In a recent study published in Emerging Infectious Diseases, researchers used dogs to investigate the histopathologic alteration caused by the severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2) on mammalian neurological systems.
Study: Neurologic Effects of SARS-CoV-2 Transmitted among Dogs. Image Credit: el-ka/Shutterstock.com
Background
Their findings reveal that infection by SARS-CoV-2 results in significant damage to the brain, especially concerning the blood-brain barrier.
They further discovered the presence of phosphorylated tau, a protein linked to neurodegenerative alternations in mitochondria. Researchers observed these changes in both symptomatic and asymptomatic dogs.
Given the similarity between the histopathology of SARS-CoV-2 across both human and non-human hosts, these findings may provide insights into the neurological effects of the coronavirus disease 2019 (COVID-19) on the brains of its human survivors.
COVID-19 and its impacts on non-human hosts
Since the emergence of the coronavirus disease 2019 (COVID-19) in late 2019, most severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2), its causative pathogen’s hosts, have been human.
However, given that the angiotensin-converting enzyme 2 (ACE2) receptor of many mammals is highly conserved and very similar to those of humans, animals such as cats, non-human primates, rodents, and dogs have been infected.
The World Health Organization (WHO) reports almost 750 million confirmed cases of COVID-19 in humans, 7 million of which were lethal.
Scientific and clinical studies conducted on cohorts of survivors reveal that, in addition to COVID-19 symptoms that persist for months or even years following infection (‘long COVID’), some survivors present neurological symptoms, including fatigue, headaches, and cognitive dysfunction.
Magnetic resonance imaging (MRI) of some of these individuals’ brains has revealed white matter hyperintensities indicative of blood-brain barrier (BBB) damage.
Other research has identified markers of neuroinflammatory responses, damage to the brain’s coagulatory systems, and microglial cell activation. Similar patterns of ARs-CoV-2 pathology have been reported in murine models.
The etiology of neuropathologic changes following COVID-19 infection, however, remains elusive. Protein modeling has confirmed that ACE2 in dogs can bind to human SARS-CoV-2, which epidemiological and genetic studies have discovered allows cross-species transmission from humans to their four-legged friends.
“Mutant strains of SARS-CoV-2 in dogs cause histopathologic changes in lung tissues and increased expression of muscle damage markers in the blood.”
Given their close similarity, the patterns of neuropathologic damage in humans and dogs are hypothesized to be alike. Studies on the effects of COVID-19 on dogs may provide glimpses into the mechanisms underpinning SARS-CoV-2-induced neural damage in humans.
About the study
In the present study, researchers used canine transmission models to elucidate dogs’ susceptibility to the Delta variant of SARS-CoV-2. Fifteen 6-month-old beagles (female) were divided into three cohorts – Control (n = 3), infection (n = 6), and contact (n = 6).
Dogs in the ‘infection’ cohort were intranasally inoculated (105 PFU of Delta variant) and subsequently placed in cohabitation with a dog from the ‘contact’ cohort to mimic natural transmission models in humans.
Once the infection in the contact cohort was established, researchers used quantitative reverse-transcription PCR to investigate SAR-CoV-2 nucleic acid in the dogs’ brains and immunofluorescence assays to observe the presence of viral particles.
To monitor the continuous spectrum of changes accompanying COVID-19 infection, nasopharyngeal, fecal swab, oropharyngeal, and blood samples were collected at 10, 12, 14, 38, 40, and 42 days postinfection (dpi) for use in experiments across day 4 through 35 dpi.
The complete cohort of tests included immunohistochemistry, immunofluorescence staining, quantitative reverse transcription PCR, enzyme-linked immunosorbent assay (ELISA), and the plaque reduction neutralization test. All experiments were conducted in triplicate. Finally, statistical analyses involved using plotted dose-response curves and Student t-tests.
Study findings
The present research found no significant change between dogs’ body weight and temperature before and after Delta variant infection. All of the dogs were neurologically and respiratorily asymptomatic for COVID-19 infection.
Quantitative reverse-transcription PCR and immunofluorescence assays detected the presence of viral particles in the brain during the early- but not late stages of the infection period.
“Our observations indicate that SARS-CoV-2 may infect the brain during the early infection period and may be cleared by the later infection period.”
Modified immunofluorescence assay using antibodies specific to the blood-brain barrier (BBB) compartments revealed loss of matrix (laminin and collagen IV) and tight junction (claudin 5) proteins.
Platelet-derived growth factor receptor-beta (PDGFR- β) densities were lower during the late stages of viral infection, suggesting damage to the BBB.
“Last, infiltration of CD4+ T cells was found in the brain by trespassing into the BBB matrix protein layer, suggesting severe damage to the BBB integrity and subsequent recruitment of these cells into the brain. Those observations indicate that SARS-CoV-2 infection could induce pathologic changes in the structural and functional integrity of the BBB.”
Together, these damages would allow immune cells and peripheral molecules to cross the BBB, resulting in small vessel disease (SVD).
Glial activation investigative staining revealed significant increases in the brain white matter of dogs from the infected cohorts compared to the controls, suggesting the COVID-19 infection could instigate neuroinflammatory responses, thereby promoting neurodegenerative pathology.
Tau is a family of major microtubule-associated proteins (MAPs) of a normal mature neuron and a hallmark of the Alzheimer’s condition. To evaluate tauopathy in COVID-19-infected brains, phosphorylated tau staining was employed.
“…we detected phosphorylation of tau at Ser202/Thr205 by using an AT8 antibody in the brains of dogs in the infection group during the early period and dogs in the infection and contact groups during the late period. Those results suggest that SARS-CoV-2 infection could induce accumulation of the pathologic form of tau in a site-specific manner.”
Finally, decreases in the number of neuronal cells in the infected cohorts during later infection periods reinforce damage to the BBB and SVD.
Conclusions
The present research uses canine models to gain insight into the neurodegenerative changes occurring in mammalian brains as a proxy for human neuronal impairment.
Their findings reveal that the Delta variant of SARS-CoV-2 can infect dogs by direct contact and dog-to-dog transmission. This study also suggests that long-COVID-19-like symptoms may plague canine survivors.
Their findings highlight viral damage to the BBB and that neuroinflammatory responses are prominently localized to the white matter, not the grey matter.
“Overall, these data can be used as translational research data to interpret the potential neuropathologic changes that may be observed in humans.”
Genetic risk factors for long-COVID uncovered in a large multi-ethnic study