6-week-old male BALB/c mouse (Mus musculus)The mouse was infected by intracerebral inoculation with a lethal dose of rabies virus (genotype 1).

Gross Description:  

No gross lesions were observed.

Histopathologic Description:

Cerebrum: Within the gray matter of the cortex, thalamus, hippocampus and hypothalamus regions (these sections were submitted to the confrence) there is low to moderate numbers of neurons with shrunken cell bodies, and hypereosinophilic cytoplasm frequently accompanied by karryorhexis or karyolysis (neuronal degeneration and necrosis). Occasionally in these cells, but significantly more often in normal appearing neurons, especially within the hippocampus, there are one to several intracytoplasmic, pale eosinophilic inclusion bodies up to 7 μm in diameter (Negri bodies). Some affected neurons are surrounded by lymphocytes and glial cells (neuronophagia). Focally distributed throughout the neuropil, aggregates of glial cells and lymphocytes are present (glial nodules). Perivascularly, there is a slight lymphocytic and histiocytic infiltration (perivascular cuffing) and few lymphocytes and histiocytes are found in submeningeal regions. In some sections, ventricles contain low amounts of lymphocytes and histiocytes.

Morphologic Diagnosis:  

Encephalitis, nonsuppurative, minimal, with neuronal degeneration and necrosis and neuronal eosinophilic intracytoplasmic inclusion bodies, mouse, etiology consistent with rabies virus infection.

Lab Results:  

Further laboratory investigations were not performed.



Contributor Comment:  

Rabies virus (RV) belongs to the genus Lyssavirus in the Rhabdoviridae family of the order Mononegavirales. The characterization of the rabies virus isolates based on monoclonal antibodies grouped the rabies-associated viruses into four serogroups, with classical rabies forming serogroup 1, the African lyssaviruses forming serogroup 2 (Lagos bat virus), serogroup 3 (Mokola virus), and serogroup 4 (Duvenhage virus). DNA sequencing was used to further distinguish the members of each serogroup, which in turn became genotypes (see table).(21) The European bat lyssaviruses were classified into two groups (EBLV-1 and EBLV-2) and designated genotypes 5 and 6.(14) A further genotype has been identified in Australia, Australian bat lyssavirus.(11)

The genome of RV comprises five genes encoding the viral proteins nucleoprotein, phosphoprotein, matrix protein, glycoprotein and the viral RNA polymerase. The replication of RV proceeds in the presence of the host immune response, including the antiviral type I interferon (IFN) system. RV is known for its IFN-sensitivity and therefore must encode mechanisms that prevent IFN expression.5 It is hypothesized that the viral phosphoprotein is involved in this mechanism.(1)

Rabies occurs in more than 150 countries and territories worldwide. Dogs are the source of infection in all of the estimated 55, 000 human rabies deaths annually in Asia and Africa. Bats are the source of most human rabies deaths in the United States of America and Canada. Bat rabies has also recently emerged as a public health threat in Australia, Latin America and Western Europe. Infections due to contact to rabid foxes, raccoons, skunks, jackals, mongooses and other wild carnivore are very rare. Human-to-human transmission by bite has never been confirmed.(23)

RV is the causative agent of the classical rabies and is responsible for the vast majority of all human rabies cases. The transmission occurs via the bite of rabid animals shedding virus within their saliva, or direct contact of infectious material (i.e. saliva, cerebrospinal liquid, nerve tissue) to mucous membranes or skin lesions. The virus cannot penetrate intact skin. Rabies virus replicates in striated muscle cells or infects nerve cells, followed by a retrograde transport of virions to the central nervous system (CNS). Within the CNS, virus replication induces pathologic effects on nerve cell physiology. The anterograde transport of virus to secretory tissues of salivary glands leads to viral shedding via saliva.(24)

In animals, RV infection brings out neurological signs which might differ slightly depending on the species and the time point infected. The incubation period for rabies is typically 13 months, but may vary from <1 week to >1 year.(23) After this so-called prodromal stage, the onset of clinical symptoms follows, which lasts for about 1-3 days with aggressiveness, daytime activities in nocturnal animals or abnormalities in appetite. Potentially, the prodromal stage is followed by the excitative phase with severe agitation and aggressiveness. Further, in the paralytic phase, the infected animals are unable to swallow, leading to a typical sign of foaming saliva around the mouth. A complete paralysis occurs and is followed by death.(24)

Many neurotropic viruses use apoptosis as a mechanism of neuropathogenicity.(16) The induction of apoptosis by rabies virus has been a controversial topic, but increasing evidence indicates that pathogenic rabies virus strains do not induce apoptosis.(8)

In natural rabies, pyknotic chromatolytic neurons are seen throughout the CNS most frequently in the brainstem, and periaqueductal gray matter, cervical spinal cord, thalamus and less frequently the cerebral cortex.(4) In general, these degenerated neurons do not harbor Negri bodies but are positive for viral antigen and may be accompanied by inflammatory reaction.(6) As in other viral encephalitides, neuronophagia and glial nodules are seen often in areas where neuronal degeneration and inflammatory cell infiltration are conspicuous. Topographic dissociation between location of inflammatory reactions and of Negri body has been reported in an autopsy study of 49 cases. Inflammatory changes were most frequently found in medulla, pons and spinal cord followed by thalamus and less frequently in cerebellum and hippocampus. Negri body bearing neurons were rare where inflammation was dense.(10) Perivascular infiltrates are composed of lymphocytes and monocytes intermingled with small numbers of granulocytes and plasma cells, depending on the stage and severity of infection.(7)

Two different mechanisms have been proposed for the transport of rabies virus through the axon to the cell body: transport of either the rabies virus capsid alone or transport of the whole virion. A recent study showed that the whole virion is transported to the cell body in an endosomal vesicle, although the exact mechanism by which this occurs remains unclear.(12) The rabies virus glycoprotein mediates the entry of the virion into the vesicle. However, as the rabies virus glycoprotein is inside the vesicle, it should not be able to interact directly with a specific transporter complex. Based on these observations, it is speculated that entry into the vesicle determines the direction and provides the driving force of rabies virus transport. This indicates that the nature of the vesicle formed dictates the transport method that follows.(19)

In cases of human rabies, the patients die with absence of protective antibodies. The immune privileged status of the CNS and the bloodbrain barrier might explain the delayed development of a protective immune response and the poor survival rate.(10) Remarkably, there is one report of a human survivor of rabies. The patient had detectable antibody and the treatment applied was aimed to allow the humoral immune response to develop. High IgG titres developed and might have contributed to the survival of this patient.(24)

The influence of the immune privileged status of the CNS on the antibody response is controversial. In contrast to RV, Herpes viruses and Borna disease virus can infect the brain, but are effectively controlled by the immune system. Several other factors are discussed, as for example that the infectious dose transmitted with a bite is too small to trigger immune responses and further enable the virus to infect sensory nerves. A final explanation for the lack of antibody could result from immunosuppression induced by the virus. It is known that the viral phosphoprotein inhibits interferon response. However, RV infection is associated with the increase in gene transcripts for interferon-inducible genes. It is speculated that interferon inhibition is transitory and provides a short delay in the host response.(10)

Table: Current classification of the Lyssavirus genus, host species and geographical distribution.(11)
Virus and Genotype (I-VII) Hosta Geographical distribution
Classical rabies virus (RABV), I Numerous chiropteran and carnivoran species Worldwide
Lagos bat virus (LBV), II Epomophorus wahlbergi (Wahleberg's epauletted fruit bat) Africa
Mokola virus (MOKV), III Unknown Africa
Duvenhage virus (DUVV), IV Miniopterus species (?) Southern Africa
European bat lyssavirus type 1 (EBLV- 1), V Eptesicus serotinus (Serotine bat) Europe
European bat lyssavirus type 2 (EBLV-2), VI Myotis daubentonii (Daubenton's bat) Western Europe
Australian bat lyssavirus (ABLV), VII Australian mega- and microchiropteran species Australia
Aravan Myotis blythi (lesser mouse-eared bat)b Kyrgistan
Khujand Myotis mystacinus (whiskered bat)b Kyrgistan
Irkut Murina leucogaster (greater tube-nosed bat)b Eastern Siberia
West Caucasian bat virus (WCBV) Miniopterus schreibersi (common bent-winged bat)b Western Caucasus Mountains
a Species from which the virus is predominantly isolated.
b Single isolations have been made.

JPC Diagnosis:  

Brain, cerebrum: Neuronal necrosis, with minimal lymphoplasmacytic perivasculitis and numerous neuronal intracytoplasmic viral inclusion (Negri) bodies.

Conference Comment:  

The contributor provided a very thorough overview of rabies virus. Following invasion and replication in rhabdomyocytes, virions enter the extracellular space of the neuromuscular junction and neurotendinal sensory stretch receptors, and enter the neurons via the acetylcholine receptor at the neural synapses. Once in the central nervous system, the virus concentrates in the limbic system and generally spares the neocortex, which is the cause of the furious stage of infection. Once in the central nervous system, virions can bud from the neuronal cell body and process plasma membranes and directly infect neighboring neurons. Negri bodies are found in greatest concentration in large neurons, such as in the pyramidal cells of the hippocampus, ganglionic neurons of the pontine nuclei, and cerebellar Purkinje cells. Negri bodies represent the accumulation of rabies virus nucleocapsids in cells due to the defective assembly of virions. Negri bodies are found in large numbers in salivary epithelial cells of carnivores, and the virus can be detected in chromaffin cells of the adrenal medulla, basal epithelial cells of the nasal mucosa, cornea, epidermis, and external root sheath of hair follicles. Infection of these sites is by peripheral nerves(2).

Australian bat lyssavirus, mentioned by the contributor, is a virus closely related to rabies that was discovered in Australia in 1996. It occurs in various species of flying foxes and occasionally in other species of bats and is responsible for a few human fatalities. Lesions are similar to rabies and consist of nonsuppurative meningoencephalomyelitis and ganglioneuritis with Negri bodies(6).

Pseudo-Negri bodies, which may be confused with rabies and are a reason that rabies diagnosis should not be based solely on the presence of Negri bodies, are found in cats, skunks, and dogs and are nonspecific, 1.5 um, homogenous inclusions in the pyramidal cells of the hippocampus. They present in cats as nonspecific inclusions in the lateral geniculate neurons; in dogs as cytoplasmic lamellar bodies in the thalamic neurons and Purkinje cells; in aged sheep and cattle as nonspecific 1 um, brightly eosinophilic, angulated inclusions in the large neurons of the medulla and spinal cord; in Japanese brown beef cattle; in mice as hippocampal inclusions; and in woodchucks as inclusions in the brainstem(3, 15, 17, 18, 20, 22).


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3-1. Brain, hippocampus

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