A double-protease-resistant variant of transmissible gastroenteritis virus and its ability to induce lactogenic immunity.

A virus resistant to 2 major intestinal proteases (trypsin and alpha-chymotrypsin) was derived from the attenuated Purdue strain of transmissible gastroenteritis virus. Its enzymatic stability was confirmed, in vitro, by exposure to proteolytic enzymes and to porcine intestinal fluids. Vaccination of 5 seronegative pregnant sows with the variant virus by a series of 2 oral and 1 IM inoculations resulted in high titers of neutralizing antibody in serum and colostrum. The mean antibody titer in milk whey decreased 44-fold within 1 week after parturition. At 3 days of age, the 40 pigs delivered by these sows were challenge exposed orally with virulent transmissible gastroenteritis virus. Pigs nursing the 5 vaccinated sows underwent a relatively mild clinical course of illness. The average mortality of these 40 pigs was 33%. Thirty-six pigs which had been raised by 4 nonvaccinated sows had a more severe illness, greater daily weight loss, and higher mortality (92%).

Elaboration of chemotactic substances by alveolar cells: possible mechanisms for the initial neutrophilic response in feline caliciviral pneumonia.

In vitro experiments established that the interaction of feline calicivirus (FCV) with alveolar macrophages and pneumocytes results in the generation of chemotactic factors that produce directed migration of neutrophils in Boyden chambers. Factors were produced independent of immune mechanisms and of discernible serum factors. Lysates of noninfected alveolar macrophages and pneumocytes did not possess chemotactic activity, indicating that chemotactic factors were not preformed in these cells. Noninfected alveolar macrophages also elaborated neutrophil chemotactic factors in culture; however, activity was greater when macrophages were infected with FCV. The results of this study suggested that the neutrophilic response in the peripheral portion of the lung of cats exposed to aerosols of FCV was the result of the elaboration of complete chemotactic factors from FCV-infected alveolar cells.

Ultrastructural morphogenesis of acute viral pneumonia produced by feline calicivirus.

The ultrastructural morphogenesis of acute viral pneumonia was studied in pathogen-free cats exposed to aerosols of a virulent strain of feline calicivirus. Electron microscopy and fluorescence microscopy indicated that the virus had a marked tropism for alveolar pneumocytes. Viral-induced necrosis of pneumocytes was observed as early as 12 hours after exposure and was associated with acute serofibrinous and neutrophilic inflammation in the distal air exchange tissue. The acute exudative phase abated by 168 hours after exposure. The regenerative phase that followed was characterized by hyperplasia of type II pneumocytes; influx of monocytes, lymphocytes, and plasma cells; and interalveolar and intraalveolar proliferation of fibroblasts. The decrease in pneumocyte injury coincided with the increase in alveolar macrophages and immunocytes. With minor exceptions, feline calicivirus-induced alveolar damage and the subsequent repair resembled toxic alveolar injury produced by oxidant gases. Feline caliciviral pneumonia, therefore, is a naturally occurring example of viral-induced diffuse alveolar damage.

In vitro interaction of alveolar macrophages and pneumocytes with feline respiratory viruses.

Feline alveolar macrophages and feline pneumocytes were inoculated in vitro with low multiplicities of either feline calicivirus or feline viral rhinotracheitis virus. Pneumocytes were permissive for both viruses. High titers were attained, and characteristic cytopathic effects developed. Alveolar macrophages were permissive for feline viral rhinotracheitis virus, although the cycle of replication was delayed. Infection of macrophages with feline calicivirus resulted in the production of viral antigens and cytopathic effects; however, viral particles were not detected by electron microscopy, and viral infectivity titers rose only slightly and then fell to undetectable levels by 96 h. The differences in viral susceptibility between these two peripheral pulmonary cell populations that were demonstrated in vitro probably contribute to the differences in pathogenesis of viral rhinotracheitis and calicivirus infections in cats.

Experimentally induced feline chlamydial infection (feline pneumonitis).

Cats exposed to aerosols of feline Chlamydia psittaci developed a disease characterized principally by conjunctivitis. Signs of conjunctivitis appeared between postexposure days (PED) 5 and 10, were often unilateral initially, and persisted for 22 to 45 days. Fever followed the onset of conjunctivitis (PED 11 to 15) and persisted for 3 to 8 days. Signs of mild rhinitis (occasional sneezing and mild serous nasal discharge) occurred in some cats between PED 8 and 37. Neither signs of lower respiratory tract disease nor significant pulmonary lesions were produced by the feline pneumonitis agent. Small foci of pneumonia were detected microscopically in 3 of 6 cats examined between PED 7 and 14. Chlamydiae were identified between PED 7 and 14 in the cytoplasm of epithelial cells in stained conjunctival smears. Conjunctivitis persisted for at least 18 days after chlamydiae no longer were detectable in conjunctival smears. Low levels of chlamydial infectivity, however, still were present in conjunctiva and lung on PED 45.

Diffuse alveolar damage in cats induced by nitrogen dioxide or feline calicivirus.

The ultrastructural morphogenesis of pulmonary lesions was studied in cats exposed to either aerosols of feline calicivirus (FCV) or high concentrations of NO2. Both directly injured alveolar lining cells, particularly type I cells. Necrosis of pneumocytes attended by an acute exudative response in the air exchange tissues was evident from 0 through 24 hours after exposure of cats to NO2 and from 12 through 96 hours after infection with FCV. The reparative process following alveolar injury was characterized by regenerative hyperplasia of type II pneumocytes, proliferation of stromal cells, and infiltration of mononuclear cells. Differences in the lesions produced by NO2 and FCV also were encountered. Endothelial necrosis was detected only after NO2 injury, whereas a marked infiltration of neutrophils and immunocytes was observed only after FCV injury. The FCV/NO2 experimental system in cats is well suited for studies of diffuse alveolar damage of toxic and viral etiology.

Feline caliciviral disease: experimental immunoprophylaxis.

An attenuated feline calicivirus (FCV) was administered intramuscularly to specific-pathogen-free cats. Vaccination did not cause signs of illness. Oropharyngeal replication of attenuated FCV was not detected, nor was there evidence of virus transmission to contact-control cats. Antiviral neutralizing antibody was present in the serum of all vaccinated cats 7 days after they were given the 2nd intramuscular dose of immunogen. Vaccinated and control cats were challenge exposed to aerosols of a virulent FCV strain. All controls developed severe pneumonia and died within 7 days after this challenge exposure. In the vaccinated cats, signs of illness were absent or minimal; pulmonary lesions were milder and less extensive than those in the control cats. Feline calicivirus was isolated from ocular, nasal, and oropharyngeal swabbings from both control and vaccinated cats after viral challenge. Results indicate protective immunity to FCV disease can be induced by intramuscular administration of an attenuated FCV.

Comparison of canine distemper viral strains: an electron microscopic study.

A canine distemper (CD) viral strain, designated R252, originally obtained from a dog with demyelinating encephalomyelitis has been shown to reproduce this disease in gnotobiotic dogs in a high incidence in contradistinction to other CD viral strains which produce an acute fatal disease. Because comparision of R252 strain with the Snyder Hill (SH) and Onderstepoort (Ond) strains revealed differences in in vitro behavior, the 3 viruses were ultrastructurally investigated. The results revealed differences among the 3 viruses: Cytoplasmic nucleocapsid aggregates were characteristic of R252, diffuse aggregates of nucleocapsids were characteristic of SH, and although budding viral particles were recognized in Ond-infected cells, few nucleocapsids were seen in the cytoplasm. Cytoplasmic fibrillar structures were characteristic of R252- and Ond-infected cells. The budding viral particles observed with R252 and SH were similar, whereas Ond budding particles seemed to contain fewer nucleocapsids. Intranuclear inclusions recognized in R252- and SH-infected cultures appeared as areas of clearing of nucleoplasm along with disruption of the nucleolus. The presence of nucleocapsid-like structures was variable. Ultrastructurally, the cytoplasmic inclusions in cells infected with all 3 viruses progressed from focal aggregates of nucleocapsids to electron-dense bodies.

Induction of immunity to feline caliciviral disease.

Six specific-pathogen-free cats were exposed by aerosol to a feline calicivirus of low virulence (F-9 virus). Homotypic (anti-F-9) seroconversion occurred in all cats by postexposure day 14. The serum of one cat on postexposure day 14 and four of six cats on postexposure day 35 neutralized feline picornavirus isolate no. 225 (FPV-255), a virulent feline calicivirus. Homologous antiviral activity was detected before the appearance of heterologous (anti-FPV-255) activity and always was present in higher titer. Protective immunity was evaluated on postexposure day 35 by aerosol challenge with FPV-255. The pyrexia, depression, dyspnea, oral ulcers, and severe pneumonia produced in two susceptible specific-pathogen-free cats by exposure to FPV-255 did not occur in the cats that had been infected previously with F-9 vir. The study demonstrates that heterotypic protective immunity to feline calicivirus disease can be induced by prior infection with feline calicivirus of low virulence.

Biological properties of a canine distemper virus isolate associated with demyelinating encephalomyelitis.

A canine distemper virus (CDV), DESIGNATED R252, originally recovered from a dog with demyelinating encephalomyelitis was shown to reproduce this disease in gnotobiotic dogs with a high incidence as compared to other CDV strains, which produced an acute fatal infection. In this investigation, R252 was propagated for the first time in Vero cells and compared to two known strains of CDV, Snyder-Hill (SH) and Onderstepoort (Ond). The results of this study revealed that intracellular R252 accumulated more slowly than either SH or Ond. There was extensive destruction of Vero monolayers infected with either R252 or SH. Each virus induced the formation of intracytoplasmic and intranuclear inclusions. Ond infection resulted in minimal cytopathic changes and intracytoplasmic inclusions. Immunofluorescence studies indicated that the spread of R252 infection within the monolayers was intermediate between the rapidly spreading SH and slowly spreading Ond. R252-infected cells developed characteristic immunofluorescent cytoplasmic inclusions. Initially, each stained homogeneously and later appeared as a non fluorescent body surrounded by a fluorescent ring. This characteristic pattern of fluorescence was observed only infrequently in thelate stage of SH infection and was absent in Ond-infected cultures. Reciprocal neutralization studies indicated that the three strains are of one serotype. These findings suggest that R252-CDV has biological properties which differ from two other strains of CDV and which may have bearing upon the in vivo capability of this virus to produce demyelinating encephalomyelitis.

Experimentally induced feline calicivirus infection: clinical signs and lesions.

Sixty-six specific-pathogen-free cats were allotted to 10 groups and exposed by aerosol to 10 feline calicivirus (FCV) isolates. Viruses of different virulence were identified. The more virulent FCV caused pyrexia, depression, dyspnea, pneumonia, vesicles, or ulcers of the tongue and ulceration of the hard palate and nostrils. The FCV of low virulence caused similar lesions of the tongue, palate, and nostrils but little or no malaise, pyrexia, or pneumonia. Lesions produced by FCV usually were confined to the oral mucosa, tonsils, and lungs. Lesions in the nasal or trachea were associated with 2 of the 10 FCV tested.

Differences in acute and convalescent-phase antibodies of cats infected with feline picornaviruses.

Complement fixation inhibiting and complement-fixing antibodies were demonstrated in sera from cats during the acute and convalescent stages, respectively, of feline picornavirus infections. Complement-fixing antibody activity was present in 19S and 7S globulins, whereas complement fixation inhibiting antibody activity was confined to the 7S globulins. Sera with complement-fixing antibody and sera with complement fixation inhibiting antibody were also shown to have neutralizing and precipitating antibodies. In a study of antibody responses to homotypic and heterotypic strains of feline picornavirus, less serological cross-reactions were observed between strains of feline picornavirus in sera from cats in the acute phase of feline picornavirus infection than in the convalescent phase. The temporal relationship of complement fixation inhibiting and complement-fixing antibody responses of cats after acute viral respiratory infections is contrasted with previously observed antibody response in cats to feline leukemia virus.