Public Health Importance

Three important pathogens are transmitted to humans by body lice. These are the agents of epidemic typhus, trench fever, and louse-borne relapsing fever. Today, the prevalence and importance of all three of these louse-borne diseases are low compared to times when human body lice were an integral part of human life. However, trench fever has emerged as an opportunistic disease of immunocompromised individuals, including persons who are positive for human immunodeficiency virus (HIV).

Epidemic Typhus

Epidemic typhus is a rickettsial disease caused by infection with Rickettsia prowazekii. It is also known as louse-borne fever, jail fever, and exanthematic typhus. The disease persists in several parts of the world, most notably in Burundi, Democratic Republic of Congo, Ethiopia, Nigeria, Rwanda, and areas of northeastern and central Africa, Russia, Central and South America, and northern China. Epidemic typhus is largely a disease of cool climates, including higher elevations in the tropics. It thrives in conditions of widespread body louse infestations, overcrowding, and poor sanitary conditions. Epidemic typhus apparently was absent from the New World until the 1500s, when the Spanish introduced the disease. One resulting epidemic in 1576-1577 killed 2 million Indians in the Mexican highlands alone.

The vector of R. prowazekii is the human body louse. Lice become infected when they feed on a person with circulating R prowazekii in the blood. Infective rick-ettsiae invade cells that line the louse gut and multiply there, eventually causing the cells to rupture. Liberated rickettsiae either reinvade gut cells or are voided in the louse feces. Other louse tissues typically do not become infected. Because salivary glands and ovaries are not invaded, anterior-station and transovarial transmission do not occur. Infection of susceptible humans occurs via louse feces (posterior-station transmission) when infectious rickettsiae are scratched into the skin in response to louse bites. R. prowazekii can remain viable in dried louse feces for 60 days. Infection by inhalation of dried louse feces or by crushed lice are less frequent means of contracting the disease.

Transmission of R. prowazekii by body lice was first demonstrated by Charles Nicolle, working at the Institut Pasteur in Tunis in 1909. During these studies, Nicolle accidentally became infected with epidemic typhus, from which he fortunately recovered. He was awarded the Nobel prize in 1928 for his groundbreaking work on typhus. Several other typhus workers also were infected with R. prowazekii during laboratory experiments. The American researcher Howard T. Ricketts, working in Mexico, and Czech scientist Stanislaus von Prowazek, working in Europe, both died from their infections and were recognized posthumously when the etiologic agent was named.

Infection with R. prowazekii is ultimately fatal to body lice as progressively more and more infected gut cells are ruptured. Infective rickettsiae are first excreted in louse feces 3—5 days after the infective blood meal. Lice usually succumb to infection 7—14 days after the infectious blood meal, although some may survive to 20 days.

The disease caused by infection with R. prowazekii and transmitted by body lice is called classic epidemic typhus because it was the first form of the disease to be recognized. Disease onset occurs relatively soon after infection by a body louse in classic epidemic typhus. Symptoms generally appear after an incubation period of 10—14 days. Abrupt onset of fever, accompanied by malaise, muscle and head aches, cough, and general weakness, usually occurs at this time. A blotchy rash spreads from the abdomen to the chest and then often across most of the body, typically within 4—7 days following the initial symptoms. The rash rarely spreads to the face, palms, and soles, and then only in severe cases. Headache, rash, prostration, and delirium intensify as the infection progresses. Coma and very low blood pressure often signal fatal cases. A case fatality rate of 10—20% is characteristic of most untreated epidemics, although figures approaching 50% have been recorded. Diagnosis of epidemic typhus involves the demonstration of positive serology, usually by microimmunofluorescence. DNA primers specific to R. prowazekii can also be amplified by polymerase chain reaction from infected persons or lice. One-time antibreak biotic treatment, especially with doxycycline, tetracycline, or chloramphenicol, usually results in rapid and complete recovery. Vaccines are available but are not considered to be sufficiently effective for widespread use.

Persons that recover from epidemic typhus typically harbor R. prowazekii in lymph nodes or other tissues for months or years. This enables the pathogen to again invade other body tissues to cause disease seemingly at any time. This form of the disease is called recrudescent typhus or Brill—Zinsser disease. The latter name recognizes two pioneers in the study of epidemic typhus: Nathan Brill, who first recognized and described recrudescent typhus in 1910, and Hans Zinsser, who demonstrated in 1934 that it is a form of epidemic typhus. Zinsser's (1935) book Rats, Lice, and History is a pioneering account of the study of epidemic typhus in general.

Recrudescent typhus was widespread during the 19th and early 20th centuries in some of the larger cities along the east coast of the United States (e.g., Boston, New York, and Philadelphia). At that time, immigrants from regions that were rampant with epidemic typhus, such as eastern Europe, presented with Brill—Zinsser disease after being infected initially in their country of origin. Some of these patients experienced relapses more than 30 years after their initial exposure, with no overt signs of infection with R. prowazekii between the two disease episodes. Because infestation with body lice was still a relatively common occurrence during that period, the lice further disseminated the infection to other humans, causing local outbreaks. The last outbreak of epidemic typhus in North America occurred in Philadelphia in 1877. Today, even recrudescent typhus is a rare occurrence in North America. However, this form of typhus is still common in parts of Africa, Asia, South America, and, occasionally, in eastern Europe.

The southern flying squirrel (Glaucomys volans) has been identified as a reservoir of R. prowazekii in the United States, where it has been found to be infected in Virginia during vertebrate serosurveys for Rocky Mountain spotted fever. Since the initial isolations from flying squirrels in 1963, R. prowazekii has been recorded in flying squirrels and their ectoparasites in several states, especially eastern and southern states. Peak seroprevalence (about 90%) in the squirrels occurs during late autumn and winter, when fleas and sucking lice are also most abundant on these hosts. Although several ectoparasites can imbibe R. prowazekii when feeding on infected flying squirrels, only the sucking louse Neohaematopinus sciuropteri (Fig. 4.7C) is known to maintain the infection and transmit the pathogen to uninfected squirrels.

Several cases of human infection have been documented in which the patients recalled having contact with flying squirrels, especially during the winter months when these rodents commonly occupy attics of houses. To distinguish this form of the disease from classic and recrudescent typhus, it is called sporadic epidemic typhus or syl-vatic epidemic typhus. Many details, such as the prevalence and mode of human infection, remain unresolved. Because the louse N. sciuropteri does not feed on humans, it is speculated that human disease occurs when infectious, aerosolized particles of infected louse feces are inhaled from attics or other sites occupied by infected flying squirrels.

Except for flying squirrels in North America, humans are the only proven reservoirs of R. prowazekii. Widespread reports published in the 1950s to 1970s that various species of ticks and livestock animals harbored R. prowazekii have since been disproved.

Historically, epidemic typhus has been the most widespread and devastating of the louse-borne diseases. Zinsser (1935) and Snyder (1966) have documented the history of this disease and highlighted how major epidemics have influenced human history. For example, the great outbreak of disease at Athens in 430 BC, which significantly influenced the course of Greek history, appears to have been caused by epidemic typhus. Napoleon's vast army of 1812 was defeated more by epidemic typhus than by opposing Russian forces. Soon thereafter (ca. 1816— 1819), 700,000 cases of epidemic typhus occurred in Ireland. Combined with the potato famine of that period, this encouraged many people to emigrate to North America; some of these people carried infected lice or latent infections with them. During World War II, several military operations in North Africa and the Mediterranean region were hampered by outbreaks of epidemic typhus. One epidemic in Naples in 1943 resulted in over 1400 cases and 200 deaths. This outbreak is particularly noteworthy because it was the first epidemic of the disease to be interrupted by human intervention through widespread application of the insecticide dichlorodiphenyltrichloroethane (DDT) to louse-infested persons.

Today, epidemic typhus is much less of a health threat than it once was. This is largely because few people, especially in developed countries, are currently infested by body lice. Higher sanitary standards, less overcrowding, regular laundering and frequent changes of clothes, effective pesticides, and medical advances have contributed to the demise of this disease. Nevertheless, epidemic typhus has the potential to re-emerge. This is evidenced by the largest outbreak of epidemic typhus since World War II that affected about half a million people living in refugee camps in Burundi in 1997—1998. Similarly, more than 5600 cases were recorded in China during 1999. Additional information about epidemic typhus is provided by the Pan American Health Organization/World Health Organization (1973), McDade (1987), and Azad (1988).

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