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VIRAL TRANSMISSION HOST RANGE AND TROPISM, © Viruses depend entirely on living hosts for their replication and survival. O The ability of to spread, infect specific cells, and adapt to different species is governed by three interrelated concepts: 1. Viral transmission, 2. Viral tropism, and 3. host range CO These concepts help explain how; 1. How viruses cause disease. 2. How viral outbreaks occur, and 3. why some viruses remain confined to one species while others jump across species barriers and cause epidemics or pandemics VIRAL TRANSMISSION © This refers to the routes and mechanisms by which viruses move from one host to another. O Transmission routes determine how effectively a virus spreads at the population level: (sino | transmissionMode | pesciption | examples orvinises | 1 Direct Contact Touching, kissing, sexual contact HSV, HIV. 2 Indirect Contact / Fomites | Contaminated surfaces or objects Adenoviruses, Norovirus 3 _| Airborne / Respiratory Droplets or aerosals Influenza, Measles virus 4 Fecal-Oral Route Contaminated water or food Rotavirus, Enteroviruses 5 Vector-Borne Transmission via mosquitoes, ticks. Denque, Zika, West Nile virus 6 Vertical Transmission Mother-to-child: in utero, perinatal, breastfeeding | CMV, HIV, HBV, Zika 7 Zoonotic Transmission From animals to humans. Rabies, Coronaviruses © The mode and mechanism of viral transmission are distinct processes that influence spread of virus and other pathogens. 1. Mode of viral transmission: * It is the route or pathway by which a virus moves from an infected host to a susceptible host * It is also referred to as the “high-level route” of spread. * Determines how rapidly and widely a virus can spread in a population. * Modes can overlap such that a virus may use multiple modes. 1. Modes of viral transmission include; a) Direct Contact b) Indirect Contact / Fomites c) Airborne / Respiratory d) Fecal-Oral Route e) Vector-Borne (Biological) f) Mechanical Transmission g) Vertical Transmission h) Zoonotic Transmission i) Sexual Transmission j) Bloodborne Transmission k) Parenteral Transmission 1) Iatrogenic / Nosocomial Transmission m) Environmental Transmission n) Autoinoculation / Self-Inoculation 2. Mechanism of viral transmission: # Refers to the biological or physical processes that allow a virus to successfully infect a new host via its mode of transmission. * The explains how the virus enters and establishes infection. * It can involve molecular interactions, survival strategies, or cellular entry mechanisms. © Common Mechanism of viral transmission Facilitates Spillover (Zoonosis & Revese Zoonos's) Virus can move between species, causing new outbreale in ‘lumans or animals. Influenza A (birds to pigs to humans); ‘SARS-CoV-2 (humans to/from mink) Creates Reservoir Hoss ‘Same species maintain the virus long-term without severe disease, enabling reintroduction into others. Pats (coranavinises), Birds (West Nile. virus) Increeses Viral Genetic Diversity Infection of mulzple species promotes recombination or reassorment with related viruses. Influenca reassoriment in pigs; Coronavirus recombination (bat-civet-human) Enhances Ecological Survival Virus can persist even if one host species declines, because It} cycles through others. ‘West Nile virus (bircs, horses, humans, mosquitoes) Drives Host Immune System Evolution Hosts evolve stronger or specialized antiviral defenses in response tw cross-spedes viruses. Mammalian interferon diversity shaped by viral pressure Expands Transmission Opportunities New species can serve as amplife's or intermediate hosts, neveasing spread amplify arboviruses; Pigs (Causes Variable Disease Severity in Different] Hosts ‘Same virus may d€ mid in one species but deadly in another. Rables (mild in bats, fatal in humans/dogs) A. Species Host Range (Inter-Species Host Range). W This specifies the range of hosts a virus can infect; C Host range can be subdivided into three categories based on the species being infected: * Human papillomavirus (HPV) infects humans only Narrow Species Host 1 Virus infects only one species. Range * Feline leukemia virus (FeLV) infects cats only « Influenza A viruses infeects humans, pigs, 3 Intermediate Species Host | Virus infects a limited group of birds Range dosely related species. Lyssavirus (rabies virus) infects many mammals, especially carnivores and bats * West Nile virus infects birds, horses, humans © Vesicular stomnatitis virus (VSV) infects many mammals and insects Virus infects multiple, sometimes unrelated, species. 5 |Broad Species Host Range B. Tissue or Organ Range (Tissue Tropism). OC This refers to which tissues or organs within a host a virus can infect. © Determinants of tissue tropism include; 1. Virus infect cells with its specific receptor. 2. Some tissue must have a co-receptor be infected e.g HIV uses CCR5/CXCR4 of CD4+cells. 3. Availability of cell processes for virus replication. 4. Virus exposure to immune. Some infect immune-privileged tissues for persistent infection, 5. Anatomical barriers e.g mucus, pH (torelance to low pH), enzymes, temperature, tight junctions 6. Viral tropism factor to recognize host cell receptor before infection. B. Tissue or Organ Range (Tissue Tropism). C Tissue host can be grouped into two categories; C. Cellular Range. O This refers refers to the specific cell types within a host organism that a virus can infect. © Determinants of cell tropism include; 1. Receptor availability on the cell surface. 2. Presence of co-receptors. 3. Intracellular factors that support viral replication. 4. Cellular environment (pH, enzymes, metabolic state). © Different cells may be susceptible or resistant to virus infection depending on these factors. Cc. Cellular host Range C Cellular host range can be grouped into two categories; A. Narrow cellular host range: W infects only one or a few specific cell types. O This occurs Even if other cells are physically present in the same location. It has strict receptor requirements There is dependence on specialized co-receptors Virus requires specific intracellular machinery (eo Examples: HIV infects CD4+ T cells and macrophages (both with CD4 receptor and a co-receptors, CCR5 or CXCR4). A. Broad cellular range: © The virus can infect many different cell types in the same host. O Characteristics. 1. Virus uses widely expressed receptors. 2. Has flexible replication requirements. 3. Able to function in varied cellular environments O Examples: Herpes simplex virus (HSV) infects epithelial cells, neurons, fibroblasts, and others. 
