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This review summarizes the results of a study on rotaviruses isolated from calves affected by neonatal diarrhea. The results indicated that rotavirus infection is widespread and supported the evidence for an etiologic role of these viruses in neonatal diarrhea. Differences in virulence among bovine rotaviruses appeared also to be confirmed. Conventionally reared calves were fully susceptible to the experimental infection induced by rotaviruses originating from heterologous hosts, i.e. monkeys, pigs and rabbits. When rotavirus strains of bovine, simian and rabbit origin were compared by cross neutralization tests, it was found the simian and porcine strains were indistinguishable and both appeared to relate antigenically to the bovine strain. Finally, it was proven that feeding newborn calves with colostrum and first milk of their dams, previously vaccinated with an inactivated adjuvanted rotavirus vaccine, could prevent the neonatal diarrhea from occurring.

An inactivated bovine rotavirus vaccine, prepared with an adjuvant which gives a water-in-oil emulsion (Montanide ISA 50, adjuvant) was used in field trials for prevention of calf neonatal diarrhoea. The vaccine, which was designated as Romovac 50, when injected into cows in their last month of pregnancy, proved to be as effective as the traditional vaccine prepared with Freund's incomplete adjuvant. Thus, the incidence of the naturally occurring neonatal diarrhoea was significantly reduced in calves which were fed mammary secretion from their vaccinated dams, compared to the calves delivered from the unvaccinated cows. Romovac 50 also has the advantages over the traditional Freund's adjuvanted vaccine in that it is less viscous and thus more readily injected and less irritant to tissues.

Field trials were conducted using an inactivated rotavirus vaccine for prevention of calf neonatal diarrhea. For the trials, 458 pregnant cows from 26 herds were involved. In each herd, cows which had been inseminated within a period of two months were selected and randomly subdivided in two groups. Cows in one group (248 head in total) were vaccinated 6 weeks before calving and again 4 weeks later; cows in the other group (210 head in total) were left as unvaccinated controls. At calving, colostrum was collected from each cow and stored at -30 degrees C until used for feeding calves. The newborn calves, beginning the second day of life and for the next 7-10 consecutive days, each was fed a daily supplement of 400 ml of colostrum from its dam. The diarrhea occurred in 86 (40.9%) calves that had received colostrum from unvaccinated dams (normal colostrum), and in 7 (2.8%) calves which were fed colostrum from vaccinated dams (immune colostrum). The disease was very severe in the normal colostrum-fed calves and 52 of them died. Those calves which survived the disease underwent a significant loss of condition. By contrast, the 7 immune colostrum-fed calves displayed a rather mild enteric condition, and all recovered without any sequela being observed.

This presentation summarizes the results of a comprehensive study on rotaviruses isolated in Italy from calves and rabbits affected by neonatal diarrhea. The results clearly indicated that rotavirus infection is widespread and supported the evidence for an etiologic role of these viruses in neonatal diarrhea. The evidence of differences in virulence among bovine rotaviruses appeared also to be confirmed. Conventionally reared calves were fully susceptible to the experimental infection induced by three rotaviruses originating from heterologous hosts, i.e. monkeys, pigs and rabbits, respectively. When rotavirus strains of bovine, simian, porcine and rabbit origin were compared by cross neutralization tests, it was found the simian and porcine strains were indistinguishable and both appeared to relate antigenically to the bovine strain. On the other hand, a reciprocal antigenic correlation was found between bovine and rabbit isolates. Finally, it was proven that feeding newborn calves with colostrum of their dams, previously vaccinated with an inactivated rotavirus vaccine, could prevent the neonatal diarrhea from occurring.

Pregnant cows were given the first injection of an inactivated bovine rotavirus vaccine approximately 4 weeks before calving and a second injection just before calving. This led to the enhancement of rotavirus antibody titers in their colostrum as well as in the milk for at least 5 days after parturition. Thus, when newborn calves were fed with the mammary secretions obtained from the vaccinated cows daily for 5 consecutive days they were fully refractory to experimental infection with 81/36F bovine rotavirus. By contrast, the calves which were given the mammary secretions from unvaccinated cows, had clinical signs consistent with rotavirus infection and viral shedding. Based on these results it is suggested that vaccination of cows according to the scheme followed in this experiment, i.e., two injections within the last month of pregnancy, might be a valid approach which depending on confirmation under field conditions, could help reduce the incidence of rotavirus-induced diarrhea in newborn calves.

An inactivated rotavirus vaccine was prepared with an adjuvant which gives a water-in-oil emulsion. The vaccine when injected into cows in their last month of pregnancy, proved to be as effective as the traditional vaccine, prepared with the Freud's incomplete adjuvant, in enhancing the lactogenic immunity. Thus, feeding experimentally infected calves with colostrum and first milk from vaccinated cows prevented diarrhea and reduced significantly rotavirus shedding. Because of its low grade viscosity the water-in-oil emulsifier facilitates inoculation of the vaccine and is therefore recommended as an adjuvant in the preparation of inactivated rotavirus vaccine.

Four cytopathic strains (81/32F, 81/36F, 81/40F, 82/80F) of bovine rotavirus were shown to be pathogenic for conventionally reared newborn calves. Calves were infected orally, using 3 calves for each isolate. All became febrile, were depressed and diarrhoeic. Two calves, one of which in the group of those infected with 81/36F isolate, and the other infected with strain 81/40F, were killed when moribund. A 3rd calf from the 81/36F infected group, died. At necropsy localized lesions of the small intestines, which are considered to be typical of rotavirus infection, were found. Virus was consistently isolated from the fecal samples of the inoculated calves up to 13 days post-inoculation. It was speculated that some differences existed in the virulence of the bovine rotaviruses tested. The cross protection tests revealed that 1 strain (81/36F) might be antigenically more complex than the others.