Rotaviral Diarrhea in Pigs
Rotavirus (RV) infections are a prevalent cause of diarrhea in suckling and weaned pigs in conventional swine herds leading to substantial economic losses to the pork industry. There are 5 porcine rotavirus groups: A, B, C, E and H. In one study, Groups A, B, C and H rotaviruses were found in up to 62%, 33%, 53% and 15% of piglets in the US (Marthaler, Homwong et al. 2014, Marthaler, Rossow et al. 2014). Group E rotaviruses are rarely reported and considered of low pathological relevance. Group A and C rotaviruses are most common in post-weaning and nursing piglets, respectively (Vlasova, Amimo et al. 2017). Nearly 100% of pigs of market age are rotavirus A and C seropositive. Some human rotavirus strains are of suspected porcine origin. There is cross-reactivity (and therefore at least partial cross-protection) within (but not between) each rotavirus group.
- Discuss the prevalence and clinical importance of porcine rotaviruses
- Explain existing tools and methods for porcine rotavirus diagnostics
- Discuss methods for treatment and prevention of porcine rotaviruses
History of Discovery
Rotavirus A was first detected in pigs suffering from diarrhea in 1975 (McNulty, Allan et al. 1976). Group B and C rotaviruses were first described in piglets in the 1980s and were referred to as rotavirus-like viruses or pararotaviruses (Saif, Bohl et al. 1980, Theil, Saif et al. 1985). An atypical group E porcine RV was only reported in United Kingdom (UK) swine in pigs older than 10-weeks (Chasey, Bridger et al. 1986). Most recently, rotavirus H strains were described in pigs in Japan, Brazil and in the United States (US), where they were reportedly circulating since at least 2002 (Wakuda, Ide et al. 2011, Marthaler, Rossow et al. 2014, Molinari, Lorenzetti et al. 2014).
Rotaviruses are characterized by their wheel-like appearance when viewed using the electron microscope. Rotaviruses are resistant to low pH, lipid solvents, and many commonly used disinfectants enabling them to survive for long periods under normal environmental conditions.
Rotavirus, like transmissible gastroenteritis virus (TGEV) and porcine epidemic diarrhea virus (PEDV), replicate in mature cells, enterocytes, that line the small intestine. These enterocytes cover the millions of long fingerlike projections, called villi, which make up the inside lining of the small intestines (Fig. 2, A and D). When these cells are infected and ultimately destroyed by rotavirus, villi become short and blunt (Fig. 2, B and E) or shrink, and nutrients are incompletely absorbed (Lecce and King 1978).
In suckling pigs, much of the ingested milk will pass through the gut without being digested or absorbed. The passage of undigested food into the lower small and large intestines has two effects: 1) It provides a nutritional substrate for various bacteria leading to secondary disease; 2) It can have an osmotic effect in the intestine which may exacerbate diarrhea and dehydration. This can result in diarrhea, loss of water, electrolytes, body weight and sometimes death. Villous atrophy occurs very rapidly, within 24 to 36 hours, after rotavirus infection of the intestinal cells, and coincides with the onset of diarrhea. However, regeneration of the intestinal villi and recovery of normal digestive capacities will take about 7 to 10 days in suckling pigs (Moon, 1971). This is then the most critical time to prevent malabsorption diarrhea and secondary bacterial infections.
Rotaviruses have dual serotype/genotype designation identified as G (for surface glycoprotein VP7) and P (for protease sensitive hemagglutinin VP4) types based on virus neutralization or genotyping assays (Estes and Greenberg 2013). This is important because rotaviruses with distinct G and P types generally have low cross-protection, so vaccines need to contain the dominant G and P types associated with disease on farm. Out of 12 G and 16 P types found in pigs, 5 G types of group A rotaviruses are considered most common in swine including: G4 and G5 (prototype Gottfried and the OSU strains, respectively), G3, G9 and G11. These G types are combined with multiple P genotypes (P-P, P and P most commonly) that vary with pig age and region (Vlasova, Amimo et al. 2017).
Over the last decade improved diagnostic tools have allowed identification of the widespread group B and C rotaviruses in pigs in the US and revealed their significant genetic diversity with at least 26 and 9 G genotypes identified for group B and C rotaviruses, respectively (Marthaler, Rossow et al. 2013, Shepherd, Herrera-Ibata et al. 2018). Although, no G/P typing system is established for group H porcine rotaviruses, recent data suggest that they are also genetically diverse (Marthaler, Rossow et al. 2014). Pigs infected with one group or genotype are still susceptible to infection with another group or genotype. It is noted that rotavirus H is most frequently identified from pigs co-infected with group A, B and C rotaviruses (Marthaler, Rossow et al. 2014). Up to 30% of healthy carrier sows may be fecal shedders during the periparturient period, exposing their offspring to infection.
Clinical Signs and Epidemiology
Rotaviral diarrhea can occur in suckling pigs shortly after birth and is a cause of the clinical syndrome referred to as milk scours, white scours, or pre-wean scours (Bohl, Kohler et al. 1978). The age of peak incidence varies for the different rotavirus groups and under different management conditions. Probable reasons for the peak occurrence of group A rotavirus infections at preweaning age include the decline in milk antibody levels coupled with the dilution of this antibody as a result of the pigs ingesting creep feed and water. In contrast to rotavirus A, group C rotavirus infections are dominant in suckling pigs and cause diarrhea mostly in piglets born to gilts that generally have lower levels of antibodies in their milk than sows (Chepngeno, Diaz et al. 2019).
Rotavirus diarrhea, similar to other types of neonatal diarrhea, can appear as white or yellow stool which, at the onset, is liquid; but after a few hours or a day, in uncomplicated cases, it becomes creamy and then pasty before returning to normal (Bohl, Kohler et al. 1978). At necropsy, undigested milk is often evident in the intestinal contents, and the stomach can be distended with milk curd. Diarrhea may persist for only a few hours or several days. Vomiting may or may not be detected, but it occurs much less frequently than with enteric coronavirus infections including TGEV and PEDV. Under ideal conditions, pigs remain active and lose little weight. Present information suggests that rotaviral infections are common may result in either no clinical signs of disease or only a mild disease characterized by short-term diarrhea. However, the severity of the disease and mortality may be increased by simultaneous infections with Echerichia coli (colibacillosis), TGEV or other causes (other enteric viruses such as PEDV and sapelovirus, clostridia, coccidiosis), by inadequate intake of immune milk, or by environmental stressors such as chilling. The disease is more severe in younger pigs, while sows usually do not show clinical signs. Diarrhea is more profuse and more noticeable in pigs that ingest a large amount of milk. In many respects, rotaviral diarrhea is similar to enzootic TGE (persistence of TGE infection in a herd). Usually, the duration of diarrhea is longer, and dehydration and death losses are greater in enzootic TGE and PED than in rotaviral diarrhea.
As some pigs initially develop rotaviral diarrhea, the farm environment becomes heavily contaminated with virus, which leads to exposure of younger pigs to high doses of virus often exceeding the protective capacity of the milk antibodies present in these pigs’ intestines (Fig. 1). Subsequently diarrhea may occur routinely in suckling pigs. To break this cycle of infection, an “all in all out” management system should be practiced in farrowing and nursery units. Housing units should be designed with floors and all surfaces that can be thoroughly cleaned and disinfected between groups. Examples of disinfection methods and management practices to reduce infections with rotavirus include reduced pig handling, disinfection of processing equipment between litters, reduced piglet movements, frequent glove changes and others.
Pigs that have had rotavirus diarrhea during the nursing period may have another episode postweaning because of the loss of protective antibodies provided in the sow’s milk or infection with other rotavirus groups or serotypes/genotypes. Two studies have shown that rotavirus infection shortly after weaning leads to intestinal damage favoring the colonization of the gut with enteropathogenic E. coli (Lecce, Balsbaugh et al. 1982, Deprez, Van den Hende et al. 1986). Results of both studies suggest that pigs infected with two agents develop a more severe diarrhea than that produced by each agent alone. Diet might also play an important role in weanling diarrhea.
Clinical and laboratory diagnosis of rotaviral diarrhea also requires evaluation for the presence of E. coli, Isospora suis (coccidiosis), coronaviruses including enzootic TGEV, PEDV, porcine deltacoronavirus (PDCoV), and other agents that can cause a similar diarrhea syndrome (Biehl and Hoefling 1986, Malik, Verma et al. 2019). E. coli, but also group C rotavirus diarrhea commonly occurs in suckling pigs, or postweaning, whereas enzootic TGE, coccidiosis and rotavirus group A diarrhea often occur in pigs after 1 week-of-age. However, since disease caused by rotavirus in younger pigs is usually more severe, producers might think the pigs have colibacillosis unless they submit pigs for a complete diagnosis.
Laboratory diagnosis requires the submission of feces or intestinal sections collected early (24 hours. or less) after the onset of diarrhea. Laboratory methods that are helpful in making a diagnosis (when used in combination), include: histopathology, immunohistochemistry (IHC), in situ hybridization (ISH), fluorescence in situ hybridization (FISH), reverse-transcription polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA) (Biehl and Hoefling 1986, Will, Paul et al. 1994).
Rotaviruses from different groups can be differentiated using group-specific primers in RT-PCR (Marthaler, Homwong et al. 2014) and serologic tests such as ELISA. Currently, commercial ELISA reagents are available only for detection of group A rotaviruses. ELISA can be done on feces or intestinal contents and has the advantages of high sensitivity, requirement for minimal amounts of sample, and rapid results (6 to 24 hours). RT-PCR/qRT-PCR tests can also be performed after rotaviral nucleic acid is extracted from a suspended fecal sample (Marthaler, Homwong et al. 2014). If rotavirus nucleic acid is present in the sample (it can only be present if rotavirus is present), it will be recognized as rotavirus-specific DNA fragments that are amplified in a positive reaction when enough rotavirus nucleic acids are present in the sample. Amplified rotavirus nucleic acid can be visualized under UV light using a special fluorescent dye. Blood samples aid little in serologic diagnosis, since most swine are positive for rotavirus antibodies.
Immunity and vaccines
Because most, if not all sows are positive for rotavirus antibodies (Saif and Fernandez 1996), they transfer a variable amount of passive immunity to their nursing pigs via colostrum and milk. Studies on immunity to PEDV and TGEV show that effective protection depends, not on blood antibody levels, but on the almost continual presence of milk antibodies and other immune factors in the intestine of the pig, such as occurs following frequent nursing (Saif and Fernandez 1996, Langel, Paim et al. 2016). This type of “lactogenic immunity” is also important in rotavirus infections for protection of susceptible intestinal cells. Various factors which may interfere with this balance between passive immunity and rotavirus clinical infections include: 1) failure of the piglet to nurse at frequent intervals shortly after birth, failure of the sow to provide milk, and low antibody titers in milk (group C rotavirus)(Chepngeno, Diaz et al. 2019), all of which may lead to severe rotavirus diarrhea in pigs under one-week-old; 2) high doses of virus as a result of a heavily contaminated environment may exceed the level of protective antibodies in the milk, leading to rotavirus diarrhea in nursing pigs; 3) ingestion of creep feed and water by 2 to 3-week-old nursing pigs may dilute the level of protective antibodies leading to rotavirus diarrhea; and 4) weaning, which results in complete loss of protective milk antibodies, may cause severe diarrhea and mortality to younger pigs which are weaned in a contaminated environment. Parenteral (intramuscular or subcutaneous) rotavirus immunization of rotavirus antibody positive sows shortly before or after farrowing can increase rotavirus antibody levels in colostrum and milk (Saif and Fernandez 1996).
The practical application of these immunization methods might be to enhance passive lactogenic immunity, thereby delaying the onset of rotavirus diarrhea in herds with a history of severe rotavirus diarrhea and high mortality in young piglets. Protection of weanling pigs against rotavirus diarrhea requires active immunization, probably via the oral route, prior to weaning. Multiple groups and serotypes/genotypes of porcine rotavirus and interference by maternal antibodies make this type of potential vaccine a less feasible prospect at present.
Currently, there are two live modified (ProSystem Rota and ProSystem RCEMerck) federally licensed vaccines for porcine group A rotavirus. They both incorporate two serotypes (G4 and G5) of porcine group A rotavirus and are to be administered orally and intramuscularly to pregnant swine or orally to nursing piglets. In theory, administration of a rotavirus vaccine to pregnant swine should boost colostrum and milk antibodies providing increased lactogenic immunity to nursing pigs (Saif and Fernandez 1996). However, there are no reported controlled studies on the efficiency of these vaccines for boosting rotavirus antibodies in colostrum and milk or for preventing rotavirus-associated diarrhea in nursing pigs. Current knowledge indicates that group A rotavirus vaccine failure can be due to co-circulation of multiple rotavirus serotypes/genotypes other than the prototype vaccine strains or interference by maternal antibodies with active immunization of pigs. Although it is an increasing problem in swine herds, especially in pigs under one week-of-age, there are no licensed vaccines for non-group A rotaviruses such as group C rotavirus.
Sows are often given rotavirus containing fecal material approximately 2-to-5 weeks before their expected farrowing date in an attempt to boost their immunity and enhance transfer of maternal immunity to the piglets. Efficacy of this feed-back approach has been variable and it may spread other enteric pathogens throughout the herd.
There are no specific treatments for rotaviruses other than supportive therapy. Although most studies suggest that rotavirus infections cannot be prevented, their severity can probably be moderated by optimal management conditions. These include “all in/all out” systems in farrowing and nursery units. Careful and thorough cleaning and disinfection of the premises should be done routinely since high viral doses may lead to earlier onset of and possibly more severe infections in nursing pigs. Disinfectants which are effective to various degrees against rotavirus include: 3.7% formaldehyde, chloramine T (Multichlor®), 5% Lysol, hexachlorophene (Septisol®), lime; and triclosan (Triclosan® hand soap). It is likely that fecal material may further reduce the effectiveness of many of these rotavirus disinfectants, necessitating complete cleanliness to achieve maximal disinfection. Attention should be given to providing adequate heat to suckling and weaned pigs since this affects their clinical response to rotaviruses and other enteric infections. It is also important to keep in mind that the younger the pig, the more vulnerable it is to dehydration and energy and weight losses caused by rotavirus. Pigs with diarrhea caused by rotaviruses or other infections that damage the villi do not absorb nutrients well and are more susceptible to chilling. Although villous repair should occur within a few days, chilling and other stresses delay this, and the pig may develop multiple nutritional deficiencies and become stunted or a chronic “poor-doer.” It is essential to ensure that neonatal pigs receive adequate colostrum and milk.
Control of weanling diarrhea may depend on factors such as: 1) feeding newly weaned pigs small quantities of feed at frequent intervals for the first few days postweaning; 2) dividing pigs into small groups of similar ages since mixing pigs of various ages at weaning may lead to stress and favor transmission of infection from older to younger pigs; 3) emptying and disinfecting the premises between groups; 4) weaning age—younger weaned pigs usually are more severely affected than older pigs; thus later and gradual weaning is preferred; 5) meeting critical temperature needs of pigs; and 6) ventilating for minimal levels of noxious gases (ammonia). It is helpful to provide adequate water to maintain hydration in weaned pigs with diarrhea. Antibiotics or other drugs are not effective against rotaviral infections and would be of no value in treatment unless there is a concurrent bacterial infection, such as with pathogenic E. coli.
The following points can be summarized from this fact sheet:
- Infections of swine with rotaviruses are very common with most (if not all) swine herds infected.
- Porcine rotaviruses are genetically diverse and constantly evolving
- Rotavirus is associated with a diarrhea in nursing and post-wean pigs.
- Laboratory diagnosis is completed by examining the intestinal tissues through histopathology, IHC, ISH, FISH, serology through ELISA or RT-PCR tests completed on feces.
- Mortality is usually low unless there are complications owing to concurrent infections or stress such as chilling. Reasons for the widespread presence and deaths associated with group C rotaviruses in pigs <1 week-of-age are unknown, but insufficient lactogenic immunity provided by gilts is associated with increased diarrheic disease in suckling piglets.
- Control measures rely on good management including provision of adequate colostrum and milk at an early age, good sanitation (incl. control of other pathogens), and keeping piglets warm and hydrated.
References and citations
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