Because it was known that ticks carried RMSF, one way to eradicate the disease was to keep people from getting bitten by infected ticks. That could mean changing the way that ranchers, shepherds, woodsmen, and others did their jobs.
There was another approach to tick control: eradicating the rodents and other small animals in an area where RMSF outbreaks occurred, often by using poison. The attempt to eradicate small animals in areas infested with RMSF failed; it wasn’t clear which animal or animals gave the infections to ticks, and new animals kept moving in.
To protect the workers most in contact with animals that could harbor ticks, Dr. Robert Cooley, the Montana State Entomologist and head Entomologist at the Canyon Creek Schoolhouse laboratory, recommended dipping livestock in disinfectants to kill the ticks, as was being done in this photo.
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Image: Office of NIH History and Stetten Museum, 1520
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Developing a Vaccine
One of the surest ways to stop the spread of a disease is to develop a vaccine against it. At the Canyon Creek Schoolhouse laboratory, bacteriologists (Dr. Roscoe Spencer) and entomologists (Dr. Ralph Parker) worked together to that end. Despite the limited technology and understanding of bacteriology of the 1920s, once Spencer and Parker began to work together in 1921, a vaccine was developed in less than three years.
As illustrated by this photo of two RMSF researchers, the development of a RMSF vaccine was only possible because of cooperation between state and federal agencies, scientific disciplines, and the research staff.
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Image: Office of NIH History and Stetten Museum, 1114
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Inventing the Vaccine
Dr. Roscoe Spencer came to the Canyon Creek Schoolhouse laboratory from Washington, D.C., in spring 1921, when the ticks were out. After the laboratory’s field workers collected ticks, Spencer would test the ticks for RMSF by taping them to a guinea pig; if the guinea pig came down with RMSF, the ticks were infected. But this process was messy and dangerous and slow, so he decided to grind up the ticks and inject them under the skin of the guinea pigs. Still, none of them got sick. Then he took samples from guinea pigs that were sick with RMSF. He injected the samples into guinea pigs that had been injected with ground-up ticks and into those guinea pigs that had not. The guinea pigs that had previously been injected with the ground-up ticks did not get sick.
To be sure of the results, Spencer took some unfed ticks, attached them to a sick guinea pig to feed, ground up the ticks, and injected them into healthy guinea pigs. They all died. As Lucy Salamanca dramatically wrote:
“They had proved it was the meal of blood that had turned a harmless tick into an agent of death!”
Image: Office of NIH History and Stetten Museum, 1465-3
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Spencer took Lot 2351-B in a pillbox to the Hygienic Laboratory (precursor to the NIH) in Washington, D.C. to test them. These ticks were known to carry RMSF. After they were warmed up to get the pathogen active, they were fed on infected guinea pigs, so that they had been exposed to RMSF two times. They proved to be particularly virulent after they were fed, with more infectious material per weight than guinea pigs could produce.
Image: Office of NIH History and Stetten Museum, 1490-1
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It was from a pan full of engorged, doubly-infected ticks like the one shown here that Spencer decided to try to make a vaccine by grinding the ticks with phenol (also known as carbolic acid, a strong disinfectant). He injected the ground-up ticks into healthy guinea pigs to vaccinate them. Then he infected both the vaccinated guinea pigs and unvaccinated guinea pigs with RMSF; the vaccinated guinea pigs did not get sick, while unvaccinated ones died.
Image: Office of NIH History and Stetten Museum, 1465-1
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After experimenting with different combinations of fed vs. unfed ticks and ticks in different stages of their life cycle to get the highest concentration of RMSF in the vaccine, Spencer and Parker were ready for the next step. In February 1925, they conducted an experiment in 18 monkeys to see if the vaccine was effective and safe. None of the vaccinated monkeys died; all of the unvaccinated monkeys did.
Image: Office of NIH History and Stetten Museum, 1487-1
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3. After the eggs hatch, feed the whole group of larvae on infected rabbit. Do the same after they molt and become nymphs (as shown in the tubes) on a non-infected rabbit.
4. After each feeding, inject a few ticks into a guinea pig’s abdomen to see if the guinea pig gets RMSF. Then you know the ticks are infected too.
5. After the ticks molt to become adults, keep them a month to let the RMSF infection grow in them.
Image: Office of NIH History and Stetten Museum, 1486
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6. Feed the adult ticks on an infected guinea pig. Then eviscerate the ticks and grind them in a mortar for 10-15 minutes with sterile sand and a bit of salt solution. This will separate their internal organs from their exoskeletons to create an emulsion.
7. Dilute the emulsion with salt solution so 1 cubic centimeter of emulsion equals two or more tick viscera.
8. Test the emulsion on two guinea pigs to find the minimal infectious dose. Both guinea pigs have to get RMSF to consider the emulsion for the vaccine.
9. Dilute the emulsion again to equal one tick. Add phenol so that the final product has 0.5% phenol (preservative-disinfectant).
