MONOCELLULAR ANIMALS-THE PROTOZOA 



131 



phases are spent in the body of a plant or 

 an animal, and throuo;h transfer from one 

 to the other the cycle is kept going. Many 

 diseases are caused by this group of Proto- 

 zoa, the most significant of which is malaria. 



The malarial parasites (Plasmodium vi- 

 vax is the most common ) infect large num- 

 bers of warm-blooded vertebrates besides 

 man. In fact, the life cycle was worked out 

 oriejinally on birds by Ronald Ross in 1898. 

 The widespread occurrence of the disease 

 in human populations is indicated by the 

 fact that over 300 million people are in- 

 fected all the time. It has been estimated 

 that 3 million die of malaria each year — 

 over half of all the deaths in the world. 

 This certainly places it in the number one 

 position among deadly diseases. These fig- 

 ures come as a surprise to most Americans 

 because we now have the disease under 

 control, although a hundred years ago it 

 was responsible for a great many deaths in 

 the South. During World War II it once 

 again became a very important health prob- 

 lem for men in the tropics, and the large 

 number of men continuing to suffer from 

 the disease attests to the fact that we were 

 not altogether successful in our preventive 

 program. 



Two factors are necessary for the propa- 

 gation of malaria, a large population of the 

 appropriate species of Anopheles mosquito 

 and infected humans (Fig. 7-25). Only fe- 

 male mosquitoes bite. In order to become 

 infective the female anopheles must bite a 

 person suffering from the disease and with- 

 draw blood that contains the parasite in 

 a particular stage called the gametocyte. 

 There are two kinds of gametocytes, male 

 and female, both of which must be taken 

 into the stomach of the mosquito, where 

 each type of cell undergoes certain modifi- 

 cations. One remains pretty much as it 

 is, producing a single large macrogamete, 

 whereas the other produces 6 or 8 smaller 

 motile, threadlike cells or microgametes. 

 The macro- and microgametes unite in pairs 

 to form zygotes which are able to bore 



through the stomach wall under their own 

 power. In the outer part of the stomach 

 wall each zygote multiplies many times, 

 producing a great many tiny infective sporo- 

 zoites. These swollen zygotes protrude from 

 the outside walls of the mosquito's gut like 

 tiny beads. They puzzled Ross when he first 

 saw them, and one can imagine his surprise 

 when he squeezed them and saw thousands 

 of spindle-shaped bodies emerge. Normally 

 they burst into the blood which fills the 

 space between the gut and the body wall, 

 and via the blood the sporozoites make 

 their way into the salivary glands. With 

 each bite of the mosquito from this time on 

 sporozoites are injected into the blood of 

 the next host. 



After entering man's blood stream the 

 sporozoites seem to disappear for a few 

 days. This fact has puzzled biologists for 

 many years until the recent discovery that 

 they undergo their early multiplication 

 stages in various tissues of the body, notably 

 certain cells of the liver. At any rate, within 

 ten days some of the parasites are in the 

 blood stream, each entering a red blood 

 cell where it grows and multiplies asexu- 

 ally. After a remarkably regular period of 

 time — namely, 48 hours in P. vivax, the 

 most common form of malaria — the in- 

 fected red cells burst, each releasing 10-20 

 tiny oval bodies called merozoites. These 

 immediately enter other cells, and so the 

 infection keeps increasing in intensity. 

 When there is a sufficient number of in- 

 fected cells, the person suffers alternate 

 chills and fever with the bursting of the red 

 cells at 48-hour intervals. The symptoms be- 

 come more intense for the next two weeks 

 when either the person is unable to combat 

 the infection and dies, or he is able to and 

 lives, although intermittent chills and fever 

 continue for a lono; time, sometimes for sev- 

 eral years. During this time some of the 

 merozoites become modified into gameto- 

 cytes. If some of these are taken up by the 

 mosquito with its blood meal, they pass to 

 the stomach and thus complete the cycle. 



