Transmissible gastroenteritis (TGE) and rotavirus are the most important viral causes of diarrhea in U.S. swine.
TGE is a highly contagious disease. It can affect swine of all ages and cause high death loss in pigs less than 2 weeks of age.
TGE has been the focus of intensive disease control efforts for many years. Diagnostic laboratory records indicate that these efforts have resulted in a large decrease in TGE diagnoses in suckling and nursery pigs with diarrhea, from 26% in 1988 to 6-7% in 2002.
Rotavirus is a highly resistant virus that is present in most herds. Rotavirus most commonly causes a low-level infection without clinical signs (subclinical infection) or mild diarrhea.
How These Viral Agents Work
TGE and rotavirus infect cells covering finger-like projections (villi) lining the intestine (Figure 1). Viral infection leads to loss of these cells and an atrophic enteritis (the finger-like villus structures become shortened, blunted and fused), resulting in decreased digestion and absorption with subsequent diarrhea, dehydration and weight loss (Figures 2 and 3).
Both viruses multiply in the intestine, are shed in feces, and are transmitted to other pigs primarily when they ingest infective fecal material.
No Magic Bullet
One of the difficulties in dealing with intestinal viruses of swine is that there are no medications that will eliminate them. Once a pig is infected, supportive care can decrease mortality, but the pig's immune system must eliminate the virus.
Because swine intestinal viruses cause an atrophic enteritis, recovery is often delayed while intestinal villi regenerate.
Clinical disease due to TGE and rotavirus is most common in suckling and nursery pigs. The primary clinical sign with both viruses is diarrhea, leading to dehydration and weight loss. Other common clinical signs with TGE include fever, lethargy, piling and vomiting.
The incubation period for both agents is short (1-3 days), so there is the potential for rapid spread, particularly with TGE. It isn't uncommon for pigs to become sick very soon after delivery. This makes it seem like the farm of origin was the source of infection, when in fact, the source may often be the delivery vehicle.
Rotavirus is present in most herds, but diarrhea is most common in pigs 4 to 41 days of age. Infections are most often undetectable or result in mild diarrhea.
TGE is more common in the winter from November through March. Potential reasons for this seasonality include:
The virus survives well in the cold but is inactivated at warm temperatures, and
Chilling and temperature fluctuations predispose the pig to development of TGE. Disease due to TGE occurs in two relatively distinct forms: epizootic (outbreak) or enzootic (ongoing problem).
Epizootic TGE occurs when the virus is introduced into a naïve herd. The typical clinical signs in piglets are brief periods of vomiting, accompanied by severe, watery diarrhea with rapid weight loss, dehydration and a high incidence of sickness and death loss in pigs under 2 weeks of age. Most pigs under 7 days of age will die, while most pigs over 3 weeks of age will survive, but are likely to remain unthrifty for a period of time. Getting these pigs back on feed as soon as possible and minimizing body weight loss are the primary treatment objectives in order to minimize economic losses.
Clinical signs of TGE in grow/finish and adult swine are usually limited to diarrhea and loss of appetite for a few days, with vomiting seen in an occasional animal.
Enzootic TGE is a milder form of this enteric disease. Pigs generally develop diarrhea 1-2 weeks after weaning, as passive antibody levels decline; mortality is typically low.
TGE and rotavirus cause changes in the intestine that can predispose the animal to developing concurrent Escherichia colibacillosis (E. coli) or Clostridium perfringens Type A enteritis, which can increase the severity of the diarrhea. If these pigs are treated with antibiotics effective against the secondary bacterial infection, they may show clinical improvement and decreased mortality. However, treatment will not completely eliminate the diarrhea.
Determining Disease Severity
Differences in clinical signs are not generally due to different strains of virus. The severity of disease is determined by the level of immunity from ingested colostrum, exposure dose, presence or absence of concurrent infections (E. coli) and the age at exposure.
While explosive outbreaks of TGE occur when the virus infects non-immune herds, the enzootic form of TGE occurs when sows pass colostral immunity against TGE onto their offspring. This immunity protects pigs during the critical first few weeks of life. But as immunity begins to wane, and partially immune pigs are exposed to the virus, they develop milder forms of the disease.
Experimentally, when newborn pigs are exposed to rotavirus, they develop severe diarrhea with significant mortality. Yet most field infections are subclinical or mild.
Because rotavirus is extremely common, most sows are immune, and as with enzootic TGE, subclinical infection or mild diarrhea occurs.
Clinical rotavirus problems occur most commonly in “start-up” herds or herds with a disproportionately large number of gilts. These herds may lack adequate protective immunity, and rotavirus infection in suckling pigs will cause higher mortality and greater reduction in weaning weights than is typically seen in more mature herds.
|Disease||1-7 days||7-21 days||Nursery|
|Clostridium perfringens Type A||13%||2%||—|
|Clostridium perfringens Type C||6%||4%||—|
|*Iowa State University Veterinary Diagnostic Laboratory Data, 2000-2002.|
For both TGE and rotavirus, there is an age-related resistance — the older the pig, the less severe the illness. Also, modern slotted flooring has greatly reduced exposure to fecal material and to these viral organisms. Low-level exposure of older, partially immune pigs leads to a milder or subclinical form of disease with either virus.
As described, an explosive outbreak of diarrhea with vomiting, affecting all age groups with large losses in young pigs, would be indicative of TGE. A low morbidity, low mortality scour in suckling pigs that is not responsive to antibiotics would be more suggestive of rotavirus. However, there is considerable overlap in clinical signs between agents that cause pig diarrhea.
If a diagnosis cannot be made on the basis of clinical signs and response to treatment, producers or veterinarians should select live, acutely affected, untreated pigs as the specimen of choice for complete diarrhea diagnostics.
Do not pull a pig off the dead pile for this diagnostic work, because the intestine degrades rapidly; in less than 30 minutes (especially when hot), the gut is useless for a number of diagnostic tests.
Do not select stalled-out pigs that have had diarrhea for several days or pigs that have been treated. Stalled-out pigs may still have diarrhea due to residual intestinal damage (atrophic enteritis). But these pigs are now recovering from infection and the virus is no longer detectable. If pigs have been treated with antibiotics, the viruses may still be detectable, but therapy may prevent the identification of concurrent bacterial infections (antibiotics do not affect viruses.)
Porcine Epidemic Diarrhea (PED) is an important cause of viral diarrhea in Europe and Asia, but has not been detected in North America. PED cannot be reliably differentiated from TGE on the basis of clinical signs or gross or microscopic lesions. If clinical signs and lesions are suggestive of TGE, but extensive testing fails to identify TGE, samples (feces or fresh intestine) can be forwarded to the National Veterinary Services Laboratory in Ames, IA, for PED testing.
Treatment and Control
The early treatment of viral enteritis is important in getting pigs back on track as quickly as possible. Due to the nature of viral infections, treatment must focus on supportive care and suppression of secondary bacterial infections. Because of profuse diarrhea and vomiting, fluids and electrolytes are quickly lost from the body. Supplying the pigs with oral, water-soluble electrolytes helps reverse this process. Electrolytes can be provided to pigs using a medicator.
Because pigs are often lethargic and chilled due to fever, anti-pyretic/anti-inflammatory compounds, such as aspirin, are also helpful. If pigs are chilled, additional zonal heating or increasing room temperature can improve pig comfort.
The goal of treatment is simply to convince pigs to feel normal with the hope they will soon return to eating and drinking normally.
In sow herds, TGE will typically cause very high death loss in nursing pigs. To bring the disease under control, it is imperative to expose the whole herd as quickly as possible to reduce the length of time that weaning piglet quality and numbers are affected. It is prudent to stop the introduction of new breeding stock to allow the infection to “burn itself out.”
It is possible to eradicate TGE from a herd through planned exposure to the virus and by halting the introduction of na've animals. This can be done in a number of ways. It's best to consult with your veterinarian regarding protocols for specific farms.
Because rotavirus is very hardy and stable, it exists on the vast majority of swine farms. Through proper management of colostral immunity, rotavirus-positive farms are generally not an economic burden.
However, this is not the case for TGE. Epizootic TGE can be and often is devastating to a sow herd's productivity and economic health. The best way to control it is to prevent entry into a farm by paying special attention to several cornerstones of good biosecurity:
Never underestimate the potential for an unclean transport vehicle to cause problems. Trucks and trailers need to be cleaned of all organic matter, disinfected and thoroughly dried before being allowed on a farm site. Frozen wash water on trailer floors is unacceptable and may in fact be the perfect way to preserve TGE virus.
Use of a shower-in, shower-out or Danish entry protocol is wise. The Danish entry protocol consists of washing hands and changing outerwear before entry into facilities. Clothing and footwear worn off the farm should not be worn on the farm.
Delivery and feed trucks should be kept as far away from buildings as possible. It is best if these vehicles don't come to your farm directly from other farms, but this is very difficult to control.
Equipment-sharing among farms is generally not recommended.
Visitors to your farm should understand and abide by your biosecurity protocols. Reducing the number of visitors to a farm is often prudent, especially in cold weather months. Generally, reducing the traffic across your farm is wise as well.
Once a downstream site has been diagnosed with viral enteritis (especially TGE), it is important not to track it back upstream to the nursery or sow herd. One-way flow of pigs and people through the system is advisable.
As with any disease problem, it is important to get a specific diagnosis as quickly as possible so that the proper control measures can be implemented.
Pork production has turned into a battle of numbers. Controlling economic loss due to infectious disease is critical to financial performance. Viral enteritis may not be as threatening as some of the newer, emerging diseases. Nonetheless, it still has the capacity to sting you where it counts most — in the pocketbook.