Definition

Smallpox is a highly contagious and virulent, often fatal infectious disease caused by variola virus, a large orthopoxvirus of the family Poxviridae, subfamily Chordopoxvirinae. The 2 classic varieties of smallpox are variola major and variola minor, each of which confers immunity against the other. Variola minor is less virulent and is found mainly in South America and West Africa. Hemorrhagic smallpox is a severe and highly fatal variety of the disease in which hemorrhages develop in the skin and mucous membranes. Flat smallpox is a severe variety in which lesions do not project above the skin surface.

Synonyms

Smallpox is also known as variola or variola vera. Variola minor has many synonyms and local names, such as alastrim, amaas, cottonpox, milkpox, whitepox, and Cuban itch. Smallpox is often subdivided according to clinical presentation into variola vera discreta (discrete papules) and variola vera confluens (confluent papules). Hemorrhagic smallpox, also known as fulminant smallpox, is subdivided clinically into purpura variolosa and variola pustulosa haemorrhagica (4). Malignant smallpox is another name for flat smallpox.

General Considerations

Four species of the genus Orthopoxvirus cause infection in humans: variola virus, vaccinia virus, cowpox virus, and monkeypox virus. Vaccinia virus is essentially a laboratory virus grown in chick embryos and used to vaccinate humans against smallpox (22). Cowpox virus is a virus of rodents that is transmitted to humans by cows or cats (21). In the absence of smallpox, the most important orthopoxvirus that causes human disease is monkeypox (21). Monkeys may transmit the disease to humans (7), but the major reservoirs are several species of forest squirrels. Most cases of monkeypox have been reported in forested regions of Central and West Africa (7). Monkeypox is clinically indistinguishable from smallpox, but there are significant differences between the diseases. Interhuman transmission of monkeypox is relatively poor: approximately 10% in susceptible family contacts, compared to 25% to 40% for smallpox. The mortality rate for monkeypox is substantially lower than for smallpox, and interhuman spread to a fourth generation is rare (5).

Variolation against smallpox began as early as the 10th century through intranasal insufflation of dried crusts of smallpox lesions. In 18th-century Europe, the method consisted of subcutaneous injection of fluid from smallpox pustules or scabs (23). Variolation caused a mild form of smallpox, but prevented significant morbidity and mortality. In England there was a popular concept that dairy maids who had caught cowpox were thereafter immune to smallpox. In 1796, Jenner established the validity of this folkloric concept by his discovery that subcutaneous injection of cowpox virus conferred immunity against smallpox (21,24). In 1803 the term vaccination was coined from the Latin for cow (vacca) (24). Jenner's observations also are consistent with the now recognized importance of cell-mediated immunity in resistance to smallpox. In 1967, the World Health Organization undertook a global program of smallpox vaccination. At that time, 10 to 15 million cases of the disease occurred each year, with more than 2 million deaths. The last case of endemic smallpox was reported in Somalia in 1977. In 1980, endemic smallpox was considered eradicated, vaccination ceased worldwide, and the WHO requested that all smallpox virus stocks be destroyed (6,26). The long-term consequence of eradication is that much of the world's population is now unvaccinated and at risk of infection. In the United States, routine vaccination of the civilian population ended in 1972. The most current statistics indicate that approximately 41% of the resident U.S. population is under the age of 30, most of whom have not been vaccinated against smallpox (47). The immune status of those who were vaccinated 30 or more years ago has not been satisfactorily established, but there is some evidence of residual immunity (3). Reports from the late 19th century indicate that vaccination 20 to 30 years previously may not protect against infection, but will often prevent death (20). There are no conclusive studies on whether people with residual immunity can transmit smallpox to nonvaccinated individuals (36).

Variola major has been endemic in India for at least 2000 years, and has spread to China, Japan, Africa, and the Americas (29). Beginning in the 20th century, the less virulent form of smallpox, variola minor (or alastrim), spread from South Africa to Florida and the Americas, and then to Europe (12). The mortality rate from smallpox in unvaccinated patients is 25% to 50%, and significant morbidity ranges up to 90%. Infectivity begins with the onset of rash. Virions in ulcers of the mouth and throat enter saliva and are released in oropharyngeal aerosols or droplets, the most common route of viral transmission (35). The virus can also be transmitted by contact with an infected patient or with contaminated clothing or linens (12,13,15,40). The virus can cross the placenta, but incidence of congenital smallpox is not high (39,42). Infectivity wanes in 7 to 10 days when scabs form over lesions, binding the virus in fibrin matrix (27). Incidence is highest in winter and early spring (27,34); age distribution depends on acquired immunity. Vaccination immunity declines over time and is probably lost in all but recently vaccinated populations (22).

Morphologic Description

Smallpox virus particles, or virions, are visible by light microscopy. By cryoelectron microscopy, virions appear as smooth, rounded rectangles and measure approximately 302 to 350 nm by 244 to 270 nm (17,38,48). The smallpox genome consists of a single linear double-stranded DNA molecule with a hairpin loop at each end (37), and has 186 kilobase pairs (kbp), which is 4, 6, and 34 kbp fewer than monkeypox, vaccinia, and cowpox, respectively. The DNA genome of variola minor has 2 more HindIII cleavage sites than variola major (22). Virion replication is cytoplasmic and uses viral-associated DNA-dependent RNA polymerase. Viral envelopes are made of modified Golgi membranes containing viral-specific polypeptides, including hemagglutinin. Both enveloped and nonenveloped virions are infectious (22,37).

The evolution of the ordinary variety of smallpox may vary from patient to patient, depending on immune status and severity of disease. The following is an outline of the general course of disease (13,42).

Incubation Period

The incubation period lasts approximately 12 to 14 days (range: 7-21 days). During this time, virions multiply in the reticuloendothelial system. Once inhaled, variola virus invades the oropharyngeal or respiratory mucosa, migrates to regional lymph nodes, and begins to multiply. A few days later, the virus enters the bloodstream and begins a second wave of multiplication in the spleen, bone marrow, and lymph nodes. Finally, the virus reenters the blood in leukocytes, producing fever and toxemia, and then passes from leukocytes to adjacent cells in small blood vessels of the dermis and beneath the oropharyngeal mucosa.

Evolution of Rash

Preeruptive Stage

The preeruptive syndrome lasts approximately 2 to 6 days and is so typical of smallpox, it is a must for diagnosis. It is characterized by sudden onset of high fever (102º F to 105º F), prostration, encephalitis, severe headache, backache, pain in the limbs and abdomen, and sometimes vomiting. Virus is most likely to be found in the blood in the first few days of the preeruptive fever, particularly in severe infections (14).

Eruptive Stage

Day 3 or 4 of fever: Minute reddish spots appear on mucous membranes of the mouth, tongue, palate, and pharynx, and temperature falls to near normal. Mucous membranes of the lower trachea, bronchi, and lower digestive tract are affected less frequently (4).

Day 4 to 6: Rash develops on the skin 24 to 48 hours after lesions on the mucous membranes appear. Typically, macules first appear on the forehead, then rapidly spread to the whole face, proximal portions of extremities, the trunk, and lastly to distal portions of extremities. The process takes no more than 24 to 36 hours, after which no new lesions appear.

Day 6 to 8: Lesions on mucous membranes have evolved into papules and vesicles and broken down.16 Because mucous membranes lack a keratinized layer, damaged epithelial cells are shed from these lesions early in the disease, and the patient is most infective at this time (13). Macules on the skin become papules 24 to 48 hours after forming. Papules fill with an opalescent fluid to become vesicles in another 48 hours (figs. 1, 2, 3).

Day 8 to 10: Fluid in the vesicles becomes opaque, giving them the appearance of pustules; however, the so-called pustules are filled with tissue debris, not pus. There is some recurrence of fever during pustulation (figs. 4 and 5).

Day 10 to 14: Pustules mature, increase in size, and become umbilicated. Crusting begins and temperature again returns to near normal (figs. 6, 7, 8, 9, 10).

Day 16 to 20: Scab formation is complete (fig. 10).

Day 21 to 28: With the exception of those embedded in the palms of the hands and soles of the feet, most crusts have separated, leaving depigmented scars.

Hemorrhagic smallpox produces a dusky erythema, petechiae, and hemorrhages in the spleen, kidney, serosa, muscle, and, rarely, the epicardium, hepatic capsule, testes, and urinary bladder (4). Patients in the early stage of disease show a decrease in platelets, prothrombin, and accelerator globulin, and an increase in circulating antithrombin. Patients in the late stage have significant thrombocytopenia; however, deficiency of coagulation factors is less severe. Some in the late stage also show increased antithrombin (33,34). Hemorrhagic smallpox leads to certain death 5 or 6 days following the onset of rash. In the often fatal flat, or malignant, form of smallpox, confluent vesicles develop slowly, remain flat and velvety to the touch, appear fine-grained, and may cause large areas of the epidermis to slough (figs. 11 and 12) (41).

In variola minor, rash erupts more slowly, with smaller vesicles and rare umbilication or scarring. Mortality is 0.5% to 1% (11,42).

Complications and Sequelae

Complications of smallpox arise most commonly in the respiratory system and range from simple bronchitis to fatal pneumonia. Other complications include encephalitis (1 in 500 patients), which is more common in adults and may cause temporary disability; permanent pitted scars, most notably on the face; and ocular complications (2% of all cases). Pustules can form on the eyelid, conjunctiva, and cornea, leading to complications such as conjunctivitis, keratitis, corneal ulcer, iritis, iridocyclitis, and optic atropy. Blindness results in approximately 35% to 40% of eyes affected with keratitis and corneal ulcer. Hemorrhagic smallpox can cause subconjunctival and retinal hemorrhages (19,42). In 2% to 5% of young children with smallpox, virions reach the joints and bone, causing osteomyelitis variolosa. Lesions are symmetrical, most common in the elbows, tibia, and fibula, and characteristically cause separation of an epiphysis and marked periosteal reactions. Swollen joints limit movement, and arthritis may lead to ankylosis, malformed bones, flail joints, and stubby fingers (10,42).

The following survey of the histopathologic features of smallpox is based on observation of specimens submitted to the AFIP, and on several comprehensive review articles (4,9,31,49).

Skin

Capillaries in the papillary dermis dilate, the lining endothelium swells, and lymphocytes and histiocytes infiltrate the epidermis. In the middle layer of the overlying epidermis, cells enlarge, vacuolate, and degenerate (figs. 13, 14, 15, 16, 17, 18). Affected cells contain round or oval inclusion bodies, called Guarnieri's bodies, that measure 2 to 8 µm. Guarnieri's bodies are slightly basophilic or acidophilic and lie close to the nuclei in cell cytoplasm (figs. 18, 19, 20). Guarnieri's bodies are composed of viral particles and proteins; each body is the locus of viral replication and assembly. Later, viral inclusions occupy large portions of cytoplasm, especially in the base of a vesicle or pustule.

Cells proliferate in the lower epidermis and may lose normal orientation, condense, detach, hyalinize, and form the base of a vesicle. Cellular degeneration spreads in the middle layer of the epidermis. Epithelial cells that ring a vesicle proliferate, thickening the epithelium (fig. 16). The vesicle's center forms as membranes of dying cells rupture and edema increases. Septa from incompletely destroyed cells form loculations (fig. 15). Neutrophils enter vesicles and septa disappear, producing a pustule. Pustular fluid is eventually absorbed, the contents dry, and epithelial cells surrounding the pustule grow under the exudate (figs. 21 and 22). Cells flatten, are parakeratotic, and link with the adjacent unaffected keratohyalin layer. The remaining crust sheds.