Reply To: A Hello to Virology
“A Filterable Virus, The Cause of Infectious Laryngotracheitis of Chickens”
The summary and conclusions are given here:
“1. Experiments have shown that tracheal exudate from two strains of laryngotracheitis of chickens from New Jersey and two from California when suspended in bouillon and passed through Berkefeld V filters will produce the disease. Two of six Berkefeld N filters allowed
the etiological agent to pass, whereas four did not. Attempts to produce the disease with Seitz filtrates were unsuccessful. These results demonstrate that laryngotracheitis is caused by a filtrable virus that because of its size or some other property does not pass readily through the finer filters.
2. It has been shown that the sera from fowls that have recovered from an infection with one of the New Jersey viruses will neutralize the same strain and also the one California strain tested. In order to demonstrate neutralization conclusively it was necessary to titrate samples of dried virus and in the tests to use approximately ten infecting doses.
3. The virus dried over calcium chloride for 10 days and then stored in the refrigerator for 60 days produced disease. Kept over calcium chloride for a month it was still active and when dried by Swift’s method it remained alive for 5 months.”
Essentially, they made chickens sick with artificial infection with previously collected virus, took exudate from the tracheae of the sick chickens, and purified that with centrifugation and various filters. They then took those filtrates and attempted to infect other chickens, with some success, depending on the filter. The control bacteria they added to the centrifuged fluid, however, were always filtered. No bacteria could be cultured from the filtrates.
Berkefeld filters, by the way, are simply made from diatomaceous earth, with the “N” variety having smaller pores than the “V” variety. They are still used to remove bacteria and purify water. (*Mycoplasma, which is a very small bacterium, may sometimes pass through these filters.)
Now, studies of this era are historical and usually not as rigorous, comprehensive or definitive as later work. There would be no claimed electron micrographs of a virus until 1941 (the first commercial transmission electron microscope became available in 1938).
Studies such as this, however, demonstrate, at very least, successful attempts to purify viruses (in some capacity) from diseased organisms, show that they cause specific diseases, and that they differ from bacteria and other causes of illness in their properties.