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Viruses are simple in structure and function compared to bacteria or unicellular parasites, yet their presence often leads to identifiable disease patterns as well as nonspecific symptoms for both humans and animals. Like other pathogens, viruses are detected and characterized from patient samples using various testing techniques.
Introduction
Viruses are infectious particles that rely on suitable hosts for their own survival. They must be able to evade a host’s immune response long enough to invade the target cells and successfully replicate. Some viruses accomplish this feat by co-evolving with a host over time, whereas others may diverge into a new species by entering a novel host and infecting them when the right conditions are met. SARS-CoV-2, the novel coronavirus that is responsible for the ongoing COVID-19 pandemic, is a good example of a virus’ capacity for adaptation.
SARS-CoV-2 has been detected in people across the world since first being isolated from a patient in China in December of 2019. Although coronaviruses are not new to causing illness in humans, this particular one has proven to be more efficient than its predecessors. Its high transmissibility even led some to question whether it was bioengineered to specifically infect humans. However, when researchers look at the evolutionary pattern of SARS-like viruses, it is apparent that SARS-CoV-2’s development was a natural occurrence due to random genetic mutations and our relationship with animals. In a previous blog post, we discussed how scientists believe this occurs with bats as an example.
SARS-CoV-2 causes the respiratory disease COVID-19, but case reports have included a wide variety of symptoms occurring singularly or together, including gastrointestinal symptoms, skin symptoms, neurological symptoms involving the sense of smell, and more. Nonspecific symptoms that could be from many diseases plague any diagnosis, but they are especially difficult with SARS-CoV-2, when physicians such as dermatologists may be exposed to a patient who at first is not suspected to be infected.
Viruses Transmitted by Ticks
There are other viruses that exist in reservoirs we often cross paths with, like rodents and deer, that are transmitted to humans by vectors such as fleas or ticks. Powassan virus, which causes POW virus disease, is considered rare, with less than 50 cases reported each year. It is primarily spread by the same Ixodes tick species that can transmit the bacteria that cause Lyme disease (Borrelia burgdorferi). It causes non-specific symptoms like fever, nausea, and weakness, but can lead to severe neuro-invasive disease that is fatal in 10% of POW virus disease cases. A former North Carolina senator, Kay Hagan, died in October 2019 after a three-year struggle with encephalitis caused by Powassan virus.
Heartland virus, which was first identified as a human pathogen in 2012, is another virus that tick-borne disease patients may have heard of. Heartland, the disease that the virus causes, leads to many non-specific symptoms and often mimics other tick-borne illnesses such as anaplasmosis and babesiosis. A report from this year stated that 88% of confirmed cases between 2012-2017 resulted in hospitalization. Unlike Powassan virus, Heartland virus has only been shown to be transmitted by Lone Star ticks (Amblyomma americanum).
Virus Structure and Function
There are about 150 families of viruses, each containing numerous species and subspecies, that fill a specific niche in their relative environment(s) and host(s). The Powassan virus is a Flavivirus, whereas the Heartland virus is considered a Phlebovirus. What do these names actually mean or refer to though?
Just like bacteria, scientists use viruses’ identifying structures and how they are genetically related to classify them:
Capsid
This protein shell provides a protective barrier that keeps the genetic material intact. The receptors that are needed to attach and invade target cells are also integrated into the capsid. These structures often give viruses a distinct shape which helps researchers identify and name them. For example, coronaviruses are named for “crown” because of the spike proteins that give them their crown-like shape and play a role in pathogenicity. Flavivirus capsids have an icosahedral shape (see below), which is what many people may visualize when they think of a virus.
The capsid plays an important role in antibody testing. When the host immune system mounts a response using IgM and IgG antibodies, specific structures found on the capsid are often their target. These antibodies can typically be detected in patient blood serum about 1-2 weeks after infection.
Genetic Material (DNA or RNA)
Like bacteria, viruses contain genetic material (nucleic acids) that encodes the genes necessary for protein production and viral replication. Since viruses rely on the machinery of the host cell for many of their functions, however, their genomes tend to be much smaller and more efficient. Further, viruses can contain either DNA or RNA. This affects many factors, but most importantly how likely they are to mutate. For example, RNA viruses are more likely to mutate than DNA viruses due to having a less stable structure and their lack of “proofreading” mechanisms during replication. Mutations that occur during DNA/RNA replication allow viruses to infect new hosts or better evade the defenses of current hosts.
The genetic material of viruses is vital for detecting active infections in patients. Just like with bacterial infections such as Lyme disease or bartonellosis, PCR is a standard method used to detect DNA from infectious organisms in blood, tissue, etc. Viral RNA requires an extra step using an enzyme (Reverse-Transcriptase) to convert it into a molecule called cDNA before a standard PCR can be performed.
Envelope
Some viruses have an additional outer membrane made of lipids that is similar to the plasma membrane of bacteria or animal cell membranes. It not only provides additional protection from the environment, but it can help them “stick” to certain environments and persist longer in the environment. Furthermore, this envelope can lead to differences in pathogenicity by assisting in evasion of the host immune response.
Conclusion
Viral infections, like bacterial infections, can cause classic disease cases and also lead to hard-to-diagnose nonspecific symptoms. PCR testing is one of the tools that can identify the infection and point toward the correct treatment. Unfortunately, there are currently few treatments for SARS-CoV-2, but knowing the infection status of the patient can explain the symptoms and protect healthcare workers.
References
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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
Jaslow, R. (2020). Clue to infection. Harvard Medical School News & Research [website]. Available at: https://hms.harvard.edu/news/clue-infection
Andersen, K. G. et al. (2020). The proximal origin of SARS-CoV-2. Nature Medicine, 26, 450-452. doi:10.1038/s41591-020-0820-9 Available at: https://www.nature.com/articles/s41591-020-0820-9
Centers for Disease Control and Prevention. (2019). Powassan virus statistics & maps. Available at: https://www.cdc.gov/powassan/statistics.html
Jewell, M. (2020). A primer on and conversation about the biology and evolution of SARS-CoV-2, the virus that causes Covid-19. Applied Ecology News [North Carolina State University website]. Available at: https://cals.ncsu.edu/applied-ecology/news/a-primer-on-and-conversation-about-the-biology-and-evolution-of-sars-cov-2-the-virus-that-causes-covid-19/
Staples, J. E. et al. (2020). Investigation of Heartland virus disease throughout the United States, 2013-2017. Open Forum Infectious Diseases, ofaa125. doi:10.1093/ofid/ofaa125 https://academic.oup.com/ofid/article/doi/10.1093/ofid/ofaa125/5819209
Centers for Disease Control and Prevention. (2018). Heartland virus disease (Heartland) symptoms and treatment. Available at: https://www.cdc.gov/heartland-virus/symptoms-treatment/index.html
Simmonds, P., & Aiewsakun, P. (2018). Virus classification – where do you draw the line? Archives of Virology, 163(8), 2037-2046. doi:10.1007/s00705-018-3938-z https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6096723/