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Circovirus Control Remains Critical

With virtually no hog farms in the world free of the virus, except two U.S. herds, porcine circovirus has become established as one of the top three economically important swine diseases behind PRRS

With virtually no hog farms in the world free of the virus, except two U.S. herds, porcine circovirus has become established as one of the top three economically important swine diseases behind porcine reproductive and respiratory syndrome (PRRS) and Mycoplasmal pneumonia, according to a leading researcher at Iowa State University’s (ISU) Veterinary Diagnostic Laboratory.

ISU pathologist Darin Madson, DVM, says university research suggests pigs take one of two distinct paths when infected with porcine circovirus type 2 (PCV2). Either they become clinically infected and develop porcine circovirus-associated disease, which can produce 70-80% mortality in finishing pigs, or they develop a subclinical infection with impaired growth and reduced immunity to other disease pathogens or co-infections.

Co-infections are amazingly common, Madson noted during a World Pork Expo seminar sponsored by Intervet Schering-Plough Animal Health.
Figure 1 illustrates that out of 500 cases of PCV2 investigated over the last four years by ISU’s diagnostic laboratory, just nine cases were comprised solely of PCV2. Many others were mixed viral or bacterial infections along with PCV2.

Figure 2 on page 30 shows the close relationship between PCV2 and PRRS infections. That association stayed close until 2008, when reported cases of PCV2 fell sharply in relationship to an increasing number of PRRS cases.

“That’s an indicator that more people were vaccinating for PCV2. That’s the way it is with PCV2. It waits for other diseases like PRRS or mycoplasma to come along, and then the results are magnified,” Madson observes.

So it’s important to control co-infections and improve immunity and the environment to keep pigs healthy.

Vaccination Vital

“Probably the most important thing we can do to combat the clinical, and even the subclinical disease, is to vaccinate for PCV2,” Madson continues. No matter what type of circovirus is in your herd, the vaccines will provide immunity and reduce lesions, and reduce morbidity and mortality while improving performance.

Proper vaccination is critically important for protection, adds Brad Thacker, DVM, technical service manager with Intervet Schering-Plough Animal Health.

“If you quit vaccinating or you vaccinate too late, infection can be a problem. This is an unforgiving disease. If you miss a pig during vaccination, that pig is at high risk for infection, and that is something that we are not used to.

“It’s not like mycoplasma. If you get 70-80% of the pig population vaccinated for mycoplasma, then you will control the disease. But it doesn’t work that way for circovirus,” Thacker points out.

Farms that stop vaccinating risk up to 20% death loss when exposed to the virus, similar to mortality levels before vaccines became available in 2007, he adds. Vaccinated pigs that don’t develop immunity are also at risk of developing the disease. For example, a 10% vaccination failure rate can result in up to a $2 loss for every pig in a group, Thacker calculates.

The economic benefit of vaccination ranges from $3-15 for every dollar invested in vaccine. Cost benefit on individual pigs can vary widely. Cost benefit is based on several company studies and customer testimonials, Thacker says. Vaccine costs also vary depending on the operation.

A Canadian vaccine study at the University of Guelph showed that Intervet Schering-Plough’s Circumvent PCV provided protection all the way to market, with pigs remaining free of viremia and maintaining a steady rate of gain until being sold, Thacker reports.

Signs of Infection

Virtually all pigs get infected with circovirus, but only a few may show clinical signs, Madson says. With circovirus, pigs appear unthrifty, and wasting and diarrhea mimic signs of ileitis or swine dysentery. Along with diarrhea, a thickened small intestine may also look like ileitis. “You’ve got to watch out, because PCV2b (the most common circovirus genome causing disease) may be the reason you have clinical diarrhea in your finishing herd.”

PCV2b generally strikes pigs at 7-15 weeks of age, when maternal antibodies wane; it is the smallest of viruses.

“It is very, very hard to inactivate, and the only disinfectant we have found to kill this virus is a combination of bleach and Virkon S (Antec International, Ltd.),” Madson says.

It’s also important to have thorough removal of gross debris, application of an initial detergent and appropriate drying following disinfecting, says Abby Patterson, DVM, fellow ISU diagnostician.

It’s also important to have thorough removal of gross debris, application of an initial detergent and appropriate drying following disinfecting, says Abby Patterson, DVM, fellow ISU diagnostician.

Difficulty of Diagnosis

Diagnosis is difficult. There are three requirements for diagnosis. First, pigs must show clinical signs of disease. Six fundamental signs include wasting, dyspnea (respiratory distress), enlarged lymph nodes, pallor, diarrhea and jaundice. Pneumonia can also cause lots of problems. The second requirement is loss of lymphocytes based on analysis of tissues. Third, the actual virus must be confirmed in the lesions of grow-finish pigs, Madson says.

Subclinical Disease

The most common way to diagnose subclinical disease is using a polymerase chain reaction (PCR) test.
Submit tissue, feces, serum or oral fluid. Virus isolation using tissue or serum is a less common means of identification. A number of serology tests are also available to confirm infection.

It’s well known that the virus transmits one of two ways — nasal-oral (sow-to-piglet) and fecal-oral (pig-to-pig).

In the pregnant sow, PCV2 is capable of crossing the placenta during infection or viremia. PCV2 is also shed in sow colostrum/milk for at least 27 days post-farrowing, Madson says. PCR test results have shown that colostrum can contain more PCV2 than milk.

“Pig-to-pig is probably the more common means of transmission. PCV2 is shed in anything that comes out of the pig — saliva, semen, urine, feces, etc.,” Madson says.

What is not known is the role of transmission by fomites or inanimate objects, such as needles, clothing and boots that could carry disease-causing organisms. It’s known that PCV2 can replicate in mice and spread to other mice, but spread from rodents to pigs has not been confirmed. Likewise, the virus is found in the air/dust outside of hog barns, but infectivity is unknown, he says.

The rate of infection is quick, taking about eight days for pigs to start shedding virus. “Viremia — the presence of virus in the blood — is an important method of disease shedding in a pig, and it averages 18.4 days, based on sentinel pig studies,” Madson says.

But shedding time is variable. Patterson recently reported shedding to 209 days in nasal secretions and feces of infected pigs. Shedding can be intermittent as the infection progresses.
Once infected, some pigs become immune to circovirus and stop shedding, while others can turn into chronic shedders.

An easy way to detect chronic shedders is to take samples using nasal or rectal swabs and send them to the diagnostic lab, Madson says. Chronic shedding of the virus, which generally corresponds with viremia, can last a lifetime if vaccine doesn’t provide protection.

Environmental Study

The objective of Potter’s study was to demonstrate the applicability of swabbing and testing using PCR (polymerase chain reaction) as a means of monitoring PCV2 viral loads on a variety of surfaces in swine production units. “Although detection by PCR does not indicate viral infectivity, this testing procedure does allow viral nucleic acid to be detected and quantified,” she says.

The team began by swabbing inside surfaces of barns at KSU’s swine teaching and research facility in Manhattan where circovirus had been identified, but clinical signs of wasting and pneumonia were not prevalent, she explains. No PCV2 was found in the environment.

Then a series of nurseries and finishers at the Kansas hog operation sites were tested. Only a couple of the sites had reported clinical problems with PCV2. But all of the rooms in both nursery and finisher sites showed detectable levels of the virus.

Potter says a key objective of the study — detecting circovirus in the environment — was achieved. “But we were surprised that we detected the virus in every room at the commercial farm,” she says. “As far as we know, this is the first time this type of testing has been done on the farm for environmental presence of this virus.”

Cleaning Protocol

Once it was determined there was environmental contamination, the next objective was to learn if cleaning and disinfection made a difference in the viral load inside the barns of the infected sites.
Previously, Synergize (Preserve International) had been used to disinfect the barns between pig groups. In the latest study, there were three cleaning protocols for nursery and finisher rooms:

1. Rooms not washed but empty;

2. Rooms washed, disinfected with Synergize, disinfected with Virkon S (Antec International) and washed again; and

3. Rooms washed, disinfected with Synergize, disinfected with Virkon S, washed again and disinfected with Synergize again.

Rooms were swabbed and tested. Results indicated that the unwashed room had higher levels of virus than did the other two rooms. Rooms in the second protocol had somewhat higher levels of virus than did the third room, Potter says.

“It appears that we are having some effect by mechanical cleaning and disinfection in lowering viral levels, but we are still able to detect presence of viral DNA regardless of protocol,” she notes.

The hardest places to clean were around fans and heaters, which when swabbed had the highest concentration of virus, Potter notes. Next in order of concentration were floor slats, gates, feeders and water sources.

“So this suggests you may want to concentrate on thorough cleaning and disinfection and focus on specific areas,” she says.

The research bears out that environmental swabbing and testing is a practical method of monitoring for circovirus in the environment. The next step in the KSU research is to determine if the virus detected in the environment with PCR is infectious, Potter concludes.

Once that is learned, researchers will better understand if more rigorous cleaning programs are warranted.