Immunologic phenomena in the effusive form of feline infectious peritonitis

The effusive form of feline infectious peritonitis (FIP) was reproduced by injecting 12- to 16-week-old kittens intraperitoneally with a cell-free inoculum derived from the tissues of infected cats. The kittens used for the study were either positive for FIP virus-reacting antibodies before inoculation or they were seronegative. Seropositive kittens were obtained from a cattery where the natural infection was enzootic, and seronegative kittens were obtained from a specific-pathogen-free cattery. Only about half the kittens that were seronegative before inoculation developed disease or serum antibodies to the tissue-derived virus. Seronegative kittens that developed disease showed no signs of illness until 8 to 10 days after inoculation, and they lived for 7 to 14 days after clinical signs appeared. The onset of clinical disease coincided with the appearance of serum antibodies. In contrast, all of the seropositive kittens became ill within 36 to 48 hours after inoculation, and died within 5 to 7 days. If seronegative kittens were treated with immune serum or immunoglobulin (Ig)G, they developed disease with the same frequency, acuteness, and severity as seropositive kittens. Foci of hepatitis and serositis in seropositive kittens contained viral antigen, IgG bound to antigen, and complement. Serum complement activity also decreased several days before death in seropositive kittens inoculated with tissue-derived FIP virus. The temporal relationship of clinical disease and the appearance of serum antibodies, the more acute and severe nature of the disease produced in seropositive kittens, and the presence of antibody and complement in the lesions indicated that effusive FIP is immunologically mediated.     

Pathogenicity studies of feline coronavirus isolates 79-1146 and 79-1683

Two feline coronavirus isolates were characterized by their disease-causing potential in cats. The 79-1683 feline coronavirus isolate caused an inapparent-to-mild enteritis when given oronasally to specific-pathogen-free kittens and was not a cause of feline infectious peritonitis (FIP). Target tissues for the virus were the mature apical epithelium of the small intestine, mesenteric lymph nodes, tonsils, thymus, and (to a lesser extent) the lungs. Inoculated kittens shed high numbers of virus in their feces for 14 to 17 days, but remained infectious to susceptible kittens for longer periods of time, as evidenced by contact-exposure studies. Because the 79-1683 isolate induced only enteritis, it was designated feline enteric coronavirus (FECV) 79-1683. The 79-1146 feline coronavirus isolate induced effusive abdominal FIP in specific-pathogen-free kittens after oronasal and intraperitoneal inoculation. Clinical signs of disease appeared within 12 to 14 days in almost all inoculated kittens. Because this isolate caused FIP, it was designated FIP virus (FIPV) 79-1146. Cross-protective immunity was not induced by the various coronavirus infections. Kittens preimmunized with the UCD strain of FECV (FECV-UCD) or with FECV-79-1683 were not immune to infection with FIPV-79-1146. Likewise, kittens previously inoculated with FECV-79-1683 were not immune to infection with FIPV-UCD1. In fact, preexisting heterologous FECV-79-1683 immunity often accelerated and enhanced the severity of disease caused by inoculation with FIPV-UCD1.(ABSTRACT TRUNCATED AT 250 WORDS)     

Antibody-mediated enhancement of disease in feline infectious peritonitis: Comparisons with dengue hemorrhagic fever

Non-immune kittens passively immunized with feline serum containing high-titered antibodies reactive with feline infectious peritonitis virus (FIPV) developed a more rapid disease after FIPV challenge than did kittens pretreated with FIPV antibody-negative serum. Antibody-sensitized, FIPV challenged—kittens developed earlier clinical signs (including pyrexia, icterus, and thrombocytopenia) and died more rapidly than did non-sensitized, FIPV-challenged kittens. Mean survival time in sensitized kittens was significantly (P < 0.05) reduced compared to non-sensitized kittens (mean ± SEM, 10.0 ± 0.6 days vs. 28.8 ± 8.3 days, respectively). Lesions induced included fibrinous peritonitis, disseminated pyogranulomatous inflammation and necrotizing phlebitis and periphlebitis. FIPV antigen, immunoglobulin G, complement (C3) and fibrinogen were demonstrated in lesions by immunofluorescence microscopy.The pathogenesis of dengue hemorrhagic fever (DHF) in persons bears striking resemblance to that of FIP in experimental kittens. In both FIP and DHF, non-neutralizing antibody may promote acute disease by enhancement of virus infection in mononuclear phagocytes or by formation of immune complexes, activation of complement and secondary vascular disturbances.     

Disseminated intravascular coagulation in experimentally induced feline infectious peritonitis

Disseminated intravascular coagulation was induced in kittens by intraperitoneal inoculation of feline infectious peritonitis virus (FIPV). Kittens seronegative to FIPV survived significantly (P less than 0.05) longer than those seropositive to FIPV. Pyrexia, anemia, icterus, hyperbilirubinemia, and elevated concentrations of liver-specific enzymes were detected in the inoculated cats. Lesions induced included disseminated fibrinonecrotic and pyogranulomatous inflammation, hepatic necrosis, and widespread phlebitis and thrombosis. Localization of FIP viral antigen and immunoglobulin G was demonstrated in foci of heptic necrosis by immunofluorescence miroscopy. Lymphopenia, thrombocytopenia, hyperfibrinogenemia, and increased quantities of fibrin-fibrinogen degradation products were present in cats after the onset of clinical illness. Depression of factor VII, VIII, IX, X, XI, and XII plasma activities and prolongation of prothrombin and partial thromboplastin times also developed in infected cats. The accelerated onset of clinical disease and mortality in seropositive kittens vs seronegative kittens and the association of virus and antibody in multiple foci of hepatic necrosis suggest an immune-mediated component is involved in the pathogenesis of this disease.     

An enteric coronavirus infection of cats and its relationship to feline infectious peritonitis

An enteric coronavirus that is antigenically closely related to feline infectious peritonitis virus (FIPV) is ubiquitous in the cat population. This virus has been designated feline enteric coronavirus to differentiate it from FIPV. The virus is shed in the feces by many seropositive cats; in catteries it is a cause of inapparent to mildly severe enteritis in kittens 6 to 12 weeks of age. The virus may produce a more severe enteritis in young specific-pathogen-free kittens. Feline enteric coronavirus selectively infects the apical columnar epithelium of the intestinal villi, from the caudal part of the duodenum to the cecum. In severe infections, there are sloughing of the tips of the villi and villous atrophy. Many cats recovering from the disease remain carriers of the virus. Recovered cats, observed for 3 to 24 months, remained healthy and did not develop peritonitis, pleuritis, or granulomatous disease. The relationship of feline enteric coronavirus and FIPV was studied. Although the viruses were antigenically similar, they were distinctly different in their pathogenicities. The enteric coronavirus did not cause feline infectious peritonitis in coronavirus antibody-negative cats inoculated orally or intraperitoneally nor in coronavirus antibody-positive cats inoculated intraperitoneally or intratracheally. Serologic tests, using FIPV, canine coronavirus, and transmissible gastroenteritis virus of swine as substrate antigens in fluorescent antibody procedures may not accurately identify FIPV infection. These tests do not appear to distinguish between FIPV and this feline enteric coronavirus.     

Antibody, immune complexes, and complement activity fluctuations in kittens with experimentally induced feline infectious peritonitis

Humoral changes were studied in 6 specific-pathogen-free kittens during experimental infection with feline infectious peritonitis virus. Although the incubation period and the duration of the disease differed widely, a similar pattern of rectal temperatures, serum complement values, circulating immune complexes, and antibody titers was found for all kittens during the last 15 to 20 days of life. Antibody formation started 8 to 13 days before death and was accompanied by the appearance of circulating immune complexes with subsequent increased concentrations of complement followed by complement depletion. The data are discussed as evidence for an immune complex pathogenesis of feline infectious peritonitis.     

Attempted immunization of cats against feline infectious peritonitis, using avirulent live virus or sublethal amounts of virulent virus

Kittens vaccinated with an avirulent biotype of the Black strain of feline infectious peritonitis virus (FIPV; given oronasally) developed both indirect fluorescent and virus-neutralizing antibodies, but were not protected against oronasal challenge exposure with virulent virus. In fact, kittens vaccinated with avirulent virus were more readily infected than were nonvaccinated cats. A proportion of kittens could be immunized to FIPV by giving sublethal amounts of virulent virus. This technique, however, was too inconsistent and hazardous to have clinical relevance. The results of these studies indicated that humoral immunity was not protective in FIPV infection. There was no correlation between fluorescent and virus-neutralizing antibodies and either disease or immunity. Immune serum from FIPV-resistant cats failed to passively protect susceptible animals against virulent virus given intraperitoneally or oronasally, and as expected, actually sensitized them to infection. It was concluded that cell-mediated immunity was probably responsible for protection.     

Clinical disease in kittens inoculated with a pathogenic strain of Bartonella henselae

OBJECTIVE: To evaluate disease in kittens inoculated with Bartonella henselae strain LSU16. ANIMALS: Eighteen 12-week-old specific-pathogen-free kittens. PROCEDURE: Kittens were inoculated with B henselae strain LSU16 or saline (0.9% NaCl) solution. Blood samples were collected from kittens on alternate weeks, and bacteremia, clinical signs, and antibody concentrations were monitored for 6 months after inoculation. RESULTS: Kittens developed raised, erythematous areas at the site of inoculation within 72 hours. Swelling peaked at 14 days and resolved by 28 days after inoculation. Fever had a biphasic pattern, with an episode of 1- to 3-days' duration beginning 6 to 7 days after inoculation followed by an episode of 3- to 8-days' duration beginning 11 to 13 days after inoculation. Kittens were bacteremic by day 14 with peak bacteremia at days 14 to 28. Strong antibody responses to B henselae were detected. Clinical disease resolved before bacteremia became undetectable, but signs of disease correlated with the highest degree of bacteremia. Regional lymphadenopathy also was evident. CONCLUSION AND CLINICAL RELEVANCE: Clinical disease in kittens was similar to that in adult cats infected with B henselae strain LSU16, except that lethargy and anorexia were less severe in kittens, and a biphasic pattern of fever was detected in kittens. Clinical disease after inoculation with B henselae may be strain-dependent. To limit transmission of Bartonella organisms, appropriate flea prevention should be instituted. IMPACT FOR HUMAN MEDICINE: Kittens that are febrile, anorectic, lethargic, and that have lymphadenopathy should be tested for Bartonella organisms, and contact with immunocompromised owners should be discouraged.     

The sites of early viral replication in feline infectious peritonitis

The sites of early replication of feline infectious peritonitis virus were studied following oral inoculation of specific-pathogen-free (SPF) cats with virus grown in cell cultures. Viral antigen was first detected by immunofluorescence in the tonsils and small intestine within 24 h of inoculation, and was later found in caecum, colon, mesenteric lymph nodes and liver. However, histological changes in the gut did not appear until relatively late in the course of infection. Virus was recovered from the oropharynx and the faeces from as early as the second or third day after inoculation, and shedding continued until euthanasia.     

A study of naturally occurring feline coronavirus infections in kittens.

Feline coronavirus is a common infection in cats, as indicated by the high prevalence of antibodies against the virus, especially in multicat households. Approximately 5 to 12 per cent of seropositive cats develop classical feline infectious peritonitis. A survey of kittens born into households of seropositive cats demonstrated the existence of healthy coronavirus carriers. Seronegative animals did not appear to excrete virus. No specific antibody titre could be linked to carrier status and some carrier cats subsequently became seronegative. The management of the kittens strongly influenced whether they became infected, and some degree of protection appeared to be conferred by maternally derived antibody. At present, feline infectious peritonitis virus and feline enteric coronavirus can only be differentiated by their different clinical histories in infected catteries. In this survey, cases of feline infectious peritonitis occurred in kittens from households where the initial presentation had been enteritis and vice versa. Therefore no difference in epidemiology could be found.     

Increased plasma levels of leukotriene B4 and prostaglandin E2 in cats experimentally inoculated with feline infectious peritonitis virus

Specific-pathogen-free kittens experimentally infected with feline infectious peritonitis virus (FIPV) subsequently demonstrated increased plasma levels of the arachidonic acid metabolites, leukotriene (LT) B4 and prostaglandin (PG) E2. Significant increases (P<0.025) in LTB4 plasma levels occurred in all (5/5) FIPV-inoculated kittens on postchallenge-exposure days (PCD) 7 and 14 vs PCD 0. Significant increases (P<0.05) in PGE2 plasma levels occurred in 80% (4/5) of FIPV-infected kittens on PCD 7 and 14. Maximal mean plasma levels of LTB4 and PGE2 occurred on PCD 7 (502.5±45.6 pg/ml and 1108.0±247.9 pg/ml, respectively). A positive correlation was found between LTB4 plasma levels and body temperature (r=0.609, P<0.01). Mean survival time in FIPV-inoculated kittens was 19.4±3.2 days. Gross lesions, including peritoneal or pleural effusions (or both) and connective tissue edema, indicated an increased vascular permeability in the FIPV-infected kittens. Histologically, lesions were characterized by pyogranulomatous inflammation. Immunofluorescent studies of tissues from FIPV-infected kittens demonstrated foci of polymorphonuclear leukocytes and FIPV-positive macrophages oriented around dilated blood vessels. Seemingly, arachidonic acid metabolites, including LTB4 or PGE2 released from macrophages, neutrophils or other cells, may be involved in the pathogenesis of FIP vascular and inflammatory lesions and in some of the clinical disease manifestations.     

Attempted immunisation of cats against feline infectious peritonitis using canine coronavirus

Specific pathogen free kittens were vaccinated with an unattenuated field isolate of canine coronavirus (CCV) either by aerosol or subcutaneously, and received boosting vaccinations four weeks later. Aerosolisation elicited a homologous virus-neutralising (VN) antibody response that increased steadily over a four-week period and levelled off one to two weeks after revaccination. The initial aerosolised dose produced an asymptomatic infection with excretion of CCV from the oropharynx up to eight days after vaccination; virus shedding was not detected, however, after the second inoculation. Cats vaccinated subcutaneously developed low VN antibody titres after the first CCV dose and experienced a strong anamnestic response after the second dose. Neutralising antibody titres then levelled off one to two weeks after revaccination at mean values somewhat lower than in cats vaccinated by aerosol. CCV was not isolated from the oropharynx after either subcutaneous dose. Four weeks after CCV boosting inoculations, vaccinated cats and sham-vaccinated control cats were divided into three subgroups and challenged by aerosol with the virulent UCD1 strain of feline infectious peritonitis virus (FIPV UCD1) at three different dosage levels. Five of six cats (including sham-vaccinated controls) given the lowest challenge dose showed no signs of disease, while all other cats developed lesions typical of feline infectious peritonitis (FIP). The five surviving cats developed FIP after subsequent challenge with a fivefold higher dose of FIPV. Thus heterotypic vaccination of cats with CCV did not provide effective protection against FIPV challenge.     

Intratracheal inoculation as an efficient route of experimental infection with maedi-visna virus

Maedi-visna virus (MVV) spreads horizontally via the respiratory route. In order to establish an experimental mucosal infection route, we compared intranasal and intratracheal inoculation using the infectious MVV molecular clone KV1772-kv72/67. For intranasal infection 0.5 x 10(3)-0.5 x 10(7) TCID50 of virus was sprayed into the nostrils of the sheep. For the intratracheal infection 10(0)-10(6) TCID50 of virus was injected into the trachea. Successful infection was indicated by development of MVV specific antibodies and virus isolation over a period of 6 months. In the intranasal infection, only the sheep receiving the highest dose i.e., 0.5 x 10(7) TCID50, became infected, suggesting that intranasal application was not an efficient mode of infection. In the intratracheal infection, the sheep infectious dose 50% was 10(1) TCID50 and virus could be isolated from the central nervous system 4 months post infection with 10(4) TCID50. Therefore it is concluded that intratracheal infection is a very efficient route for experimental inoculation with MVV.     

Efficacy of a Feline Leukemia Virus Vaccine in a Natural Exposure Challenge

A commercial feline leukemia virus (FeLV) vaccine was evaluated in a natural exposure system. All kittens were negative for FeLV antigen on two enzyme-linked immunosorbent assay (ELISA) tests and one indirect immunofluorescence antibody (IFA) test before vaccination or exposure. Twenty-three kittens were vaccinated subcutaneously at nine and 12 weeks of age. The vaccinated kittens and 14 unvaccinated littermates were housed in an infected environment starting at 14 weeks. The kittens were exposed for 24 weeks by living in a large room with one feline leukemia virus-positive, asymptomatic adult cat for each five kittens. Sixty-four percent of the unvaccinated kittens and 70% of the vaccinated kittens became infected as determined by ELISA. Forty-three percent of unvaccinated kittens and 39% of vaccinated kittens died. There was no difference between the infection and mortality of vaccinated kittens that developed antibodies to anti-FeLV glycoprotein 70-envelope antigen and those that did not. Consideration should be given to evaluation of feline leukemia virus vaccines using "street" virus in a natural exposure system.     

Humoral immune response of foals to experimental infection with Rhodococcus equi

Humoral immune response to Rhodococcus equi in experimentally infected foals was studied with the enzyme-linked immunosorbent assay (ELISA) method. Class-specific antibodies were measured by ELISA in the sera of foals after intratracheal or oral inoculation with R. equi ATCC 6939 or T 48 and in the lung washings of a foal after intratracheal inoculation or of normal horses. After intratracheal or oral inoculation with R. equi, serum antibodies were first detected in immunoglobulin G (IgG) followed by IgM and IgA classes, but significant levels of IgM and IgA developed only in the foal infected intratracheally with R. equi T 48. Only the foal infected intratracheally with T 48 developed pneumonia. Anti-R. equi IgG and IgA antibodies appeared in lung washings of the intratracheally infected foal. There were differences in the antibody response to R. equi among the intratracheally infected foals, the orally infected foal and the naturally infected foal. These results suggest that the humoral immune response to R. equi may be affected by the type of R. equi strain and the route and extent of R. equi exposure.     

Kitten mortality in the United Kingdom: a retrospective analysis of 274 histopathological examinations (1986 to 2000)

The postmortem findings in 274 kittens were reviewed. The kittens were grouped by age at death: perinatal (< one day), neonatal (one to 14 days), preweaning (15 to 34 days) and postweaning (35 to 112 days); 203 (74 per cent) of the kittens were postweaning and 38 (14 per cent) were preweaning. Infectious disease was identified in 55 per cent of the kittens, and 71 per cent of the infectious disease was viral and detected significantly more frequently in rescue shelter kittens than in kittens from private homes. Twenty-five per cent of all kitten mortality was due to feline parvovirus (FPV). During the neonatal and preweaning periods, the main viral infections were feline herpesvirus and calicivirus. Feline infectious peritonitis caused the death of 17 kittens in the postweaning period. The rescue shelter kittens were significantly younger than the kittens from private homes (median survival 49 and 56 days) and were more likely to have FPV. The non-pedigree kittens were significantly younger than the pedigree kittens (42 v 56 days), and the pedigree kittens were significantly less likely to originate from rescue shelters. There was no significant difference between the age distribution of the male and female kittens. No diagnosis could be found in 33 per cent of the kittens, and this failure was correlated significantly with the submission of tissue samples as opposed to the whole carcase.     

Virus shedding and immune responses in cats inoculated with cell culture-adapted feline infectious peritonitis virus

Eight specific pathogen-free cats were inoculated orally or parenterally with a cell culture-adapted strain of feline infectious peritonitis virus (FIPV). Faeces and oropharyngeal swabs were monitored daily for infectious virus by inoculation of feline embryo lung cells. Virus was recovered from both sites for approximately 2 weeks after inoculation, before clinical signs of disease developed. Peripheral blood lymphocytes collected from these cats were tested in an in-vitro blastogenic assay using concanavalin A (con A) and FIPV antigen. All cats showed a profound suppression of the response to con A which only recovered to pre-inoculation levels in 2 cats, one of which survived. These 2 cats also responded to FIPV antigen on the 21st day after infection, the greater response being in the survivor. The other cats, surviving 16-18 days, developed no response to FIPV antigen. Antibody titres, measured by immunofluorescence and by virus neutralization, rose rapidly to very high levels in all cats, regardless of the route of inoculation.     

Pathogenesis of feline infetious peritonitis: pathologic changes and immunofluorescence

Feline infectious peritonitis (FIP) was experimentally induced in FIP virus (FIPV) antibody-positive and antibody-negative kittens after challenge exposure to live-virus aerosol. Seropositive kittens developed antiviral immunofluorescence and lesions more rapidly after challenge exposure than did seronegative kittens. In seropositive kittens, FIPV antigen was present in macrophages and large mononuclear cells in tracheobronchial lymph nodes, lungs, and trachea on postchallenge-exposure day (PCD) 2; in liver and spleen on PCD 3; in kidneys and omentum on PCD 4; and subsequently in nasal turbinates, thoracic and abdominal lymph nodes, thymus, bone marrow, parotid salivary gland, eyes, and brain. Initial antiviral immunofluorescence on PCD 2 coincided with the onset of viremia and vascular lesions. Systemic lesions characterized by perivascular necrotizing pyogranulomatous inflammation, phlebitis and thrombosis, fibrinous serositis, and generalized lymphoid necrosis developed on PCD 3 and 4. Coronavirus-like particles were observed by electron microscopy in cytoplasmic vacuoles or smooth endoplasmic reticulum of degenerating macrophages in inflammatory lesions. In seronegative kittens, antiviral immunofluorescence in tracheobronchial lymph nodes was first detected on PCD 5, and viremia occurred on PCD 6. Systemic necrotizing lesions, comparable with those observed in seropositive kittens on PCD 3 or 4, did not occur in seronegative kittens until PCD 13 or 16. In both groups of kittens, initial viral infection in regional lymphoreticular tissue was followed by viremia and infection of macrophages in reticuloendothelial organs (liver, spleen, lymph nodes) and perivascular locations. The accelerated onset of infection and lesions indicative of an Arthus-type reaction in challenge-exposed seropositive vs seronegative kittens further supports the immune-mediated pathogenesis of FIP.     

Effects of anesthesia and surgery on serologic responses to vaccination in kittens

OBJECTIVE: To determine the effects of anesthesia and surgery on serologic responses to vaccination in kittens. DESIGN: Prospective controlled trial. ANIMALS: 32 specific-pathogen-free kittens. PROCEDURES: Kittens were assigned to 1 of 4 treatment groups: neutering at 7, 8, or 9 weeks of age or no neutering. All kittens were inoculated with modified-live virus vaccines against feline panleukopenia virus (FPV), feline herpesvirus (FHV), and feline calicivirus (FCV) at 8, 11, and 14 weeks of age and inactivated rabies virus (RV) at 14 weeks of age. Serum antibody titers against FPV, FHV, and FCV were determined at 8, 9, 11, 14, and 17 weeks of age; RV titers were determined at 14 and 17 weeks of age. RESULTS: Serologic responses of kittens neutered at the time of first vaccination (8 weeks) were not different from those of kittens neutered 1 week before (7 weeks) or 1 week after (9 weeks) first vaccination or from those of kittens that were not neutered. In total, 31%, 0%, 69%, and 9% of kittens failed to develop adequate titers against FPV, FCV, FHV, and RV, respectively, by 17 weeks of age. CONCLUSIONS AND CLINICAL RELEVANCE: Neutering at or near the time of first vaccination with a modified-live virus vaccine did not impair antibody responses in kittens. Many kittens that were last vaccinated at 14 weeks of age had inadequate antibody titers at 17 weeks of age. Kittens may be vaccinated in the perioperative period when necessary, and the primary vaccination series should be extended through at least 16 weeks of age.