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Bartonella species are complex bacteria that cause non-specific, multi-systemic symptoms in people and animals. The presence of these bacteria as well as many other infectious pathogens leads to differing symptoms between individuals because of variations in host responses. These differences in severity and duration of symptoms have also been observed in patients infected by SARS-CoV-2.
Tick-borne disease patients are well-aware of the impact of the host response. The host response initiates an immune reaction against a pathogen, but this response itself can cause symptoms and lead to damage to vulnerable areas of the body. For example, inflammation that the immune system causes in response to an infection can damage an organ without the pathogen ever being present in that organ. An example we’ve written about previously is the central nervous system (brain and spinal cord).
Patients infected with SARS-CoV-2, the novel coronavirus, also have a variety of host responses. From a study of 23 hospitalized patients, it appears that the viral load goes very high at first. Between day 10 and day 21 of infection, the immune system begins to produce IgG and IgM antibodies that bind to the surface spike receptor on the virus, initiating an immune process that neutralizes the virus.
Infection vs. Disease
Some patients are asymptomatic throughout this process, that is, they truly have no symptoms or have symptoms that are so minor they don’t think anything of them. It is unclear how many people are asymptomatic and research varies widely, but it may be about half of infected people.
Some asymptomatic people are what is called pre-symptomatic, that is, they are infected but haven’t developed symptoms yet. People who develop symptoms have the disease called COVID-19.
(We wrote about the variety of names a pathogen and a disease can have previously.)
COVID-19 can present with a variety of symptoms. Although COVID-19 is generally considered a respiratory disease, about 50% of infected people have gastrointestinal symptoms like diarrhea. Half of those have only gastrointestinal symptoms and never develop respiratory symptoms. These symptoms may also be affected by the gut microbiome.
Many, but not all, people with COVID-19 have a fever, a dry cough and fatigue. Additional symptoms such as a headache or chills occur in a smaller percentage of people. About 15% of people develop severe respiratory symptoms requiring hospitalization.
Underlying health conditions, such as diabetes mellitus or being immunocompromised, seem to play a role in how individuals present with COVID-19. A study from the CDC found that of patients who became sick enough to require ICU care for COVID-19, 78% had at least one underlying health condition or risk factor.
The virus uses ACE-2 receptors to attack cells. These receptors are high in organs like the intestines and kidneys. This receptor may sound familiar because it is targeted by certain blood pressure medications. Once the cells are damaged, they send out signals that lead to inflammation, particularly in the lungs. If the inflammation is severe, doctors may try to suppress the host response in order to protect the lung tissue from the body’s own immune response.
Immune suppressing treatment may take a variety of forms depending on what part of the immune system is overreacting and causing damage. While macrophages and granulocytes are part of the initial inflammatory reaction, the immune reaction in question may also be cytokine storm syndrome, which can then also lead to yet other reactions that need to be controlled.
Conclusion
Even though everyone that becomes infected with SARS-CoV-2 experiences the same pathogen, the host response means they can have a wide variety of symptoms. Some people have no symptoms, while others experience severe respiratory symptoms and even death. Stimulation of a variety of parts of the immune reaction has been identified as the cause of some COVID-19 symptoms. For doctors to manage the disease in very sick people, they must identify the disease as well as the specific host response to find the best treatment.
References
Heneghan, C. et al. (2020). COVID-19: What proportion are asymptomatic? [Blog post]. CEBM.net. Available at: https://www.cebm.net/covid-19/covid-19-what-proportion-are-asymptomatic/
Shi, Y. et al. (2020). COVID-19 infection: The perspectives on immune responses. [Online publication] Cell Death & Differentiation. doi:10.1038/s41418-020-0530-3 https://www.nature.com/articles/s41418-020-0530-3
Pan, L. et al. (2020). Clinical characteristics of COVID-19 patients with digestive symptoms in Hubei, China: a descriptive, cross-sectional, multicenter study. [online pre-print version]. Available at: https://journals.lww.com/ajg/Documents/COVID_Digestive_Symptoms_AJG_Preproof.pdf
World Health Organization-China Joint Mission. (2020). Report of the WHO-China Joint Mission on coronavirus disease 2019 (COVID-19). Available at: https://www.who.int/docs/default-source/coronaviruse/who-china-joint-mission-on-covid-19-final-report.pdf
Chen, Y., & Li, L. (2020). SARS-CoV-2: Virus dynamics and host response. [Online]. The Lancet Infectious Diseases. doi:10.1016/S1473-3099(20)30235-8 https://www.thelancet.com/action/showPdf?pii=S1473-3099%2820%2930235-8
Centers for Disease Control and Prevention. (2020). Information for healthcare professionals: COVID-19 and underlying conditions. Available at: https://www.cdc.gov/coronavirus/2019-ncov/hcp/underlying-conditions.html
Centers for Disease Control and Prevention COVID-19 Response Team. (2020). Preliminary estimates of the prevalence of selected underlying health conditions among patients with coronavirus disease 2019 – United States, February 12-March 28, 2020. MMWR: Morbidity and Mortality Weekly Report, 69(13), 382-386. doi:10.15585/mmwr.mm6913e2 https://www.ncbi.nlm.nih.gov/pubmed/32240123