TGE is a very important and highly contagious disease of pigs caused by a coronavirus.TGE is a very important and highly contagious disease of pigs caused by a coronavirus.
Coronaviruses take their name from the halo-like array of envelope proteins that surround the capsid; they're visible in a TEM negative stain (far left) as spiky processes forming a "crown" around the RNA core. The coronaviruses are important pathogens of humans and animals, with a fair degree of specificity for hosts and target cells. More than one coronavirus can affect pigs: there is one similar in structure that infects the respiratory system, and there have been reports of TGE transmission by aerosol, but the usual method of transmission of this pathogen is via fecal-oral contamination.
TGE virus enters the pig by mouth and multiplies in the absorptive cells of the villi of the small intestine. Uptake is very rapid and the presence of the virus causes destruction of the villi. This can be appreciated grossly in a "wet mount" (right) of normal vs affected intestines: the villi are visibly missing in the infected animal. This takes place in 24 to 48 hours and is followed by vomiting and a very severe acute diarrhea with very high mortality in young animals. Undigested milk and necrotic debris in the lumen of the intestine ferment, producing gas and bloating the thinned walls. Mortality in piglets in their first two weeks may be 100%. This decreases in pigs over 3 weeks of age but morbidity is high.
The virus multiplies in the intestine and is shed in feces, the major source of transmission. Contaminated feces can come from an introduced asymptomatic carrier or indirectly through 
mechanical transmission. The virus is killed by sunlight within a few hours but will survive for long periods outside the pig in cold or freezing conditions. This farmer undoubtedly received an asymptomatic carrier from his supplier, and had the bad luck to time his breeding and farrowing for the winter months. The clinical picture in acute disease is almost diagnostic, but ultimate diagnosis of TGE must be made in the laboratory from the intestine of a fresh dead pig using fluorescent antibody tests.
When it's introduced into a herd, TGE spreads rapidly, affecting affects all age classes. Adult animals show varying degrees of inappetence and usually recover, but in suckling piglets there's a very acute watery diarrhea with almost 100% mortality within 2 to 3 days, due to severe dehydration and electrolyte imbalance. Disease will persist in the farrowing houses over a period of 3 to 4 weeks until sows have developed sufficient immunity to protect the piglets.
Once the piglets are weaned, the influence of maternal antibodies wanes, and young pigs become infected. They then shed the virus, contaminating the weaner rooms and infecting pigs being weaned after them. Weaned pigs are far less likely to die, but many of them suffer permanent injury to the digestive tract, as a result of erosion of the gastric mucosa and exposure of the underlying lamina propria. This causes fusion of the villi, decreased absorptive surface, and chronic malabsorption. Even after recovery, their impaired ability to take up nutrients makes them "poor doers" whose growth and weight gain won't match those of unaffected animals.
There is no treatment for TGE. The only thing that can be done is to provide affected animals with supportive care (warmth and rehydration) and hope for the best. Once the epidemic has burned itself out (in small herds it usually will: in large ones it may become endemic) the premises have to be thoroughly disinfected. This is easily done because TGE virus is susceptible to disinfectants. This farmer is probably going to find it necessary to start over again with new animals from a herd that has been proven to be serologically negative for TGE.
References:
Geering, W.A. Forman, A.J., and Nunn, M.J. 1995. Exotic Diseases of Animals. Australian Government Publishing Service (Canberra) pp. 252-258
Saif, L.J. and Wesley, R.D. 1999. "Transmissible Gastroenteritis and Porcine Respiratory Coronavirus." In: Diseases of Swine. Iowa state University Press (Ames) pp. 295-325
"Transmissible Gastroenteritis." In: Merck Veterinary Manual (8th Edition) National Publishing Inc. (Philadelphia) pp 257-258
"Transmissible Gastroenteritis of Pigs." 1997. In: Veterinary Medicine. Saunders (London) pp. 1009-1015