One of my pet peeves about historical accounts of plague is the simplistic ways they look at plague transmission. It is true that fleas should always be considered a primary route of transmission, but they are far from the only method. It is not correct to think that once pnuemonic plague occurs in one patient that most transmission thereafter will be pneumonic. In reality at least three of the four methods of transmission are present in any large epidemic at the same time.
The four methods of transmission are:
- Flea bite causing bubonic plague (or less commonly septicemic plague, but they all become septicemic before death).
- Inhalation of respiratory droplets causing pneumonic or bubonic plague.
- Blood to blood contact causing bubonic (mostly axillary) or septicemic plague.
- Ingestion causing primarily bubonic plague (mostly cervical).
I think it hard for us to guess how plague transmission dynamics would have been different because we know so little of the little things in the community that matter (like conditions for rodents, sanitation, mortuary practices, etc) and how it would change when the scale of infection reaches a level that modern medicine has never experienced. Routes that seem minor today, or improbable, may have had a more significant role in propelling and maybe more importantly sustaining epidemics over 1000 years ago.
Between 1977 and 1998, there were 297 human cases of the plague in the western United States. Of these “nearly 20%” came from direct contact with animals that either had the plague or were contaminated with the plague. The great majority of the 297 cases did indeed come from flea bites, but 20% (1 out of 5) is a significant number. Of these 297 cases, cats transmitted 23 cases (7.7%).
There is an interesting degree of variability in cat-transmitted plague. Of the 23 cases there were 17 bubonic, 5 pneumonic, and 1 septicemic. Further, all but one of the bubonic cases produced axillary (arm pit) buboes and the remaining a cervical (neck) bubo. Although the cats got the plague during a rodent epizootic (animal epidemic), human cases still didn’t follow flea seasonality. There were cases in every month except January and February, but no increase during top summer flea months. Typical cat prey that would have transmitted plague to cats include prairie dogs, ground squirrels, wood rats, and chipmunks and their fleas. Cats frequently have cervical bubos probably suggesting an oral route of infection. It does pass cat-to-cat through scratches and bites when they fight. One of these people contracted the plague from the scratch of a cat who had got in a fight with another cat that had a cervical bubo. The cat that infected the human apparently never contracted the plague, it passed it on in its claws. One of the human pnuemonic cases was a child who slept with her sick cat.
Plague critters can be an unexpected lot. Camels have been repeatedly associated with modern cases of plague. Modern cases are associated with eating an infected camel. Although this is a relatively minor source of plague today we should keep in mind that the Plague of Justinian began in Egypt during a major famine. In times of famine, people will turn to unusual food sources. In a time of famine, I wonder how many people ate the rats! In a recent case from Jordan, 12 people were infected from eating off of one camel carcass. Not only might infected camels have a role to play in the Mediterranean spread of the plague, it shows that humans can be infected by the ingestion route. Like cases associated with cats, these are unlikely follow flea-seasonality. Many mammals, including rabbits, can be infected by the plague. Cats are not the only predators either. Plague (or plague antibodies) have been found in dogs and coyotes. This makes me wonder if wolves and foxes can be infected. Humans contract plague from such animals by skinning them and getting blood into one of their own wounds/scratches. In medieval times, wolf and fox furs were valued. Actually that reminds me of a famous saying by St. Dunstan about bishops who wear cat fur, so there is another way for furriers to contract plague.
A very important point to remember about plague transmission from these non-tranditionaly associated animals is that even in modern times the vast majority of these cases were initially misdiagnosed. In some cases, patients died because they were not properly diagnosed fast enough and some were diagnosed upon autopsy. My point is that modern medicine still usually initially misses the plague when its found in non-epidemic levels even with modern lab medicine techniques. All types of plague eventually become a septicemia if fatal and septicemias can be difficult to differentially diagnose. Black death gets its name from the blacking of the skin (blood clotting in the skin) that occurs during the septicemic phase of the disease, but the blackening doesn’t always occur.
Gage KL, Dennis DT, Orloski KA, Ettestad P, Brown TL, Reynolds PJ, Pape WJ, Fritz CL, Carter LG and Stein JD. (2000). Cases of Cat-Associated Human Plague in the Western US, 1977-1998. Clinical Infectious Disease, 30: 893-900.
Arbaji, A., Kharabsheh S, Al-Azab S, Al-Kayed M, Amr ZS, Abu Baker M & Chu MC. (2005). A 12-case outbreak of pharyngeal plague following the consumption of camel meat, in north-eastern Jordan. Ann. Tropical Medicine and Parasitology, 99 (8), 789-93.