Comprehensive Study Notes on the Epidemiology of Viral Hemorrhagic Fevers (VHF)

Introduction to Viral Hemorrhagic Fever (VHF)

• Definition and General Manifestations: Viral hemorrhagic fevers represent a group of significant human diseases characterized by a wide spectrum of clinical symptoms. These illnesses often culminate in abnormal vascular regulation and plasma leakage.
• Historical Context: The term "viral hemorrhagic fever" was first adopted in the early 1950s. It was used to designate an illness occurring sporadically among soldiers in the Korean War. This specific illness was later determined to result from exposure to the excretions of local rodents.
• Pathobiological Impact: VHFs affect multiple organ systems within the body. While the name implies bleeding, hemorrhage or bleeding from various body orifices is an accompaniment that occurs in some, but not all, cases.
• Causative Families: All viruses causing VHF belong to four distinct families of RNA viruses:
  • Flaviviruses
  • Bunyaviruses
  • Arenaviruses
  • Filoviruses

Characteristics of Infective Agents

• Genomic Structure: All hemorrhagic fever viruses possess single-stranded RNA genomes ($ssRNA$). However, they differ in polarity and segmentation:
  • Flaviviruses: Positive sense ($+$-sense), consisting of one segment.
  • Filoviruses: Negative sense ($$-sense), consisting of one segment.
  • Arenaviruses: Negative sense ($$-sense), consisting of two segments.
  • Bunyaviruses: Negative sense ($$-sense), consisting of three segments.
• Viral Envelope: All these viruses are enveloped. They feature surface glycoprotein molecules anchored in a lipid bilayer, which is derived from the host cell membranes.

General Epidemiology of Viral Hemorrhagic Fevers

• Zoonotic Nature: Almost all types of VHF are zoonoses. Their causative agents primarily infect animal species and are transferred to humans via:
  • Direct contact with an animal.
  • Exposure to animal excretions.
  • The bite of a blood-feeding arthropod.
• Reservoirs: Nearly all VHFs have non-human reservoirs. One exception is Dengue fever, for which humans can now be considered a reservoir.
• Transmission Routes:
  • Animal-to-Human: The primary source of infection.
  • Human-to-Human: Requires direct contact with virus-containing body fluids. Note that Yellow Fever and Dengue Fever are exceptions as they are specifically arthropod-borne.
• Occurrence Patterns: In endemic areas, these diseases manifest as single cases or small clusters of infections that typically "burn out" quickly.
• Importance of VHFs:
  • Increasing endemicity in certain regions.
  • Potential for rapid spread and large-scale epidemics.
  • High case fatality rates.
  • A general lack of widespread vaccines (with specific exceptions).
  • Challenges in maintaining effective infection control.

Classification and Geographical Distribution

• Arenaviridae: Examples include Argentine HF, Bolivian HF, Sabia-associated HF (Brazilian HF), Lassa fever (discovered 1969), and Lymphocytic choriomeningitis (LCM).
• Bunyaviridae: Examples include Crimean-Congo HF (CCHF), Rift Valley fever, Hantavirus pulmonary syndrome (HPS), and HF with renal syndrome (HFRS).
• Filoviridae: Examples include Ebola HF (identified 1976) and Marburg HF (identified 1967 in Marburg, Germany and Belgrade, Yugoslavia).
• Flaviviridae: Examples include Yellow fever, Kyasanur forest disease, Omsk HF, Tick-borne encephalitis, and Dengue HF.

Epidemiology and Stages of Yellow Fever

• Definition: An acute arboviral infectious disease characterized by fever, severe intoxication, thrombohemorrhagic syndrome, and significant damage to the liver and kidneys. The name "yellow" stems from jaundice, which causes yellowing of the eyes and skin.
• Transmission and Vectors: Human infection starts with the deposition of viral particles through the skin via the saliva of an infected arthropod. Mosquitoes are the primary vectors for transmission from forest monkeys to humans and between humans.
• Transmission Cycles:
  • Sylvatic (Jungle) Yellow Fever: Occurs when monkeys are the virus bearers. Aedes mosquitoes bite monkeys and سپس transmit the virus to humans entering the forest.
  • Intermediate Yellow Fever: Occurs in rural areas of Africa (humid or semi-humid savannahs). Increased human contact with infected mosquitoes leads to small epidemics. This is currently the most common outbreak type in Africa and can shift to an urban epidemic if vaccination rates are low.
  • Urban Yellow Fever: Occurs when migrants introduce the virus into high-density populations where the Aedes aegypti mosquito is present. These can be large and severe outbreaks.
• Clinical Presentation:
  • Incubation Period: Typically $3$ to $6$ days.
  • Period of Infection (Acute Phase): Characterized by fever, muscle pain (notably backache), headache, shivers, nausea, vomiting, and loss of appetite. A clinical hallmark is Faget’s sign, where a high fever is associated with a paradoxically slow pulse. Symptoms usually disappear after $3$ to $4$ days.
  • Period of Remission: Lasts several hours up to $1$ to $2$ days. The temperature drops and the patient feels better.
  • Period of Intoxication (Toxic Phase): Develops within $24$ hours of remission. Fever returns, jaundice appears, and abdominal pain with vomiting occurs. Bleeding may manifest from the mouth, eyes, nose, and stomach (blood in vomit/feces). Kidney function fails (ranging from proteinuria to anuria). Approximately $50\%$ of patients in this phase die within $14$ days.

Yellow Fever Diagnosis, Treatment, and Prevention

• Laboratory Diagnosis:
  • Isolation of the virus from serum.
  • Polymerase Chain Reaction (PCR) for viral genome demonstration.
  • Antigen detection via monoclonal antibody-enzyme immunoassay.
  • Serological tests (IgM enzyme immunoassay).
  • Blood/Urine Analysis: Leukopenia and thrombocytopenia in early stages; leukocytosis in the toxic phase. Proteinuria and hematuria are common.
  • Postmortem: Histopathological liver examination showing midzonal necrosis and eosinophilic degeneration of hepatocytes known as Councilman bodies.
• Management: No specific cure. Treatment is symptomatic and supportive, involving fluid replacement, blood derivatives for severe cases, and dialysis for renal failure.
• Prevention:
  • Vaccine: A live-virus vaccine requiring one shot. A booster is required every $10$ years if remaining in endemic areas.
  • Immunization Schedule: In Nigeria, it is given as routine immunization at $9$ months of age.
  • Yellow Card: The International Certificate of Vaccination becomes valid $10$ days after the shot and lasts for $10$ years.
  • Other Measures: Use of bed nets, repellents, insecticides, larvicides, legislations against forest encroachment, and surveillance.

Epidemiology of Ebola Virus Disease (EVD)

• Origin and Taxonomy: First discovered in $1976$ near the Ebola River in the Democratic Republic of Congo (DRC/Zaire). It is a member of the Filoviridae family and order Mononegavirales.
• Virology: Single-strand negative polarity RNA genome. It is structurally similar to the Marburg virus but antigenically distinct.
• Ebola Species:
  • Zaire Ebolavirus (EBOV): The most aggressive and deadly species.
  • Sudan Ebolavirus (SUDV).
  • Ivory Coast Ebolavirus (Tai Forest Ebolavirus - TAFV): Associated with epizootic infection; only one reported human case in $1994$.
  • Bundibugyo Ebolavirus (BDBV).
  • Reston Ebolavirus (RESTV): Found in non-human primates; can infect humans but is not pathogenic to them.
• Infectivity Stability: The virus is destroyed by heating for $30$ minutes at $60\,^{\circ}\text{C}$, UV/gamma irradiation, lipid solvents, bleach, and phenolic disinfectants.
• Mortality and Reservoir: Case fatality rate ranges from $25\%$ to $90\%$. While no definitive reservoir is confirmed, bats are the likely candidates due to asymptomatic high viral loads.
• Historical Outbreaks: The $2014\text{–}2016$ West Africa outbreak was the largest, involving approximately $2,240$ cases and $1,229$ deaths.

EVD Transmission and Clinical Progression

• Human Transmission: Occurs via direct contact with blood/secretions of infected individuals or contact with contaminated objects (fomites like needles). Common sources include bush meat, fruit contaminated by bats, stool/urine, and dead bodies.
• Risk Categorization:
  • Very Low/No Risk: Casual contact with an ambulant, self-caring patient (e.g., public transport).
  • Low Risk: Close face-to-face contact with an ambulant patient (e.g., physical exams).
  • Moderate Risk: Face-to-face contact without PPE with a patient who is coughing, vomiting, or has diarrhea.
  • High Risk: Percutaneous or mucosal exposure to contaminated fluids or tissues from severely ill patients.
• Symptoms: Incubation is $2\text{–}21$ days (average $8\text{–}10$). Common symptoms include fever, headache, muscle aches, weakness, and GI distress. Progressive symptoms include a maculopapular hemorrhagic rash, red eyes, hiccups, and internal/external bleeding. Approximately $11\%$ of infections may be subclinical.
• Standard Case Definition (IDSR 2010):
  • Suspected Case: Fever not responding to usual treatments plus one sign of bleeding (gum, skin, eyes, urine, or bloody diarrhea).
  • Epidemic Alert/Action Threshold: A single suspected case for alert; a single laboratory-confirmed case for action.
• Laboratory Handling: Testing must occur in Biosafety Level 4 (BSL-4) containment. Diagnostic methods include ELISA (antigen and antibody), PCR, and virus isolation.

Prevention, Control, and Safe Burial of Ebola

• Prevention Strategy: Elimination of reservoirs is impossible; thus, focus is on isolating cases, contact tracing, and a $21$-day quarantine for contacts.
• Transmission Interruption: Barrier nursing (gloves, goggles, gowns, boots), sterilization of equipment, and avoiding bush meat.
• Safe Burial Protocols:
  • Family must be counseled that they cannot touch the body.
  • Preparation: Use double gloves (with the outer pair being thick rubber). Spray the body and surroundings with a $1:10$ bleach solution. Use a mortuary sack/body bag, spray it again with $1:10$ bleach. If no bag exists, use two layers of cotton cloth soaked in $1:10$ bleach, then plastic sheeting.
  • Transportation: Shortest route possible. Staff must wear protective clothing. Carry a sprayer with $1:10$ bleach for accidents.
  • Burial: Grave depth must be at least $2$ meters. No viewing of the body allowed.
  • Vehicle Decontamination: Rinse interior with $1:10$ bleach, soak for $10$ minutes, then rinse thoroughly with clean water to prevent corrosion. Air-dry the vehicle.

Epidemiology of Lassa Fever

• Background: Discovered in Nigeria in $1969$. It is an Arenavirus endemic to West Africa (Nigeria, Liberia, Sierra Leone, Guinea). Roughly $300,000\text{–}500,000$ infections occur annually, resulting in $5,000$ deaths.
• The Infective Agent: An enveloped virus ($50\text{–}300\,nm$ diameter) with two RNA segments. It is inactivated by heating to $56\,^{\circ}\text{C}$, pH extremes ($<5.5$ or $>8.5$), UV/Gamma irradiation, and detergents.
• The Reservoir: The "multimammate rat" (Mastomys genus). Transmission occurs when food or household items are contaminated by rat urine or feces, or through handling/consuming the rats.
• Modes of Transmission:
  • Rodent-to-Human: Ingestion of contaminated food, catching/preparing rats as food, or potential aerosol inhalation.
  • Human-to-Human: Direct contact with fluids, needlestick injuries, or aerosolized virus (potential).
• Clinical Presentation: Only $20\%$ of cases are symptomatic. Onset is insidious with fever, headache, myalgia, and malaise. Progressive symptoms include nausea, vomiting, conjunctival hemorrhage, retrosternal pain, sore throat, and a maculopapular rash at $7\text{–}9$ days.
• Severe Complications: Liver failure, ascites, pulmonary/cerebral edema, renal failure, and irreversible hearing impairment.
• Differential Diagnosis: Malaria, Typhoid, Streptococcal pharyngitis, Yellow fever, and meningitis.

Treatment and Prevention of Lassa Fever

• Ribavirin Treatment: The only known effective drug; most effective if given within the first $6$ days of illness. Administrated intravenously for $10$ days.
  • Loading Dose: $33\,mg/kg$ body weight.
  • Days 1-4: $16\,mg/kg$ every $6$ hours.
  • Days 5-10: $8\,mg/kg$ every $8$ hours.
  • Side Effects: Reversible mild anemia. It is technically contraindicated in pregnancy.
• Lassa Prophylaxis: Ribavirin $500\,mg$ orally every $6$ hours for $10$ days.
• Community Prevention:
  • Rat-proofing: Quality housing and rodent-proof food storage.
  • Hygiene: Dispose of garbage far from homes; rodent trap usage ("Kill, but don't eat").
  • Health Education: Avoiding contact with fluids of the sick and changing burial practices involving washing corpses.
• Hospital Control: Strict isolation and barrier nursing. Surveillance of all close contacts (those who lived with or cared for the patient) during the three weeks after illness onset.