Pathogen Safety Data Sheets: Infectious Substances – Variola virus



NAME: Variola virus(Footnote 1,Footnote 2).

SYNONYM OR CROSS REFERENCE: Smallpox, VARV, and variola minor virus strains (alastrim, amass, or kaffir viruses)(Footnote 1,Footnote 2,Footnote 3).

CHARACTERISTICS: A member of the family Poxviridae , subfamily Chordopoxvirinae , genus Orthopoxvirus. Virions are shaped like bricks on electron micrographs and measure approximately 300 x 250 x 200 nm(Footnote 2,Footnote 4). Orthopoxviruses have an outside envelope and a second membrane underneath. Instead of a capsid, poxviruses have a nucleosome which contains DNA, and is surrounded by its own membrane(Footnote 5). They contain single, linear, double-stranded DNA molecules of 130 to 375 kb pairs and replicate in the cell cytoplasm(Footnote 4). Variola virus is the most complex of the orthopoxvirus genus, having many different strains(Footnote 6,Footnote 7).


PATHOGENICITY/TOXICITY: Smallpox is an acute, contagious disease with two main forms, variola major and variola minor, both of which cause similar lesions(Footnote 1). There are 4 types of variola major infection presentations.

Ordinary variola major: The most common form, which accounts for more than 90% of cases and has a fatality rate of approximately 30% among unvaccinated individuals, and 3% for vaccinated individuals(Footnote 8). The cause of death is usually bronchopneumonia, although in about 3% of cases, fatal haemorrhages occur. The prodromal phase consists of sudden onset of influenza-like symptoms, characterized by fever, malaise, headache, prostration, severe back pain, and sometimes abdominal pain and vomiting. Two to 3 days later, the temperature falls and the patient feels somewhat better, at which time a characteristic rash appears, first on the face (starting as small red spots) then on the tongue, mouth, nose, and hands. After a few days, the rash progresses to the trunk where fewer lesions occur. Lesions in the mucous membranes of the nose and mouth ulcerate quickly, releasing large amounts of virus into the mouth and throat. Lesions progress from macules to papules to vesicles to pustules, and at 8 to14 days, the pustules form scabs which leave depressed depigmented scars upon healing. All lesions in a given area progress through these stages together(Footnote 1,Footnote 8).

The milder or "modified" variola major: Accounts for 2% of cases in unvaccinated persons and for 25% in previously vaccinated persons. Cases are rarely fatal with fewer, smaller, and more superficial lesions than those in patients with the ordinary type(Footnote 4).

Haemorrhagic variola major: A rare form of variola major which is always fatal and involves haemorrhages in the mucous membranes and the skin(Footnote 1,Footnote 4).

Flat variola major: Another rare form of variola major that is almost always fatal and is characterized by lesions that do not develop to the pustular stage, but remain soft and flat(Footnote 2,Footnote 4).

Variola minor: The other main form of smallpox (also known as alastrim), which is a milder illness with a fatality rate of less than 1%(Footnote 1).

Another type of smallpox, variola sine eruptione , occurs in previously vaccinated contacts or in infants with maternal antibodies. Affected persons are asymptomatic or have a brief rise in temperature, headache, and present influenza-like symptoms. The transmission of clinical smallpox has not been documented with variola sine eruptione(Footnote 4). Overall, 65% to 80% of smallpox survivors have pockmarks, mostly on the face(Footnote 1).

EPIDEMIOLOGY: As a result of a successful worldwide vaccination program, smallpox was eradicated, with the last natural infection occurring in Somalia in 1977(Footnote 1,Footnote 8).

HOST RANGE: Humans(Footnote 1). Monkeys are also susceptible to infection(Footnote 8).

INFECTIOUS DOSE: Viruses in an aerosol suspension can spread widely, and infect at a very low dose (10 to 100 organisms)(Footnote 8,Footnote 9,Footnote 10).

MODE OF TRANSMISSION: Transmission occurs via respiratory droplets (primary route of transmission), or via fine-particle aerosol, or skin inoculation. The conjunctiva or placenta may be occasional portals of entry(Footnote 1). Respiratory droplets (i.e., coughing, sputum, and saliva) have a range of likely no more than 2 metres and are, therefore, a threat only to persons in the immediate vicinity of the affected patient(Footnote 11).

INCUBATION PERIOD: Can range from 7-19 days(Footnote 12). Typically, onset of illness occurs after 10-14 days and 2-4 more days for onset of rash to occur(Footnote 4,Footnote 9,Footnote 12).

COMMUNICABILITY: The highest risk of transmission is during the appearance of the earliest lesions; however, the period of communicability is from the development of the earliest lesions to the disappearance of all scabs (about a 3 week period)(Footnote 1). Some infected people shed the virus without ever showing signs of illness(Footnote 5).


RESERVOIR: No known animal or environmental reservoir. Currently, the virus isolates are maintained only in World Health Organization (WHO) designated laboratories(Footnote 1).




DRUG SUSCEPTIBILITY: The acyclic nucleoside phosphonate analogue cidofovir has been shown to have activity against variola virus in cell culture, and in animal models, including animals where therapy was delayed until the first signs of smallpox(Footnote 13,Footnote 14).

SUSCEPTIBILITY TO DISINFECTANTS: Inactivation can be achieved using apolar lipophilic solvents (chloroform) and quaternary ammonia compounds(Footnote 15). Other disinfectants used for standard hospital infection control such as 0.5% or 200 ppm hypochlorite, 40% ethanol, 30% isopropyl alcohol, formalin, formaldehyde, phenol, cresol, surface-active detergents (e.g. 100 ppm benzalkonium chloride), chlorobenzene mixtures, and 75 ppm iodophor may be effective(Footnote 15,Footnote 16,Footnote 17,Footnote 18,Footnote 19).

PHYSICAL INACTIVATION: Inactivated by heat (incineration and autoclaving)(Footnote 18).

SURVIVAL OUTSIDE HOST: The virus can be propagated in a monkey kidney cell line(Footnote 17). Specimens of blood, scrapings from skin lesions, saliva, pustular fluid, and crusts can be transported and stored for short periods without refrigeration. Materials from smallpox patients (dried fluid and crusts) containing virus remain infectious at room temperature for approximately 1 year. The infectivity of the virus is maintained at 4°C for several months and at -20 to -70°C for years(Footnote 15). Based on the behaviour of vaccinia virus, it is believed that aerosolized variola can retain its infectivity for up to 24 hours if not exposed to UV light, and if temperatures are cool (10°C to 11°C) and humidity low (20%). Variola can be almost completely destroyed within 6 hours in an atmosphere of high temperature (31°C to 33°C) and humidity (80%). At cooler temperatures (26°C), variola virus has survived for 8 weeks at high relative humidity and 12 weeks at a relative humidity less than 10%. Virus has been isolated from scabs that had been sitting on a shelf for 13 years(Footnote 9). It was also found to be viable in bread, salad, sausages and gauze bandages stored at 4°C for up to 2 weeks, and in storm water kept at 4.5 °C for up to 166 days(Footnote 5). Samples of freeze-dried virus in a laboratory have been revived after storage for 20 years(Footnote 20).


SURVEILLANCE: In the past, smallpox was sometimes confused with chickenpox. The centrifugal distribution of lesions, mostly on the face and extremities than on the trunk, is a distinctive diagnostic feature of smallpox which serves to distinguish it from chickenpox, which is characterized by much more superficial lesions that are concentrated mostly on the trunk, as opposed to the face and extremities(Footnote 1,Footnote 8).

There are several methods for confirming the diagnosis. Some are specific for variola virus, and others are for orthopoxviruses in general. Haemadsorption with susceptible chicken erythrocytes is an early detection method for infection with smallpox virus. Giemsa-stained smears of material from skin lesions may show Guarnieri inclusion bodies. The soluble antigens in blood, vesicle fluid, pustule fluid, and saline extracts from crusts or scrapings in certain stages of disease can be detected via complement fixation, haemagglutination inhibition, immunofluorescence, and Ouchterlony techniques. Serologic response is variable in partially immune patients, who may present clinically with variola sine eruptione(Footnote 15). Specimens such as vesicular or pustular fluids or scabs can be examined directly for the presence of virions by electron microscopy, and viral antigen can be identified by immunohistochemical studies. Isolation of the virus in live-cell cultures, followed by PCR, or growth on chorioallantois, is confirmatory; however, PCR diagnostic techniques are more accurate(Footnote 4). The results of serologic testing do not differentiate among orthopoxvirus species, and paired serum samples are required to distinguish recent infection from vaccination in the remote past. Newer methods, that detect IgM responses, may enhance the sensitivity and specificity of serological test(Footnote 4). Other diagnostic methods such as immunodiffusion technique, ELISA, restriction fragment-length polymorphisms, and in situ hybridization have been suggested by various laboratories for confirming the presence of Variola(Footnote 1,Footnote 8,Footnote 9,Footnote 21).

Note: All diagnostic methods are not necessarily available in all countries.

FIRST AID/TREATMENT: If possible, a suspected case of smallpox should be managed in a negative-pressured room, and the patient should be vaccinated, particularly in an early stage of illness. Strict respiratory and contact isolation is imperative. When there are many patients, an isolation hospital or other facility should be designated. Penicillinase-resistance antimicrobial agents should be used if smallpox lesions are secondarily infected, if bacterial infection endangers the eyes, or if the eruption is very dense and widespread. Daily eye rinsing is required in severe cases. Patients need adequate hydration and nutrition. Topical idoxuridine should be considered for the treatment of corneal lesions, although its efficacy is unproved for smallpox(Footnote 4,Footnote 15). Leukocyte transfer from immuno-compromised persons, and methisazone have been used to treat Vaccinia gangrenosa (or progressive vaccinia), which is characterized by a slowly progressive enlargement and necrosis or gangrene of the skin at the site of smallpox vaccination that can be fatal if treated with non-specific measures only (antibiotics or steroids alone)(Footnote 22).

IMMUNIZATION: The vaccine consists of a live Vaccinia virus , which is a "pox"-type virus related to smallpox. There are significant side effects and risks associated with this vaccine, such as skin complications (eczema vaccinatum) which may occur in people with pre-existing eczema, allergic reactions at site of vaccination, vaccinia gangrenosa (or progressive vaccinia), eye infections (spread of virus from site of vaccination), postvaccinal encephalitis, intrauterine vaccinia, and viremia(Footnote 15). Thus vaccination should be administered only to those exposed to the virus or facing a high probability of exposure(Footnote 4). A successful primary vaccination confers full immunity to smallpox in more than 95% of persons for perhaps 5 to 10 years, and successful revaccination probably provides protection for 10 to 20 years or more(Footnote 4,Footnote 23).

The recently recognised risk of myopericarditis with both first and second generation vaccinia vaccines serves as a reminder that larger-scale studies of newer vaccines are necessary to further define their safety profiles and relative roles in protection against smallpox(Footnote 24). Two attenuated vaccine strains have also been isolated and tested: modified vaccinia Ankara (MVA), which has been effectively used in more than 1900 people with few adverse effects(Footnote 25); and a Japanese strain (LC16m8), which is licensed in Japan, and was safely used on more than 50,000 children in the 1970(Footnote 26).

PROPHYLAXIS: Vaccination very early in the incubation period (within 3 to 4 days of exposure) can markedly attenuate or even prevent clinical manifestations of smallpox. Full protection occurs after successful vaccination(Footnote 4). Vaccination at 4 to 7 days after exposure likely offers some protection from disease or may modify the severity of disease(Footnote 23). Unless directly exposed, the vaccine is not advised for groups of people at greater risk for serious side effects, including pregnant and nursing women, children younger than 12 months of age, immuno-compromised patients, HIV patients, or patients with a history of eczema(Footnote 4,Footnote 23). Vaccinia immune globulin has also been administered (0.6 ml/kg IM) within 3 days of exposure(Footnote 4,Footnote 10).


LABORATORY-ACQUIRED INFECTIONS: Except for a laboratory-associated smallpox death at the University of Birmingham, England, in 1978, no further cases have been identified. All known variola virus stocks are held under security at WHO collaborating centers located at Centers for Disease Control and Prevention, Atlanta, United States, or the State Research Centre of Virology and Biotechnology, Koltsovo, Novosibirsk Region, Russia(Footnote 1,Footnote 4). The possession and use of variola viruses are restricted to the WHO collaborating centers(Footnote 27).

SOURCES/SPECIMENS: Scrapings of skin lesions, scab materials, papular, vesicular, or pustular fluid, crusts, bodily fluids including blood and urine, respiratory secretions, pharyngeal and tonsillar swabbings(Footnote 1,Footnote 4,Footnote 8,Footnote 19,Footnote 27).

PRIMARY HAZARDS: Ingestion, parenteral inoculation, and droplet or aerosol exposure of mucous membranes or broken skin with infectious fluids or tissues(Footnote 27).

SPECIAL HAZARDS: Genetically engineered recombinant vaccinia viruses pose a potential risk to laboratory personnel, through direct contact or contact with clinical materials from infected volunteers or animals(Footnote 8,Footnote 27).


RISK GROUP CLASSIFICATION: Risk Group 4(Footnote 28).

CONTAINMENT REQUIREMENTS: Containment Level 4 facilities, equipment, and operational practices for work involving infectious or potentially infectious materials, animals, or cultures.

PROTECTIVE CLOTHING: Personnel entering the laboratory must remove street clothing, including undergarments, and jewellery, and change into dedicated laboratory clothing and shoes, or don full coverage protective clothing (i.e., completely covering all street clothing). Additional protection may be worn over laboratory clothing when infectious materials are directly handled, such as solid-front gowns with tight fitting wrists, gloves, and respiratory protection. Eye protection must be used where there is a known or potential risk of exposure to splashes(Footnote 29).

OTHER PRECAUTIONS: All activities with infectious material should be conducted in a biological safety cabinet (BSC) in combination with a positive pressure suit, or within a class III BSC line. Centrifugation of infected materials must be carried out in closed containers placed in sealed safety cups, or in rotors that are loaded or unloaded in a biological safety cabinet. The integrity of positive pressure suits must be routinely checked for leaks. The use of needles, syringes, and other sharp objects should be strictly limited. Open wounds, cuts, scratches, and grazes should be covered with waterproof dressings. Additional precautions should be considered with work involving animal activities(Footnote 29).


SPILLS: Allow aerosols to settle and, wearing protective clothing, gently cover spill with paper towels and apply appropriate disinfectant, starting at the perimeter and working towards the centre. Allow sufficient contact time before clean up (30 min)(Footnote 29).

DISPOSAL: Decontaminate all materials for disposal by steam sterilisation, chemical disinfection, and/or incineration(Footnote 29).

STORAGE: In leak-proof containers that are appropriately labelled and locked in a Containment Level 4 laboratory(Footnote 29).


REGULATORY INFORMATION: The import, transport, and use of pathogens in Canada is regulated under many regulatory bodies, including the Public Health Agency of Canada, Health Canada, Canadian Food Inspection Agency, Environment Canada, and Transport Canada. Users are responsible for ensuring they are compliant with all relevant acts, regulations, guidelines, and standards.

UPDATED: September 2010.

PREPARED BY: Pathogen Regulation Directorate, Public Health Agency of Canada.

Although the information, opinions and recommendations contained in this Pathogen Safety Data Sheet are compiled from sources believed to be reliable, we accept no responsibility for the accuracy, sufficiency, or reliability or for any loss or injury resulting from the use of the information. Newly discovered hazards are frequent and this information may not be completely up to date.

Copyright ©
Public Health Agency of Canada, 2010

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