Comparative Pathogenesis, Genomics and Phylogeography of Mousepox

Ectromelia virus (ECTV), the causative agent of mousepox, has threatened laboratory mouse colonies worldwide for almost a century. Mousepox has been valuable for the understanding of poxvirus pathogenesis and immune evasion. Here, we have monitored in parallel the pathogenesis of nine ECTVs in BALB/cJ mice and report the full-length genome sequence of eight novel ECTV isolates or strains, including the first ECTV isolated from a field mouse, ECTV-MouKre. This approach allowed us to identify several genes, absent in strains attenuated through serial passages in culture, that may play a role in virulence and a set of putative genes that may be involved in enhancing viral growth in vitro. We identified a putative strong inhibitor of the host inflammatory response in ECTV-MouKre, an isolate that did not cause local foot swelling and developed a moderate virulence. Most of the ECTVs, except ECTV-Hampstead, encode a truncated version of the P4c protein that impairs the recruitment of virions into the A-type inclusion bodies, and our data suggest that P4c may play a role in viral dissemination and transmission. This is the first comprehensive report that sheds light into the phylogenetic and geographic relationship of the worldwide outbreak dynamics for the ECTV species.

Comparison of Host Gene Expression Profiles in Spleen Tissues of Genetically Susceptible and Resistant Mice during ECTV Infection

Ectromelia virus (ECTV), the causative agent of mousepox, has emerged as a valuable model for investigating the host-Orthopoxvirus relationship as it relates to pathogenesis and the immune response. ECTV is a mouse-specific virus and causes high mortality in susceptible mice strains, including BALB/c and C3H, whereas C57BL/6 and 129 strains are resistant to the disease. To understand the host genetic factors in different mouse strains during the ECTV infection, we carried out a microarray analysis of spleen tissues derived from BALB/c and C57BL/6 mice, respectively, at 3 and 10 days after ECTV infection. Differential Expression of Genes (DEGs) analyses revealed distinct differences in the gene profiles of susceptible and resistant mice. The susceptible BALB/c mice generated more DEGs than the resistant C57BL/6 mice. Additionally, gene ontology and KEGG pathway analysis showed the DEGs of susceptible mice were involved in innate immunity, apoptosis, metabolism, and cancer-related pathways, while the DEGs of resistant mice were largely involved in MAPK signaling and leukocyte transendothelial migration. Furthermore, the BALB/c mice showed a strong induction of interferon-induced genes, which, however, were weaker in the C57BL/6 mice. Collectively, the differential transcriptome profiles of susceptible and resistant mouse strains with ECTV infection will be crucial for further uncovering the molecular mechanisms of the host-Orthopoxvirus interaction.

Interaction between unrelated viruses during in vivo co-infection to limit pathology and immunity

Great progress has been made in understanding immunity to viral infection. However, infection can occur in the context of co-infection by unrelated pathogens that modulate immune responses and/or disease. We have studied immunity and disease during co-infection with two unrelated viruses: Ectromelia virus (ECTV) and Lymphocytic Choriomeningitis virus (LCMV). ECTV infection can be a lethal in mice due in part to the blockade of Type I Interferons (IFN-I). We show that ECTV/LCMV co-infection results in decreased ECTV viral load and amelioration of ECTV-induced disease, likely due to IFN-I induction by LCMV, as rescue is not observed in IFN-I receptor deficient mice. However, immune responses to LCMV in ECTV co-infected mice were also lower compared to mice infected with LCMV alone and potentially biased toward effector-memory cell generation. Thus, we provide evidence for bi-directional effects of viral co-infection that modulate disease and immunity.

[A comparative study of the antiviral activity of chemical compounds concerning the orthopoxviruses experiments in vivo]

In the experiments using intranasal (i/n) infection of mice with the ectromelia virus (EV) in a dose 10 LD50/head (10 x 50% lethal doselhead) or with the monkaypox virus (MPXV) in a dose 10 ID50/head (10 x 50% infective dose/ head) it was demonstrated that the antiviral efficiency of chemical compounds - the condensed derivatives of pyrrolidin-2,5-dion, as well as their predecessors and the nearest analogues, synthesized in Novosibirsk Institute of Organic Chemistry of the Siberian Branch of the Russian Academy of Sciences (NIOCH SB RAS) was observed. As a positive control we used the antipoxvirus chemical preparation ST-246 available from SIGA Technologies Inc. (USA), synthesized in NIOCH SB RAS by the technique suggested by the authors. It was demonstrated that the compound NIOCH-14 (7-[N'-(4-Trifluoromethylbenzoil)-hydrazidecarbonil]-tricyclo[3.2.2.02,4]non-8-en-6-carbonic acid) possessed comparable with ST-246 antiviral activity concerning EV and MPXV on all indicators used. Therefore, at infection of mice with EV (strain K-1) and peroral administration of NIOCH-14 and ST-246 in a dose 50 mkg/g of mouse weight (12-14 g) within 10 days the survival rate and average life expectancy of mice authentically exceeded the control levels. EV titers in lungs through 6 days after infection in the same groups were lower than in the control. In addition to that, after 7 days of infection of mice with MPXV (strain V79-1-005) and daily peroral administration of NIOCH-14 and ST-246 in a dose 60 mkg/g of mouse weight (9-11 g) authentic decrease in a part of infected animals and MPXV titers in lungs was observed.

Mousepox, a small animal model of smallpox

Ectromelia virus infections in the laboratory mouse have emerged as a valuable model to investigate human orthopoxvirus infections to understand the progression of disease, to discover and characterize antiviral treatments, and to study the host-pathogen relationship as it relates to pathogenesis and the immune response. Here we describe how to safely work with the virus and protocols for common procedures for the study of ectromelia virus in the laboratory mouse including the preparation of virus stocks, the use of various routes of inoculation, and collection of blood and tissue from infected animals. In addition, several procedures are described for assessing the host response to infection: for example, measurement of virus-specific CD8 T cells and the use of ELISA and neutralization assays to measure orthopoxvirus-specific antibody titers.

Obligatory requirement for antibody in recovery from a primary poxvirus infection

To understand the correlates of protective immunity against primary variola virus infection in humans, we have used the well-characterized mousepox model. This is an excellent surrogate small-animal model for smallpox in which the disease is caused by infection with the closely related orthopoxvirus, ectromelia virus. Similarities between the two infections include virus replication and transmission, aspects of pathology, and development of pock lesions. Previous studies using ectromelia virus have established critical roles for cytokines and effector functions of CD8 T cells in the control of acute stages of poxvirus infection. Here, we have used mice deficient in B cells to demonstrate that B-cell function is also obligatory for complete virus clearance and recovery of the host. In the absence of B cells, virus persists and the host succumbs to infection, despite the generation of CD8 T-cell responses. Intriguingly, transfer of naive B cells or ectromelia virus-immune serum to B-cell-deficient mice with established infection allowed these animals to clear virus and fully recover. In contrast, transfer of ectromelia virus-immune CD8 T cells was ineffective. Our data show that mice deficient in CD8 T-cell function die early in infection, whereas those deficient in B cells or antibody production die much later, indicating that B-cell function becomes critical after the effector phase of the CD8 T-cell response to infection subsides. Strikingly, our results show that antibody prevents virus from seeding the skin and forming pock lesions, which are important for virus transmission between hosts.

Involvement of Fas and FasL in Ectromelia virus-induced apoptosis in mouse brain

In this study we showed that the virulent Moscow strain of Ectromelia virus (ECTV-MOS) infection leads to induction of apoptosis in the BALB/c mouse central nervous system. ECTV-MOS-infected cells and inflammation sites were found in brain parenchyma between 5 and 15 days after footpad infection with ECTV-MOS. Infected cells consisted of microglia and monocytes, astrocytes and oligodendrocytes and these type of cells underwent apoptosis within 5-15 days post infection (d.p.i.). The highest number of apoptotic cells was found at 5 and 10 d.p.i. and represented mainly microglia (61.4% and 38.6% of apoptotic cells, respectively) and astrocytes (21% and 8.9%, respectively). The number of apoptotic oligodendrocytes was 5.4% and 4.5%, respectively. Fluorometric assays demonstrated involvement of caspase-1, -3 and -8 but not caspase-9 in apoptosis in ECTV-MOS-infected mouse brains. Expression of Fas/FasL was significantly increased on ECTV-MOS-infected cells between 5 and 15 d.p.i., whereas Fas was up-regulated also on the surrounding, non-infected cells. Taking together we may conclude that ECTV-MOS infection of microglia and astrocytes leads to local inflammation resulting in Fas/FasL up-regulation and apoptosis, which limits mouse central nervous system infection with ECTV-MOS.

Apoptosis during ectromelia orthopoxvirus infection is DEVDase dependent: in vitro and in vivo studies

Ectromelia virus (EV), which causes mousepox, is a member of the orthopoxviruses that are defined as being able to suppress apoptosis. Caspase-3 is one of the key effector proteases which regulates the apoptotic cascade and which is responsible for DNA fragmentation observed during apoptosis. It is well known that viruses, especially poxviruses, can inhibit caspase activity. Here, we report that EV can regulate apoptosis in vitro, suppressing the activity of caspases recognizing the DEVD (Asp-Glu-Val-Asp) motif (caspase-3 and -7) before successful virus replication is completed. Caspase-3 activity measurement showed that an increase in caspase-3 activity preceded the peak of DNA fragmentation demonstrated by TUNEL staining of L929 and RK-13 cells. By using specific caspase inhibitors (Ac-DEVD-CHO, Ac-IETD-CHO and zVAD-fmk), we showed that caspase-3 and -7 (DEVDases) are major effector caspases during EV-induced apoptosis in permissive L929 and RK-13 cell cultures. Apoptosis in vivo seems to play an important role during viraemia as well as during the clearance of EV from genetically susceptible BALB/c (H-2(d)) mice. However, as shown by measurement of caspase-3 activity, caspase-3 protein detection and M30-antibody staining, both DEVDases seem to play an important role during EV clearance from draining lymph nodes and conjunctivae at 15 days p.i. up to 20 days p.i., whereas in the liver and spleen DNA fragmentation coexisted with viral multiplication and secondary viraemia. Apoptosis was DEVDase dependent only in the liver, while spleen DNA fragmentation observed between 5 and 10 days p.i. was caspase independent. Therefore, we conclude that DEVDase- (caspase-3- and caspase-7-) dependent apoptosis is an important mechanism regulating the resolution of EV infection.

Enhanced resistance in STAT6-deficient mice to infection with ectromelia virus

We inoculated BALB/c mice deficient in STAT6 (STAT6(-/-)) and their wild-type (wt) littermates (STAT6(+/+)) with the natural mouse pathogen, ectromelia virus (EV). STAT6(-/-) mice exhibited increased resistance to generalized infection with EV when compared with STAT6(+/+) mice. In the spleens and lymph nodes of STAT6(-/-) mice, T helper 1 (Th1) cytokines were induced at earlier time points and at higher levels postinfection when compared with those in STAT6(+/+) mice. Elevated levels of NO were evident in plasma and splenocyte cultures of EV-infected STAT6(-/-) mice in comparison with STAT6(+/+) mice. The induction of high levels of Th1 cytokines in the mutant mice correlated with a strong natural killer cell response. We demonstrate in genetically susceptible BALB/c mice that the STAT6 locus is critical for progression of EV infection. Furthermore, in the absence of this transcription factor, the immune system defaults toward a protective Th1-like response, conferring pronounced resistance to EV infection and disease progression.

Mousepox resulting from use of ectromelia virus-contaminated, imported mouse serum

Mousepox was identified in a single mouse-holding room in early 1999 after a group of 20 CAF1/Hsd mice were inoculated SC with a killed murine spindle cell tumor line, S1509A. The cell line had been used without complications multiple times and was determined to be free of viral contamination on the basis of results of mouse antibody production testing. Of the 20 mice inoculated, 12 mice died by postinoculation day 8. Severe lymphoid and hepatic necrosis was observed in select mice subjected to histologic examination. Ballooning degeneration of epithelial cells with intracytoplasmic eosinophilic inclusion bodies was observed in the skin overlying the inoculation site of the single mouse from which this tissue site was evaluated. Presence of ectromelia virus was confirmed by use of immunohistochemical and polymerase chain reaction analyses, and the virus was isolated after serum, pooled from 5 of the index cases, was inoculated into an immune-naive mouse. Investigation into the source of virus contamination included inoculating mice with aliquots of various S1509A freeze dates; chemically defined media and supplements, including fetal bovine serum; and two lots of pooled commercial mouse sera, after heat inactivation at 56 degrees C for 30 minutes used as a medium supplement. One lot of pooled commercial mouse serum was identified as the source of ectromelia virus. This lot of serum was inadvertently used to feed S1509A cells that were subsequently inoculated into mice. We determined that the contaminated serum, which was purchased in late 1998, originated from China. The serum was imported into the United States as a batch of 43 L in early 1995. The serum was blended into a single lot and filtered (0.2 microm) before distribution to major suppliers throughout the country. The serum was sold or further processed to obtain a variety of serum-derived products. Because murine serum is generally sold in small aliquots (10 to 50 ml), we speculate that several thousand aliquots may have been derived from this batch of serum and, if inoculated into mice, would likely result in additional mousepox outbreaks.

Homing studies on distribution of ectromelia (mousepox) virus-specific T cells adoptively transferred into syngeneic H-2d mice: paradigm of lymphocyte migration

Mousepox (infectious ectromelia) may be used as a model for studies on the cellular immune response and pathogenesis of generalized viral infections. Ectromelia virus (EV) initially replicates in the footpad (f.p.) skin at the site of infection, next in draining lymph nodes, and then in the spleen and liver where the virus may induce extensive necrotic process with inflammatory reaction. We show in this study that after recipient BALB/c mice (H-2d) f.p. infection with EV prior to the adoptive transfer of syngeneic donor EV-specific cytotoxic T lymphocytes interferon-gamma-positive (IFN-gamma-+), interleukin-2-positive (IL-2+), and IL-4+ of both phenotypes, CD8+ approximately 70%, and CD4+ approximately 30%) preferentially migrated to the inguinal and auxiliary lymph nodes, spleen, liver, and skin at the site of infection (f.p.). Many particles of EV with the morphology characteristic for orthopoxviruses and virus-specific immunofluorescence within the cells of inguinal and auxiliary lymph nodes, liver, spleen, and skin have been observed using high-resolution transmission electron microscopy and fluorescence antibody technique, respectively. Results presented in this article support the concept that immune T cells adoptively transferred into infected recipient mice are able not only to specific migration in the host and homing in the sites of virus replication, but also to develop immunoprotection in the transferred animals.

Differential pathogenesis of lethal mousepox in congenic DBA/2 mice implicates natural killer cell receptor NKR-P1 in necrotizing hepatitis and the fifth component of complement in recruitment of circulating leukocytes to spleen

Innate resistance of C57BL/6 (B6) mice to lethal mousepox is controlled by multiple genes. Previously, four resistance genes were localized to specific subchromosomal regions and transferred onto a susceptible DBA/2 (D2) background by serial backcrossing and intercrossing to produce congenic strains. Intraperitoneally inoculated ectromelia virus was uniformly lethal and achieved similar titers in B6 and D2 mice but elicited differential responses in liver, spleen, and circulating blood leukocytes. The distribution of these response phenotypes in congenic strains linked control of phenotypes with specific subchromosomal regions. D2.R1 mice, which carried a differential segment of chromosome 6, exhibited a B6 liver response and intermediate spleen and circulating leukocyte responses. D2.R2 and D2.R4 mice, which carried differential segments of chromosomes 2 and 1, respectively, exhibited a D2 liver response, a B6 spleen response, and an intermediate circulating leukocyte response. The localization of control of liver response phenotypes to chromosome 6 implicates cells that express natural killer (NK) cell receptor NKR-P1 alloantigens. The localization of control of spleen and circulating leukocyte responses to chromosomes 1, 2, and 6 implicates NK cells, the fifth component of complement, and a gene near the selectin gene complex in recruitment of circulating leukocytes to spleen.

Mousepox outbreak in a laboratory mouse colony

Mousepox was diagnosed in and eradicated from a laboratory mouse colony at the Naval Medical Research Institute. The outbreak began with increased mortality in a single room; subsequently, small numbers of animals in separate cages in other rooms were involved. Signs of disease were often mild, and overall mortality was low; BALB/cByJ mice were more severely affected, and many of them died spontaneously. Conjunctivitis was the most common clinical sign of disease in addition to occasional small, crusty scabs on sparsely haired or hairless areas of skin. Necropsy findings included conjunctivitis, enlarged spleen, and pale liver. Hemorrhage into the pyloric region of the stomach and proximal portion of the small intestine was observed in experimentally infected animals. In immune competent and immune deficient mice, the most common histologic finding was multifocal to coalescing splenic necrosis; necrosis was seen less frequently in liver, lymph nodes, and Peyer's patches. Necrosis was rarely observed in ovary, vagina, uterus, colon, or lung. Splenic necrosis often involved over 50% of the examined tissue, including white and red pulp. Hepatic necrosis was evident as either large, well-demarcated areas of coagulative necrosis or as multiple, random, interlacing bands of necrosis. Intracytoplasmic eosinophilic inclusion bodies were seen in conjunctival mucosae and haired palpebra. Ectromelia virus was confirmed as the causative agent of the epizootic by electron microscopy, immunohistochemistry, animal inoculations, serologic testing, virus isolation, and polymerase chain reaction. Serologic testing was of little value in the initial stages of the outbreak, although 6 weeks later, orthopoxvirus-specific antibody was detected in colony mice by indirect fluorescent antibody and enzyme-linked immunosorbent assay procedures. The outbreak originated from injection of mice with a contaminated, commercially produced, pooled mouse serum. The most relevant concern may be the unknown location of the source of the virus and the presence of a reservoir for this virus within the United States.

The inflammatory and immune response to mousepox (infectious ectromelia) virus

The ectromelia virus (EV) has been recognized as the etiological agent of a relatively common infection in laboratory mouse colonies around the world, i.e., Europe (including Poland), USA and Asia. Due to widespread use of mice in biomedical research, it is important to study the biology of strains characteristic for a given country. This is particularly significant for the diagnosis, prevention and control ectromelia. In severe epizootics, approximately 90% morbidity is observed within colonies and mortality rate exceeding 70% is observed within 4 to 20 days from the appearance of clinical symptoms. The resistance to lethal infection is mouse strain-dependent. Several inbred strains of mice, including C57BL/6 and AKR are resistant to the lethal effects of EV infection, while others, such as A and BALB/c are susceptible. Recent studies indicate that (1) T lymphocytes, NK cells and interferon (IFN)-dependent host defenses must operate for the expression of resistance, (2) virus-specific T-cell precursors appear earlier in regional lymph nodes of resistant than susceptible mice, and (3) resistance mechanisms are expressed during early stages of infection. Over the past several years, (1) induction of anti-EV cytotoxic CD8+ T lymphocytes (CTL) responses in vivo in the absence of CD4+ (T helper) cells, (2) importance of some cytokines e.g., IFN-gamma in EV clearance at all stages of infection, and (3) induction of nitric oxide (NO) synthase, which is necessary for a substantial antiviral activity of IFN-gamma, have been demonstrated.(ABSTRACT TRUNCATED AT 250 WORDS)

[The virucidal properties of neoaquasept]

The paper provides the evidence suggesting that the disinfectant neoaquasept (NA) obtained by using sodium salt of dichlorisocyanuric acid has virucidal activity against ectromelia virus in BALB/c mice. Three concentrations of NA were tested. The recommended application rate and concentration of the disinfectant is 3 and 10 times higher. NA was demonstrated to decrease the degenerative changes of the liver and spleen and increases the survival of the animals infected with ectromelia virus. It is proposed to use the water disinfected with NA as a preventive agent in the spread foci of various infections.

Studies on poxvirus infection in cats

The development of clinical disease and the pathogenesis of cowpox were studied in domestic cats inoculated by a variety of routes. Intradermal titration in two cats demonstrated that as little as five pfu of cowpox virus caused a primary skin lesion. Intradermal inoculation of greater than or equal to 10(5) pfu cowpox virus resulted in severe systemic disease. Large amounts of virus (greater than or equal to 10(3) pfu/g) were isolated from skin lesions and the turbinates of cats killed at eight and 11 days post-inoculation (dpi). Lesser amounts of virus (congruent to 10(2) pfu/g) were isolated from lymphoid tissues and the lung, and small amounts of virus were isolated from various other tissues. A white cell-associated viraemia was detected from 5 dpi onwards. Skin scarification with 10(3) or 50 pfu cowpox virus enabled reproduction of the naturally-acquired disease. Cat-to-cat transmission was demonstrated from cats inoculated by skin scarification, but caused only subclinical infection in sentinel cats. Oronasal inoculation resulted in transient coryza and milder generalized disease than skin inoculation, and no transmission to sentinel cats. Preliminary investigations showed vaccinia virus (Lister strain) to be of low infectivity in cats while inoculation of ectromelia virus (Mill Hill strain) did not cause any clinical signs.

Mousepox in inbred mice innately resistant or susceptible to lethal infection with ectromelia virus. I. Clinical responses

Clinical responses to infection with ectromelia virus strain NIH-79 were determined in several strains of inbred mice. All mice were equally susceptible to infection, but mortality was strain dependent. BALB/c AnNCr, A/JNCr, DBA/2NCr and C3H/He/NCr MTV- mice were highly susceptible to lethal infection whereas AKR/NCr and SJL/NCr mice were moderately susceptible and C57BL/6NCr mice were highly resistant. Death rates were influenced strongly by virus dose and by route of inoculation. High doses were associated with early and high mortality. For a given dose, intraperitoneal inoculation resulted in the highest mortality and death rates were progressively reduced in mice inoculated by the footpad, subcutaneous and intranasal routes. Footpad swelling was prominent in resistant mice and in survivors among susceptible strains. Deaths among AKR and SJL mice were sporadic and often occurred late irrespective of virus dose. It is suggested that this pattern could be influenced by secondary contact infections or by immunologic injury associated with host responses to ectromelia virus.

Mousepox in inbred mice innately resistant or susceptible to lethal infection with ectromelia virus. II. Pathogenesis

The pathogenesis of mousepox due to infection with ectromelia virus strain NIH-79 was characterized in genetically susceptible (BALB/cAnNCr) and genetically resistant (C57BL/6NCr) mice. BALB/c mice inoculated subcutaneous (s.c.) or intranasally (i.n.) had high mortality. Most mice died within 7 days from severe necrosis of the spleen and liver. Necrotic foci in livers of BALB/c mice that survived beyond 7 days often were accompanied by mononuclear cell infiltrates and by hyperplasia of lymphoid tissues. C57BL/6 mice inoculated by either route remained asymptomatic and necrotic lesions were mild or absent, whereas focal non-suppurative hepatitis and lymphoid hyperplasia were prominent. Infectious virus and viral antigen were distributed widely in tissues of BALB/c mice, but had limited distribution in C57BL/6 mice. Both mouse strains had infection of the respiratory tract, genital tract, oral tissues and bone marrow, and BALB/c mice also had infection of the intestines. Both strains also developed serum antibody to vaccinia virus antigen after infection. The results show that ectromelia virus occurs in tissues conducive to mouse to mouse transmission and that the severity and character of mousepox lesions correlate directly with resistance and susceptibility to infection. They also support the concept that cellular immunity contributes to survival from infection.

Immunologic requirements for the adoptive transfer of ectromelia virus meningitis

Intracerebral infection with the Hampstead egg strain of ectromelia virus in mice produced a meningitis with clinical signs and death. A single injection of cyclophosphamide three days after infection delayed the onset of pathology and clinical signs. Adoptive transfer of immune spleen cells into preinfected recipients could induce the meningitis earlier. These donor-immune cells were virus-specific in their action. The Ig- population, deficient in B lymphocytes, could transfer meningitis; but donor-immune cells depleted of T lymphocytes by anti-theta treatment or the Ig+ population, deficient in T-cells, were unsuccessful. Studies with labeled cells showed a preferential accumulation of labeled donor cells in cerebrospinal fluid in the first 24 hours post-transfer, whereas labeled recipient cells accumulated between 24 and 48 hours. The results demonstrated that the adoptive transfer of ectromelia virus meningitis was a cell-mediated inflammatory process.

The role of the major histocompatibility complex in the adoptive transfer of ectromelia virus meningitis

Adoptive transfer of ectromelia virus meningitis was most efficient when donor-immune spleen cells and recipients were compatible in the K region of the H-2 gene complex. Weak responses could be obtained with H-2D region compatibility, but none occurred with H-2I region compatibility. Spleen cells used in adoptive transfers and cells found in cerebrospinal fluid from infected mice were found to have identical H-2-imposed restriction in their in vitro cytotoxic activity. This suggests a significant role for the major histocompatibility complex in the generation of virus-specific T lymphocytes and the recognition of virus-infected tissue in the central nervous system by these cells.

Clinical, pathologic, and serologic features of an epizootic of mousepox in Minnesota

An epizootic of mousepox in two colonies of mice was described. Clinical signs, morbidity and mortality rates, and necropsy lesions were substantially influenced by husbandry practices, intercurrent diseases (Sendai pneumonia, mouse hepatitis virus), and total body x-irradiation. In one colony, 54 mice were necropsied; 19 mice had cutaneous or visceral lesions of mousepox although none had combined visceral and cutaneous lesions. In the other colony, 20 mice were necropsied, and 10 mice had cutaneous lesions; none had visceral lesions. Sera from 24 mice with the cutaneous form of the disease had no demonstrable HI antibody titer. Experimental mice injected with spleen/liver homogenates from infected colony mice developed typical visceral lesions with numerous poxvirus profiles on electron micrographs and positive HI titers. Mice immunized with vaccinia virus were not susceptible to the disease.

Pathology and diagnosis of mousepox

The pathologic changes of mousepox were studied during an outbreak at the National Institutes of Health in 1979. The most consistent lesions were necrosis of lymphatic tissues, especially the spleen, lymph nodes, and Peyer's patches. Hepatic necrosis and jejunal hemorrhage also were found. In two transmission studies, the disease was experimentally induced in BALB/cAnN and C3H/HeN-nu mice. Athymic mice were found to be highly susceptible, and they developed fulminant disease. The diagnosis was confirmed by demonstration of pox virions in infected tissues by electron microscopy, staining of viral antigen by immunoperoxidase methods, and by isolation of the virus in chorioallantoic membranes of hen's eggs and in cultures of chick embryonic cells.

Clinical manifestations of mousepox in an experimental animal holding room

A study of the clinical aspects of mousepox was conducted during the 1979-80 outbreak at the National Institutes of Health. The disease was detected serologically in a room located adjacent to the index room. The index room received animals prior to this outbreak from a noncommercial colony which later was found to be infected with mousepox. The infection was present in the room for at least 6 weeks prior to the completion of the study. The paucity of clinical signs and low mortality were striking when compared to previous descriptions of mousepox in the United States. Only 27 of the 939 mice in the room were infected, and only one of these had typical skin lesions. A few of the mice had non-specific signs such as ruffled hair coat and hunched appearance. Minimal spread of the disease was evidenced by clustering of infected cages on one of five animal racks in the room.