Herpes Simplex Virus (HSV) is very prominent worldwide and known since ancient times. It’s a neuroinvasive virus which infects humans causing mild to very complicated lethal mucocutaneous infection.^1,2 HSV is a hypoxia-inducible pathogen frequently causing lifelong latent infection in humans. It has a linear, enveloped double-stranded DNA genome which is surrounded by an icosapentahedral capsid.³ HSV-I manifest latency after initial infection in sensory neurons terminating virion production along with lytic cycle gene expression of virus. However, only Latency Associated Transcript (LAT), 5 well-defined microRNAs (miRNAs), and two small non-coding RNAs (sncRNA1 and sncRNA2) are abundantly expressed.⁴ Physical or emotional stress, cerebral insults, immunosuppression are the various factors which are responsible for HSV-1 infection in the patient.^5,6

Alzheimer’s disease (AD) is one of the most common causes of dementia in elderly population affecting millions of people worldwide. Two different types of proteins are present ie., amyloid-β (Aβ) and tau. Amyloid-β peptides are proteolytic fragments which are synthesized from amyloid precursor protein (APP) which accumulate into oligomers and fibrils to form Aβ-containing plaques measuring 200-650 μm² in size.^7,8 Tau proteins are mostly microtubule-associated proteins found in neurons and are associated with microtubules stabilization. However, in case of AD tau protein gets accumulated resulting in formation of neurofibrillary tangles (NFTs) due to hyperphosphorylation. Their size generally ranges from 10 to 500 μm² .⁹

Association of HSV-1 in AD
There is many evidence suggesting that the HSV-1 is linked with AD. Aβ and Tau proteins are two biological hallmarks of AD which gets accumulated in brain leading to inflammation and dementia.^10,11 Thus, it has been observed that HSV-1 virus once enters the lytic phase due to immunosuppression, physical and emotional stress, it causes viral reinfection leading to brain impairment and inflammation which is alike early AD clinical findings. A clinical investigation was carried out in a group of AD patients which showed greater abundance of HSV-1 latency-associated transcripts (LATs) in older patients.¹² HSV-1 virus can further lead to different mechanisms like neuroinflammation, oxidative stress, mitochondrial damage, synaptic dysfunction, and neuronal apoptosis which can be linked to AD pathologies.  APOE is a gene that generally codes for apolipoprotein E protein (apoE) which is a risk factor for developing AD. There are 3 versions of APOE gene: ε2, ε3, and ε4 allele. Carriers with ε4 allele has the strongest risk factor of developing AD in elderly stage of their life or late onset alzheimer disease (LOAD) symptoms of which generally starts to appear after the age of 65. Apolipoprotein E (APOE), phosphatidylinositol binding clathrin assembly protein (PICALM), complement receptor 1 (CR1) and clusterin (CLU) are different genes susceptible to AD are also involved in HSV-1 life cycle. The polymorphisms may affect brain susceptibility to HSV-1 infection.^13,14

Once the HSV-1 virus starts to replicate in the oral, corneal, olfactory epithelial cells, the virus will start to utilize neuronal retrograde transport machinery and will outstretch to Trigeminal ganglion trigeminal ganglion and olfactory bulb where HSV-1 virus will enter latency and further leads to stress-induced reactivation. HSV-1 will be reactivated due to anterograde axonal transport to penetrate the brain. After that from trigeminal ganglion HSV-1 will travel to brainstem pons, which links our brain to our spinal cord and finally moves to limbic system where it inhibits the hippocampal memory system. HSV-1 alternately can infect hippocampal-entorhinal circuit directly via olfactory bulb a part of the limbic system¹⁵ (Figure 1 ).

Diagnosis
Serological Assays
Immunofluorescence assays, Western blot assays, and enzyme-linked immunosorbent assays (ELISAs) are the various types of serological tests are used for the detection and diagnosis of anti-HSV-1 and anti-HSV-2 antibodies. Irrespective of HSV-1 or HSV-2 infection, antibodies react with almost equal efficiency to both subtypes and leads to the cross reactivity. Limited sequence homologies and elicit type specific virus response are usually present in gGs of both HSV-1 & HSV-2.^16,17

Molecular Assays
The real time PCR has setup a milestone in detecting a wide variety of nucleic acids through its mechanism. The concept of synthetically made oligonucleotides or primer itself has showed impeccable results in detecting load of pathogenic nucleic acids in varities of samples. It’s a chain reaction process where primer binds to the open chain of DNA and with the help of enzyme mostly (Taq or Thermus Aquaticus) starts the process of adding dNTPs and making new complementary strand of template DNA. The complete process is divided into three steps:

1. Denaturation: The template double stranded DNA starts denaturing and separates out at 90°C to 95°C. which helps the DNA strand free for the primer to get binds on open strand.
2. Annealing: Primer anneals to the denatured single stranded DNA at 55°c to 60°c and binds specifically to the binding region.
3. Extension: Addition of dNTPs with the help of enzyme takes place at 72°c to 75°c just after the annealing of primer.

Recently, shotgun sequencing and modified Viromescan bioinformatics analysis are also applied to detect initial and low levels of HSV-1 gene expression between its reactivation and latent stage.¹²

RNAscope Technology
RNAscope hybridization is like traditional RNA ISH which is based on the concept that RNA probes can be organized in a way so that it can hybridized specifically to RNA of interest. For the detection of target RNA, a pair of ‘Z’ probes are used other than traditional ISH where a single RNA sequence is allowed to conjugate with labelled fluorophore or a digoxigenin.^18,19 The ‘Z’ probe contains three major elements- bottommost region that hybridizes to RNA molecule. The bottommost region with ‘Z’ probe tail is joined by linker sequence. And the tail which binds to the pre-amplifier region (Figure 2A). Through series of sequential actions signal amplification is achieved when the lower portion (RNA specific sequence) of double ‘Z’ probe binds to their target sequence. Conjugation occurs when chromogenic or fluorescent labelled probes binds specifically on the binding site of amplifier molecules. The complementary base pairing multiple amplifier sequences binds subsequently to the preamplifier at the top of each double ‘Z’ pair (Figure 2B). ²⁰

RNAscope probes are designed in such a way that it binds specifically to each transcript. In contrast to serological or RT-PCR assays, this method plays an important role in detection of HSV-1 reactivation status in the brain. One of the most important features of RNA scope is that it acts on multiple RNA targets and can detect neuronal HSV-1 latent LAT and lytic UL37 transcripts in mouse brain tissues. This combination detection tool can also be used to study different diseases in human brain samples such as HSV-1 induced neurodegeneration.²¹ Molecular and serological based HSV-1 detection assays are extensive techniques for its detection in clinical diagnosis. Low level of HSV-1 transcripts detection is a very challenging task. Therefore, to detect the low expressed multiple HSV-1 latent or lytic transcripts in infected brains with high specificity and sensitivity, RNAscope technology has proved to open a new technology in the field of neurodegenerative disorders.²²

Viral infections such as HSV is not completely curable, but it can be prevented through a mix of life-style changes and dietary supplements. Since the beginning of mankind, naturally grown fruits and vegetables are widely used for different nutritional features.^23-25 It has been observed that there are different Plant-Derived Antiviral Peptides (AVP) which plays an important role against DNA Viruses such as HSV-1 & HSV-2. From Sorghum bicolor seeds (Poaceae family), mostly found in Africa, a cationic, amphipathic peptide which is a 2 kD peptide is isolated which plays an important role by exerting HSV-1 virucidal activity by targeting viral replication cycles. This peptide plays an important role in covering the viral envelope protein and thereby making them inactive. From the leaves of Melia azedarach L., also called “Chinaberry tree, a Plant-Derived Antiviral Peptides (AVP), Meliacine was isolated. Meliacine contains polypeptides which can stop the replication cycle of HSV-1 polypeptides and viral DNA in the infected cell. The treatment of 50 µg/mL of meliacine in the infected patient cell was found to stop the spread of viral particles significantly. ²⁶

Diets for Alzheimer’s Disease
The occurrence of Alzheimer’s disease is also greatly depended on the nutrition and healthy diet. So, saturated fatty acids such as meat is strictly prohibited in the patients with AD.^36,37 This diet also slower the progression of AD by reducing the loss of brain volume.^38,39 A specified type of diet named DASH (Dietary Approaches to Stop Hypertension) which involves high consumption of fruits, vegetables, whole grains and low-fat dairy products.⁴⁰ So, a combination of DASH along with Mediterranean diet also known as MIND (Mediterranean-DASH Intervention for Neurodegenerative Delay) is designed which improves cognitive performance thereby reducing the risk of developing AD. The following diet combinations have significant correlation with less chance of cognitive disorder [Table].⁴¹

Table:
Antiviral properties of edible fruit extracts and fruit juices against HSV-1.²⁷

Prevalance of Alzheimer disease in India
As per the recent study, 3.74 million people, out of which 2.93 million people from India are affected by Alzheimer disease.^42,43 According to Clinical Dementia Rating (CDR), about 0.8% in individual’s ≥55 years of age and 1.3% in individuals ≥65 years of age was found to be affected by dementia.⁴⁴ In the study conducted in 52,377 participants from Kolkata, showed dementia is the major disorders found in 1.0% in individuals aged ≥60 years.^45-47 According to survey report, 5.3 million Indians aged >60 years is suffering from dementia till 2022 which is expected to exceed 14 million by 2050.^48-50