Introduction

For many years Mycoplasma hyopneumoniae (M. hyo) has been recognized as one of the primary initiators of the porcine respiratory disease complex (PRDC), which inflicts significant production losses in swine herds world-wide. As a result, aggressive efforts to control M. hyo infection have been implemented through routine management practices such as vaccination, medication, and all-in/ all-out pig flow, as well as efforts to maintain Mycoplasma-negative herds.

Only in recent years, however, has a new global health threat become recognized: postweaning multisystemic wasting syndrome (PMWS). The most commonly implicated pathogen causing this syndrome is a virus, porcine circovirus type 2 (PCV2). By itself, PCV2 typically triggers only mild, transient disease signs. However, research has demonstrated that a mixed infection of PCV2 with PRRSV, or with parvovirus, or with M. hyo, may induce severe respiratory disease and lesions consistent with PRDC and PMWS.^1,2

PMWS

A PMWS outbreak is typically characterized by the acute onset of illthrift, weight loss, paleness, and labored breathing in pigs between 6 and 14 weeks of age. Some of the notable features of PMWS include enlarged lymph nodes (especially in the inguinal region), non-collapsed lungs with interstitial pneumonia, and enlarged pale kidneys. The morbidity rate varies between 5 and 30%, but the mortality rate of affected pigs is usually in excess of 80%.^3-8

The simple detection of PCV2 infection does not necessarily mean a pig has PMWS. Furthermore, because wasting or ill thrift can accompany many diseases, and because of the non-specific nature of some of the microscopic lesions associated with PMWS, researchers have identified 3 specific criteria that must be demonstrated to establish a PMWS diagnosis:^9,10

1. wasting, weight loss, ill thrift, or failure to thrive;
2. microscopic lymphoid depletion or monocyte-macrophage associated inflammation in any organ;
3. detection of PCV2 in the affected tissues.

M. hyo Vaccination and PMWS

Numerous research trials conducted in the past few years have assisted practitioners in making sensible therapeutic choices on how to deal with PMWS. While these research efforts have generated much insight about the PMWS disease complex, they have also created some confusion about how to incorporate this knowledge into practical health programs under commercial production conditions.

One source of confusion has centered on a speculated link between routine M. hyo vaccination programs and PMWS. Early research suggested that use of commercial M. hyo bacterins may exacerbate PMWS incidence or severity.¹¹ If true, this link was disturbing because M. hyo vaccination is viewed as a management practice critical for prevention of PRDC, which may inflict devastating financial losses. Adding to the discomfort was the fact that early research efforts seemed to implicate oil-adjuvanted M. hyo bacterins in particular, which research has shown to be extremely effective products for PRDC control.¹² As a result, some practitioners feared that the prudent control of PRDC with a M. hyo bacterin may inadvertently predispose herds for PMWS.

Fortunately, more recent studies with refined protocols have been completed that specifically investigated these initial concerns.^13-15 Study results have identified the optimal timing of M. hyo vaccination in PCV2-affected herds, demonstrated that oil-adjuvanted M. hyo bacterins do not affect PMWS any differently than other formulations, and confirmed that M. hyo vaccination continues to deliver valuable benefits to swine production.

STUDY 1: Best Timing of M. hyo Vaccination

Experiment Design

A study was conducted to determine whether the timing of M. hyo vaccination has any effect on PCV2 replication or PMWS disease.¹³ The study involved 78 pigs randomly assigned to 8 treatment groups. Six of the treatment groups were vaccinated with M+Pac® (Schering-Plough), an oil-adjuvanted M. hyo bacterin. As shown in Table 1, vaccinations were administered either as a two-dose regime (groups 1-4) or a single-dose regime (groups 5-6) at different time points relative to PCV2 challenge infection. All pigs (with the exception of negative controls, group 8) were intranasally inoculated with PCV2 at 8 weeks of age, receiving 5 mL of PCV2 at a dose of 10^5.1 TCID₅₀ (isolate ISU-40895, passage 6). Pigs were evaluated daily for clinical signs and weighed at weekly intervals.

Necropsy was performed on all pigs 42 days after PCV2 inoculation.

Samples from lymphoid tissues, spleen, tonsil, thymus, kidney, lung, liver, heart, and intestine were evaluated microscopically in a blinded fashion for presence and severity of lesions.

Results and Discussion

None of the challenged pigs developed clinical PMWS, regardless of vaccine timing or dosage regimen. Clinical signs were limited to mild respiratory disease in PCV2-infected pigs, plus macroscopic lesions comprised of enlarged lymph nodes.

Microscopic lesions in PCV2-challenged pigs were characterized as mild-to-moderate lymphoid depletion and granulomatous inflammation in lymph nodes, mild-to-moderate lymphohistiocytic hepatitis, mild-to-moderate lymphohistiocytic myocarditis, and/or mild focal-to-diffuse interstitial pneumonia. Notably, microscopic lesions tended to be more severe in groups vaccinated at the time of or 2 weeks before PCV2 challenge.

Study results indicate that PCV2- associated lesions tend to be absent or minimal if pigs are vaccinated with a M. hyo bacterin no closer than 2 weeks before expected PCV2 replication.

Researchers concluded that producers with recurrent PCV2-associated disease in their herds should estimate the likely time of PCV2 infection and complete M. hyo vaccination at least 2 weeks prior to that time.

In most herds, maternal antibodies to PCV2 are present and follow a predictable decay curve. Maternal antibodies typically interfere with viral replication until falling below a 0.6 S/P¹⁶ around 8 weeks of age, which explains why PMWS often appears in the late nursery or early finishing phase. Therefore, M. hyo vaccination regimes should typically be timed for completion by 6 weeks of age or younger in order to avoid the subsequent period of PCV2 replication. This is consistent with a widely recognized industry practice to avoid vaccinating piglets immediately before or concurrent with disease challenges (i.e., PRRSV, PCV2, SIV, PRV), and the 2-week interval has become the industry standard for M. hyo vaccine timing relative to PCV2 infection.

STUDY 2: Oil-Adjuvanted M. hyo Bacterins and PMWS

Experiment Design

An extensive study was designed to evaluate the effects of M. hyo vaccination on PMWS severity.^14,15 The trial involved 272 weaned (3-week-old), M. hyo-negative, PCV2-negative barrows that were randomly allotted to 4 treatment groups, 68 pigs per group. Three commercial USDA licensed M. hyo bacterins were evaluated in the study; two used oil-based adjuvant formulations while one employed an aqueous-based adjuvant.

All products were administered at 3- and 5-weeks of age per label directions, which also accommodated the vaccination timing recommendations derived from the previous study.¹³

A placebo vaccine regimen was administered to pigs comprising the fourth treatment group, according to the same schedule. An additional 24 pigs were maintained as nonvaccinated, nonchallenged controls.

After vaccination, all animals were subjected to 2 challenge infections. A M. hyo (strain 232) lung homogenate was administered on day 35 of the study, followed by challenge with a PCV2 cell culture (ISU infectious clone 40895) on day 49. The experimental design is summarized in Figure 1. below. Pigs were observed daily, and animals showing signs of wasting or illness were euthanized and necropsied. Blood samples and body weights were throughout the 131-day study, allowing determination of PCV2 and M. hyo antibody titers, PCV2 DNA copy numbers in sera (viremia), and weight gain data. Fourteen pigs from each group were scheduled for necropsy at days 63 and 77. At necropsy, macroscopic lesions and the percent of lung involvement were scored, bronchial alveolar lavage (BAL) samples were collected for M. hyo quantitation, bronchial swabs were collected for routine culture, and fixed tissue samples were collected for scoring of microscopic lesions and immunohistochemistry (IHC) staining for the presence of PCV2 antigen. Veterinary and laboratory staff were blinded to bacterin and treatment group identities.

Primary variables analyzed from the collected data were body weights and average daily gain (ADG), as well as percent lung lesions on days 63 and 77. Secondary variables included antibody titers, PCV2 and M. hyo quantitation, post-challenge clinical signs, “sort loss” at close out (low end weight), macroscopic and microscopic lesions at necropsy, and the incidence of clinical case-defined PMWS. The sort-loss parameter was defined as pigs weighing less than 230 lb at close-out. The case definition of PMWS was met if a pig showed clinical signs of wasting/weight loss/ill thrift, microscopic lesions of lymphoid depletion and/or lymphohistiocytic inflammation, plus the presence of PCV2 antigen confirmed by IHC.

Results and Discussion

Clinical results from the placebo treatment group confirmed that the challenge infections were virulent.

The M. hyo challenge produced severe macroscopic and microscopic lesions, while PCV2 co-infection was confirmed by the presence of typical macroscopic and microscopic lymphoid lesions, PCV2 nucleic acid in serum, and PCV2 antigen in lymphoid and lung specimens.

No evidence of increased PMWS in vaccinated pigs was observed. In regard to the primary study variables, all vaccinated pigs, regardless of the type of adjuvant formulation, demonstrated significantly (P ≤ 0.05) higher mean body weights and ADG at days 100 and 131 (close-out) compared to the placebo controls (Table 1).

Furthermore, all vaccine treatment groups had significantly (P ≤ 0.05) lower percent lung scores (least squares mean) than the placebo controls (19.9%) at necropsy on day 63 (lung lesions fell to only 5.3% in controls by the day 77 necropsy).

Concentrations of M. hyo DNA in BAL were consistent with macroscopic lung lesions. Pigs in the oil adjuvant groups also experienced significantly (P ≤ 0.05) fewer days that animals were scored for the presence of any clinical sign of respiratory disease.

Results relating to PCV2 showed no significant differences between treatment groups (PCV2 viremia; macroscopic lesion scores; microscopic lesion scores of tissues examined) (Table 2). Although PCV2-associated lesions in lung and lymphoid tissues were severe in some pigs in all infected groups, no differences were detected between treatment groups in the number of animals that developed clinical PMWS (5.6% meeting specific case definition criteria) or pigs categorized as sort loss at close-out (low endweight).

Study results were consistent with previous reports that co-infections with M. hyo and PCV2 result in severe lesions in the respiratory tract and lymphoid tissues.^1,2 However, the early premise that M. hyo vaccination enhances PMWS disease and lesions was refuted in this study. Compared to challenged controls, vaccinated pigs showed no increases in PCV2 antigen in serum or tissues, or microscopic lesions associated with PCV2 infection. In other words, when administered at the proper time relative to PCV2 challenge, no potentiation of PCV2 disease by any of the bacterins was observed, regardless of adjuvant type (oil-based or aqueous-based).

Conclusions

When the timing of M. hyo vaccination is appropriately separated from PCV2 viremia/infection, neither M. hyo vaccination nor the type of adjuvant formulation used in a M. hyo bacterin (oil- or water-based), increases the incidence or severity of PMWS.

In a study designed to determine the optimal timing of M. hyo vaccination in pigs exposed to PCV2, results indicate that PCV2-associated lesions tend to be absent or minimal if pigs are vaccinated with a M. hyo bacterin no closer than 2 weeks before anticipated viral replication. Researchers concluded that producers with recurrent PCV2-associated disease in their herds should estimate the likely time of PCV2 infection and complete M. hyo vaccination at least 2 weeks prior to that time.

A second study evaluated the impact of M. hyo bacterins formulated with either oil- or water-based adjuvants on the severity of PMWS in pigs co-infected with M. hyo and PCV2.

Results revealed that M. hyo vaccinates experienced significantly better ADG compared to placebo controls, with no differences between adjuvant formulations in regard to the number of pigs developing PMWS or the number of pigs with low end-weights at close-out. All bacterins successfully controlled M. hyo challenge infection (i.e., percent lung involvement at necropsy, M. hyo DNA level in BAL, clinical signs after challenge). In fact, only bacterins with oil-based adjuvants significantly (P ≤ 0.05) reduced the number of days with clinical signs of respiratory disease.

Regardless of the adjuvant system employed in a M. hyo bacterin, the use of appropriately timed M. hyo vaccination remains a powerful, critical management tool for reducing production losses associated with PRDC, especially in herds where PCV2 is circulating or PMWS is diagnosed.

Further Information

For further information on PCV2 and PMWS visit our PMWS Focus Section

References

1 Opriessnig T, Yu S, Meng XJ, et al. PCV2 and Mycoplasma hyopneumoniae coinfection model. Proc 18th IPVS Cong 2004; 1:95.

2 Opriessnig T, Thacker E, Yu S, et al. Experimental reproduction of postweaning multisystemic wasting syndrome in pigs by dual infection with Mycoplasma hyopneumoniae and porcine circovirus type 2. Vet Pathol 2004; 41:623-640.

3 Segalés J, Rosell C, Domingo M. Pathological findings of postweaning multisystemic wasting syndrome (PMWS) affected pigs. Proc Conf SsDNA Viruses of Plants, Birds, Pigs and Primates 2001; Sep. 24-27, St. Malo, France; p 69.

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11 Opriessnig T, Yu S, Gallup M, et al. Effect of vaccination with selective bacterins on conventional pigs infected with type 2 porcine circovirus. Vet Pathol 2003; 40:521- 529.

12 Thacker E, Thacker B, Boettcher TB, et al. Comparison of antibody production, lymphocyte stimulation, and protection induced by four commercial M. hyopneumoniae bacterins. Swine Health Prod 1998; 6:107- 112.

13 Opriessnig T, Halbur PG, Thacker EL, et al. Effects of timing of administration of Mycoplasma hyopneumoniae bacterin on the development of lesions associated with porcine circovirus type 2. Vet Rec 2006; 158:149-154.

14 Halbur P, Opriessnig T, Thomas P. Best practices for control of PCV2-associated diseases. In Proc Swine Disease Conf for Swine Practitioners 2005; 13:98-107.

15 Halbur PG, Rapp-Gabrielson VJ, Hoover T, et al. Effect of Mycoplasma hyopneumoniae vaccination in swine experimentally coinfected with M. hyopneumoniae and porcine circovirus type 2. Proc Amer Assoc Swine Vet 2006; 447-449.

16 Harms PA, Sorden SD, Halbur PG, et al. Role of maternal immunity to PCV2 and PRRSV co-infection in the pathogenesis of PMWS. Proc Amer Assoc Swine Vet 2002; 307-312.

June 2006