Bovine Herpesvirus 4 infections and bovine mastitis

Abstract

Mastitis is an often occurring disease in dairy cattle with an enormous economic impact for milk producers worldwide. Despite intensive research, which is historically based on the detection of bacterial udder pathogens, still around 20-35% of clinical cases of bovine mastitis have an unknown aetiology. Due to the high number of unknown causes of clinical mastitis, studies were undertaken to gain more insight into the role of viruses in this important disease. For the first time, we found that BHV4 may be involved in the aetiology of bovine mastitis, because it was isolated from milk of 3 (5%) out of 58 cows with clinical mastitis and not from the 58 matched control cows. Concomitant with the isolation of BHV4, an increase in antibody titre against BHV4 was noted in two of these three mastitis cows from which BHV4 was isolated. The use of the bovine umbilical cord endothelial cells, a cell type seldom used in bovine virology, proved to be a good choice. Susceptibility studies showed that BHV4 grew to much higher titres in bovine umbilical cord endothelial cells than in the routinely used Madin Darby bovine kidney cells. The bovine umbilical cord endothelial cell type was not only highly susceptible to BHV4, but it also proved to be susceptible to other bovine herpesviruses, such as BHV2 and BHV5. For the detection of BHV4 antibodies only one commercial indirect-ELISA was available. We developed and evaluated an immunoperoxidase monolayer assay (IPMA) for the detection of antibodies directed against BHV4. After validation, the IPMA proved to be a reliable test, and it was found more sensitive for the early detection of BHV4 antibodies than the indirect-ELISA. In addition, a serological survey showed that the estimated seroprevalence of BHV4 in Dutch cattle was 16-18% and that the percentage of BHV4 seropositive cattle varied by age category between 6 and 43%. A polymerase chain reaction for the detection of BHV4-glycoprotein-B (gB) DNA was developed and validated, and a nested-PCR was modified to detect BHV4-thymidine kinase (TK) DNA in bovine milk samples. Both methods proved to be rapid and reliable tests for the screening of BHV4 DNA in milk. In a second case-control study, using these newly developed diagnostic tools, BHV4 and BHV4-gB DNA was detected in milk from 2 (4%) out of 54 cows with clinical mastitis, whereas no BHV4 was detected by virus isolation and PCR in 54 matched controls. An experimental study was performed to examine whether a simultaneous intramammary and intranasal inoculation of lactating cows with BHV4 induced clinical mastitis. No clinical mastitis was noted in the four inoculated lactating cows, but the somatic cell counts increased significantly in milk of 50% of the BHV4 inoculated quarters, compared to the non-inoculated quarters of the same cows and quarters of mock-inoculated cows (control group) on days 8, 9, and 11 post-inoculation. Another interesting finding was that a bacterial infection, Streptococcus uberis infection, triggered BHV4 replication. Based on the studies performed, we conclude that BHV4 may play a role in the aetiology of bovine mastitis, albeit likely a minor one in clinical mastitis. Studies suggest rather a role in subclinical mastitis and an indirect role for BHV4 in the aetiology of bovine mastitis. More research is warranted to investigate its indirect role as a result of e.g. immunosuppression.

In a review study, the role of viruses in the aetiology of bovine mastitis has been noted. Bovine herpesvirus 1, bovine herpesvirus 4, foot-and-mouth disease virus, and parainfluenza 3 virus have been isolated from milk from cows with clinical mastitis. Intramammary inoculations of bovine herpesvirus 1 or parainfluenza 3 virus induced clinical mastitis, while an intramammary inoculation of foot-and-mouth disease virus resulted in necrosis of the mammary gland. Subclinical mastitis has been induced after a simultaneous intramammary and intranasal inoculation of lactating cows with bovine herpesvirus 4. Bovine leukaemia virus has been detected in mammary tissue of cows with subclinical mastitis, but whether this virus was able to induce bovine mastitis has not been reported. Bovine herpesvirus 2, vaccinia, cowpox, pseudocowpox, vesicular stomatitis, foot-and-mouth disease viruses, and bovine papillomaviruses can play an indirect role in the aetiology of bovine mastitis. These viruses can induce teat lesions, for instance in the ductus papillaris, which result in a reduction of the natural defence mechanisms of the udder and indirectly in bovine mastitis due to bacterial pathogens. Bovine herpesvirus 1, bovine viral diarrhoea virus, bovine immunodeficiency virus, and bovine leukaemia virus infections may play an indirect role in bovine mastitis, due to their immunosuppressive properties.

Finally, based on the studies performed during this thesis and the review study on viral infections and bovine mastitis, we may conclude that viruses can play a direct or indirect role in the aetiology of bovine mastitis.