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16 CHAPTER 2

Pemphigus Rhinitis Trophozoite
Pica Schistosomiasis Undulating membrane
Pinworms Strongyloidiasis Vacuole
Pneumocystosis Subcutaneous Vincent’s angina
Pneumonia Toxocariasis Volutin
Precystic form Toxoplasmosis Whipworms
Pseudopod Trichinosis

EPIDEMIOLOGY one area, whereas nearby, another entirely different spe-
cies of parasite will be the dominant finding. Therefore,
Humans are hosts to nearly 300 species of parasitic by passing the organisms within families and to those
worms and over 70 species of protozoa, some derived
who live nearby, the closest condition to an epidemic
from our primate ancestors and some acquired from the is created. These individuals may also share certain as-
animals we have domesticated or come in contact with
pects of life, such as a common water source (Figure 2-1)
during our relatively short history on Earth. Acquiring or fields and gardens fertilized by human waste, which
knowledge of parasitic infections extends into antiquity,
could spread the organisms(s) between groups closely
as evidenced from some of the earlier writings. Descrip- related in their daily routines. There are even differences
tions and drawings of parasites and of the effects of para-
in the dispersal area of certain parasitic species for those
sitic infections are common topics of the earliest writings. in urban versus rural areas, with the rural areas normally
This written evidence has largely been confirmed by the
encompassing a smaller area. This is possibly due to less
finding of parasites in archaeological material obtained daily contact in the rural areas with a sparser population
from explorations of ancient civilized areas.
density than that found in urban environments.
The systematic study of parasites began to gain
impetus with the rejection of the theory of spontane- Epidemiological Surveillance
ous generation and the promulgation of the germ theory.
Thereafter, the history of human parasitology proceeded Most epidemiological surveys are performed by govern-
along two lines. First, the discovery of a parasite occurred mental agencies and most often are conducted through
along with its subsequent association with disease. Sec- global organizations of countries, such as the United
ond, the recognition of a disease led to the subsequent Nations (UN), the World Health Organization (WHO),
discovery that it was caused by a parasite. This review is and others (Figure 2-2). As shown previously by the lim-
concerned with the major helminth (worm) and proto- ited nature of parasitic epidemiology, studies of human hel-
zoan infections of humans. Many exotic-sounding names minth (worm) infections show that there is seldom if ever a
will be used in the following chapters dealing with classes wholesale distribution of parasites, but that they are aggre-
of parasites. Some of these are as follows (see Table 2-1), gated among given pockets of population. Even in endemic
and will be discussed individually in most cases. areas, the vast majority of parasitic populations are usually
concentrated in only a small representation of the host pop-
PATTERNS OF PARASITIC ulation. Contracting of the parasites found only in certain
OUTBREAKS areas of the globe is due to a number of factors, including
those of an environmental nature. The presence of risk fac-
Parasite infections tend to be concentrated within small tors among the indigenous population and the presence of
geographic areas, within families, and within a short dis- reservoirs of infection as well as vectors for transmitting the
tance (in miles) where large numbers of those suffering organisms, if applicable, lead to pockets of infections.
from parasitic infections reside. Sometimes the overall Determination of these small-scale infestations is easily
parasitic infections will encompass large numbers of indi- accomplished today due to the availability of global position-
viduals; however, certain species will be predominant in ing satellites and handheld devices, where specific locales