Pathogen elimination through

Herd Closure and Pathogen Exposure

Example - Porcine Reproductive and Respiratory Syndrome

Eradication without Depopulation

Science Assumptions re PRRSv
PRRSv viral particle	No long term carried status for PRRSv in sows or boars
	PRRSv particles are excreted for less than 100 days following infection (see note at bottom)
	Piglets less than 14 days of age are protected by maternal colostrum deprived antibodies
	Spread of PRRSv is difficult/unlikely over 500 metres
	It is not present in other animals (excluding some ducks)
Difficulties
	PRRSv is not excreted in many body fluids consistently
	Reproductive problems of PRRSv may be accentuated by the treatment advised
Technique
	Purchase sufficient young gilts to provide breeding animals for 100 days
	Close the farm to all inputs, excluding PRRSv free semen
Infect all animals on the farm – Pathogen exposure
	Vaccinate all sows, gilts and boars with a suitable PRRSv vaccine; a live vaccine is acceptable if no previous exposure.
	Obtain tonsilar scrapes from all animals with acute signs. This is made up to vaccinate all sows, gilts, boars and young future breeding stock.
	Practice feedback of faecal materials from acutely ill animals, aborted materials; macerate piglets that die with clinical signs. Feed this material for 14 days.
	At the end of the infection period, throw away all used needles and syringes.
2 weeks later	Vaccinate all sows, gilts and boars with a dead PRRSv vaccine to reduce viral shedding.
	At the end of the infection period, throw away all used needles and syringes.
Herd Closure
	For 100 days minimum, the farm must be totally closed (excluding PRRSv free semen)
	All piglets over 14 days of age are weaned off the farm for 100 days
	Enhance biosecurity measures
Clean farm
	At 90 days post-infection, disinfect the entire farm with a suitable disinfectant. Spray the walls, water and air. Wash all clothing and boots. Throw out all used needles and syringes.
Check the effect of the eradication
1	Purchase 20 PRRSv free gilts.
2	Introduce gilts into the farm and place the animals all around the farm
3	After 21 days, bleed the 20 gilts
4	After 35 days, rebleed the 20 gilts.
5	If the gilts are negative, declare the farm free of PRRSv and allow the weaning age to increase.
6	If any of the gilts are positive, all the gilts are removed. The farm remains closed for another 30 days and the test repeated.
Post-Control
	All gilts and boars introduced into the farm through an adequate isolation area are PRRSv negative.
	Ideally practice on-farm AI on the farm
	Do not use a live vaccine on the introduced animals
	Consider dead vaccine use if proved to be effective
	Continue enhanced biosecurity measures.

Porcine Reproductive and Respiratory Syndrome virus Elimination

Calendar of Events

Week 1	Infect all sows and boars – dead vaccine and own material from farm Ensure all staff well aware of biosecurity measures Isolation animals – infect all with dead vaccine and own materials Stop live vaccines Move materials from the isolation area? Purchase new stock for 100 days and ensure all animals are exposed Close the herd
Week 2	Continue feed back for 14 days
Week 3	Throw away all used needles and syringes Start 100 day countdown All piglets older than 14 days weaned off farm Limit or cease cross-fostering
Week 4-14	All piglets older than 14 days weaned off farm Limit or cease cross-fostering
Week 14	All piglets older than 14 days weaned off farm Limit or cease cross-fostering Disinfect walls, floors, air and water. Vehicles and utensils. Throw away all clothing, boots, etc. Throw away needles and syringes. Order 20 PRRSv negative gilts.
Week 18	Introduce the 20 gilts into the isolation. Order 20 PRRSv negative gilts.
Week 21	Bleed gilts. If negative, go to next week. Move additional 20 PRRSv free gilts into main farm. If gilts are positive – immediately remove from the isolation area. Close farm for 30 days and re-start checking program.
Week 24	Bleed gilts in isolation and main farm. If negative, go to next week. If any gilts are positive – immediately remove from the isolation area. Close farm for 30 days and re-start checking program.
Week 25	Start weaning as normal
Week 27	Bleed all 40 gilts again. If negative. Restart gilt introduction program Declare the farm free of PRRSv

Note:

This above programme has successfully eliminated PRRSv from a number of farms. As science progresses, PCR technology has revealed PRRSv virus in tonsilar tissue for 200 days post-exposure. Therefore, in designing the herd closure programme the health team must consider the relative risks. In certain circumstances a 200 day closure programme may be adopted rather than the 100 day programme discussed.

Herd closure programmes may also work for Swine Influenza elimination