Medical Microbiology and Public Health

12 May

Despite  the antiquity  of concepts  that  seem to be associated  with the germ theory of disease, microbiology  was not established as a scientific discipline until the end of the nineteenth  century.  In the process, scien- tists and  medical reformers  often  cast their  arguments  in terms  of an opposition  between contagion theory and miasma theory. Although  the miasma theory of disease was the primary  stimulus to the public health campaigns  of the nineteenth  century,  closer inspection  of the evolution and  usage  of  these  terms  in  earlier  periods  suggests  that  they  were not  necessarily seen as mutually  exclusive. Sharp  distinctions  between contagion  and  miasma  models  might  be considered  rather  misleading and anachronistic when applied to the period between Girolamo Fracastoro’s On Contagion (1546) and triumph  of microbiology  at the end of the nineteenth  century.  That  is, Renaissance  authors  and those who  followed  them  often  switched  back  and  forth  between  the  two terms. When contagion  was defined loosely enough  to include harmful material  that  was indirectly,  as well as directly transmitted, it was not incompatible   with  equally  vague  definitions   of  miasma  as  disease- inducing noxious, contaminated air. Thus, when nineteenth-century bacteriologists expressed  their  interest  in Fracastoro as the  precursor of germ theory,  they were probably  interpreting his views in a manner very different from the way in which Fracastoro and other Renaissance physicians saw them.

During   the  seventeenth   century,   microscopists   established   the

existence of tiny  ‘‘animalcules,’’ infusoria,  the  capillary  network,  and certain  kinds  of cells. Antoni  van  Leeuwenhoek  (1632–1723), one  of the most ingenious microscopists of that period, described molds, proto- zoa, bacteria, sperm cells, and other ‘‘little animals.’’ Nevertheless, most physicians  and  natural philosophers regarded  the  notion  of ‘‘disease- causing  animalcules’’  as  little  better  than  ancient  superstitions about elf-shot, worms, and flying venom. Moreover,  there was little evidence available  to  decide  between  the  hypothesis   that  the  minute  entities

Girolamo Fracastoro.

Girolamo Fracastoro.

observed by microscopists were the product of disease, putrefaction, and fermentation and the alternative  hypothesis  that  they were the cause of these phenomena.

The idea that  disease, impurity,  or corruption can be transmitted

by contact  is an ancient  folk belief. On Contagion (1546) by Girolamo Fracastoro is  generally  regarded   as  the  earliest  exposition   of  germ theory,  but  it was Giovanni  Cosimo  Bonomo  (1663–1696) who  pro- vided the first convincing demonstration that a contagious human disease was caused by a minute  parasite  close to the threshold  of invisibility. Bonomo  proved  that  scabies,  commonly  known  as  ‘‘the itch,’’ was caused  by  a  tortoise-like  mite  (now  known  as  Sarcoptes  scabiei var. hominis) just  barely  visible to  the  naked  eye. When  the  female  mite burrows  into the skin and lays her eggs, the unfortunate host develops

a rash  and  intense itching. The mites can be transferred directly from person  to person  or by means of bedding  or clothing  used by ‘‘itchy’’ persons. Sarcoptes scabiei can also affect cats, dogs, horses, cattle, pigs, and  wild animals,  but  the condition  is generally referred  to as mange. The itch mite, however, was regarded  as an interesting  curiosity rather than  an example that  might apply to other  diseases.

Further evidence for contagion  theory appeared  in studies of silk- worm diseases. Agostino Bassi (1773–1857) found that he could transfer the disease called muscardine  to healthy silkworms by inoculating  them with material taken from worms that had died of the disease. According to Bassi, muscardine  was caused by a minute  living plant  or parasitic fungus. Bassi suggested that  other contagious  diseases might be caused by  similar  parasites.   The  fungus  that   causes  muscardine   was  later named  Botrytis  bassiana  in honor  of Bassi. Johann  Lucas  Scho¨ nlein’s (1793–1864) search for the cause of ringworm was influenced by Bassi’s work  on  muscardine.  In  1839, Scho¨ nlein,  a  professor  of medicine  at Zurich,  reported  finding a fungus in the pustules  of ringworm.  Unlike the prolix  Bassi, Scho¨ nlein set forth  his case for a causal  relationship between parasite  and disease in barely two hundred  words.

When Jacob Henle (1809–1895), Professor  of Anatomy  at Zurich, published On Miasmata  and Contagia in 1840, several examples of microparasites as putative  agents of disease had been added  to scabies and muscardine.  Critically evaluating  the experimental  evidence, Henle discussed the nature  of the proofs that  would be required to establish a causal  relationship  between microbes  and  disease. Although  it is pos- sible to link Fracastoro’s account  of contagion  and miasma to Henle’s hypothesis,  the  context  in which  they  worked  and  the  centuries  that separated  them  infused  very different  meanings  into  their  use of the terms miasma and contagion.

Henle  argued  that  physicians  blamed  disease on  miasma,  which

they defined  as something  that  mixed with  and  poisoned  the air,  but no one had  ever demonstrated the existence of miasma  with scientific instruments. Miasma  was only presumed to exist, by exclusion, because no other  cause could be demonstrated.

According  to Henle’s hypothesis,  contagia  animata  (living organ- isms) caused contagious  diseases because  whatever  the morbid  matter of  disease  might  be,  it  obviously  had  the  power  to  increase  in  the afflicted individual.  Given the fact that  a small inoculum  of pus from smallpox pustules could be used to infect a multitude  of people, the con- tagion must be an animate entity that multiplies within the human body. Chemicals,  toxins, and  venoms remain  fixed in amount.  By definition, only living things have the power of growing and multiplying.

One could most logically explain the natural history  of epidemics

by assuming  that  a living agent  excreted  by sick individuals  was the cause.  If  this  agent  were excreted  by  the  lungs,  it  might  easily  pass

to others  through  the air. If excreted by the gastrointestinal system, it would enter sewers and wells. Acknowledging  the lack of rigorous  evi- dence for the germ theory  of disease, Henle argued  that  science could not wait for unequivocal  proofs,  because scientists could only conduct research in ‘‘the light of a reasonable  theory.’’ Although  Henle’s theory was generally  ignored  by his contemporaries, after  the  establishment of microbiology,  his essay on contagion  was awarded  the status  of a landmark.

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