Persistent infection of cattle with the herpesvirus of malignant catarrhal fever and observations on the pathogenesis of the disease

Febre catarral maligna em bovinos no Rio Grande do Sul: transmissão experimental para bovinos e caracterização do agente etiológico

São relatados dois surtos de febre catarral maligna (FCM) em bovinos de duas propriedades rurais (A e B) do município de Santiago, Rio Grande do Sul (RS), a transmissão da doença a bovinos suscetíveis e a detecção de DNA viral de herpesvírus bovino-2 (OvHV-2) em tecidos de bovinos afetados. Os dois surtos ocorreram de novembro de 2001 a fevereiro de 2002 (Propriedade A) e de janeiro a fevereiro de 2003 (Propriedade B). O número de bovinos sob risco, as taxas de morbidade e de letalidade foram, respectivamente, 170, 10,59% e 83,33% na Propriedade A e 500, 2,4% e 100% na Propriedade B. Em ambas as propriedades havia contato de ovinos com os bovinos afetados, mas somente na Propriedade A havia ovelhas em parição. Nos bovinos afetados nas duas propriedades, a duração do curso clínico, os achados de necropsia e a histopatologia foram semelhantes. A maioria dos bovinos afetados morreu ou foi submetida à eutanásia in extremis após um curso clínico de 2 a 8 dias. Os sinais clínicos incluíam febre (40,5 e 41,5°C), corrimento nasal e ocular, opacidade da córnea, conjuntivite, salivação, erosões e ulcerações em mucosas, diarréia, hematúria e distúrbios neurológicos. Foram realizadas onze necropsias (nove na Propriedade A e duas na Propriedade B). Lesões macroscópicas incluíam erosões e úlceras nas mucosas dos cornetos nasais, cavidade oral e tratos gastrintestinal e urogenital; hemorragia e necrose da ponta das papilas bucais, aumento de volume dos linfonodos, múltiplos focos brancos no córtex renal e hiperemia das leptomeninges. Microscopicamente, havia arterite e degeneração fibrinóide em artérias de médio e pequeno calibre e em arteríolas de múltiplos órgãos e tecidos, necrose e inflamação em várias superfícies mucosas, ceratite, conjuntivite, uveíte, nefrite intersticial e encefalite. A transmissão experimental foi tentada em cinco bezerros (E1-E5) através da inoculação de cada um deles, por via intravenosa, com 500 ml de sangue total heparinizado oriundo de bovino afetado por FCM. A transmissão foi conseguida em pelo menos três (E1-E3) dos bezerros experimentais que adoeceram após um período de incubação de 15 a 27 dias. Quatro dos bezerros do experimento morreram ou foram submetidos à eutanásia in extremis após um curso clínico que durou de 3 dias a 8 semanas. O bezerro experimental remanescente (E5) recuperou-se após uma doença branda e foi submetido à eutanásia 14 semanas após a inoculação. Os cinco bezerros foram necropsiados. Sinais clínicos, achados de necropsia e histopatologia de três bezerros (E1-E3) eram característicos de FCM. O DNA viral de OvHV-2 foi detectado pela técnica de reação em cadeia de polimerase (PCR) em tecidos emblocados em parafina de sete dos 11 bovinos espontaneamente afetados por FCM e em tecidos emblocados em parafina de três bezerros experimentais (E1-E3). A técnica de PCR resultou negativa nos restantes quatro dos 11 bovinos testados nos casos espontâneos de FCM e em dois (E4-E5) dos cinco bezerros usados nos experimentos de transmissão. Testes de imunoistoquímica realizados em cortes de tecido linfóide do bezerro E4 resultaram negativos para antígeno do vírus da diarréia viral bovina. A transmissão experimental de FCM de bovino para bovino e a caracterização do agente etiológico da doença em bovinos como OvHV-2 foi conseguida pela primeira vez no Brasil.

The pathology of "sheep-associated" malignant catarrhal fever in the hamster

Lesions induced in hamsters by inoculation with the "sheep-associated" agents of malignant catarrhal fever (SA-MCF) isolated from a red deer (Cervus elaphus), designated D/1 and of bovine origin (C/2), are described. Clinical signs in hamsters inoculated with the D/1 isolate occurred as early as 13 days after infection although the mean incubation period in animals that developed signs was 27 days. Increased numbers of polymorphonuclear leucocytes were present in the blood of clinically affected hamsters. Gross lesions included erosions of epithelium in the buccal cavity, haemorrhage of the forestomach, dilated fluid-filled intestines and enlargement of the mesenteric lymph node. Microscopic lesions were widespread throughout the body but had a predilection for epithelial surfaces. They consisted of hyperplasia of certain lymph nodes, vasculitis and interstitial accumulations of mononuclear cells of lymphoid appearance in non-lymphoid tissues. Cytolysis was also seen. Lesions produced by the C/2 isolate were similar and both isolates produced disease comparable with that seen in naturally occurring cases in cattle and deer. It is suggested that disease might arise through a dysfunction of the immune system following infection of host large granular lymphocytes by the SA-MCF agent, in a way similar to that suggested for the rabbit.

Course of malignant catarrhal fever in immunosuppressed and immunostimulated rabbits

Rabbits pretreated with cyclophosphamide for 7 days (CY+) and control rabbits (CY-), were infected with cell-associated malignant catarrhal fever virus (MCFV), and CY treatment of the CY+ group was continued to day 20 p.i. The time of onset and degree of antibody formation to bovine serum albumin was significantly suppressed in the CY+ group whilst the humoral antibody response to MCFV appeared to be delayed rather than suppressed. Although incubation periods of the disease were comparable in the CY+ and CY- groups, the disease course was much shorter in the CY- group, 100% mortality occurring within a mean of 4.5 days, as opposed to a mean of 15 days in CY+ animals. Mortalities did not occur in the CY+ group until a mean of 5.5 days following withdrawal of CY. In further experiments it was shown that pre-existing antibody to MCFV had no effect on the incubation period and course of MCF in rabbits. The results thus suggest that MCF may not be an immune complex disease.

Immune complexes associated with infection of cattle by the herpesvirus of malignant catarrhal fever

Associated with the fatal lymphoid proliferation induced by the herpesvirus of African malignant catarrhal fever (MCFV) was the deposition of immune complexes comprising immunoglobulin, complement and conglutinin in the kidneys of cattle with terminal pyrexia due to infection with this virus. Such deposits were common in glomeruli and were also seen in renal arteries with an without lesions. Absence of lytic complement and a significant depletion of conglutinin in terminal sera confirmed the activation of complement. However, virus antigen was only rarely detected and virus-specific antibody could not be demonstrated in the deposits. These disparities and the importance of the observations are discussed.

Immunoglobulins, haemolytic complement and serum C3 in cattle infected with malignant catarrhal fever herpesvirus

Serum levels of the third component of complement (C3) were reduced only in the terminal stages in malignant catarrhal fever (MCF) virus-infected steers. Haemolytic complement and immunoglobulin (IgM, IgG1 and IgG2) levels were not altered. C3 and immunoglobulin deposits were also not demonstrated in the vascular lesions induced by MCFV. MCF virus infection of cattle is probably not a typical immune complex-mediated disease as previously suggested.

The pathomorphology of malignant catarrhal fever. II. Multisystemic epithelial lesions

Epithelia from 18 Holstein-Fresian calves with experimentally induced malignant catarrhal fever and two calves with naturally occurring disease had severe destructive inflammatory epithelial lesions. Calves were necropsied at early, mid and late clinical stages. Multifocal, degenerative and necrotic epithelial lesions were associated with lymphoid cells. Ultrastructurally, numerous lymphocytes, lymphoblasts and macrophages were accumulated between epithelial cells and in lamina propria, vessels and perivascular tissues. Epithelial cells in such lesions had variably severe degenerative changes or were necrotic but there was no proliferative response. Lesions were widespread in all calves and affected oral, ocular, gastrointestinal, ductal, urinary tract, choroid plexus and other epithelial tissues. Calves killed in late clinical stages had more severe lesions than those killed early. The predominantly lymphocytic, invasive-destructive pattern seen in tissues of cattle with malignant catarrhal fever was similar to that seen in such conditions as contact hypersensitivity and graft versus host disease. Viral structures were not seen.

The pathomorphology of malignant catarrhal fever. I. Generalized lymphoid vasculitis

A histologic, ultrastructural and immunohistochemical study of the vasculitis in the rete mirabile of 18 calves with experimental and two calves with spontaneous malignant catarrhal fevel showed lesions similar to those reported in graft rejection and some other lymphocyte-associated vasculopathies. The calves were examined at early, mid and late stages of clinical disease. Vascular lesions in the carotid rete were representative of those in other tissues examined. The magnitude of the arterial wall mononuclear cell infiltrate and the degree of arterial wall necrosis and degeneration increased with progression of the clinical disease. The predominant site of mononuclear cell accumulation was the adventitia. There was less medial and intimal involvement. The mononuclear infiltrate was composed of lymphocytes and macrophages with neutrophils and plasma cells rarely present. Endothelial hyperplasia, fibrinoid change and thrombosis were uncommon. Many changes contrasted with those recognized in an active Arthus reaction where adventitial change was minimal and most inflammatory cells were neutrophils or plasma cells. IgG and C3 rarely were detected in vessel walls with fluorescent antibody techniques. Serum immunoglobulins as measured by radial immunodiffusion were not elevated. Viral structures were not seen in tissues examined ultrastructurally.