Reovirus Type 3

Reovirus type 3 was initially isolated from humans in the 1950s and has since been detected in a variety of mammals. The name "reovirus" is derived from a combination of the first letters of its English descriptors: respiratory, enteric, and orphan. Reovirus type 3 (Reo-3) serves as the prototype strain of the genus Orthoreovirus, which encompasses both mammalian and avian reoviruses. Mammalian reoviruses are classified into three serotypes—Reo-1, Reo-2, and Reo-3—each sharing a common complement-binding antigen that can be identified using neutralization and hemagglutination-inhibition assays. Reo-3 is one of the mandatory tests required by national standards for SPF-grade laboratory rats, mice, guinea pigs, and hamsters. Regular sampling and testing of these animals are crucial for preventing and controlling Reo-3 infections within rodent colonies. Notably, the seroprevalence of mammalian Reo-3 virus in laboratory mice remains remarkably low; for instance, according to our company’s testing data from July 2021 to July 2023, the positive rate for Reo-3 was 0.14%, while the suspicious rate stood at 0.03%, based on a total of 20,796 samples tested.

Reovirus type 3 has a double-stranded RNA genome, and its viral particles are spherical, non-enveloped, with a single core enclosed by a double-layered capsid. The complete virion measures approximately 76 nm in diameter, while the core itself is about 52 nm across. Under an electron microscope, the surface of the virus particle is clearly visible, featuring shell-like structures—specifically, the outer capsid consists of 92 shell units, composed of pentamers and hexamers: 80 hexamers and 12 pentamers. Each shell unit is an empty, prism-shaped structure measuring roughly 10 nm in length and 8 nm in width. Meanwhile, the inner capsid exhibits icosahedral symmetry with fivefold rotational axes. In vitro, treatment with proteases can degrade the intact viral particle into two distinct subviral particles (SVPs): the intermediate subviral particle (ISVP) and the core.

Reo-3 is primarily transmitted via respiratory, airborne, and fecal-oral routes, with no obvious seasonality. Relevant studies suggest that the virus's ability to stably survive in the environment is a major factor contributing to viral contamination. Reo-3 has a remarkably broad host range, infecting humans, mice, monkeys, cattle, dogs, chickens, and more—furthermore, antibodies against Reo-3 have been detected in rats, pigs, horses, sheep, rabbits, marsupials, and even reptiles. Interestingly, susceptibility to infection varies slightly among mouse strains, with NIH mice, BALB/C mice, and Kunming mice exhibiting high susceptibility.

The main clinical manifestations of Reo-3 include an oily coat effect and fatty diarrhea. Acute cases are primarily observed in neonatal and weaned mice, while chronic cases typically occur in mice older than 28 days. During the acute phase—defined as within 28 days—the disease presents with visible lesions: the liver becomes enlarged, its color darkens, and it develops focal, ring-like yellow areas, each up to 3 millimeters in diameter but varying in size. The small intestine may appear reddened or dilated. In isolated cases, small, ring-shaped lesions also emerge on the epicardium of the heart, and occasionally, hemorrhagic areas are detected in the lungs. Additionally, the brain may exhibit swelling and congestion. In younger mice, the intestinal contents often take on a lemon-yellow hue. During the chronic phase, affected mice become emaciated, sometimes developing jaundice. Hair loss may occur in localized areas, and the liver, though mildly enlarged, appears dark in color, with faintly yellowish lesions visible beneath the capsule. Peritoneal congestion is common, accompanied by occasional serous effusions. Toward the later stages of the disease course, the spleen may show moderate enlargement. Relevant experimental findings indicate that Reo-3 infects neonatal mice via oral administration. By day 4, mononuclear cell infiltration is already evident in the liver. By day 7, hepatocytes exhibit significant swelling—expanding 3 to 4 times their normal size—and Kupffer cells undergo marked proliferation. By day 14, necrotic cells begin to dissolve, with surrounding cells undergoing myofibroblastic transformation around the necrotic foci. Meanwhile, the pancreas shows cytoplasmic vacuolization as early as 3 days post-infection, progressing to widespread pancreatic necrosis by days 10 to 14. In the heart, one week after infection, degenerative changes and necrosis are observed in the left ventricular papillary muscles, accompanied by edema and prominent macrophage infiltration. In the lungs, diffuse hemorrhages and pulmonary edema become apparent. By day 9 post-infection, neurons in the central nervous system begin to degenerate, and by day 10, vascular cuffing becomes markedly evident, with neutrophil infiltration of the meninges. By day 14, encephalitis lesions worsen and spread extensively, with small hemorrhagic spots appearing within the necrotic regions. Notably, lymphatic vessels in the gastrointestinal lamina propria also exhibit dilation during this stage.

Reo-3 can still survive after being exposed to 56°C for 2 hours or 60°C for 30 minutes, and its infectivity remains stable for at least 2 months at 4°C. It exhibits resistance to hydrogen peroxide, 10% phenol, 0.3% formaldehyde, and 20% lysol; however, it is unaffected by ether treatment and remains infectious even after exposure to chloroform, though the latter does destroy its hemagglutinin. The virus is stable across a pH range of 2.2 to 8.0. Notably, prolonged exposure to 70% ethanol effectively inactivates Reo-3, while periodic acid treatment can also rapidly eliminate the reovirus. Research has shown that Mg²⁺ at 50°C can enhance the titer of Reo-3—specifically, incubation in 2 mol/L MgCl₂ for 5 to 15 minutes boosts the viral titer by 4 to 8 times. This effect may be attributed to Mg²⁺ converting inactive viral particles into active ones. Additionally, ultraviolet light has been found to disrupt the virus, yet intriguingly, numerous instances of viral reactivation have been observed even in specimens where the virus had already been inactivated by UV exposure.

Reo-3 can replicate in primary cultures of rhesus monkey, cat, pig, and dog kidney cells, as well as in passaged cell lines such as L cells, BHK-21 cells, FL cells, BS-C-1 cells, and KB cells. Characteristic cytopathic effects and eosinophilic intracytoplasmic inclusion bodies become visible between 7 and 14 days post-infection. Treatment of viral samples with trypsin or other proteolytic enzymes—such as ficin, papain, pepsin, or streptokinase—can enhance viral titers, likely because these enzymes facilitate faster virus transmission from one cell to another.

Mice infected with Reo-3 can develop symptoms such as hepatitis, encephalitis, and pancreatitis, often disrupting experimental studies and leading to significant waste of both human and material resources. Infection with Reo-3 in mice results in reduced activity of pancreatic amylase and lipase, while trypsin activity increases. Additionally, the virus can damage the β-cells of the pancreatic islets, impairing insulin secretion and triggering metabolic and pathological changes reminiscent of diabetes. Moreover, Reo-3 plays a crucial role in modulating the host's response to environmental carcinogens, acting as an immune stimulant. Notably, this virus ranks second among viruses detected in laboratory mice, posing certain challenges for the testing of exogenous pathogens—such as blood products, monoclonal antibodies, and cell cultures—as well as for animal experiments.

Currently, the primary methods used for detecting enterovirus type 3 include histopathological examination, virus isolation and identification, enzyme-linked immunosorbent assay (ELISA), immunofluorescence assay (IFA)—which are all serological techniques—as well as molecular biology-based detection methods. Among these, ELISA is the preferred method for serum testing of enterovirus type 3, often followed by repeat testing or confirmation with IFA results. IFA is generally regarded as the gold standard. For pathogen detection in immunodeficient mice, molecular biology techniques can be employed.

References

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2. Song Yu, Zhou Jie, Gao Cheng, et al. Expression of the S1 Gene of Reovirus Type 3 in Sf9 Insect Cells [J]. Journal of Shanghai Jiao Tong University (Agricultural Sciences Edition), 2011, 29(01):28-32.

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