Comparative investigation of tissue alterations and distribution of BVD-viral antigen in cattle with early onset versus late onset mucosal disease

[Persistent form of bovine viral diarrhea]

The review provides an analysis of literature data on the persistent form of Bovine Viral diarrhea/Mucosal disease (BVD) and is focused on virus and host factors, including those related to immune response, that contribute the persistence of the virus. BVD is a cattle disease widespread throughout the world that causes significant economic damage to dairy and beef cattle. The disease is characterized by a variety of clinical signs, including damage to the digestive and respiratory organs, abortions, stillbirths and other failures of reproductive functions.

The Molecular Biology of Pestiviruses

Pestiviruses are among the economically most important pathogens of livestock. The biology of these viruses is characterized by unique and interesting features that are both crucial for their success as pathogens and challenging from a scientific point of view. Elucidation of these features at the molecular level has made striking progress during recent years. The analyses revealed that major aspects of pestivirus biology show significant similarity to the biology of human hepatitis C virus (HCV). The detailed molecular analyses conducted for pestiviruses and HCV supported and complemented each other during the last three decades resulting in elucidation of the functions of viral proteins and RNA elements in replication and virus-host interaction. For pestiviruses, the analyses also helped to shed light on the molecular basis of persistent infection, a special strategy these viruses have evolved to be maintained within their host population. The results of these investigations are summarized in this chapter.

Clinical appearance and pathology of cattle persistently infected with bovine viral diarrhoea virus of different genetic subgroups

Bovine viral diarrhoea (BVD) is an economically important cattle disease with a world-wide distribution that is caused by BVD virus, a pestivirus of the flaviviridae family. BVD viruses are genetically highly variable. They are classified into two genetic species (BVDV-1 and -2) that are further divided into numerous subgroups, particularly for BVDV-1. The complexity of these viruses is also reflected in their interaction with the host animals. Infections are either transient or persistent and can cause a wide spectrum of clinical signs, from no or very mild disease to severe forms, reminiscent of viral haemorrhagic fevers. In this work, we have analysed the clinical signs and the pathology of BVD viral infections in a cattle population where different subgroups of BVDV-1 genotype viruses are endemic. In addition, we have examined potential virulence properties of BVDV-1 subgroups during persistent infection by comparing the viral subgroups present in clinical cases with those detected in persistently infected (PI) animals sampled for epidemiological criteria, irrespective of their health condition. Furthermore, the clinical and postmortem findings were compared with respect to genetic characteristics of the viruses isolated from these animals. Our results indicate that the BVDV positive animals fall roughly into two categories, depending on the primary organ affected and the age, with lung-centred pathology occurring mainly in young animals and mucosal pathology predominantly in older animals. Furthermore, we found a markedly higher proportion of representatives of the BVDV-1e subgroup in stillborn calves and aborted foetuses originating from epidemically unrelated cattle herds, suggesting that BVDV-1e may play a special role in prenatal and perinatal losses.

Systemic reactive angioendotheliomatosis-like syndrome in a steer presumed to be persistently infected with bovine viral diarrhea virus

Unusual proliferative intravascular lesions were seen in multiple organs of a 2-year-old Corriente steer presumed to be persistently infected with bovine viral diarrhea virus (BVDV), based on widespread immunohistochemical detection of BVDV antigen. Proliferations of spindle cells, which were immunohistochemically positive for von Willebrand factor-related antigen, partially-to-completely occluded vessel lumens and were supported by cells that were immunohistochemically positive for smooth muscle actin. Distribution and character of the intraluminal proliferations are strikingly similar to those described in feline systemic reactive angioendotheliomatosis, a rare entity of unknown cause. The presence of occasional intravascular thrombi suggests that the proliferative vasculopathy was associated with an underlying thrombotic process with immunohistochemical similarities to thrombotic thrombocytopenic purpura of humans. Death of the steer was due to hemorrhage from a castration wound, which may indicate thrombocytopenia or platelet dysfunction. The role of persistent BVDV infection in the formation of the intravascular lesions is unknown.

Co-existence of genetically and antigenically diverse bovine viral diarrhoea viruses in an endemic situation

Bovine viral diarrhoea virus (BVDV) is an important cattle pathogen that causes acute or persistent infections. These are associated with immunotolerance to the viral strain persisting in animals that became infected early in their intrauterine development. To this date, the epidemiology of BVD in Switzerland runs virtually undisturbed by control measures such as restrictions on animal traffic or vaccination. Here, we analysed the viral genetics of 169 Swiss isolates and carried out crossed serum neutralisation tests to assess the antigenic spectrum of BVDV strains present in the cattle population. Besides confirming the presence of BVDV type 1 subgroups b, e, h and k, a single "orphan" BVDV-1 virus was detected that does not belong to any known BVDV-1 subgroup. No BVDV type 2 viruses were detected, suggesting that they are rare or not present in the cattle population. Antigenic comparison revealed significant differences between the different subgroups, with anti-1k immune serum having up to tenfold lower neutralising activity against 1b, 1e and 1h subgroup viruses, which however may still suffice to protect 1k-immune animals against superinfection by viruses of those other subgroups. Serum from routinely vaccinated animals revealed generally low titres but good cross-neutralisation. A geographic information system revealed that the viruses of the different subgroups are distributed in an apparently randomised fashion in the cattle population. This geographic distribution pattern may reflect peculiarities of the management practice in the Swiss cattle industry that, especially through annual transhumance of up to 25% of the entire population in the alpine region, tend to optimise the spread of BVDV.

Oesophagoscopy and detection of viral nucleic acids in oesophageal biopsies--A contribution to BVDV diagnosis

The endoscopic appearance of the oesophagus of animals infected with bovine virus diarrhoea virus (BVDV) but without signs of acute mucosal disease (MD) was investigated for any common or 'early warning' lesions. Thirty-seven BVDV-infected animals [36 persistently infected (PI) and one transiently infected] were examined clinically and endoscopically for typical erosions of the oronasal and oesophageal mucosa, respectively. During oesophagoscopy, mucosal biopsies were taken and tested for pestivirus-specific nucleic acids by reverse transcription polymerase chain reaction (RT-PCR). The results were compared with those of RT-PCR with various sample materials (blood, nasal and conjunctival swabs and faecal samples) usually used for routine diagnosis of BVDV infection. The average age of the 36 PI animals was 13.5 months, eight animals were 24 months or older. None of the PI animals had clinically visible erosions of the oronasal mucous membranes. During oesophagoscopy, all animals showed a reddening but unexpectedly no erosions or ulcerations of the oesophageal mucosa. Although in 21 of 36 PI animals all samples tested positive, the oesophageal biopsies were with no exception constantly RT-PCR positive in contrast to all other materials investigated. Remarkably, in the transiently infected animal all samples tested negative except for the oesophageal biopsy. Hence, this infection would have been missed by conventional diagnostic sampling.

Lesions and tissue distribution of viral antigen in severe acute versus subclinical acute infection with BVDV2

Differences in the distribution and spread of viral antigen, development of lesions and correlation between presence of viral antigen and lesions were compared between a low and highly virulent strain of BVDV2. Two groups of two-week- to two-month-old colostrum-deprived calves were inoculated intranasally with the naturally occurring low virulent BVDV2 strain 28508-5 or the highly virulent strain 1373. To study the sequence of virus spread and lesion development, calves were necropsied at days three, six, eight-nine and 12 to 14 post inoculation (pi). Viral antigen was detected by the indirect immunoperoxidase method in cryostat sections and lesions were evaluated in H&E-stained paraffin sections. Clinical signs and changes in lymphocyte and thrombocyte numbers confirmed the difference in virulence between the two strains. Both strains showed comparable initial infection and spread at day three pi. At day six pi, they were found widespread in lymphoid tissues and multifocally in intestinal mucosa. Lesions were very mild despite the large amount of antigen in the lymphoid tissues. After day six pi, differences between the low and highly virulent strains became more prominent. The strain of low virulence was cleared from the tissues, but there was a transient phase of depletion. The highly virulent strain continued to spread to different organs and there was severe depletion of lymphoid tissues without recovery.