Rotavirus occurs worldwide and affects pre-weaning piglets and weaners. There is human risk involved. The effects are in appetence, diarrhoea, vomiting, stunting.
- Causes of Rotavirus >
- Effects of Rotavirus >
- Diagnosis of Rotavirus >
- Treatment & Control of Rotavirus >
Causes of Rotavirus
Rotaviruses are double stranded RNA viruses with the RNA arranged in 11 segments. They have a characteristic wheel shaped appearance and can be classified by their viral proteins and nucleic acids into groups. Groups A, B, C and E have been found in pigs. Group A is usually the first to infect a piglet and is most common in pre-weaning diarrhoea.
Rotaviruses are relatively resistant, surviving 60°C for 30 minutes in faeces and 7-9 months at 12-20°C and are stable between pH 3 and 9. Rotaviruses may be cultured in calf and kidney cells but require exposure to trypsin and agitation for continued subculture.
Infection occurs in piglets which have not taken colostrum and when maternal antibody in milk declines from 2 weeks onwards. Infection is oral and the virus replicates in the cells lining the middle small intestine from upper jejunum to lower ileum.
Replication is most intense 24-96 hours after infection but later becomes restricted to villus tips. Mature absorptive cells are shed and villous atrophy, crypt hyperplasia and villous fusion occur. Lactase levels fall but lactose digestion may not be affected. The resulting failure to absorb nutrients leads to diarrhoea containing virus particles. Recovery may be complete within 7 days.
Effects of Rotavirus
The incubation period is 18-24 hours after which depression, inappetence and reluctance to move are noted. Vomiting may be seen. A few hours later, profuse diarrhoea develops and, in milk-fed pigs, this is yellow with floccules floating in a whey-like fluid, while in others it may be yellow or dark grey. There is a rapid los of condition. Inappetence continues for 24-72 hours after which appetite returns.
Clinical signs regress 4-6 days after infection but loose yellow faeces may persist for 7-14 days. Thirty-three percent of affected young pigs may die in field outbreak. Weaned pigs may also be affected, but in them no diarrhoea or only transient diarrhoea (mean duration 3 days) results when rotavirus is demonstrated in the faeces.
The difference in disappearance of immunity means that individuals in a group do not all develop the disease at once and disease may continue for days of weeks in the group. Rotavirus diarrhoea may recur as immunity to one type does not prevent infection with others. The effects on growth rate range from nothing, to at least 5 days to 25kg.
Diagnosis of Rotavirus
The ubiquity of rotavirus infection should lead to its consdieration as a component of most diarrhoeas of sucking and recently-weaned piglets. Pathological findings suggesting rotavirus include dehydration, a stomach filled with milk and a small intestine distended with creamy fluid contents. The villi lining the last three quarters of the small intestine are stunted and are visible using a hand lens or dissecting microscope. There is no necrosis of the epithelium in uncomplicated rotavirus infections.
The presence of rotavirus can be confirmed by the electron microscopical examination of faeces or colon contents for the characteristic particles or by polyacrilamide gel electrophoresis to detect the characteristic double-stranded RNA. These two methods detect all groups of rotavirus. Modifications of the Polymerase Chain Reaction (PCR) can detect viral nucleic acid.
There are commercial reversed passive latex agglutination tests (RPLA) and immunochromatic tests for virus which can be used on-farm. Unfortunately, up to 25% of outbreaks of diarrhoea in pigs may contain rotavirus particles and mixed infections are common so that it is up to the clinician to decide the importance of rotavirus in each case. Passive antibody levels persist for 2-7 weeks and active antibody levels increase from 5-8 weeks.
Treatment & Control of Rotavirus
There is no specific treatment. Bacterial and protozoal complications should be controlled. Fluid replacement using glucose:glycine electrolyte solutions may be of value and water should always be available. Growth depression in weaned pigs is reduced if high protein rations are given, but milk protein is better than vegetable protein.
Prevention may be achieved by stimulating maternal immunity by feed-back of faeces from affected piglets at least 14 days prior to farrowing or by vaccinating. Vaccines are currently not available in many countries. Live vaccines given intramuscularly or orally have been described and may contain serotypes A1 and A2 human rotavirus, or viral proteins from serotypes 4 and 5 for protection against both serotypes.
Performance may improve from vaccination of the sow 4 and 2 weeks prior to farrowing followed by oral or i/m vaccination of piglets at 2 or 4 weeks. Viral excretion may not be prevented by dead vaccines and transient diarrhoea may occur.
Protection against other rotavirus serotypes or groups may be poor. Cow colostrum containing antibodies to bovine rotavirus protects if fed to piglets. Disinfection can be carried out using hypochlorite on clean surfaces and proprietary disinfectants such as a mixture of surfactant, organic acid, oxidising agents and buffers.
