Keeping Out Transmissible Gastro-Enteritis - By Jake Waddilove MA, VetMB, MRCVS, England - Transmissible Gastro-enteritis (TGE) is a highly infectious disease of pigs leading to vomiting and/or diarrhoea in pigs of all ages. It spreads very rapidly around a pig unit or system.

Mortality is mainly associated with dehydration and acute cases in a naïve herd mortality reaches near 100% in pigs under 7 days. Deaths rapidly decrease with age such that in pigs over 3 weeks deaths are low and in pigs over 6 weeks they are rare. In an acute outbreak the typical death loss is 1.5 times the total number of sows. In spite of this the greatest problem is chronic TGE, especially in large herds and multiple site systems. This persists as a low-level problem causing mortality and slow growth in farrowing and especially nursery units. Damage to the intestinal wall in grow/finish pigs can predispose them to other diseases (e.g. Ileitis, and Salmonella).

A Peculiar Disease:

From a biosecurity and disease control viewpoint TGE is a peculiar disease. The prime target in prevention is to exclude the disease from any pig production system. As the virus is highly infectious this is a considerable biosecurity challenge. However, the unique situation is that, once an acute outbreak occurs, we need to spread the disease rapidly around the farm to encourage immunity to develop in all stock as quickly as possible. Once infected, all animals will either die from the disease or will recover and become immune. At this point the virus will only survive in the herd for about 90 days after clinical signs. If during this period there are no fresh susceptible pigs introduced the virus should be eliminated and chronic disease prevented through effective terminal biosecurity.

Why can the above method of controlling chronic TGE fail?

1. New non-immune stock are purchased
2. Vaccination is used to produce immunity – unlike natural infection the duration of vaccinal immunity is not life long
3. The virus is not spread throughout the farm or system in the early acute stages
4. Piglets from immune sows will have maternal antibody protection. After weaning this drops off and if the virus is still present they will become infected and perpetuate viral survival in nurseries
5. Some pigs are partially immune, possibly related to concurrent infection with Porcine Respiratory Coronavirus (PRCV)
6. In nursery, grower or finisher barns terminal biosecurity is inadequate and when naïve pigs come through from the breeding herd they are re-exposed to virus
7. Pigs are immuno-compromised by such agents as PRRSV, PCV2, or PRV/ADV.

Spread of Infection:

From the above it is obviously vital to protect pig units from infection in the first place. The prime method of spread is pig to pig, and the most common way for new infection to enter a farm is through the introduction of sub-clinically infected pigs. Large amounts of virus are excreted in the faeces of infected pigs.

Other methods of transmission are obviously important in closed herds and spreading infection in a closely infected area. These include:

1. On the clothing or equipment of people visiting the facility
2. On vehicles – especially those which visit many units
3. Dogs, foxes and cats shed the virus in their faeces for 2-3 weeks.(McLurkin et al)1
4. Birds, especially starlings can mechanically spread the virus. (Pilchard)2
5. Possibly by insects. (Gough et al) 3 (Groocock et al)4

It is important to remember that the virus is relatively fragile and is susceptible to U-V light, so spread is much more likely in the winter.

Preventing Infection Entering the Unit:

CCP IN THE PREVENTION OF INFECTION WITH TGE Incoming Pigs Purchase from free source Isolate for as long as possible Ensure biosecurity of isolation Infection Via People Exclude all necessary personnel Shower and change Or change and clean boots and hats. Foot dip at entry and on unit. Hand hygiene Infection Via Equipment Entering Unit Exclude all possible equipment Empty all boxes – clean and disinfect Clean and immerse contents in disinfectant If not possible wipe with Virkon S or cloths Infection Via Transport Clean and disinfect all transport prior to arrival Wheel dip or spray at approach Exclude vehicles from perimeter Ensure drivers change clothes and footwear Infection Via Other Animals Bird proof all buildings Rodent control programme Protect pig food from contamination Stock proof perimeter fence Prohibit keeping of pets on unit or in transport

This requires a stringent external biosecurity programme such as the DAHS Swine Biosecurity Programme. TGE virus is susceptible to a range of disinfectants (Brown)5 , but it is important to remember that biosecurity is ongoing and dynamic involving numerous pathogens, so use a proven broad-spectrum disinfectant – Virkon S (DAHS).

Being active at lower dilutions there is an increased safety margin with this product. Equally remember that TGE is mainly spread in the winter, and so a disinfectant that is active in low temperatures is needed (Virkon S is active, many others are not).

In a breeding farm incoming pigs should be from a proven TGE negative source. They should be isolated in a separate isolation unit and looked after by non-farm staff, but still with high levels of biosecurity. Preferably isolation should be 60 days. Consider serological testing for TGE prior to moving them to the main farm.

The farm should be fenced within a stock proof fence. Non-essential visitors should be excluded. All personnel entering the unit should ideally shower and change but as a minimum should change all outer clothing and boots and wear hats. At entry to the farm clean boots and use a footdip with Virkon S, repeat this at the entry to every building. The fact that good biosecurity measures can prevent the entry of the virus on personnel has been well demonstrated in a study where personnel who either showered and changed, or washed hands and changed, did not transmit TGE whereas those who did not do this transmitted TGE. (R.M. Alvarez et al)6

Vehicles present a significant risk especially in large production systems. Adopt a complete transport biosecurity programme (DAHS Transport Biosecurity Programme). Pig transporters, including tractor units must be thoroughly cleaned after each batch of pigs.

Using a heavy-duty detergent (Biosolve – DAHS) facilitates and speeds this process up. Once all organic material has been removed disinfect with a broad-spectrum disinfectant active against TGE virus (Virkon S). Other vehicles visiting the farm should be treated similarly. At the approach to the farm vehicles should pass through a wheel dip or be sprayed with a dilution of Virkon S. All unnecessary vehicles should be excluded from the farm and deliveries should be made over the fence.

After the hard lessons learnt from the UK's FMD crisis of 2001, we know that stock vehicles play a devastating role in the spread of infection. With the routine use of disinfectants, animal

transporters raised concerns that disinfectants could cause corrosion to vital vehicle metalwork. Reassuringly, an independent study by MIRA - the UK's official vehicle engineering and testing body - confirmed that Virkon S has no significant long-term effects on the materials used in vehicle construction.

All facilities must be bird proofed and pig feed must be protected from birds and rodents. Prevent the keeping of other animals on the unit. Institute a thorough rodent control programme (IPM – DAHS). Operate an effective fly control programme. Do not allow pets on unit or in visiting transport.

Any equipment, which has to enter the farm, must be thoroughly cleaned and disinfected with Virkon S.

Terminal Disinfection in TGE:

The Activity of Virkon S against the Pathogens Discussed in this Article Agent Virkon-S Activity Test Country TGE virus 1:450 UK Salmonella species 1:100-1:200 UK/USA Ileitis/ Lawsonia 1:100 UK PRRSV 1:700 UK PCV2 1:100 USA PRV / ADV 1:100 USA The Activity of Farm Fluid S Agent Farm Fluid S Activity Test Country TGE virus 1:1000 UK

As stated above, once a farm or system is infected, the aim is to spread infection to all pigs as soon as possible to prevent chronic disease. While doing this external biosecurity must be maintained to prevent infection with any other pathogens. It is also necessary to have good terminal disinfection in any multiple site batch system to prevent infection spreading from one batch to the next.

The terminal disinfection required would include removal of gross contamination, thorough cleaning (Biosolve) and then disinfection (Virkon S). Water and feed systems must also be cleaned and disinfected – Virkon S works well for both these. Any equipment used in the buildings or site must be similarly cleaned and disinfected. Operatives moving around the farm or system must take particular care.

To Summarise:

TGE can cause dramatic losses in the acute phase, but the chronic phase causes much the greater long-term economic losses. The best method of control is to prevent infection by means of a complete biosecurity programme. If infection does enter a system or farm control is aimed at promoting immunity to stop virus replication and survival in addition to ensuring good terminal disinfection of buildings.

Further Biosecurity Information

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References:

1. McLurkin, A. W., Stark, S. L., and Norman, J. O. Transmissible gastroenteritis (TGE) of swine: the possible role of dogs in the epizootiology of TGE. Canadian Journal of Comparative Medicine 34, 347-349. 1970
2. Pilchard, E. I. Experimental transmission of transmissible gastroenteritis virus by starlings. American Journal of Veterinary Research 26(114), 1177-1179. 1965
3. Gough, P. M. and Jorgenson, R. D. Identification of porcine transmissible gastroenteritis virus in house flies (Musca domestica Linneaus). American Journal of Veterinary Research 44(11), 2078-2082. 1983.
4. Groocock, C. M., Hess, W. R., and Gladney, W. J. Experimental transmission of African swine fever virus by Ornithodoros coriaceus, an argasid tick indigenous to the United States. American Journal of Veterinary Research 41(4), 591-594. 1980
5. Brown, T. T. Laboratory evaluation of selected disinfectants as virucidal agents against porcine parvovirus, pseudorabies virus, and transmissible gastroenteritis virus. American Journal of Veterinary Research 42(6), 1033-1036. 1981.
6. R.M. Alvarez, S.F. Amass, C.D. Anderson, D. Ragland, L.A. Grote, C.A.Dowell, L.K. Clark, G.W. Stevenson and P.M. Spicer: Evaluation of biosecurity protocols to prevent mechanical transmission of Transmissible Gastro-Enteritis virus of swine by pork production unit personnel. T he Pig Journal Vol 48 – 2001, 22-33
7. MIRA Report Number 01-550630/02. Materials Engineering Dept., Watling Street, Nuneaton, Warwickshire CV10 0TU.

Source: Jake Waddilove MA, VetMB, MRCVS - April 2003