Cats—both big and small—are an enormous part of our lives and have significantly contributed to history and society as we know it. I am sure at least everyone has encountered a feline at some point in their life, and some have even had the opportunity to cuddle or work with one. So, it is no surprise that cats are now a part of our everyday lives, and often provide much-needed support and therapy. However, we need to keep in mind that the animal companions that we hold near and dear could also pose a health risk.
Now, let me back up to give us an understanding of what I am referring to. In recent years researchers have begun to look more into the correlation between zoonotic infectious agents and human disease. A zoonotic agent is transmitted from an animal species to a human host; one of the most notorious of these is the bacterium Yersinia pestis, which causes the plague (1)—infamous in history as the Black Death, which has led to multiple severe epidemics, including in 14th century Europe. There is a fascinating story here, as brilliantly portrayed by the historian Simon Schama in his latest book (2). Traditionally, the was thought to be spread by infected fleas living on rodents, such as the black rat (Rattus rattus). However, the reservoir of Y. pestis is more likely part of the wider ecosystem, being maintained in rodents such as ground squirrels, prairie dogs, and wood rats—but also able to infect rodents that thrive in modern urban environments such as Norway rats (Rattus norvegicus) and house mice (Mus musculus).
Cats have been, and still are, a notable source of human exposure to Y. pestis, but, coincidentally, they have also been a major preventive for the disease. Ships and many populations routinely kept a cat or two around to ward off the offending rodent populations to help limit the spread of the disease. In today’s world, the parasite that spreads Y. pestis not only resides in the fur of rodents but also in the fur of feral and outdoor/ indoor domestic cats.
The plague can cause disease in the human body in three ways. The first and most common is bubonic, which is an infection of the body’s lymph nodes, causing very painful swelling (or “buboes”). After the host receives a bite from an infected flea, Y. pestis enters the body and travel through the lymphatic system towards the nearest lymph node. Another, deadlier, version of the plague that can be contracted is septicemic plague, which occurs when the swelling bursts and the plague bacteria begin to multiply and create a blood infection. The rarest is pneumonic (infection of the lungs). Pneumonic plague can occur when the Y. pestis makes its home in the lungs; this is the form that cats seem to harbor and can be spread through aerosol droplets when our pets breathe near or on us.
In contrast to how domestic cats are sometimes unfortunately portrayed (3), the vast majority of felines present little to no risk of contracting the plague, and modern antibiotics can effectively treat the disease once contracted—but vigilance and good medical management are critical, especially in parts of the world where the rodent reservoir is better maintained. The island of Madagascar, off the coast of Africa, presents a recent such example (4), but with Western states of the US also at risk; for instance, in 2023, health officials in Colorado announced a fatal plague infection in a resident of Archuleta County, located in the southwest corner of the state.
Research is currently being conducted to create an mRNA-LNP vaccine in mouse models to protect against the plague—inspired by the development of the COVID-19 vaccine. The research performed in a recent article titled “A single dose F1 based mRNA-LNP vaccine provides protection against the lethal plague bacterium” outlines how the F1 based mRNA-LNP vaccine could provide immunity after just one dose (5). Researchers were able to construct a GC-rich mRNA construct, and a low-GC construct, which could encode the same amino acid sequence. This is important because we know that there is a platform to create mRNA vaccines that are more capable of protecting the host compared to a natural infection, for example, there is a strong correlation between the increase of GC construct, which could possibly help with strengthening the immune response to Y. pestis. When conducting this experiment, researchers saw that “50% of the animals developed high IgG (antibody titer in which the specific antibody is being detected) titers against F1”, thus providing the immunity to “survive the lethal dose of the virulent strain”. Having been able to create a mRNA-LNP vaccine that could potentially help prevent the spread of Y. pestis. This discovery could function as a game changer on how the virus is transmitted and how it is treated.
In conclusion, Y. pestis relies on parasite-hosts, i.e., rodents and felines, interactions to spread. A vaccine for high-risk populations could potentially be used to help prevent its spread. However, a lot more can be done by cat lovers to help prevent the spread of Y. pestis worldwide. Some of these methods include controlling the ever-increasing overpopulation of feral animals, and by vaccinating both indoor and outdoor domestic cats. There are programs in place in communities all over the United States that focus on Trap-Neuter-Release (TNR) feral cats. Although the TNR program has helped slow the spread of other infectious diseases; such as rabies, feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV), we have to understand that a single vaccination at the time of spay-neuter may not fully prevent the spread of these infectious diseases entirely, and cats often need booster shots.
Infectious diseases and zoonotic diseases will continue to mutate in the environment to survive. We must take the proper action to slow the spread of these diseases and determine the most effective treatments and vaccinations to keep humans and our beloved animal companions safe.
Contributed by Faith Morris – edited by G. Whittaker
References
- Barbieri R, Signoli M, Chevé D, Costedoat C, Tzortzis S, Aboudharam G, Raoult D, Drancourt M. 2020. Yersinia pestis: the Natural History of Plague. Clin Microbiol Rev 34.PMC7920731
- Schama S. 2023. Foreign Bodies: Pandemics, Vaccines, and the Health of Nations. Simon & Schuster.
- Marra PP, Santella C. 2016. Cat Wars: The Devastating Consequences of a Cuddly Killer. Princeton University Press.
- Randremanana R, Andrianaivoarimanana V, Nikolay B, Ramasindrazana B, Paireau J, Ten Bosch QA, Rakotondramanga JM, Rahajandraibe S, Rahelinirina S, Rakotomanana F, Rakotoarimanana FM, Randriamampionona LB, Razafimbia V, De Dieu Randria MJ, Raberahona M, Mikaty G, Le Guern AS, Rakotonjanabelo LA, Ndiaye CF, Rasolofo V, Bertherat E, Ratsitorahina M, Cauchemez S, Baril L, Spiegel A, Rajerison M. 2019. Epidemiological characteristics of an urban plague epidemic in Madagascar, August-November, 2017: an outbreak report. Lancet Infect Dis 19:537-545.PMC6483974
- Kon E, Levy Y, Elia U, Cohen H, Hazan-Halevy I, Aftalion M, Ezra A, Bar-Haim E, Naidu GS, Diesendruck Y, Rotem S, Ad-El N, Goldsmith M, Mamroud E, Peer D, Cohen O. 2023. A single-dose F1-based mRNA-LNP vaccine provides protection against the lethal plague bacterium. Science Advances 9:eadg1036