PARASITIC DISEASES OF CATTLE IN PUERTO MOO 6 
LIFE HISTORY OF INTERNAL PARASITES 
Protozoa multiply within the body, so that one organism may give 
rise to a large number of organisms. Adult worm parasites in their 
definite host do not multiply in the body, as a rule, as each egg or 
young parasite which gains entrance into the body develops usually 
'into only one worm and multiplication depends on the eggs or young 
worms produced by such a parasite. As most of the mature worms 
live in the digestive tract or in organs in communication with the 
digestive tract, the eggs or young worms are passed with the feces. 
It is possible, by making 'a microscopic examination of the fresh 
dung of the animal, to determine the presence of parasites and 
approximately the amount of infestation. The eggs or young worms 
as they are passed are not capable of infesting an animal, but must 
pass through a stage of development outside of the body, in water, 
-oil. or manure piles, or wherever the proper conditions are found. 
Some species must pass a period of development in such lower ani- 
mals as insects or snails. Warmth, moisture, and shade are necessary 
for this development on the ground. Shade for the microscopic 
free-living forms may be provided by any covering that conserves 
moisture and prevents the penetration of the direct rays of the sun. 
If conditions are favorable the infective stage is reached in a few 
davs or a few weeks. This infective stage is more resistant to 
unfavorable conditions than are the fresh ova or intermediate stages. 
The infective larvae of several species are capable of crawling up the 
blades of grass when these are wet with rain or dew. The host 
animal becomes infested by grazing or by ingesting contaminated 
water and green forage or by picking up the infective stages that 
have developed on the floors and walls of the pens or stables. Con- 
trol measures are concerned with breaking this cycle of development 
by preventing infestations or by eliminating the worms from the 
host. 
INFLUENCE OF CLIMATE AND TOPOGRAPHY ON PARASITES 
In the temperate zones, the freezing temperatures have a very 
appreciable effect in limiting and destroying pasture and field infes- 
tations with parasites. In the climate of Puerto Rico there are two 
important limiting factors. One is a lack of moisture and another 
is an excess of moisture. The dry season provides a limiting factor 
through the lack of moisture. The southern coast, during 6 months 
of the year, is semiarid, and parasites do not thrive under arid 
conditions. The heat and drying tend to desiccate the worm eggs 
and larvae on the ground so that, practically speaking, pastures and 
fields are sterilized as far as parasites are concerned. This is as 
unfavorable for worms as freezing temperatures elsewhere. In por- 
tions of the other coastal areas, with a few inches of rainfall dis- 
tributed over a period of several months and with the amount of 
evaporation exceeding the precipitation, similar although less notice- 
able results occur over a snorter season. In those areas, such as on 
the western coast, where the rainfall is more evenly distributed 
throughout the year and where the rate of evaporation is lower, this 
limiting factor is less apparent. 
