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The likelihood that a pathogen will overwhelm the immune system and cause a person to become ill has to do with the dose of the exposure. Scientists have explored the dose of exposure in a variety of infectious diseases such as Lyme borreliosis and – most recently – COVID-19. Scientists are learning more about how infectious dose affects whether a person will develop COVID-19, the disease caused by the novel coronavirus. This research informs the decisions we all make about where we go in nature, in public and the pets we keep in our households.
Infectious Dose
The amount of a pathogen that is required to establish an infection is called the “infectious dose.” If a person were to be exposed to exactly one copy of a SARS-CoV-2 virus or a single Bartonella species bacterium, it is unlikely they would become infected. The body’s innate immune system (the immune system that does not require identifying specific pathogens) has several different layers of activity that would neutralize that one viral or bacterial organism.
Another side to the infectious dose is how much of the pathogen is available to infect you. If the pathogen is in low levels in the area where you are exposed, you are less likely to be infected. If the pathogen is available in large quantities, you are more likely to be infected.
What does this mean for human/pet exposure and COVID-19?
When a person is infected with SARS-CoV-2, their viral load, or the amount of virus that is replicating in their bodies, goes very high for the first few days. There is some evidence that the larger the infectious dose, the higher the individual’s viral load is likely to become before their immune system is capable of controlling the infection. This high viral load means they are also shedding large amounts of virus into the air around them and is part of the reason why it is relatively easy to catch the disease from another person.
It’s also why people have on rare occasions been able to infect dogs and cats. Dogs and cats are not as good of hosts for the virus as people are, but when they are exposed to people shedding lots of virus it seems that they can be infected. Cats may also be able to infect other cats. At this point in mid-May 2020, only a handful of infected dogs and cats have been identified as infected out of thousands tested.
It is much less likely that a person could become infected from a dog or cat. Because dogs and cats seem to only be able to carry a lower viral load, even if they are infected, it is not likely that a human could receive an infectious dose from them. At this point in mid-May 2020, no humans have been identified as being infected from a dog or cat.
What does this mean for vector exposure, bartonellosis and Lyme disease?
For a vector-borne disease like bartonellosis or Lyme disease, one area of study is which vectors are “competent.” A competent vector can receive the pathogen from a human or animal, keep it alive inside their body, and transmit it to another human or animal as an infectious dose. To transmit an infectious dose to a host, the vector must be able to keep that quantity of pathogen alive in its system.
Some vectors may be exposed to a pathogen, but because their bodies are not a hospitable environment, not enough pathogen survives to create an infectious dose. Thus, determining that an attached tick contains a pathogen in no way confirms that the tick transmitted the pathogen to the human. Laboratory studies with mice suggest that the dose exposure of Borrelia burgdorferi affects Lyme disease severity and the host immune response throughout the course of the infection. When hamsters were exposed to whole dead bacteria in an attempt to vaccinate them, differing immune responses were seen at different doses.
In another example, fleas are competent vectors for various Bartonella species. Fleas acquire these bacteria via blood meals from mammalian reservoirs, such as cats or small mammals and rodents. The bacteria survive transit through the gut and end up in the flea feces, where they can remain viable for over a week. Cat scratch disease, caused by Bartonella henselae, is caused by the infectious dose that is found in flea feces. When a recently flea-infested cat scratches someone, the viable bacteria in the feces found under the nails gain access to the bloodstream.
This differing infectious dose response also suggests that the duration of a tick bite may have an effect on whether a bitten animal or human gets sick and how sick they get. The shorter the tick bite time, the less bacteria can be transmitted. Research also suggests that tick saliva itself can act synergistically with the bacteria to overcome (suppress) the host immune system. Again, the shorter the bite before the tick is removed, the less saliva (and potential pathogen) the host is exposed to. Simplistic explanations of tick bite time have concerned tick-bite disease clinicians and patients. Research studies have documented that attachment times vary substantially among tick-borne pathogens. Much more needs to be known about this topic.
Conclusion
Pathogen exposure is not a simple question of whether or not exposure occurred. The dose matters in determining whether an infection can take hold and seemingly how severe the resulting disease will be. For pathogens like SARS-CoV-2 and the bacteria that cause Lyme disease and bartonellosis, much more needs to be known about infectious dose and how people and animals receive and respond to different infectious doses. Knowing these things will help us move about in our environments with more confidence in our safety and allow clinicians to be more informed about how to respond to different kinds of exposure.
References
Hogan, A. (2020). How much of the coronavirus does it take to make you sick? The science, explained. Stat. Available at: https://www.statnews.com/2020/04/14/how-much-of-the-coronavirus-does-it-take-to-make-you-sick/
Barthold, S. W. et al. (1995). Circumvention of outer surface Protein A immunity by host-adapted Borrelia burgdorferi. Infection and Immunity, 63, 2255-2261. https://iai.asm.org/content/63/6/2255.short
Schaible, U. E. et al. (1993). Distinct patterns of protective antibodies are generated against Borrelia burgdorferi in mice experimentally inoculated with high and low doses of antigen. Immunology Letters, 36(2), 219-226. doi:10.1016/0165-2478(93)90056-8 https://www.sciencedirect.com/science/article/abs/pii/0165247893900568
Johnson, R. C. et al. (1986). Active immunization of hamsters against experimental infection with Borrelia burgdorferi. Infection and Immunity, 54, 897-898. https://iai.asm.org/content/54/3/897.short
IDEXX. (2020). Leading veterinary diagnostic company sees no COVID-19 cases in pets [press release]. Available at: https://www.idexx.com/en/about-idexx/news/no-covid-19-cases-pets/?fbclid=IwAR09pwwtuoiMQxF4iOq8logLNCFvS5bIagz9qZucgnoN-G1ATUFcJNcV5Nk