Sir Richard Doll studied the incidence of cardiac and lung disorders in workers
exposed to asbestos. He compared incidences of heart failure, pulmonary tuberculosis,
lung cancer, and other respiratory diseases between workers who were exposed to
asbestos and those who were not. Doll found that the risk of developing lung cancer
was 10 times greater in the population exposed to asbestos as compared with those
not exposed and that the incidence of chest malignancies decreased with a decrease in
duration of exposure.
Smith and Spalding (1959), working as temporary advisers to the World Health
Organization (WHO), investigated an outbreak of paralysis in Morocco. Local health
authorities had been efficient in collecting incidence data. The advisers further investigated
the outbreak, drawing conclusions from clinical and laboratory evidence, thegeographical distribution of cases, socioeconomic factors, and case follow-ups. The
clinical signs and symptoms of those affected were muscle weakness and loss of superficial
sensations in the hands and legs. The affected were diagnosed with acute peripheral
neuritis. Even though some patients suffered from fever and diarrhea, no
trace of infection was detected. All routine blood tests and cerebrospinal fluid tests
were negative. The role of infection as a cause of the epidemic was ruled out, because
the culture of secretions from those affected yielded no growth of any microorganisms.
The advisers suspected that poisoning caused the neuritis, which primarily affected
those in living in poverty. One affected family suspected the olive oil they used
for cooking was responsible for the paralysis. They fed the dog food cooked with the
dark oil and then they also ate it. After several days the family and dog developed
paralytic signs and other symptoms. On further investigation, the dark olive oil sold
in the lower socioeconomic communities was found to be contaminated with orthocresyl
phosphate, a synthetic oil used to lubricate jet engines. Similar outbreaks were
reported in Switzerland, Germany, and the United States. The prognosis of the disease
depended on the extent to which individuals were affected. Those who had spinal
damage were permanently disabled; those whose distal muscles only were affected
recovered within a year. This study, conducted by Smith and Spalding, shows that
epidemiological investigations can take long periods of time and demand teamwork, a
background in the clinical sciences, and knowledge of similar past incidents