Chapter 42: Rabies, Slow Virus Infections, and Prion Diseases

Many different viruses can invade the central nervous system and cause disease. This chapter discusses rabies, a viral encephalitis feared since antiquity that is still an incurable disease; slow virus infections; and transmissible spongiform encephalopathies—rare neurodegenerative disorders that are caused by unconventional agents called “prions.”

Rabies is an acute infection of the central nervous system that is almost always fatal. The virus is usually transmitted to humans from the bite of a rabid animal. Although the number of human cases is small, rabies is a major public health problem because it is widespread among animal reservoirs.

Properties of the Virus

A. Structure

Rabies virus is a rhabdovirus with morphologic and biochemical properties in common with vesicular stomatitis virus of cattle and several animal, plant, and insect viruses (Table 42-1). The rhabdoviruses are rod- or bullet-shaped particles measuring 75 × 180 nm (Figure 42-1). The particles are surrounded by a membranous envelope with protruding spikes, 10 nm long. The peplomers (spikes) are composed of trimers of the viral glycoprotein. Inside the envelope is a ribonucleocapsid. The genome is single-stranded, negative-sense RNA (12 kb; molecular weight 4.6 × 10⁶). Virions contain an RNA-dependent RNA polymerase. The particles have a buoyant density in CsCl of about 1.19 g/cm³ and a molecular weight of 300–1000 × 10⁶.

B. Classification

The viruses are classified in the family Rhabdoviridae. Rabies viruses belong to the genus Lyssavirus, whereas the vesicular stomatitis-like viruses are members of the genus Vesiculovirus. The rhabdoviruses are very widely distributed in nature, infecting vertebrates, invertebrates, and plants. Rabies is the major medically important rhabdovirus. Many of the animal rhabdoviruses infect insects, but rabies virus does not.

C. Reactions to Physical and Chemical Agents

Rabies virus survives storage at 4°C for weeks and at −70°C for years. It is inactivated by CO₂, so on dry ice it must be stored in glass-sealed vials. Rabies virus is killed rapidly by exposure to ultraviolet radiation or sunlight, by heat (1 hour at 50°C), by lipid solvents (ether, 0.1% sodium deoxycholate), by trypsin, by detergents, and by extremes of pH.

D. Virus Replication

The rhabdovirus replication cycle is shown in Figure 42-2. Rabies virus attaches to cells via its glycoprotein spikes; the nicotinic acetylcholine receptor may serve as a cellular receptor for rabies virus. The single-stranded RNA genome is transcribed by the virion-associated RNA polymerase to five mRNA species. The template for transcription is the genome RNA in the form of ribonucleoprotein (RNP) (encased in N protein and containing the viral transcriptase). The monocistronic mRNAs code for the five virion proteins: nucleocapsid (N), polymerase proteins (L, P), matrix (M), and glycoprotein (G). The genome RNP is a template for complementary positive-sense RNA, which is responsible for the generation of negative-sense progeny RNA. The same viral proteins serve as polymerase for viral RNA replication as well as for transcription. Ongoing translation is required for replication, particularly of viral N and P proteins. The newly replicated genomic RNA associates with the viral transcriptase and nucleoprotein to form RNP cores in the cytoplasm. The particles acquire an envelope by budding through the plasma membrane. The viral matrix protein forms a layer on the inner side of the envelope, whereas the viral glycoprotein is on the outer layer and forms the spikes.

E. Animal Susceptibility and Growth of Virus

Rabies virus has a wide host range. All warm-blooded animals, including humans, can be infected. Susceptibility varies among mammalian species, ranging from very high (foxes, coyotes, wolves) to low (opossums); those with intermediate susceptibility include skunks, raccoons, and bats (Table 42-2). The virus is widely distributed in infected animals, especially in the nervous system, saliva, urine, lymph, milk, and blood. Recovery from infection is rare except in certain bats, where the virus has become peculiarly adapted to the salivary glands. Hematophagous vampire bats may transmit the virus for months without themselves ever showing any signs of disease.

When freshly isolated in the laboratory, the strains are referred to as street virus. Such strains show long and variable incubation periods (usually 21–60 days in dogs) and regularly produce intracytoplasmic inclusion bodies. Serial brain-to-brain passage in rabbits yields a “fixed” virus that no longer multiplies in extraneural tissues. This fixed (or mutant) virus multiplies rapidly, and the incubation period is shortened to 4–6 days. Inclusion bodies are found only with difficulty.

F. Antigenic Properties

There is a single serotype of rabies virus. However, there are strain differences among viruses isolated from different species (raccoons, foxes, skunks, canines, bats) in different geographic areas. These viral strains can be distinguished by epitopes in the nucleoprotein and glycoprotein recognized by monoclonal antibodies as well as by specific nucleotide sequences. There are at least seven antigenic variants found in terrestrial animals and bats.

The G glycoprotein is a major factor in rabies virus neuroinvasiveness and pathogenicity. Avirulent mutants of rabies virus have been selected using certain monoclonal antibodies against the viral glycoprotein. A substitution at amino acid position 333 of the glycoprotein results in loss of virulence, indicating some essential role for that site of the protein in disease pathogenesis.

Purified spikes containing the viral glycoprotein elicit neutralizing antibody in animals. Antiserum prepared against the purified nucleocapsid is used in diagnostic immunofluorescence for rabies.

Pathogenesis and Pathology

Rabies virus multiplies in muscle or connective tissue at the site of inoculation and then enters peripheral nerves at neuromuscular junctions and spreads up the nerves to the central nervous system. However, it is also possible for rabies virus to enter the nervous system directly without local replication. It multiplies in the central nervous system and progressive encephalitis develops. The virus then spreads through peripheral nerves to the salivary glands and other tissues. The organ with the highest titers of virus is the submaxillary salivary gland. Other organs where rabies virus has been found include pancreas, kidney, heart, retina, and cornea. Rabies virus has not been isolated from the blood of infected persons.

Susceptibility to infection and the incubation period may depend on the host’s age, genetic background, and immune status, the viral strain involved, the amount of inoculum, the severity of lacerations, and the distance the virus has to travel from its point of entry to the central nervous system. There is a higher attack rate and shorter incubation period in persons bitten on the face or head; the lowest mortality occurs in those bitten on the legs.

Rabies virus produces a specific eosinophilic cytoplasmic inclusion, the Negri body, in infected nerve cells. Negri bodies are filled with viral nucleocapsids. The presence of such inclusions is pathognomonic of rabies but is not observed in at least 20% of cases. Therefore, the absence of Negri bodies does not rule out rabies as a diagnosis. The importance of Negri bodies in rabies diagnosis has been lessened by the development of the more sensitive fluorescent antibody and reverse transcription-polymerase chain reaction diagnostic tests.

Clinical Findings

Rabies is primarily a disease of lower animals and is spread to humans by bites of rabid animals or by contact with saliva from rabid animals. The disease is an acute, fulminant, fatal encephalitis. The incubation period in humans is typically 1–3 months but may be as short as 1 week or more than a year. It is usually shorter in children than in adults. The clinical spectrum can be divided into three phases: a short prodromal phase, an acute neurologic phase, and coma. The prodrome, lasting 2–10 days, may show any of the following nonspecific symptoms: malaise, anorexia, headache, photophobia, nausea and vomiting, sore throat, and fever. Usually there is an abnormal sensation around the wound site.

During the acute neurologic phase, which lasts 2–7 days, patients show signs of nervous system dysfunction such as nervousness, apprehension, hallucinations, and bizarre behavior. General sympathetic overactivity is observed, including lacrimation, pupillary dilatation, and increased salivation and perspiration. A large fraction of patients will exhibit hydrophobia (fear of water) or aerophobia (fear when feeling a breeze). The act of swallowing precipitates a painful spasm of the throat muscles. This phase is followed by convulsive seizures or coma and death. The major cause of death is cardiorespiratory arrest. Paralytic rabies occurs in about 30% of patients, most frequently in those infected with bat rabies virus. The disease course is slower, with some patients surviving 30 days. Recovery and survival are extremely rare.

Rabies should be considered in any case of encephalitis or myelitis of unknown cause even in the absence of an exposure history, and particularly in a person who has lived or traveled outside the United States. Most cases of rabies in the United States are in individuals with no known exposure. Because of the long incubation period, people may forget a possible exposure incident. People who contract bat rabies often have no recollection of being bitten by a bat.

The usual incubation period in dogs ranges from 3 to 8 weeks, but it may be as short as 10 days. Clinically, the disease in dogs is divided into the same three phases as human rabies.

Laboratory Diagnosis

There are no tests to diagnose rabies infections in humans before the onset of clinical symptoms. Rabies can be diagnosed from euthanized animals by direct fluorescent antibody testing of brain tissue.

A. Rabies Antigens or Nucleic Acids

Tissues infected with rabies virus are currently identified most rapidly and accurately by means of immunofluorescence or immunoperoxidase staining using antirabies monoclonal antibodies. A biopsy specimen is usually taken from the skin of the neck at the hairline. Impression preparations of brain or cornea tissue may be used.

A definitive pathologic diagnosis of rabies can be based on the finding of Negri bodies in the brain or the spinal cord. They are sharply demarcated, more or less spherical, and 2–10 μm in diameter, and they have a distinctive internal structure with basophilic granules in an eosinophilic matrix. Negri bodies contain rabies virus antigens (Figure 42-3). Both Negri bodies and rabies antigen can usually be found in animals or humans infected with rabies, but they are rarely found in bats.

Reverse transcription-polymerase chain reaction testing can be used to amplify parts of a rabies virus genome from fixed or unfixed brain tissue or saliva. Sequencing of amplified products can allow identification of the infecting virus strain.

B. Serology

Serum antibodies to rabies can be detected by immunofluorescence or neutralization tests. Such antibodies develop slowly in infected persons or animals during progression of the disease but promptly after vaccination with cell-derived vaccines. Antibodies in cerebrospinal fluid are produced in rabies-infected individuals but not in response to vaccination.

C. Viral Isolation

Available tissue is inoculated intracerebrally into suckling mice. Infection in mice results in encephalitis and death. The central nervous system of the inoculated animal is examined for Negri bodies and rabies antigen. In specialized laboratories, hamster and mouse cell lines can be inoculated for rapid (2- to 4-day) growth of rabies virus; this is much faster than virus isolation in mice. An isolated virus is identified by fluorescent antibody tests with specific antiserum. Virus isolation takes too long to be useful in making a decision about whether to give vaccine.

D. Animal Observation

All animals considered “rabid or suspected rabid” (Table 42-3) should be sacrificed immediately for laboratory examination of neural tissues. Other animals should be held for observation for 10 days. If they show any signs of encephalitis, rabies, or unusual behavior, they should be killed humanely and the tissues examined in the laboratory. If they appear normal after 10 days, decisions must be made on an individual basis in consultation with public health officials.

Immunity and Prevention

Only one antigenic type of rabies virus is known. More than 99% of infections in humans and other mammals that develop symptoms end fatally. Survival after the onset of rabies symptoms is extremely rare. It is therefore essential that individuals at high risk receive preventive immunization, that the nature and risk of any exposure be evaluated, and that individuals be given postexposure prophylaxis if their exposure is believed to have been dangerous (Table 42-3). Because treatment is of no benefit after the onset of clinical disease, it is essential that postexposure treatment be initiated promptly. Postexposure rabies prophylaxis consists of the immediate and thorough cleansing of all wounds with soap and water, administration of rabies immune globulin, and a vaccination regimen.

A. Pathophysiology of Rabies Prevention by Vaccine

Presumably the virus must be amplified in muscle near the site of inoculation until the concentration of virus is sufficient to accomplish infection of the central nervous system. If immunogenic vaccine or specific antibody can be administered promptly, virus replication can be depressed and virus can be prevented from invading the central nervous system. The action of passively administered antibody is to neutralize some of the inoculated virus and lower the concentration of virus in the body, providing additional time for a vaccine to stimulate active antibody production to prevent entry into the central nervous system. Successful postexposure prophylaxis will therefore prevent the development of clinical rabies.

B. Types of Vaccines

All vaccines for human use contain only inactivated rabies virus. Two vaccines are available in the United States, although a number of others are in use in other countries. Both rabies vaccines available in the United States are equally safe and efficacious.

1. Human diploid cell vaccine (HDCV)—To obtain a rabies virus suspension free from nervous system and foreign proteins, rabies virus is grown in the MRC-5 human diploid cell line. The rabies virus preparation is concentrated by ultrafiltration and inactivated with β-propiolactone. No serious anaphylactic or encephalitic reactions have been reported. This vaccine has been used in the United States since 1980.

2. Purified chick embryo cell vaccine (PCEC)—This vaccine is prepared from the fixed rabies virus strain Flury LEP grown in chicken fibroblasts. It is inactivated with β-propiolactone and further purified by zonal centrifugation. It became available in the United States in 1997.

A recombinant viral vaccine consisting of vaccinia virus carrying the rabies surface glycoprotein gene has successfully immunized animals following oral administration. This vaccine may prove valuable in the immunization of both wildlife reservoir species and domestic animals.

C. Types of Rabies Antibody

1. Rabies immune globulin, human (HRIG)—HRIG is a γ-globulin prepared by cold ethanol fractionation from the plasma of hyperimmunized humans. There are fewer adverse reactions to human rabies immune globulin than to equine antirabies serum.

2. Antirabies serum, equine—This is concentrated serum from horses hyperimmunized with rabies virus. It has been used in countries where HRIG is not available.

D. Preexposure Prophylaxis

This is indicated for persons at high risk of contact with rabies virus (research and diagnostic laboratory workers, spelunkers) or with rabid animals (veterinarians, animal control and wildlife workers). The goal is to attain an antibody level presumed to be protective by means of vaccine administration prior to any exposure. It is recommended that antibody titers of vaccinated individuals be monitored periodically and that boosters be given when required.

E. Postexposure Prophylaxis

Although few (0–5) cases of human rabies occur in the United States per year, more than 20,000 persons receive some treatment every year for possible bite wound exposure. The decision to administer rabies antibody or rabies vaccine—or both—depends on several factors: (1) the nature of the biting animal (species, state of health, domestic or wild) and its vaccination status; (2) the availability of the animal for laboratory examination (all bites by wild animals and bats require rabies immune globulin and vaccine); (3) the existence of rabies in the area; (4) the manner of attack (provoked or unprovoked); (5) the severity of the bite and contamination by saliva of the animal; and (6) advice from local public health officials (Table 42-3). Schedules for postexposure prophylaxis involving the administration of rabies immune globulin and vaccine are available from the Centers for Disease Control and Prevention and state public health offices.

Epidemiology

Rabies is enzootic in both wild and domestic animals. Worldwide, at least 50,000 deaths due to human rabies occur each year; however, rabies is grossly underreported in many countries. Almost all rabies deaths (>99%) occur in developing countries, with Asia accounting for over 90% of all rabies fatalities. In these countries, where canine rabies is still endemic, most human cases develop from bites of rabid dogs. Children aged 5–15 years are at particular risk. An estimated 15 million persons are given postexposure prophylaxis annually, the majority in China and India.

In the United States, Canada, and western Europe, where canine rabies has been controlled, dogs are responsible for very few cases. Rather, human rabies develops from bites of wild animals (especially bats, raccoons, skunks, and foxes) or occurs in travelers bitten by dogs elsewhere in the world. The most serious problem in livestock appears to be vampire bat–transmitted rabies in Latin America. The increase in wildlife rabies in the United States and some other developed countries presents a far greater risk to humans than do dogs or cats.

Primarily as a result of the successful control of rabies in domestic dogs, the incidence of human rabies in the United States declined to fewer than three persons per year during the last two decades.

Antigenic analysis with monoclonal antibodies and genotyping by nucleotide sequence analysis can distinguish rabies virus isolates from different animal reservoirs. From 2000 to 2011, there were 32 diagnosed human rabies cases in the United States, of which more than 95% of domestically acquired cases were proved to be due to bat-associated virus. Eight of nine patients with imported rabies had dog-associated strains.

Raccoons are an important reservoir for rabies in the United States and account for over half of all reported cases of animal rabies. It is believed that raccoon rabies was introduced into the mid-Atlantic region in the 1970s, when infected raccoons were transported there from the southeastern United States to replenish hunting stocks. The raccoon rabies epizootic has spread and now covers the eastern United States into Canada.

Bats present a special problem because they may carry rabies virus while they appear to be healthy, excrete it in saliva, and transmit it to other animals and to humans. Among human rabies cases in the United States attributed to bat-associated variants, the majority were caused by the silver-haired bat and eastern pipistrelle bat variants. However, only two cases were associated with a history of bat bite, as most bat exposures go undetected. Bat caves may contain aerosols of rabies virus and present a risk to spelunkers. Migrating fruit-eating bats exist in many countries and are a source of infection for many animals and humans. Bat rabies may be important in the initiation of terrestrial enzootics in new regions. Australia, long considered to be a rabies-free continent, was found in 1996 to harbor rabies virus in fruit bats. All persons bitten by bats must receive postexposure rabies prophylaxis.

Human-to-human rabies infection is very rare. The only documented cases involve rabies transmitted by corneal and organ transplants. One example involves corneal transplants—the corneas came from donors who died with undiagnosed central nervous system diseases, and the recipients died from rabies 50–80 days later. The first documented case involving solid organ transplants occurred in the United States in 2004. The liver and kidneys from a single donor were transplanted into three recipients, all of whom died of confirmed rabies 5–7 weeks later. Transmission likely occurred via neuronal tissue in the transplanted organs, as rabies virus is not spread in the blood. Theoretically, rabies could originate from the saliva of a patient who has rabies and exposes attending personnel, but such transmission has never been documented.

Treatment and Control

There is no successful treatment for clinical rabies. Interferons, ribavirin, and other drugs have shown no beneficial effects. Symptomatic treatment may prolong life, but the outcome is almost always fatal.

Historically, several key events have contributed to the control of human rabies: the development of a human rabies vaccine (1885), the discovery of the diagnostic Negri body (1903), the use of rabies vaccines for dogs (1940s), the addition of rabies immune globulin to human postexposure vaccination treatments (1954), the growth of rabies virus in cultured cells (1958), and the development of diagnostic fluorescent antibody tests (1959).

Preexposure vaccination is desirable for all persons who are at high risk of contact with rabid animals, such as veterinarians, animal care personnel, certain laboratory workers, and spelunkers. Persons traveling to developing countries where rabies control programs for domestic animals are not optimal should be offered preexposure prophylaxis if they plan to stay for more than 30 days. However, preexposure prophylaxis does not eliminate the need for prompt postexposure prophylaxis if an exposure to rabies occurs.

Isolated countries (eg, Great Britain) that have no indigenous rabies in wild animals can establish quarantine procedures for dogs and other pets to be imported. In countries where dog rabies exists, stray animals should be destroyed and vaccination of pet dogs and cats should be mandatory. In countries where wildlife rabies exists and where contact between domestic animals, pets, and wildlife is inevitable, all domestic animals and pets should be vaccinated.

An oral vaccinia–rabies glycoprotein recombinant virus vaccine (V-RG) proved effective at controlling rabies in foxes in Europe. Added to baits, the oral vaccine is being used to curtail rabies epizootics in wildlife in the United States.

Emerging Rhabdovirus Infections

A small outbreak of viral hemorrhagic fever in central Africa in 2009 was associated with a novel rhabdovirus named Bas-Congo virus. Two patients died and two health care workers survived, indicating potential person-to-person transmission. The probable animal reservoir is unknown and no additional cases have since been identified.

Borna disease, a central nervous system disease primarily of horses and sheep in certain areas of Germany, is manifested by behavioral abnormalities usually ending in death. Inflammatory cell infiltrates are present in the brain. The disorder is immune mediated.

Borna disease virus (BDV) is an enveloped, nonsegmented, negative-stranded RNA virus in the family Bornaviridae (Table 42-4). BDV is novel among nonsegmented, negative-sense RNA viruses in that it transcribes and replicates its genome in the nucleus and uses RNA splicing for regulation of gene expression. BDV is noncytolytic and highly neurotropic; it establishes persistent infections. There is a single recognized serotype of BDV. Titers of neutralizing antibodies produced in host species are usually very low.

Many species can be infected by bornaviruses, including humans. Serologic or reverse transcription-polymerase chain reaction data suggest that BDV may be associated with neuropsychiatric disorders in humans, although those findings are controversial and it remains to be established whether BDV is etiologically involved in the pathophysiology of certain human mental disorders.

Some chronic degenerative diseases of the central nervous system in humans are caused by “slow” or chronic, persistent infections by classic viruses. Among these are subacute sclerosing panencephalitis and progressive multifocal leukoencephalopathy. Other diseases known as transmissible spongiform encephalopathies—such as, Creutzfeldt-Jakob disease (CJD)—are caused by unconventional transmissible agents termed “prions” (Table 42-5). The progressive neurologic diseases produced by these agents may have incubation periods of years before clinical manifestations of the infections become evident (Table 42-5).

Slow Virus Infections

A. Visna

Visna and progressive pneumonia (maedi) viruses are closely related agents that cause slowly developing infections in sheep. These viruses are classified as retroviruses (genus Lentivirus; see Chapter 44).

Visna virus infects all the organs of the body of the infected sheep; however, pathologic changes are confined primarily to the brain, lungs, and reticuloendothelial system. Inflammatory lesions develop in the central nervous system soon after infection, but there is usually a long incubation period (months to years) before observable neurologic symptoms appear. Disease progression can be either rapid (weeks) or slow (years).

Virus can be recovered for the life of the animal, but viral expression is restricted in vivo so that only minimal amounts of infectious virus are present. Antigenic variation occurs during the long-term persistent infections. Many mutations occur in the structural gene that codes for viral envelope glycoproteins. Infected animals develop antibodies to the virus.

B. Subacute Sclerosing Panencephalitis

This is a rare disease of young adults caused by measles virus, with slowly progressive demyelination in the central nervous system ending in death (see Chapter 40). Large numbers of viral nucleocapsid structures are produced in neurons and glial cells. There is restricted expression of the viral genes that encode envelope proteins, so the virus in persistently infected neural cells lacks proteins needed for the production of infectious particles. Patients with subacute sclerosing panencephalitis have high titers of antimeasles antibody except that antibody to the M protein is frequently lacking. Reduced efficiency of measles virus transcription in differentiated brain cells is important in maintaining the persistent infection that leads to subacute sclerosing panencephalitis.

C. Progressive Multifocal Leukoencephalopathy

JC virus, a member of the family Polyomaviridae (see Chapter 43), is the etiologic agent of progressive multifocal leukoencephalopathy, a central nervous system complication that occurs in some immunosuppressed individuals. Once exceedingly rare, the disease can occur in a significant proportion (about 5%) of patients with AIDS; however, as antiviral drugs slow the progression of human immunodeficiency virus infections, fewer patients develop this disease. Progressive multifocal leukoencephalopathy is also a rare complication of some therapeutic monoclonal antibodies for diseases such as multiple sclerosis. Demyelination in the central nervous system of patients with progressive multifocal leukoencephalopathy results from reactivation and replication of JC virus when an immune system is compromised.

Transmissible Spongiform Encephalopathies (Prion Diseases)

Degenerative central nervous system diseases—kuru, Creutzfeldt-Jakob Disease (CJD), Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia of humans, scrapie of sheep, transmissible encephalopathy of mink, bovine spongiform encephalopathy (BSE) of cattle, and chronic wasting disease of deer—have similar pathologic features. These diseases are described as transmissible spongiform encephalopathies. The causative agents are not conventional viruses; infectivity is associated with proteinaceous material devoid of detectable amounts of nucleic acid. The term “prion” is used to designate this novel class of agents.

The different types of prions appear to have common mechanisms of pathogenesis. Species barriers exist for all transmissible spongiform encephalopathies, but some prions have crossed such barriers. These diseases are associated with acquisition of misfolded prion proteins that can cause misfolding and aggregation of normal cellular prion protein expressed in brain tissue.

These agents are unusually resistant to standard means of inactivation. They are resistant to treatment with formaldehyde (3.7%), urea (8 M), dry heat, boiling, ethanol (50%), proteases, deoxycholate (5%), and ionizing radiation. However, they are sensitive to phenol (90%), household bleach, ether, NaOH (2 N), strong detergents (10% sodium dodecyl sulfate), and autoclaving (1 hour, 121°C). Guanidine thiocyanate is highly effective in decontaminating medical supplies and instruments.

There are several distinguishing hallmarks of these prion diseases. Although the etiologic agent may be recoverable from other organs, the diseases are confined to the nervous system. The basic features are neurodegeneration and spongiform changes. Amyloid plaques may be present. Long incubation periods (months to decades) precede the onset of clinical illness and are followed by chronic progressive disease (weeks to years). The diseases are always fatal, with no known cases of remission or recovery. The host shows no inflammatory response and no immune response (the agents do not appear to be antigenic); no production of interferon is elicited; and there is no effect on host B-cell or T-cell function. Immunosuppression of the host has no effect on pathogenesis; however, chronic inflammation induced by other factors (viruses, bacteria, autoimmunity) may affect prion pathogenesis. It has been observed that prions accumulate in organs with chronic lymphocytic inflammation. When coincident with nephritis, prions are excreted in urine.

A. Scrapie

Scrapie shows marked differences in susceptibility of different breeds of animal. Susceptibility to experimentally transmitted scrapie ranges from zero to over 80% in various breeds of sheep, whereas goats are almost 100% susceptible. The transmission of scrapie to mice and hamsters, in which the incubation period is greatly reduced, has facilitated study of the disease.

Infectivity can be recovered from lymphoid tissues early in infection, and high titers of the agent are found in the brain, spinal cord, and eye (the only places where pathologic changes are observed). Prion protein is associated with circulating B cells in scrapie-infected sheep. Infectivity has also been detected in milk from sheep incubating natural scrapie. Maximum titers of infectivity are reached in the brain long before neurologic symptoms appear. The disease is characterized by the development of amyloid plaques in the central nervous system of infected animals. These areas represent extracellular accumulations of protein; they stain with Congo red.

A protease-resistant protein of molecular mass 27–30 kDa can be purified from scrapie-infected brain and is designated prion protein PrP. Preparations containing only PrP and no detectable nucleic acid are infectious. PrP is derived from a larger host-encoded protein, PrP^Sc, that is an altered version of a normal cellular protein (PrP^C). The protein is a glycolipid-anchored membrane protein. The level of PrP^Sc is increased in infected brains because the protein becomes resistant to degradation. Genetic susceptibility to scrapie infection is associated with point mutations in the PrP^C gene, and mice genetically altered to be devoid of PrP^C are resistant to scrapie. A conformational model for prion replication proposes that PrP^Sc forms a heterodimer with PrP^C and refolds it so that it becomes like PrP^Sc. “Strains” of prions are speculated to reflect different conformations of PrP^Sc. In the last few years, several studies have generated synthetic prions in vitro that caused disease when inoculated in vivo, further suggesting that prions are infectious proteins.

B. Kuru and Classic Creutzfeldt-Jakob Disease (CJD)

Two human spongiform encephalopathies are kuru and the classic form of CJD. Brain homogenates from patients have transmitted both diseases to nonhuman primates. Kuru occurred only in the eastern highlands of New Guinea and was spread by customs surrounding ritual cannibalism of dead relatives. Since the practice has ceased, the disease has disappeared. CJD in humans develops gradually, with progressive dementia, ataxia, and myoclonus, and leads to death in 5–12 months. Sporadic CJD is believed to be caused by the spontaneous transformation of normal prion protein into abnormal prions. This occurs with a frequency of approximately one case per million population per year in the United States and Europe and involves patients over 50 years of age. The estimated incidence is less than one case per 200 million for persons under 30 years of age. Sporadic CJD is believed to be caused by the spontaneous transformation of normal prion protein into abnormal prions. However, the new variant form of CJD linked to BSE (see below) has mainly affected people under the age of 30.

Two familial forms of CJD are Gerstmann-Sträussler-Scheinker syndrome and fatal familial insomnia. These diseases are rare (10–15% of CJD cases) and are due to inheritance of mutations in the PrP gene.

Iatrogenic CJD has been transmitted accidentally by contaminated growth hormone preparations from human cadaver pituitary glands, by corneal transplant, by contaminated surgical instruments, and by cadaveric human dura mater grafts used for surgical repair of head injury. It appears that recipients of contaminated dura mater grafts remain at risk of developing CJD for more than 20 years following receipt of grafts. There is currently no suggestion of CJD transmission by blood or blood products, although the potential is there.

C. Bovine Spongiform Encephalopathy (BSE)and New Variant CJD

A disease similar to scrapie, BSE, or “mad cow disease,” emerged in cattle in Great Britain in 1986. This outbreak was traced to the use of cattle feed that contained contaminated bone meal from scrapie-infected sheep and BSE-infected cattle carcasses. The use of such cattle feed was prohibited in 1988. The epidemic of “mad cow disease” peaked in Great Britain in 1993. It is estimated that over 1 million cattle were infected. BSE has also been found in other European countries. In 1996, a new variant form of CJD was recognized in the United Kingdom that occurred in younger people and had distinctive pathologic characteristics similar to those of BSE. It is now accepted that the new variant forms of CJD and BSE are caused by a common agent, indicating that the BSE agent had infected humans. Through 2006, over 150 people had been diagnosed with new variant CJD in England, and most had died. A particular polymorphism in the amino acid sequence of the human prion protein seems to influence susceptibility to disease.

D. Chronic Wasting Disease

A scrapie-like disease, designated chronic wasting disease, is found in mule deer and elk in the United States and Canada. It is laterally transmitted with high efficiency, but there is no evidence that it has been transmitted to humans. Infectivity has been detected in feces of deer before they become ill; the agent is retained in the soil, where it can then be ingested by other deer and elk.

E. Alzheimer Disease

There are some neuropathologic similarities between CJD and Alzheimer disease, including the appearance of amyloid plaques. However, the disease has not been transmitted experimentally to primates or rodents, and the amyloid material in the brains of Alzheimer patients does not contain PrP^Sc protein.

• Rabies is a viral encephalitis that is almost always fatal once symptoms appear. It is caused by an RNA virus classified as a rhabdovirus.

• Humans get infected with rabies by a bite from a rabid animal. The incubation period can range from 1 week to more than a year.

• Most rabies deaths worldwide occur in Asia and result from bites of rabid dogs. In the United States, most human cases are acquired from wild animals.

• Killed rabies vaccines exist for use in humans; live attenuated virus vaccines are available for animal immunization.

• There are no tests to diagnose rabies infections in humans before disease develops. There is no successful treatment for clinical rabies.

• Postexposure prophylaxis consists of administration of rabies antibody, rabies vaccine, or both, following a possible exposure.

• Subacute sclerosing panencephalitis is a rare and fatal central nervous system disease caused by measles virus.

• Progressive multifocal leukoencephalopathy is a rare, usually fatal, central nervous system disease caused by polyomavirus JC virus in immunosuppressed individuals.

• The prion diseases (transmissible spongiform encephalopathies) are caused by unconventional agents with properties of infectious proteins.

• Human prion diseases include kuru, CJD, and variant CJD.

• Prion agents are very resistant to inactivation, including formaldehyde, boiling, and radiation; they can be inactivated by bleach and autoclaving.

• The progressive neurologic diseases may have very long incubation periods, ranging from months to years.