156 • Alternatives to Animal Use in Research, Testing, and Education 
Table 7-1.— Number of Animals Needed to Detect 
Carcinogenicity in 90 Percent of All Tests 
for a Statistical Significance of 0.05 
Number of animals 
Rate of incidence caused by (3 dose levels 
test substance (percent) plus control group) 
80 48 
60 84 
40 184 
20 1,020 
15 3,304 
SOURCE: Adapted from I.F.H. Purchase, "Carcinogenicity,” Animals and Alter- 
natives in Toxicity Testing, M. Balls, R.J. Riddell, and A.N. Worden (eds.) 
(New York: Academic Press, 1983). 
duced, and the sensitivity of the method thereby 
improved, if animals were not kept under condi- 
tions that aggravate cancer (excessively nutritious 
diet, little exercise, and isolation) (42). 
Developmental and Reproductive 
Toxicity 
The effects of chemicals on human reproduc- 
tion are difficult to assess because of the complex- 
ity of the reproductive process and the many kinds 
of insults that can be inflicted before reproduc- 
tive maturity as well as during fetal development 
(8). Reproductive functions that can be harmed 
by foreign substances include the storage and 
maturation of the germ cells, fertility (including 
factors that affect sperm maturation and implan- 
tation of the fertilized egg), and the development 
of the fetus. Possible toxic effects to the fetus in- 
clude birth defects (teratogenicity), low birth 
weight, abnormal gestation time, and prenatal or 
postnatal death (7). 
There are a variety of experimental protocols 
by which these effects can be determined in ani- 
mals. Some involve more than one generation; 
others involve evaluation of a fetus before birth. 
Exposure to a substance can start before the fe- 
male ovulates or as late as some specific stage of 
fetal development. Exposure can be chronic or 
acute. The great variety of procedures available 
can lead to a certain amount of overlapping test- 
ing (2). 
Rats and rabbits are the most commonly used 
species. Mice and hamsters and other mammals 
are used as well. Three dose levels are normally 
used, the highest of which causes minimal toxicity 
in the adult female. Groups of about 20 pregnant 
females are typically used. In the OECD Testing 
Guidelines, if no teratogenic effects are observed 
at a dose of 1,000 mg/kg body weight, other dose 
levels are not necessary (38). 
Neurotoxici ty 
Neurotoxicity (damage to the nervous system) 
is observed in acute and chronic testing, but the 
range of neurotoxic effects is so great and the signs 
so varied that special tests for damage to the nerv- 
ous system are sometimes warranted. Neurotoxic 
effects that tend to be associated with acute expo- 
sure are functional, sometimes reversible changes 
in the nervous system that might not involve struc- 
tural damage or degeneration. Most chronic neuro- 
toxic effects do involve structural changes or de- 
generation and are not readily reversible (6). The 
type of neurotoxic effect tends to depend on the 
size of the dose and the duration of exposure (46). 
There are many types of nerve cells, each per- 
forming special functions. Damage can occur to 
the functioning of the cell itself, to its connections 
to other nerve cells or to muscle cells, or to the 
supporting cells. Neurotoxicity can be manifested 
in the following ways: motor disorders such as 
weakness, lack of coordination, paralysis, tremor, 
convulsions, or slurred speech; sensory disorders 
such as numbness, pain, or auditory, olfactory, 
or visual deficits; disturbances of autonomic func- 
tion such as sweating, incontinence, vomiting, im- 
potence, or tear formation; increased state of 
excitability such as hyperactivity, irritability, or 
euphoria; impairment of short - or long-term mem- 
ory, disorientation, or confusion; sleep disorders; 
psychiatric disturbances; impaired temperature 
regulation; or alterations in appetite, or weight gain 
or loss (6). 
More than any other kind of toxicity test, neuro- 
toxicity does not lend itself to standard procedures 
or in vitro tests because the range of effects is so 
broad. There are considerable differences among 
species, and little standardization of tests across 
species has occurred. Neurotoxicity tests would 
typically follow acute or chronic toxicity ones in 
which neurotoxic effects had been observed or 
were suspected (6). 
