Some contagious viral infections seen in humans are as follows: 1. Chickenpox 2. Common Cold 3. Dengue 4. Encephalitis 5. Influenza 6. Measles 7. Mumps 8. Poliomyelitis 9. Psittacosis 10. Rabies 11. Rubella 12. Smallpox 13. Yellow Fever.
Viral Infection # 1. Chickenpox (Varicella):
An acute, extremely contagious virus disease characterized by fever, mild constitutional symptoms, a cutaneous eruption involving the superficial layers of the skin, lasting 3 to 4 days, and leaving a granular scab. Vesicles tend to be more abundant on the covered parts of the body. Sometimes vesicles may be so few as to escape observation. Lesions also appear on scalp and on mucous membranes of upper respiratory tract.
Source of Infection – Secretions from respiratory tract of infected persons. Disease may be communicable before eruption is in evidence.
Mode of Transmission – From person to person by direct contact; by articles freshly soiled with discharges from infected persons.
Incubation Period – From 2 to 3 weeks; usually 14 to 16 days. Disease communicable probably not more than 1 day before nor more than 6 days after appearance of vesicles. Especially communicable in early stages of eruption.
Susceptibility and Immunity – Susceptibility to disease universal among those who have not had an attack. About 70 per cent of persons have had the disease by the time they are fifteen years of age. Recovery from the disease usually confers permanent active immunity.
Prevention and Control – Isolation of infected persons for period of communicability. Disinfection of discharges from nose and throat and articles soiled by such discharges.
Viral Infection # 2. Common Cold:
The etiological agent is one or more viruses, although at one time numerous bacteria were believed to be the cause of the infection.
Disease Produced – An acute, highly communicable, catarrhal infection of the nose, throat, larynx, sinuses, trachea, and larger bronchi, lasting usually 2 to 7 days. Infection usually accompanied by a rise in temperature on the first day and by chilly sensations with coryza and lassitude.
Other minor respiratory infections of a similar character include acute respiratory diseases (ARD) and respiratory illness-adenoidal pharyngeal conjunctival (RIAPC).
Infection has been transmitted experimentally to chimpanzees and man by a filterable agent. Reagan et al. (1954a) reported the cultivation of the filter-passing agent in the chorioallantoic cavity of embryonating eggs.
Source of Infection – Discharges from nose and throat of infected persons or from articles freshly soiled with discharges from such persons.
Mode of Transmission – Usually directly by droplets of infected saliva sprayed into the air during coughing, sneezing, and taking, or indirectly from articles freshly soiled with such discharges.
Incubation Period – Probably 12 to 72 hr. Communicability limited to early stages of disease, although the virus remains in discharges for an undetermined period.
Susceptibility and Immunity – Susceptibility to disease is universal. Temporary active immunity of approximately 1 month follows recovery from the disease.
Prevalence – Most individuals contract one or more colds each year. Both sexes about equally susceptible to disease. Incidence higher in children under five years of age and becomes less after twenty years.
Prevention and Control – Infected persons should avoid contact with others. Rest in bed during the acute stage is advisable. Nasal and mouth discharges should he kept away from others and disposed of, preferably by burning. Disinfection of eating and drinking utensils.
Viral Infection # 3. Dengue:
A disease caused by at least two immunologically distinct virus types.
Disease Produced – An acute infection of sharp onset, usually with two paroxysms of short duration, accompanied by fever, intense headache, pains in muscles and joints, and irregular eruption. Eruption appears usually 3 to 4 days after onset of fever.
Source of Infection – From blood of infected persons 1 day before and up to 5 days after onset.
Mode of Transmission – By the bite of several species of mosquitoes, including Aedes aegypti, A. albopictus, and A. scutellaris. The mosquito becomes infectious 8 to 11 days after feeding on an infected person.
Incubation Period – From 3 to 15 days, usually 5 to 6 days.
Susceptibility and Immunity – Susceptibility to disease universal. Two immunologically distinct strains known. Homologous immunity of long duration; heterologous immunity of short duration.
Prevalence – May occur wherever specific mosquitoes exist. Disease found mainly in the tropics and subtropics.
Prevention and Control – Elimination of mosquitoes and their breeding places. Use of repellents. Isolation of patients in screened rooms previously treated with insecticide.
Viral Infection # 4. Encephalitis (Arthropod-borne):
In the United States and Canada, includes Eastern and Western equine and St. Louis types of human disease; in some South American countries, includes Venezuelan equine type; in Japan, Korea, China, includes Japanese B type; in European and Siberian Russia, includes Russian spring-summer type.
Disease Produced – Mild cases of all types characterized by headache, fever, stiff neck and back, tight muscles, possibly drowsiness with changes in spinal fluid. Moderately severe cases characterized by acute onset, high fever, stupor, meningeal signs, coma, spasticity, tremors, rarely flaccid paralysis. Each form of disease caused by a specific virus.
Source of Infection – Wild and domestic birds serve as principal source of mosquito infection for U.S. types. Mites of chickens and wild birds may be infected. Man probably not a source of infection for U.S. types.
Mode of Transmission – Russian spring-summer type tick-borne; all others transmitted by mosquitoes.
Incubation Period – Usually 5 to 15 days.
Susceptibility and Immunity – Susceptibility to disease usually highest in childhood and old age. Permanent active immunity against specific virus believed to follow recovery from disease.
Prevalence – Disease occurs usually in summer or early fall. Endemic in the United States in hot valley areas, and epidemic in some dry farm areas of Middle West, South, and East.
Prevention and Control –Destroying larvae and feeding places of vector mosquitoes. Screening of sleeping quarters. Avoiding exposure during hours when mosquitoes are biting. Education of public to mode of spread and control.
Viral Infection # 5. Influenza:
Disease formerly believed to be caused by the organism Haemophilus influenzae but it is now known to be produced by a virus. In typical severe cases H. influenzae is also present but is probably of secondary importance.
Two distinct types of influenza virus, designated as types A and B, have been long identified. Type A is the older and more widely distributed; type B has usually been found in smaller and more localized outbreaks. Some outbreaks are not caused by either A or B but by a third type designated C.
Virus capable of passing through most filters, resists freezing for about 2 weeks, and retains its potency in 50 per cent glycerol for the same length of time. When dried from the frozen state (lyophilized), virus retains its potency for at least 6 weeks in the refrigerator.
Virus inactivated by mercurials, which would indicate the presence of -SH groups (cysteine), and reactivated by sodium thioglycollate. Lee strain (type B) virus inactivated at 50 to 54°C. in 15 min. and by as little as 0.05 per cent formalin in 18 hr. Virus inactivated by intense ultraviolet irradiation.
Virus sensitive to strong oxidizing agents such as iodine, salts of heavy metals, Mercurochrome, and the wetters Phemerol, Roccal, and sodium dodecyl sulfate. It is only slightly affected by reducing agents, sulfathiazole, dilute phenol solutions, glucose, ammonium sulfate, calcium chloride, and sodium thiosulfate.
Disease Produced – Influenza is an acute virus infection characterized by sudden onset, fever of 1 to 7 days’ duration, catarrh of the respiratory tract (sometimes alimentary tract), pains in the head and muscles, coryza, sore throat, bronchitis, and a tendency to pneumonic complications. The disease produces marked prostration. Virus capable of infecting mice, ferrets, and possibly swine intranasally.
Source of Infection –Discharges from nose and throat of infected persons or from articles freshly soiled with discharges from such individuals.
Mode of Transmission – By direct contact with infected persons, by droplet infection, or by articles freshly soiled with discharges from nose and throat of infected individuals.
Incubation Period –Usually 24 to 72 hr.
Susceptibility and Immunity – Susceptibility to disease is general, although some have natural immunity. Acquired immunity of short duration, possibly as long as 1 year, follows recovery from the disease and is effective only against the particular type which caused the infection.
Prevalence – Epidemics may affect up to 50 per cent of population within 4 to 8 weeks. Pandemics occur at irregular intervals.
Prevention and Control – Education of public to the dangers of droplet infection from spitting, sneezing, and coughing in the presence of others. Use of common eating and drinking utensils, towels, etc., should be avoided. Use of disposable tissue and napkins should be encouraged.
Isolation of infected persons during acute stage. Disinfection of discharges from nose and throat or articles freshly soiled by such discharges. Patients should be put to bed at the beginning of an attack. During epidemics, overcrowding should be avoided.
Some success achieved by administration of currently available vaccines, provided the infecting strain matches closely the antigenic component of the vaccine. Vaccine should be administered in advance of infection.
Antibiotics or sulfa drugs not effective but may be used to combat complications due to secondary invaders.
Viral Infection # 6. Measles (Rubeola, Morbilli):
A filterable virus capable of passing through Berkefeld N and Seitz filters. Can be preserved at -35 or -72°C. for as long as 4 weeks and for several days at 0°C. At room temperature virus remains infective for 36 hr. Virus may be dried from frozen state and remain active for at least 15 weeks. Can withstand 10 per cent ether for 40 min.
Claims that the virus of measles has been cultivated appear questionable.
Disease Produced – A specific, highly contagious disease, characterized by fever; catarrhal symptoms of the eyes, nose, and throat; an early eruption of the mouth; Koplik spots; a cutaneous rash followed by desquamation during convalescence.
Source of Infection – Secretions of nose and throat of infected persons.
Mode of Transmission – Directly from person to person; by droplets of infected saliva sprayed into the air during coughing, sneezing, and talking; by articles freshly soiled with discharges from an infected individual.
Measles is one of the most easily transmitted of the communicable diseases.
Incubation Period – After exposure, fever appears in about 10 days; rash in 13 to 15 days, occasionally shorter or longer.
Susceptibility and Immunity – Susceptibility to disease is general. Disease occurs most commonly in children between five and fourteen years of age. Permanent acquired immunity usually follows recovery from disease.
Prevalence – Universal, almost all persons (up to 90 per cent) have had an attack at some time during life. Disease common in childhood.
Prevention and Control – Immune globulin may be used for passive immunization of children under three years of age in families where cases of measles occur.
Isolation of infected persons during periods of communicability to protect them from possibility of reinfection and as protection to others. Isolation period usually 7 days from first appearance of rash. Disinfection of articles soiled with fresh discharges from nose and throat of infected persons.
Passive immunity may be transferred to healthy individuals before symptoms of measles appear by the injection of convalescent serum or serum from a person who has recovered from the disease. Such passive immunity may persist for about 4 weeks. During an epidemic, convalescent serum may either prevent the disease or modify the severity of the attack. In the latter instance a mild case of measles is usually sufficient to produce a lasting immunity.
Antibiotics or sulfa drugs not effective but may be used to combat complications due to secondary invaders.
Viral Infection # 7. Mumps (Infectious Parotitis):
Virus has been cultivated in the yolk sac, amniotic sac, and allantoic sac of the developing chick embryo. Measurements of particle size range from 90 to 340 mµ. Weil et al. (1948), by means of electron micrographs, found the virus particles to be circular and ranging in size from 106 to 282 mµ, with an average of 190 mµ.
Disease Produced – Mumps is an acute, specific, contagious disease characterized by fever and inflammation of the salivary glands. The parotid, submaxillary, and sublingual glands may be infected, although the parotid is most frequently involved. Sometimes the ovaries and testes may be attacked.
Source of Infection – Secretions of the mouth and possibly the nose.
Mode of Transmission – By direct contact with infected persons or by articles freshly soiled with discharges from mouth and nose of such individuals,
Incubation Period – From 12 to 26 days, usually 18 days.
Susceptibility and Immunity – Susceptibility to disease is general. Recovery from infection usually confers permanent active immunity. Complement-fixing antibodies regularly appear or increase in concentration in the sera of persons during an attack of mumps or during convalescence.
Prevalence – Mumps less prevalent than other diseases of childhood. Occurs more frequently during winter and spring. Occurrence of disease sporadic and epidemic. In large cities, it is endemic. Outbreaks frequently occur during periods of overcrowding.
Prevention and Control – Infected persons should be isolated until 7 days after swelling of salivary glands has subsided. Disinfection of eating and drinking utensils.
Viral Infection # 8. Poliomyelitis (Infantile Paralysis):
This virus is one of the smallest known, measuring 25 mµ in diameter.
It is a ribonucleoprotein and has been prepared in crystalline form. The ribonucleic acid may be separated from the protein and is capable of producing infections.
The virus is one of the most stable known. It resists inactivation by alcohol, phenol, formalin, chlorine, antibiotics, and by repeated freezing and thawing.
It is inactivated by ultraviolet light, by lyophilization, and by pasteurization at 62°C. for 30 min.
Disease Produced – Poliomyelitis is widely prevalent. In only a small proportion of infected persons are the disease clinically recognizable. In its recognizable form poliomyelitis is an acute, systemic infectious disease that involves the central nervous system. Disease characterized usually by fever, headache, vomiting, constipation, drowsiness alternating with irritability, almost always stiffness of neck and spine, tremor, and exaggeration of muscular reflexes.
In about half of such cases paralysis may develop in the first few days, which shows a marked tendency for improvement after it has reached its height. Diagnosis depends upon detection of a flaccid paralysis characteristically irregular in its involvement of muscular tissue. All strains fall into three immunologically distinct types designated types 1, 2, and 3.
Source of Infection – Nose and throat discharges of infected persons; also from those not suffering from clinically recognized attack of disease. Virus also present in feces.
Virus recovered from throat swabs and throat washings of poliomyelitis patients collected 3 to 13 days after onset of disease.
A study of four households attacked by poliomyelitis provided evidence of widespread distribution of virus in the members of these units. Of 20 members in these households, 16 had polio virus in their intestinal discharges, 7 had virus in the oropharynx.
Mode of Transmission – It is believed that the virus enters by way of nose and mouth, either from a carrier or from a person with a subclinical infection. There is some evidence that disease may be spread by milk, water supplies, swimming pools, food, sewage, and insects. Flies have been shown to be contaminated with the virus.
Rhodes et al. (1950) showed that a strain of human polio virus in stool survived after addition to river water for at least 188 days and retained its property of inducing paralysis in monkeys. Storage of the contaminated water was at -4°C., and the dilution of the human stool was about 1:200. The fact that the virus could survive for such a prolonged period would seem to have some epidemiological implications.
Lensen et al. (1949) found that in samples of natural waters having a pH range of 7.9 to 8.3, virus was consistently inactivated within 10 min. in presence of 0.05 p.p.m. residual free chlorine. In experiments at a higher pH range (10 to 11.25), 0.1 to 0.15 p.p.m. residual chlorine was necessary to achieve the same results.
Incubation Period – Usually 7 to 14 days; may be as early as 3 and as late as 35 days. Period of greatest communicability may be from latter part of incubation period to first week of the acute illness.
Susceptibility and Immunity – Susceptibility to infection is general. Children are believed to be more susceptible than adults, although definite proof is still lacking. Active immunity produced after recovery from the disease. Duration of immunity’ unknown, but second attacks are rare.
Prevalence – Infection widespread throughout the world. Paralytic cases more prevalent in temperate climates. Cases more numerous in summer and early fall. Children one to sixteen years of age appear to be more susceptible than adults.
Prevention and Control – Infected individuals should be isolated for 2 weeks after symptoms appear. Disinfection of nose, throat, and bowel discharges, and articles soiled with such discharges.
All children with fever should be isolated and kept in bed pending outcome of diagnosis. Protection of children from contact with others during an epidemic. Avoidance of nose and throat operations on children during an epidemic. During period when disease is prevalent, crowds should be avoided as far as possible.
Viral Infection # 9. Psittacosis (Ornithosis):
Virus particles relatively large, measuring 455 m. in diameter, and approaching the size of small bacteria. Particles visible under a light microscope. Virus filterable through membranes which retain bacteria.
Virus stains Gram-negatively, is susceptible to sulfonamides, penicillin, tetracyclines, and other chemotherapeutic agents which antagonize bacteria. However, in its mode of multiplication intracellularly, and its relationship to the metabolism of animal cells, it behaves more like the true viruses than to other classes of microorganisms.
Virus easily cultivated on the chorioallantoic membrane of the developing chick.
Disease Produced – Psittacosis is a contagious disease of parrots, parakeets, love birds, canaries, and other birds. It resembles influenza and is transmissible to man. In man disease characterized by high fever, headache, backache, thirst, changes in tongue and pharynx, stupor or depression, rapid pulse, diarrhea or constipation, enlargement of spleen; symptoms of atypical pneumonia or of a typhoidal state, with rales and cardiac dullness. Sputum light yellow in colour and of extreme viscosity. White blood count is normal or slightly increased early, with leucopenia later.
Source of Infection – Canaries, pigeons, parrots, parakeets, love birds, and other birds. Birds that appear to be well occasionally transmit the infection.
Mode of Transmission – Virus present in blood, saliva, and feces of infected birds. Disease transmitted by contact with such birds or their recent surroundings.
Incubation Period – From 6 to 15 days in human cases.
Susceptibility and Immunity – All ages susceptible. Disease more severe in higher age groups. Recovery from disease confers immunity.
Prevalence – Outbreaks usually sudden and caused by exposure to sick birds. Deaths usually confined to adults over thirty years of age. Mild cases may result from exposure to infected birds not necessarily sick.
Prevention and Control – Strict regulation of traffic in birds of the parrot family. Education of public in dangers of birds of the parrot family, particularly of those freshly imported. Homes and pet shops harboring infected birds should be quarantined.
Disease may be diagnosed by presence of virus in saliva and blood during the first week of the infection. Serum contains complement-fixing antibodies. Infected persons should be isolated during the febrile and acute clinical stage.
Masks should be worn when handling patients with coughs. Disinfection of discharges and articles soiled with such discharges. Infected birds should be disposed of by burning. Buildings in which infected birds were housed should be thoroughly cleaned and disinfected.
Specific treatment consists of administration of the tetracycline antibiotics or chloramphenicol until patient afebrile. Repeat if relapse occurs.
Viral Infection # 10. Rabies (Hydrophobia, Canine Madness):
Particles large, measuring 125 to 150 mµ in diameter. Virus capable of passing through Berkefeld and the coarser Chamberland filters, but not through Seitz filter pads. Infected tissues may be stored in undiluted glycerol for several weeks at room temperature, for several months in the refrigerator, and for 1 or 2 years at subzero temperatures.
The virus is reduced in virulence (attenuated) by drying infected tissue (spinal cords of rabbits) suspended over pellets of potassium hydroxide as a dehydrating agent, by exposure to 1 per cent phenol, and by heating to 54 to 56°C. for 1 hr. or less. Virus best preserved by drying from the frozen state followed by storage at refrigerator temperature. Repeated freezing and thawing of virus suspensions results in loss of infectivity.
Disease Produced – An, acute encephalitis caused by a neurotropic virus acquired from the bite of a rabid animal, usually the dog. Disease characterized by depression, itching at site of primary infection, and fever. Patient becomes uneasy, swallowing becomes difficult, salivation marked, followed by attacks of delirium. Paralysis of the face muscles, eyes, and tongue appears, gradually spreading to the trunk and limbs.
Cause of death may be determined by demonstrating the presence of Negri bodies in the nerve cells of brain or spinal cord, or by emulsifying a small portion of the hippocampus in sterile saline and inoculating subdurally into guinea pigs or rabbits. Death occurs in about 16 days, and Negri bodies can be demonstrated in the brain tissue.
Source of Infection – Infected animals, chiefly dogs; vampire bats also involved.
Mode of Transmission – Virus present in saliva and is usually transmitted to man by bite of a rabid animal. Infections have occurred by contact of saliva with a scratch or break in the skin.
Incubation Period – Usually 2 to 6 weeks, sometimes as long as 6 months, depending upon site of wound in relation to richness of nerve supply and distance of nerve path to brain.
Susceptibility and Immunity – Susceptibility to disease is general. Natural immunity in man and animals not known to exist. Active artificial immunity may be developed by use of vaccine.
Prevalence – Occurs throughout the world except in Australia, New Zealand, Hawaii, and other Pacific and Atlantic islands. More prevalent in dogs than in other animals. Incidence in man is low.
Prevention and Control – A dog or other animal that has bitten a person should be isolated and observed for a proper period of time. If rabies is suspected, animal should be killed and brain examined for presence of Negri bodies. If examination is positive, person bitten should be given antirabic vaccination immediately before symptoms appear.
Wound caused by bite or scratch of a suspected animal should be thoroughly cleaned and irrigated with a solution of tincture of green soap or other satisfactory antiseptic. Dogs over six months of age should be vaccinated annually.
Vaccine consists of an emulsion of infected rabbit brain in saline containing 0.25 per cent phenol, and incubated at 37°C. to kill the virus. Recommended treatment consists of daily subcutaneous injections for 14 days. Treatment is generally sufficient to produce an active artificial immunity. Vaccine useless after symptoms appear.
Use of vaccine has been instrumental in greatly reducing the mortality rate. In persons so treated, death rate has dropped to 1 per cent.
Viral Infection # 11. Rubella (German Measles):
A specific, mild virus infection, characterized by fever, a cutaneous eruption sometimes resembling that of measles, sometimes that of scarlet fever, or both. Rubella usually appears without other symptoms, but is almost always accompanied by enlargement of the postauricular, suboccipital, and cervical lymph nodes.
Source of Infection:
Secretions of mouth and nose.
Mode of Transmission:
Directly from person to person; by droplets of infected saliva sprayed into the air during coughing, sneezing, and talking; by articles freshly soiled with discharges of an infected individual.
Incubation Period:
From 10 to 20 days; usually 18 days.
Susceptibility and Immunity:
Susceptibility to disease is general in children. Permanent acquired immunity usually follows recovery from disease. Disease more prevalent in adults than measles.
Prevalence:
Occurs most commonly in children. More prevalent in winter and spring.
Viral Infection # 12. Smallpox (Variola):
Virus capable of passing through most filters, resistant to low temperatures, to 50 per cent glycerol, and to 0.5 per cent phenol; sensitive to heat, being destroyed at 55°C. or over. Cytoplasmic inclusions are characteristic of the infection. These are believed to be masses of virus particles and may be demonstrated in various tissues but are most characteristic in epithelial cells.
Disease Produced – An acute, specific infectious disease characterized by sudden onset, usually with severe chill, with rapidly rising temperature, followed by an eruption passing through papular, vesicular, and pustular stages. Permanent scars frequently remain. Eruption most abundant and earliest on face, next on forearms, wrists, and hands, favouring the limbs more than trunk. Lesions more abundant on shoulders and chest than on loins or abdomen.
Vaccinia and alastrim are milder forms of the disease, presumably caused by the same virus which has become altered in virulence.
Source of Infection – Lesions of mucous membranes and skin of infected persons.
Mode of Transmission – Contact with diseased persons; by articles soiled with discharges from such persons.
Incubation Period – From 7 to 16 days, usually 12 days. Milder types tend to have longer incubation periods.
Susceptibility and Immunity – Susceptibility to infection is universal, but not every individual exposed to virus contracts the disease. Permanent active immunity usually follows recovery from disease. Artificial active immunity may be effective for as long as 20 years or for less than 2 years.
Vaccine-immune serum has been shown to contain agglutinating, precipitating, complement-fixing, and neutralizing antibodies.
Prevalence – Distribution sporadic or endemic and occurs almost everywhere. Frequency greatest in winter; least in summer.
Prevention and Control – Isolation of infected individuals in screened wards. Disinfection of articles soiled with discharges from infected individuals. Thorough cleaning and disinfection of premises.
General vaccination in early childhood. This affords protection for about 5 years. Children on entering school should be revaccinated, and the entire population should be so treated when disease appears in severe form.
Vaccine most generally employed is prepared from calf lymph, obtained by rubbing the virus into the scarified abdomen of calves six months old. After 5 days, the scarified areas are scraped under aseptic conditions. The harvested pulp is mixed with twice its weight of water and passed through a sieve. The emulsion is preserved by the addition of glycerin and phenol to give a final concentration of 50 per cent of the former and 0.5 per cent of the latter. The preserved pulp is stored at 10°C.
Another vaccine that has been extensively employed is prepared by growing the virus in a medium composed of minced chick embryo tissue suspended in Tyrode’s solution. This is a living- tissue medium.
Viral Infection # 13. Yellow Fever:
Particles smaller than most known viruses, measuring only 22 mµ in diameter. Virus readily passes through Seitz, Berkefeld, and Chamberland filters; is readily inactivated by heat and germicides; may be preserved in 50 per cent glycerol for several months; retains its activity in the frozen state for a long time; resists desiccation from frozen state and may remain viable for many months; is inactivated by dilution in physiologic salt solution; and multiplies in tissue culture.
Disease Produced – An acute, specific virus disease characterized by sudden onset, fever, chills, prostration, headache, muscular pain, some destruction of red blood cells, congestion of mucous membranes, black vomit, mild albuminuria, and jaundice. Leucopenia is the rule. Disease of short duration. Fatality among indigenous populations of endemic regions less than 5 per cent; for others may be as high as 40 per cent.
Source of Infection – Blood of infected persons, monkeys, and probably other wild animals.
Mode of Transmission – By the bite of Aedes aegypti and other species of mosquitoes, of which Hemagogus appears to be the most important in South America.
Incubation Period – Usually 3 to 6 days, occasionally longer.
Susceptibility and Immunity – Susceptibility to disease is general. Permanent acquired immunity follows recovery from disease. Active immunity of about 6 years’ duration may be developed by inoculation of a vaccine.
Vaccine prepared by inoculating chick embryos with virus and incubating at 37°C. for 4 days. Embryos harvested and reduced to a pulp in a blender. Juice measured into ampules and desiccated from the frozen state. Ampules filled with nitrogen gas before sealing. Vaccine stored in refrigerator. For use, vaccine reconstituted with saline and injected subcutaneously.
Prevalence – Endemic among human beings and some animals, chiefly in Western and Central Africa. Epidemic among primates in South America except Uruguay and Chile. Still endemic in jungle form in Panama.
Prevention and Control – Control of breeding places of Aedes aegypti and its elimination. Active immunization of persons exposed to infection.
Isolation of infected persons during first 4 days of fever, and protection from mosquitoes. Immunization of exposed population by the use of attenuated virus only feasible method for control of yellow fever. Effective duration about 6 years.