged spleens, and 2. by determining the number of inhabitants showing malarial parasites in the blood. This index is best determined from children between two and ten years of age, as children under two show for a general average too high a proportion of parasites in the peripheral blood while those over ten years of age show too great an incidence of enlarged spleens.
Barber working in the Philippines with children from five to ten years of age obtained a spleen index of 13.3 and a parasitic index of 11.
As Before Stated there are Three Species of Malarial Parasites: 1. Plasmodium vivax, that of benign tertian—cycle, forty-eight hours; 2. Plasmodium malariae, that of quartan—cycle, seventy-two hours; and 3. Plasmodium falciparum, that of aestivo-autumnal or malignant tertian—cycle of forty-eight hours.
Multiple Infections.—Variations in cycles may be produced by infected mosquitoes biting on successive nights, so that one crop will mature and sporulate twenty-four hours before the second. This would give a quotidian type of fever. In aestivo-autumnal infections anticipation and retardation in the sporulation cause a very protracted paroxysm, lasting eighteen to thirty-six hours; this tends to give a continued or remittent fever instead of the characteristic intermittent type.
Plasmodium Vivax.—In fresh, unstained preparations, taken at the time of the paroxysm or shortly afterward, the benign tertian schizont, or nonsexual parasite, is seen as a grayish white, round or oval body, whose outlines cannot be distinctly differentiated from the infected red cell. They are about one-fifth of the diameter of the red cell and are best picked up by noting their amoeboid activity. In about eighteen hours fine pigment particles appear and make them more distinct. After twenty-four hours the lively motion of the pigment and the projection of pseudopod-like processes, in a pale and swollen red cell, make their recognition very easy. When about thirty to thirty-six hours old the amoeboid movement ceases. Approaching the merocyte stage the pigment tends to clump into one or two pigment masses and one can recognize small, oval, highly refractile bodies within the sporulating parasite.
The gametes or sexual forms do not show amoeboid movement, but the fully developed gamete, which is generally larger than the red cells, has abundant pigment, which is actively motile in the male gamete and nonmotile in the female. The male gamete is more refractile, is rarely larger than a red cell and shows yellow-brown, short rod-like particles of pigment. About fifteen minutes after the making of a fresh preparation these male gametes throw out four to eight long, slender, lashing processes, which are about 15 to 20 microns long. These spermatozoon-like bodies now break off from the useless parent cell and with a serpent-like motion glide away in search of a female gamete, knocking the red cells about in their passage through the blood plasma.
The female gamete is larger than a red cell, is rather granular and has more abundant dark-brown pigment than the male.
Stained Smears.—In dried smears, stained by some Romanowsky method, as that of Wright, Leishman or Giemsa, we note small oval blue rings, about one-fifth of the diameter of the infected yellowish-pink erythrocyte. One side of the ring is distinctly broader than the rather fine opposite end, which seems to hold a round, yellowish-brown dot, the chromatin dot, and has a resemblance to a signet ring. These small tertian rings of the nonsexual parasites (schizont) are seen about the time of the commencement of the sweating stage of the paroxysm. Two chromatin dots in the line of the ring are rare as is also true of more than one ring in a red cell.