M.C.V. Ramón Alfredo Delgado González
Universidad Autónoma Agraria Antonio Narro, Unidad LAGUNA
The infectious agents that cause diarrhea with more frequency in newborn calves kept in dairy farms are rotavirus, coronavirus and strains of E.coli enterotoxigenic, Cryptosporidium spp and Salmonella spp, which are manifested as mixed infections. However, up to date, the rotavirus infections have proved to be a common cause of diarrhea in calves, economically important throughout the world.
The Bovine rotavirus (BRV) recovered from calves with diarrhea in Nebraska, USA, in 1969. This virus was initially referred to as viruses of the neonatal diarrhea of calves, the virus of diarrhea in the calf of Nebraska, Reo-like virus, or agent reovirus-like, and the name rotavirus was finally chosen due to the appearance of a wheel of the virus in transmission electron microscopy.
The age and immune status of the animals, virulence and dose of the strain of BRV, presence of other enteropathogens and various management factors and environment influence the severity of the disease. Although the management practices can reduce the exposure of animals susceptible to BRV, high concentrations of BRV in the stool, their long-term stability in the environment, and the low doses required for infectivity suggest that the eradication of BRV in infected herds is unlikely, if not impossible. The BRV is resistant to solvents in lipids, is very stable at pH 3 and can be kept the viable virus in the stool to room temperature after 7 months.
The genome of the rotavirus (RV) encodes six structural proteins (VP1-VP4, VP6-VP7) and six non-structural proteins (NSPS1-NSP6). The RV was classified in 7 groups (A-G) and these in turn serotypes. The rotavirus group A is classified in serotypes according to the capsid proteins VP7 external (type G) and VP4 (Type P). To date, there are 14 types G the rotavirus group A with G1, G6, G8 and G10 described in isolates of BRV. The implementation of the serotyping provides epidemiological data on the distribution of the serotypes of BRV in beef cattle or dairy, an important consideration for the development of a better vaccine of BRV.
After infection by RV develops extensive cellular necrosis of the epithelium of the small intestine, which leads to the atrophy of the villi, loss of digestive enzymes, the reduction in the absorption and the increase of the osmotic pressure in the intestinal lumen and the onset of diarrhea. This is followed by a reactive hyperplasia of cells of the crypts accompanied by the increase in the secretion of fluid, which also contributes to the severity of diarrhea.
The non-structural protein NSP4 acts as a viral enterotoxin and secret from infected cells, it produces diarrhea before the detection of histological damage. The calves are susceptible to diarrhea by BRV until 8 weeks of age; by the third week of life, the susceptibility decreases with increasing age. The effects of diarrhea by BRV are large economic losses due to the reduction of the weight gain in the affected animals and the costs of treatment Clinical signs and lesions.
The abomasums typically contains milk curd and thick saliva. The content of stool in the small intestine and colon are liquid. The virus replicates in the mature epithelial cells of the villi, viral infection redirects the function of cells of the absorption for the production of virus and the digestive fluids and the partially digested milk accumulates in the intestinal lumen.
The incubation period is 12 to 13 h, or over a longer period depending on the amount of virus in the environment. The animals have depression, anorexia, excessive salivation, and diarrhea. The period of diarrhea lasts from 5 to 6 h, showing liquid stools of yellow color. The stool volume depends on the amount of milk consumed. Twenty-four hours after the onset of diarrhea, the calf seems normal breastfeeding and has stool pasty. In cases of a bad calostral, a mixed infection with E. coli and rotavirus causes a high mortality of up to 50%. Sometimes the calves die before the onset of diarrhea.
The calves are born agamaglobulinemics, and antibodies transferred in mammary secretions are absorbed into the blood by a limited time after birth (up to ~ 48 h), and from then on, continue to operate locally to provide passive immunity to the mucosa. The majority of the adult cows is seropositive to BRV and transferred several degrees of passive immunity to their descent through the mammary secretion during lactation. Due to the young calves are more severely affected by enteric viral infections, the transfer of optimal passive immunity plays a fundamental role in their protection.
There is a multi-purpose inactivated vaccine developed for the prevention and the specific prophylaxis of enteric infections caused by bovine rotavirus, bovine coronavirus and three serotypes of E. coli enterotoxigenic with protective antigen K 99. The vaccine is to be used for the immunization of pregnant cows, as live attenuated vaccines against rotavirus lack of efficacy when administered by the oral route to calves in the field because the antibodies in the colostrum inhibit the replication of the virus in the vaccine.
The presence of antibodies against rotavirus in the colostrum led to strategies for maternal vaccination against rotavirus for increasing lactogenic immunity and transfer passive antibodies to infants through the colostrum and milk. The success of the vaccines against rotavirus maternal field is influenced by the dose of vaccine, ECA, the Agentinactivation, the adjuvant, and the route of administration and the levels of environmental exposure of rotavirus. Passive immunity against BRV is associated with the frequent ingestion of colostrum and milk, which contain high levels of specific antibodies.
The antibodies absorbed by a parenteral route, unless they are present at very high titers, seem to be of lesser value than the local antibodies of colostrum and milk in the protection of enterocytes. The stimulation of immunity activated by the oral administration of live attenuated BRV to newborn calves seems to have success in experimental studies and in some field trials. However, its effectiveness was poor according to certain field studies or as reported by some of the surveys on rotavirus diarrhea in calves. Although it can occur a loss of cross protection due to infection with different serotypes of BRV, most of the researchers suggest that under field conditions, the failure of oral vaccination is mediated by the interference of the maternal antibodies. The widespread presence of these antibodies in cows and the degree of attenuation of virus needed to ensure the safety of the live vaccine in newborns limits the feasibility of this approach. An additional problem is that newborns may be exposed to a virulent field strain of BRV before it can be induced a protective immune response by the vaccine. The need to manage and to vaccinate each calf shortly after birth also raises issues of management in large herds of beef.
Recombinant vaccines have been produced of viral multiproteic subunits. Such particles of rotavirus represent a new generation of vaccines that are not infectious, stable, antigenically authentic, and highly immunogenic.
The local passive immunity is also an effective strategy to control the disease produced by BRV Technology IgY (the use of egg yolk immunoglobulin of the hen) is an economic alternative and practice to prevent diarrhea by BRV in dairy calves. In this case, antibodies avian remain intact after the passage through the gastrointestinal tract. The duodenum is the main site of secretion of BRV-specific antibodies. The components of the egg yolk modulate the immune response against infection by BRV at the level of the mucous membranes. These results indicate that supplementation of the diets of newborn calves in the first 14 days of life with the egg yolk enriched in IgY BRV-specific represents a promising strategy for the prevention of diarrhea due to BRV. On the other hand induces a strong cellular immune response in the intestinal mucosa after infection by BRV, after administration of egg yolk, without taking into account the specificity of the treatment.
Rotavirus in calves of Mexico
Studies in diarrheal stools of dairy calves with test inmunoabsorbent linked to enzymes for diagnosis of rotavirus show a 48% of positivity to the presence of the BRV in the Comarca Lagunera and 57% in Delicias, Chihuahua. Another study in Mexico, where have been analyzed calves with bloody diarrhea in herds of cattle producers of milk and beef, has demonstrated the presence of rotavirus in group A. The G and P genotypes were determined by polymerase chain reaction in samples of rotavirus-positive. We found three different combinations of genotype, G10P; G6P, and G10, P; in 67, 25 and 8 percent of the positive samples, respectively. Some animal’s rotavirus-positive had been vaccinated with an inactivated rotavirus strain of a genotype different. It would be interesting to continue with these studies in other regions of the country to determine the incidence of rotavirus infection and try to develop a national vaccine.