CONTROL OF ANIMAL PARASITES 257 



from 40 north latitude to 40 south, rendering many of the most 

 fertile valleys uninhabitable. Manson declares that malarial parasites 

 cause more death, and more predisposition to death from other causes, 

 than all other human parasites taken together. Howard estimated 

 (in 1909) that they caused 3,000,000 cases of malaria and nearly 

 12,000 deaths annually in the United States, and actually imposed a 

 yearly tax upon this country of not less than $100,000,000. 



The malarial parasite is carried to man by the bite of an anopheline 

 mosquito. The minute vermicides, or sporozoites, enter the red corpuscles 

 and grow until the substance of the corpuscles is absorbed, when they 

 divide asexually into, generally, from 16 to 24 spores, merozoites. These 

 burst out of the corpuscles, and this, probably on account of their 

 poisonous waste products set free in the blood, causes the "chill." 

 While thus unprotected in the blood plasma the parasites of our com- 

 mon, temperate-zone malarias (P. uivax and P. malar ice) may be killed 

 by heavy doses of quinine. The parasites of the malignant malarias of 

 the tropics are not affected by this drug. Our common malaria is caused 

 by P. vivax, which passes through its life cycle in the blood every forty- 

 eight hours the usual time between chills. This is also known as 

 tertian malaria. Quartan malaria, the other temperate-zone type of 

 the disease, caused by P. malarice, which requires seventy-two hours 

 to complete its life cycle, is characterized by chills every third day. 

 Some authorities distinguish two types of malignant tropical malaria, 

 the quotidian, in which the parasite completes its asexual life cycle 

 in twenty-four hours, and the tropical tertian, in which the cycle is 

 forty-eight hours. All these parasites multiply sexually within the 

 anopheline mosquitoes. 



Reasoning from the above data, we see that there are three ways by 

 which malaria may be banished from a locality : 1. Exterminate the 

 mosquitoes (see Chapter XI). 2. Prevent the mosquitoes from biting 

 healthy people. 3. Prevent mosquitoes from becoming infected by 

 biting malarial patients. As soon as every responsible member of 

 any community becomes able to grasp these simple facts, that com- 

 munity may free itself completely from the most vicious blood parasites 

 that afflict mankind. 



The Piroplasmas ; Texas fever, or bovine malaria. While the cattle 

 tick acts as carrier, the parasite of Texas fever is Piroplasma (Latin 

 pirus, " a pear ") bigeminum, which attacks the red blood cells of cattle. 

 Tick extermination is banishing this costly parasite from our South- 

 ern states (see Chapter XV). Horses, sheep, dogs, and other animals 



