Herd Health

FMD Vaccine Shows Promise

Progress is being made in the development of a safe, effective vaccine for foot-and-mouth disease (FMD) using genetic engineering.

Three experiments at USDA's Plum Island Animal Disease Center (PIADC) in Greenport, NY, have shown that a subunit vaccine may prove the best alternative to current FMD vaccines, according to Plum Island researcher Marvin J. Grubman.

Current whole virus, inactivated vaccines are used worldwide to control FMD, with more than a billion doses used annually.

However, there are several problems with the existing vaccines, explains Grubman. First, these vaccines require production of large quantities of live virus in high-containment facilities followed by virus inactivation.

Second, although the vaccines have successfully controlled FMD, some evidence exists linking outbreaks to release of virus from vaccine production plants or residual live virus in chemically inactivated vaccines.

For the United States, there is also a federal law that only allows FMD virus at the PIADC. Thus, PIADC, which houses the North American vaccine bank, can only acquire vaccine from overseas manufacturers.

The subunit FMD vaccine research supported by National Pork Board funding is one alternative approach. “This approach is based on the demonstration that empty viral capsids, virus particles lacking infectious nucleic acid, are naturally produced in infected cells and are indistinguishable from infectious virus in their ability to stimulate an immune response,” says Grubman.

Four main advantages to this approach are:

• No infectious FMD virus is required or produced;

• No large-scale, biocontainment facilities are needed for production;

• It's possible this vaccine could be produced on the U.S. mainland, and

• The ability to easily distinguish vaccinated from naturally infected animals using currently approved diagnostic tests.

Grubman's trials have shown that animals given a single inoculation of the subunit vaccine were completely protected from FMD disease when challenged with infectious virus as early as seven days later.

“These experimental results suggest that this vaccine holds much promise in the battle against FMD,” he observes.

Researcher: Marvin J. Grubman, Plum Island Animal Disease Center. Phone Grubman at (631) 323-3329 or send a fax to (631) 323-2507.

PRRS Virus Transmission in Gilts Wanes with Time

Gilts can harbor the PRRS (porcine reproductive and respiratory syndrome) virus for months after exposure. But their ability to spread the virus to penmates is limited, say South Dakota State University (SDSU) researchers.

In a study to assess the ability of infected breeding stock to spread PRRS virus, an SDSU team injected 10, 6-month-old gilts with the virus, and five gilts were given a mock injection.

To monitor spread throughout the group, tonsil biopsies were collected from gilts at 7, 14, 28, 42, 56, 70 and 84 days post-injection. One sentinel pig was mixed with each group of five experimentally infected gilts.

The tonsil biopsies revealed the presence of PRRS viral material in each of the 10 treated gilts at 7, 14 and 28 days after injection.

But, only two of 10, four of 10, four of 10 and two of 10 tonsil biopsies were positive for PRRS virus at 42, 56, 70 and 84 days post-injection, respectively.

Sentinel pigs became infected from 7 to 42 days post-injection. No transmission of the virus was detected after 56 days following inoculation.

In conclusion, gilts can carry the PRRS virus for up to 84 days, but pig-to-pig spread is minimal after 42 days post-injection.

The SDSU study also found that tonsil biopsies are a reliable tool in detecting persistently infected pigs. According to Benfield, this is an important finding as producers explore ways to control and eradicate PRRS.

Researchers: David A. Benfield, Jane Christopher-Hennings, South Dakota State University; Phone Benfield at (605) 688-4317 or e-mail David_ [email protected].

Identifying Carriers of PRRS Virus in Boars

Variation in detection and duration of PRRS (porcine reproductive and respiratory syndrome) virus shedding in boars has called into question the use of standard serum, semen or blood cell tests to identify truly negative animals.

In a challenge trial, eight boars were given PRRS virus intranasally. All boars within each breed were from the same litter and litters were within 9 days of age. The eight boars in the study were from a well-known, PRRS-free boar stud. The group included three Hampshire, three Yorkshire and two Landrace boars. Serum and white blood cell were collected twice weekly and analyzed for the presence of the PRRS virus by virus isolation and PCR (polymerase chain reaction). Semen was also collected twice weekly from seven of eight boars and analyzed by PCR.

After all samples tested negative for at least 2-3 weeks, boars were euthanized and tissues, serum, feces and urine were collected and analyzed by virus isolation. The virus was still found within the tonsils of three of the eight boars by virus isolation.

Furthermore, various tissues were positive for PRRS virus by PCR but not by virus isolation. The virus was found the majority of the time in lymphoid tissue of each boar rather than reproductive or other tissues.

This means that the boars could still be carriers of the virus and potentially spread. Extensive testing may be necessary to determine which boars are persistently infected and should be eliminated.

The research, sponsored by the National Pork Board, suggests it may be useful to produce and sell PRRS-naïve boars to prevent spread of the PRRS virus.

The data also revealed that there is variation in the duration of virus shedding. One boar shed the virus in semen for four days, 70 days in another boar. There were no breed differences.

PRRS virus was detected for a longer period in semen than in serum or blood cells in four of seven boars. Virus was also found longer in serum than in blood cells in six of eight boars.

This study suggested boars may clear the PRRS virus in time. In one boar, virus could not be detected in serum, semen, white blood cells or tissues by 88 days after infection. Researchers suggested it may be possible to eliminate persistently infected boars using a 45-day quarantine plus evaluation of a tonsil biopsy by PCR.

Researcher: Jane Christopher-Hennings, DVM, South Dakota State University; Phone (605) 688-5171 or e-mail [email protected].

PRRS Virus Persistence Confirmed in Sows

Persistently infected sows can harbor and spread PRRS (porcine reproductive and respiratory syndrome) virus to naïve, non-infected sows over extended periods of time.

University of Minnesota researcher Scott Dee, DVM, coordinated placement of 12 PRRS-naïve, non-pregnant sows that were experimentally infected with PRRS virus, in individual isolation rooms for 42 to 56 days post-infection.

Then one naïve sow was placed in each isolation room, divided by a gate, allowing nose-to-nose contact with a single infected sow. The infected sow was not shedding virus at the time of contact with the naïve sow.

Upon testing of the naïve sows, infectious PRRS virus was isolated from three of the 12 sows at 49, 56 and 86 days post-infection and was verified by polymerase chain reaction testing.

Further testing confirmed that the virus used to infect the naïve sows was the same virus that was isolated from the three infected sows at the completion of the study.

Virus persistence provides evidence that herd closure periods of three months are not adequate for ensuring elimination of PRRS virus.

Further studies need to be implemented with larger numbers to determine the length of time breeding herds need to be closed to the introduction of replacement stock.

Researcher: Scott Dee, DVM, University of Minnesota. Phone Dee at (612) 625-4786 or e-mail [email protected].

Early Iron Injections Produce Heavier Weaning Weights

Giving iron injections at an early age contributed to significantly heavier weaning weights, according to a trial conducted by scientists at the University of Guelph.

A trio of scientists wanted to find out if weaning weight was affected by the age at which piglets were given supplemental iron.

It is commonly believed that baby pigs are born with adequate iron stores for the first week of life and should not need supplemental iron until at least 5 days of age.

However, a recent study indicated that some pigs are born anemic or with iron stores too low to last five days.

Findings in this study of an 830-sow, farrow-to-wean, Canadian commercial operation suggest that iron injections produced better weaning weights when administered at an early age, 1 to 4 days of age, vs. 5 to 7 days of age.

The study involved 2,346 piglets born during a six-week period. Piglets were crossfostered to equalize litter size at birth. Pigs were processed within 72 hours of birth and randomly assigned to one of the two treatment groups. They were given 1½ cc. of 200 mg. iron dextran in the neck, processed and weighed. Piglets were weighed again 24 to 72 hours prior to weaning.

Blood samples were taken of 41 piglets: 10, 3-day-old piglets, 10, 5-day-old piglets, 12, 6-day-old piglets and nine, 10-day-old piglets.

Age when given iron, weaning age, birth weight and farrowing room were all associated with weaning weight.

Thirty percent of 3-day-old pigs, 60% of 5-day-old pigs and 92% of 6-day-old pigs showed mild to moderate anemia prior to being supplemented with iron dextran.

None of the piglets injected with iron dextran at 1 day of age were anemic when sampled at 10 days of age.

In this study, there were no negative effects from early iron injections. Delayed administration of iron produced some anemia.

The researchers suggested another study to determine if the weight differences observed in this trial were due to anemia in the older-injected group, or whether another mechanism was responsible.

They also suggested that increased weight gains in the younger-injected group could be due to immune status of the herd. Farms with a low level of disease caused by iron-dependent bacteria, such as E. coli, may experience increased weaning weights when piglets are injected with iron in the first few days of life.

Researchers: Craig Storey, Cate Dewey, DVM, and Beth Young, University of Guelph. Phone Storey at (613) 354-6615 or e-mail [email protected].

Test and Removal Plan Eliminates PRRS Virus

A test and removal (T&R) plan successfully eliminated the PRRS (porcine reproductive and respiratory syndrome) virus.

University of Minnesota Swine Center researcher Scott Dee, DVM, applied the T&R method for the elimination of PRRS virus to five Minnesota farms.

The entire breeding herd was tested for PRRS virus on one day. Animals that tested positive by ELISA (enzyme-linked immunosorbent assay) or PCR (polymerase chain reaction) were removed within two days of being tested.

Farms had to meet the following criteria at the start of the study:

• At least two years must have elapsed since the original PRRS infection;

• At least one year must have passed since the last known clinical outbreak of PRRS;

• The level of prevalence of PRRS virus in the breeding herd must be 25% or less;

• The farms must be located at least two miles from any other PRRS-infected farm, and

• The farms must not have used PRRS vaccine for two years or more prior to the study, and no vaccination during the study.

In the five farms studied, three used segregated production and two used single-site production. All farms used artificial insemination. Average farm size was 769 sows for the T&R group.

If respective nursery or finisher pigs in the test group proved positive for PRRS virus, they were depopulated before the trials began.

Following testing protocols for the two groups, breeding herds were monitored monthly for 12 months by ELISA.

Ten samples were collected monthly from 8- to 10-week-old nursery pigs and 5- to 6-month-old finishers.

During the monitoring program, if 5% of a breeding herd tested PRRS positive for three consecutive months, or if pigs tested positive postweaning, then monitoring ceased and that herd was declared PRRS positive.

Along with the T&R group, two PRRS-positive and two PRRS-negative farms were used as control groups, says Dee.

The results showed that the T&R protocol successfully eliminated PRRS from all five farms including postweaning pig groups. Those farms averaged 10% positives at the start of the study.

Partial depopulation of nursery or finisher had occurred in all five farms prior to the study.

T&R limitations include labor and cost of testing and the removal of productive sows from the herd. To reduce the impact of animal removal on herd productivity, most removed sows were taken to off-site facilities to gestate and farrow.

Dee says T&R requires further validation by use of the protocol in larger herds. But study results so far indicate, “it is a method capable of consistently eliminating PRRS virus from farms that have similar characteristics to those defined in the study.”

The study was supported by the National Pork Board and the Minnesota Pork Producers Association.

Researcher: Scott Dee, DVM, University of Minnesota; Phone Dee at (612) 625-4786 or e-mail [email protected].