Compare and contrast the biological characteristics of the following pairs, giving examples where appropriate:

(a)	live, attenuated and killed, inactivated viral vaccines.
(b)	dengue and Japanese encephalitis.
(c)	haemorrhagic fever with renal syndrome and hantavirus pulmonary 
	syndrome.
(d)	epizootic and enzootic diseases.

Suggested Answers:

(a)	Prevention of infection by the use of vaccines is important as there are few drugs which are useful or effective against viral infections. There are two types of vaccines that induce active immunity: those that contain live virus whose pathogenicity has been attenuated and those that contain killed virus.

Most live vaccines contain viruses that have been attenuated by laboratory manipulation. These attenuated viruses can infect and replicate in the recipient and produce a protective immune response without causing disease. Examples of live viral vaccines are measles, mumps, rubella, varicella and adenovirus vaccines.

Killed viral vaccines contain either whole virus particles, inactivated by chemical or physical means, or some component(s) of the virus. The traditional inactivating agent was formaldehyde, but this is being supplanted by -propiolactone and ethylenimines. Examples of killed viral vaccines are vaccines of influenza, hepatitis A, hepattis B, rabies and Japanese encephalitis.

Both types of vaccines offer various specific advantages and disadvantages in their use in clinical settings. Live attenuated viral vaccines can often confer lifelong immunity after one immunization series. However, because live viruses can multiply in the body, there is always the possibility that they may revert to a more pathogenic form. Adequate laboratory and animal testing and extensive clinical studies must be performed to assess this possibility. In addition, new recombinant technologies facilitate direct alteration of viral genetic structure, thus permitting scientists to produce attenuated viruses in which the genetic regions likely to lead to pathogenic reversion are modified or deleted. The live virus is continuously being excreted by the immunized person which may spread and infect a susceptible person such as one who is immunocompromised. As live viral vaccines are vulnerable to inactivation by high ambient temperatures, problems may be encountered in their use and storage in the tropics.

On the other hand, completely inactivated viral vaccines cannot revert to virulence to cause infection. They are also more heat-stable, so they can be easily employed in tropical climates. However, they do not generally produce lifelong immunity following one immunization series; additional doses are usually required. It is usually given intramuscularly and therefore do not stimulate a major IgA response. In addition, because killed virus does not multiply in the host, the inoculum itself must provide a sufficiently large concentration of viral antigens to induce the desired immune response. It offers a shorter duration of protection and less protection in contrast to live viral vaccines.

(b)	Both the dengue virus and the Japanese encephalitis virus belong to the family of flavivirus. They have a single-strand RNA genome and an spherical glycoprotein envelope. Though both of them are spread by the bite of an arthropod, there are many differences in their disease-causing mechanisms and clinical manifestations. 

	Dengue virus has 4 serotypes 1, 2, 3 and 4. The dengue virus is transmitted to man by the bite of the Aedes aegypti mosquito. Man is the main reservoir of the dengue virus. Dengue viruses of all four serotypes cause three distinct syndromes: classic dengue fever, dengue hemorrhagic fever, and dengue shock syndrome. Although caused by the same viruses, dengue and dengue hemorrhagic fever are pathogenetically, clinically, and epidemiologically distinct. Dengue viruses appear to replicate in macrophages at the site of the mosquito bite, in regional lymph nodes, and then throughout the reticuloendothelial system. Viremia is concurrent with clinical illness. Virus is present in the serum and in association with circulating monocytes. Severe leukopenia often is present. 
Dengue fever is characterized by sudden onset of systemic toxicity, fever, headache, vomiting, and severe myalgia or bone pain of escalating intensity. Either coincident with or following remittance of fever on days 3 to 5 of the illness, there appears a maculopapular or morbilliform rash on the trunk which spreads to the limbs and face. This phase of the illness is often accompanied by recrudescence of fever, lymphadenopathy, granulocytopenia, and thrombocytopenia. Minor mucocutaneous bleeding is occasionally manifested by petechiae, epistaxis, menorrhagia, and a positive tourniquet test. Dengue fever lasts 3 to 9 days, is self-limiting, and is rarely associated with serious sequelae.
In contrast, the clinical course of dengue hemorrhagic fever is characterized by an initial stage of fever, rash, and anorexia (lasting 3 to 5 days) followed by a shock phase in which hepatomegaly, hypotension, and a hemorrhagic diathesis occur. Complement activation and thrombocytopenia typically take place at the onset of the shock phase and reverse spontaneously after a period that ranges from hours to a few days. Dengue hemorrhagic fever results from additional pathogenetic processes not present in classical dengue fever; the most important of which are diffuse capillary leak with hemoconcentration, thrombocytopenia, and disseminated intravascular coagulation. 
In dengue shock syndrome, the decreased plasma volume which results from increased vascular permeability causes clinical shock that, if uncorrected, may lead to acidosis, hyperkalemia, and death. Ninety percent of dengue hemorrhagic fever cases occur in children experiencing multiple infections with dengue. There are four dengue serotypes that can be discriminated by neutralization assays, and persons can be infected serially or even simultaneously by two different serotypes. 
Dengue hemorrhagic fever and dengue shock syndrome arise via immunopathologic mechanisms following sequential infection of an individual with these heterologous, antigenically-related serotypes (dengue-1, -2, -3, or -4). In infants, the presence of subprotective levels of maternal anti-dengue antibody has been reported to be a factor. "Immune enhancement" is thought to play a major role in pathogenesis, whereby both homologous and heterologous antibodies binding to dengue virus can markedly enhance infection of macrophages in vitro via cellular Fc receptors. Several antigenic determinants for infection-enhancing antibodies have been found on the envelope glycoprotein. Thus, it has been postulated that cross-reacting antibodies from a previous dengue infection or maternal anti-dengue antibodies in infants enhance the entry of virus into macrophages. The increased viral replication in the macrophages then contributes to the complement activation, vascular permeability, and clotting abnormalities observed in patients, through the release of products from infected macrophages. These products may be released by increased destruction of infected macrophages. An alternative or expanded view of the "immune enhancement" phenomenon incriminates cross-reactive T cells as important mediators of immunopathology. It is speculated that T cells exacerbate the antibody-enhanced destructive cascade, as they vigorously respond to (and then destroy) antigen-presenting cells, with concomitant release of cytokines by both T cells and damaged macrophages. 
On the other hand, the Japanese encephalitis (JE) virus consists only of a single serotype and is transmitted to humans by certain species of Culex mosquito endemic to Asian rice fields. Humans and pigs are two main reservoir of hosts for the JE virus. Infection follows bite by infected mosquito. The virus replicates locally either in the skin, lymphatics or endothelium of blood vessel and there is a transient viraemia. The virus enters the nervous system by capillary seeding through endothelium into meninges and brain and infect nerve endings with subsequent axoplasmic transport to neurons. Destruction of neurons and glial cells, perivascular inflitration with lymphocytes and focal hemorrhages are the common histopathologic features. 
However, in most JE infections, there is either no apparent disease or a nonspecific febrile illness with headache. The infection resolves, and lasting immunity is produced. Viral invasion of central nervous system presents as aseptic meningitis or encephalitis. Clinical manifestations of encephalitis due to JE virus begin as fever, headache, and stiff neck, and progress to an altered level of consciousness and focal neurologic deficits (e.g., tremors, pathologic reflexes, cranial nerve palsies, nystagmus, ataxia). Paralysis is also common. Seizures occur in about 10% of JE cases, and constitute a poor prognostic sign in adults. The case fatality rate ranges from 2% in young adults to over 20% in the elderly.

(c)	Both hemorrhagic fever with renal syndrome and hantavirus pulmonary syndrome are caused by the hanta virus. The hanta virus belongs to the family bunyavirus and has a single-stranded circular RNA in three segments. It possesses a glycoprotein envelope with surface projections and many internal granules. Hanta virus is maintained in nature by infecting a single rodent species. Transmission to humans occur by inhalation of aerosols from infected rodent excreta. Hantaviruses chronically infect rodents without apparent disease, but when they are spread by aerosolized excreta to humans, two major clinical syndromes result: hemorrhagic fever with renal syndrome (HFRS) and hantavirus pulmonary syndrome (HPS). Both diseases appear to be immunopathologic, and inflammatory mediators are important in causing the clinical manifestations. In HPS, T cells act on heavily infected pulmonary endothelium, and it is suspected that gamma interferon and tumor necrosis factor are major agents of a reversible increase in vascular permeability that leads to severe, noncardiogenic pulmonary edema. HFRS has prominent systemic manifestations. The retroperitoneum is a major site of vascular leak and the kidneys suffer tubular necrosis. Both syndromes are accompanied by myocardial depression and hypotension or shock. HFRS is primarily a Eurasian disease, whereas HPS appears to be confined to the Americas; these geographic distinctions correlate with the phylogenies of the rodent hosts and the viruses that coevolved with them. 

Hemorrhagic fever with renal syndrome is a severe disease with fever, renal impairment, including proteinuria, oliguria, hemorrhages and abdominal pain. It begins abruptly with fever, chills, weakness, and dizziness. Headache, myalgia, and lumbar pain are usually prominent. The severe form of the disease, occurring primarily in Asia, may result in thrombocytopenia with petechiae and hemorrhage, while the milder form exhibits little or no hemorrhage. Both forms may result in acute renal failure. The clinical course of the disease consists of a febrile, hypotensive, oliguric and diuretic phase. 
Hantavirus pulmonary syndrome, commonly referred to as Hantavirus disease, is a febrile illness characterized by bilateral interstitial pulmonary infiltrates and respiratory compromise requiring supplemental oxygen and simulating adult respiratory distress syndrome (ARDS). It is a serious, often deadly, respiratory disease that has been found mostly in rural areas of the western United States. The typical prodrome consists of fever, chills, myalgias, headaches, and gastrointestinal symptoms. Symptoms of hantavirus pulmonary syndrome usually appear within 2 weeks of infection but can appear as early as 3 days to as late as 6 weeks after infection. The illness characterized by one or more of the following clinical features: A febrile illness (i.e., temperature >38.3C) occurring in a previously healthy person characterized by unexplained adult respiratory distress syndrome or bilateral interstitial pulmonary infiltrates with respiratory compromise requiring supplemental oxygen, developing within 72 hours of hospitalization. Other symptoms are headache; abdominal, joint, and lower back pain; sometimes nausea and vomiting. However, the primary symptom of this disease is difficulty in breathing, which is caused by fluid build-up in the lungs and quickly progresses to an inability to breathe. An unexplained respiratory illness resulting in death, with an autopsy examination demonstrating non-cardiogenic pulmonary edema without an identifiable cause. Typical clinical laboratory findings include hemoconcentration, left shift in the white blood cell count, neutrophilic leucocytosis, thrombocytopenia, and circulating immunoblasts. 

(d)	Zoonosis is an infection or infectious disease transmissible under natural conditions from vertebrate animals to man. It may be epizootic or enzootic. Epizootic disease is an outbreak of disease where there is an unusually large number of cases. This definition implies that we know what numbers constitute "normal". Epizootics are sporadic and limited in duration. They are characterized by a sudden change in prevalence and incidence. Enzootic diseases are diseases that are usually low in prevalence and constantly present. Enzootic diseases persist over long time periods and may be interrupted by epizootics. 

	Examples of epizootic diseases are the outbreak of Ebola virus in northern Gabon in 1996. The host species of the virus is unknown though it seems likely that monkeys are evolved. Ebola hemorrhagic fever is a severe febrile illness with headache, myalgia, maculopapular rash, hemorrhagic manifestations with renal and central nervous system involvement. No antiviral therapy or vaccine is available. Ebola virus is one of the most virulent pathogens, killing a very high proportion of patients (up to 70%) within 5-7 days.
Closer to home, there is outbreak of an epizootic disease between September 1998 and June 1999 among pig farmers in Malaysia. The causative agent was discovered to be a new paramyxovirus, now called the Nipah virus, that was directly transmitted to humans from pigs and their secretions. Nipah virus causes a severe febrile, rapidly progressive encephalitis with a high mortality rate. Clinical features suggest brain stem involvement. 

	One example of an enzootic disease is malaria which is endemic in tropical and subtropical areas especially in Asia, Africa, Central and South America. It is transmitted by the protozoa plasmodium which uses the anopheles mosquito as a vector and causes a febrile illness accompanied by splenomegaly, hepatomegaly, hemolysis and anemia, with renal and CNS involvement in severe cases. The filariae Brugia malayi is present in Southeast Asia, parts of India and Sri Lanka. IT causes a periodic fever and lymphadenitis of the inguinofemoral regions. Clonorchis sinensis is another enzootic disease which in endemic in China, Japan, Korea and Indochina where it affects about 20 million people. 25% of the population of Hong Kong appears to be infected. Clonorchis sinensis is a liver fluke acquired by consumption of infected fish. The acute syndrome consists of malaise, low grade fever, leukocytosis and acute pancreatitis. Chronic infections are due to the persistent presence of the flukes in the biliary tracts and it can lead to biliary stasis, chronic recurrent cholangitis and cholangiocarcinoma.