Bovine parvovirus infection

G R THOMSON

GENERAL INTRODUCTION: PARVOVIRIDAE

Introduction and aetiology

Six isolates of a previously unidentified virus were made from the faeces of clinically healthy calves on widely separated farms in Maryland (USA) in 1959.¹ This virus was initially named HADEN in view of its haemadsorbing abilities and location (HaemADsorbing, ENteric virus). It was subsequently shown that HADEN virus is a typical autonomous parvovirus (genus Parvovirus) and was then named bovine parvovirus (BPV).^{2, 17, 22}

Limited epidemiological investigation and studies into the pathogenesis of BPV infection have been conducted and there is uncertainty as to its pathogenic potential in cattle. There is no information on the presence of the agent in southern Africa, although it is possibly ubiquitous.¹⁶

Bovine parvovirus has most features that are common to other viruses within the genus Parvovirus¹⁷ (see Porcine parvovirus infection), but there are apparent differences between the structural proteins of BPV and those of most other autonomous parvoviruses. Mature virions of BPV are made up of four, as opposed to three, structural proteins and there are also small differences in molecular mass.¹⁵ However, the fourth protein of BPV appears to exist merely as the result of enzymatic cleavage of VP3.¹⁵.

Antigenic similarities between the viral proteins of a wide variety of autonomous parvoviruses have been demonstrated, but those of BPV and goose parvovirus show no such homology either with other parvoviruses or each other.14, 15 It is also possible that there may be two antigenic variants of BPV, viz. types 1 and 2.⁹

The complete nucleotide sequence of the BPV genome shows it to comprise 5 491 nucleotides with terminal regions containing non-identical, imperfect palindromic sequences of 150 and 121 nucleotides.⁶ As might be expected from the lack of antigenic similarity between BPV and other parvoviruses, the overall DNA homology between BPV and other members of the genus is low, although several small regions of high homology occur in the left and right open reading frames.⁶

Bovine parvovirus replicates in a wide variety of bovine and water buffalo (Bubalus bubalis) cell types in vitro, but foetal lung and spleen cells appear to be most suitable for diagnostic purposes.^{4, 5, 11} Continuous cell lines are generally ineffective. Plaque titration, despite the fact that the cytopathic effect produced in cell cultures is often difficult to distinguish from cell degeneration, has been achieved.⁷ The use of serum to induce parasynchrony of cells in primary cultures apparently improves virus isolation.²¹

The presence of the virus in tissues and organs can be detected and measured by haemadsorption and haemagglutination. The haemagglutinin is mainly cell-associated¹⁹ and erythrocytes of horses, sheep, goats, guinea pigs, hamsters, ducks, geese, rhesus monkeys and humans are agglutinated, but not those of mice, rabbits, chickens and cattle.^{1, 8}

Epidemiology

Serological studies show that BPV infection is widespread in the USA,^{1, 3, 5, 18, 20} with an antibody prevalence rate of between 14 and 100 per cent in infected herds.²⁰ The infection has also been identified in Japan and Algeria.^{8, 21}

Although not proven, it can be assumed that infection is transmitted predominantly by the faecal–oral route. The virus has, however, also been isolated from nasal swabs¹⁸ and from conjunctival secretions and the tonsils of infected cattle.²¹ It has been isolated intermittently from the faeces of calves which have antibodies to the virus in their sera,^{19, 21} indicating the possibility of there having been prolonged excretion.

Antibody to BPV has been detected in the sera of pigs and humans but this has been ascribed to ingestion of bovine milk or milk products containing the virus.^{12, 13} Presumably calves can also be infected in this way.

Pathogenesis, clinical signs and pathology

Viraemia, associated particularly with the leukocyte fraction of blood, lasting four to six days, has been detected in calves infected experimentally by both oral and intravenous routes.^{19, 23} Thereafter, BPV and its antigens were found in a wide range of tissues,²¹ particularly lymphoid tissues, the adrenals and throughout the intestinal tract, especially the jejunum and ileum. In the intestine, a variety of cell types is infected, fluorescent antigen having been shown in the epithelium as well as in the lamina propria. In the epithelium, cells at the tips, bases and transition zones of the villi, as well as at the crypts of Lieberkühn, all fluoresced in experimentally infected calves.¹⁹ Infection of enterocytes appears to be cytocidal.^{19, 21}

Diarrhoea in both naturally and experimentally infected calves has been reported,^{5, 18, 21} and in one instance the diarrhoea was followed by apparent poor growth in some of the calves.²¹ On the other hand, the virus has been recovered from clinically normal animals.¹ Other potentially pathogenic agents were also present in some animals where diarrhoea was associated with this infection.^{8, 18}

The detection of antibody to BPV in bovine foetal serum, as well as in foetuses after the experimental inoculation of pregnant cows, has shown that the virus is able to cross the placental barrier.²³ Experimental studies suggest that foetuses are most susceptible during the first and second trimesters of gestation. Infection has been detected in the lymphatic tissues...