Glasser's Disease (Haemophilus parasuis)

Glasser's disease is a systemic bacterial infection of swine caused by Glaesserella parasuis (formerly classified as Haemophilus parasuis), characterized by fibrinous polyserositis, polyarthritis, and meningitis. First described by Karl Glasser in 1910, the disease remains one of the most economically significant bacterial diseases affecting modern swine production worldwide. The causative organism is a small, gram-negative, pleomorphic rod belonging to the family Pasteurellaceae that colonizes the upper respiratory tract of pigs as a commensal organism but can invade systemically under conditions of immune compromise or stress to produce severe and often fatal disease.

The reclassification of the causative agent from Haemophilus parasuis to Glaesserella parasuis was formally proposed in 2020 based on phylogenomic analyses that demonstrated the organism's distinct evolutionary lineage within the Pasteurellaceae. While the new nomenclature is increasingly adopted in scientific literature, the historical name Haemophilus parasuis remains widely used in the swine industry, on vaccine labels, and in diagnostic laboratory reports. Both names refer to the same organism, and practitioners should recognize either designation. The disease itself continues to be universally known as Glasser's disease regardless of the nomenclatural change.

Glaesserella parasuis is ubiquitous in swine populations globally, with virtually all commercial pig herds harboring the organism as part of their normal upper respiratory flora. At least 15 recognized serovars of G. parasuis have been identified through immunodiffusion serotyping, with considerable variation in virulence among serovars. Serovars 1, 5, 10, 12, 13, and 14 are generally considered highly virulent, while serovars 2, 4, and 15 show moderate virulence, and serovars 3, 6, 7, 8, 9, and 11 are typically classified as non-virulent or of low virulence. However, virulence classification is not absolute, and some field isolates within traditionally non-virulent serovars have been associated with clinical disease, while virulent serovars do not invariably cause disease in all exposed populations.

The pharmaceutical and management approach to Glasser's disease encompasses both preventive vaccination and therapeutic antimicrobial intervention for clinical cases. Vaccination with commercial or autogenous bacterins represents the primary preventive strategy, while injectable and in-feed antimicrobials are used to treat affected pigs and to provide metaphylactic coverage during high-risk periods. The complexity of serovar diversity, variable cross-protection between serovars, and the interplay between pathogen virulence and host immunity make Glasser's disease management one of the more challenging aspects of swine health programs.

The pathogenesis of Glasser's disease follows a progression from commensal respiratory colonization to systemic invasion and disseminated inflammatory disease. Under normal circumstances, G. parasuis colonizes the nasal passages and upper airways of pigs within the first days of life through horizontal transmission from the sow and pen mates. In immunocompetent pigs with adequate maternal antibody protection, this colonization remains confined to the mucosal surfaces and stimulates the development of active immunity. Disease occurs when the organism breaches the mucosal barrier and enters the bloodstream (bacteremia), disseminating to serosal surfaces, joints, and meninges, where it triggers intense fibrinous inflammation.

The factors that precipitate systemic invasion include immune naivety (pigs encountering virulent serovars for the first time without prior exposure or maternal antibody protection), stress events that compromise mucosal immunity and systemic immune function, and co-infections with immunosuppressive pathogens. The classic scenario for Glasser's disease outbreaks involves the mixing of pigs from different sources with different G. parasuis serovar profiles, such as occurs during transport to nursery or growing-finishing facilities. Pigs from high-health herds that have never encountered certain virulent serovars are particularly susceptible when introduced to environments where those serovars are endemic. Porcine reproductive and respiratory syndrome virus (PRRSV) and porcine circovirus type 2 (PCV2) infections are well-recognized predisposing factors that impair immune competence and increase the risk and severity of Glasser's disease.

Clinical signs of Glasser's disease typically appear in pigs between 4 and 8 weeks of age, corresponding to the period when maternal antibody levels wane and the pig's own immune system is still maturing. Peracute cases may present as sudden death without premonitory signs, with postmortem findings of overwhelming septicemia. Acute cases develop high fever (40.5-42 degrees Celsius), depression, anorexia, reluctance to move, and signs referable to the affected body systems. Pericarditis produces muffled heart sounds and respiratory distress from cardiac compression. Pleuritis causes dyspnea and abdominal breathing. Peritonitis produces abdominal distension and pain. Meningitis manifests as incoordination, lateral recumbency, paddling, opisthotonus, and tremors. Arthritis causes joint swelling, lameness, and reluctance to bear weight.

Postmortem lesions in Glasser's disease are highly characteristic and often pathognomonic when the classic triad of polyserositis, polyarthritis, and meningitis is present. Affected serosal surfaces (pericardium, pleura, peritoneum) are covered with thick fibrinous or fibrinopurulent exudate that can be peeled from the underlying tissue in sheets. Joints contain turbid, fibrin-rich synovial fluid or fibrinopurulent material. The meninges appear congested and may be covered with fibrinous or purulent exudate, particularly over the cerebral hemispheres. In chronic cases, fibrinous exudates organize into fibrous adhesions that restrict organ motion, impair cardiac filling, and cause chronic lameness.

Differential diagnosis of Glasser's disease includes other causes of polyserositis and meningitis in young pigs, most notably Streptococcus suis infection, Mycoplasma hyorhinis polyserositis, Escherichia coli septicemia, and salmonellosis. S. suis type 2 infection is the most important differential for meningitis in nursery pigs and can co-occur with G. parasuis infection. Definitive diagnosis requires isolation of G. parasuis from affected tissues or body fluids, supported by serotyping and, increasingly, molecular characterization through PCR-based methods.

Vaccination against Glaesserella parasuis represents the cornerstone of Glasser's disease prevention in commercial swine operations. Both commercial (USDA-licensed) bacterins and custom autogenous bacterins are available, each with distinct advantages and limitations. The choice between commercial and autogenous products, along with the vaccination protocol selected, should be guided by the herd's disease history, the serovar(s) identified through diagnostic testing, and the production system's management structure.

Commercial G. parasuis vaccines available in the United States include Porcilis Glasser (Merck Animal Health), which contains inactivated serovar 5 organisms, and other products that may contain different serovars or combinations. These products are USDA-licensed, manufactured under standardized quality control conditions, and available through veterinary suppliers. The primary limitation of commercial bacterins is the restricted serovar coverage they provide. Because cross-protection between G. parasuis serovars is generally limited and unpredictable, a commercial vaccine containing serovar 5 may provide incomplete or no protection against clinical disease caused by serovars 4, 12, 13, or other types prevalent on a given farm. This serovar specificity issue is the central challenge in G. parasuis vaccination.

Autogenous (custom) bacterins are produced from G. parasuis isolates obtained from the specific farm or production system experiencing disease. The farm-specific isolate(s) are submitted to a USDA-licensed autogenous biologics manufacturer, which produces a killed vaccine tailored to the serovar(s) causing disease on that operation. Autogenous vaccines offer the advantage of precise serovar matching to the herd's disease-causing isolates, theoretically optimizing the immune response against the relevant strains. However, autogenous products are not subjected to the same efficacy testing requirements as commercially licensed vaccines, quality can vary among manufacturers, and they require ongoing isolate submission and product updating as the farm's serovar profile may change over time.

Sow vaccination protocols aim to boost maternal antibody levels in colostrum, providing passive protection to piglets during the vulnerable early nursery period. The standard approach involves vaccinating gilts with a primary two-dose series (typically 5 and 2 weeks before farrowing) and boosting sows with a single dose 2-3 weeks before each farrowing. Maternal antibodies transferred through colostrum provide protection during the first 3-5 weeks of life, after which they decline and the piglet becomes susceptible to infection. The duration and magnitude of passive protection depends on the sow's antibody titers, colostrum quality and intake, and the piglet's antibody absorption efficiency.

Piglet vaccination is used either as a standalone strategy or in combination with sow vaccination to extend protection through the nursery and early growing periods. Piglets are typically vaccinated at 1-3 weeks of age with a second dose 2-3 weeks later, though the optimal timing depends on the expected disease window on the farm and the interference of maternal antibodies with active immunization. Vaccinating piglets too early, when high maternal antibody levels are still present, can result in neutralization of vaccine antigens and poor active immune responses. Conversely, delaying vaccination until maternal antibodies have waned leaves a gap in protection when pigs may be most vulnerable to challenge during mixing and transport events.

Treatment of clinical Glasser's disease relies on early, aggressive antimicrobial therapy directed against the causative G. parasuis organism. Because the disease progresses rapidly from initial bacteremia to established polyserositis and meningitis, the timing of antimicrobial intervention is critical. Pigs treated early in the course of infection, before extensive fibrinous exudate has formed on serosal surfaces and in joints, have a substantially better prognosis than those treated after advanced lesions have developed. Once organized fibrinous adhesions form, antimicrobials cannot reverse the structural damage, and affected pigs may develop chronic lameness, reduced growth, or become non-viable despite bacteriological cure.

G. parasuis is generally susceptible to a range of beta-lactam antibiotics, including penicillin, ampicillin, amoxicillin, and ceftiofur. Ceftiofur (Excenel, Excede) is frequently the antimicrobial of choice for injectable treatment of clinical Glasser's disease due to its favorable pharmacokinetic profile in swine, including good tissue distribution to the serosal surfaces, joints, and central nervous system where the organism causes pathology. The long-acting ceftiofur crystalline free acid formulation (Excede) provides sustained therapeutic concentrations from a single injection, which is advantageous in production settings where repeated individual pig treatment is labor-intensive. Penicillin and ampicillin are effective alternatives but typically require more frequent dosing.

Fluoroquinolone antibiotics, including enrofloxacin, demonstrate excellent in vitro activity against G. parasuis and achieve good tissue penetration, including into the central nervous system. However, fluoroquinolone use in food-producing animals is subject to significant regulatory constraints. Enrofloxacin (Baytril) is approved for swine respiratory disease in certain countries but carries extra-label use prohibitions in food animals in the United States for certain routes and indications. The judicious use of fluoroquinolones must balance their clinical efficacy against antimicrobial stewardship concerns and regulatory compliance.

Metaphylactic antimicrobial administration, in which apparently healthy pen mates of clinically affected pigs are treated to prevent disease progression during an outbreak, is a common and practical approach to managing Glasser's disease in group-housed swine. Water-soluble antimicrobials such as amoxicillin or chlortetracycline can be administered through the water system to provide population-level coverage during acute outbreaks. In-feed antimicrobials require Veterinary Feed Directives (VFDs) and take longer to achieve therapeutic tissue concentrations, making them more suitable for sustained prevention during high-risk periods than for acute outbreak control. The selection of metaphylactic antimicrobials should be guided by susceptibility testing of farm-specific isolates, as antimicrobial resistance patterns vary among G. parasuis populations.

Antimicrobial resistance in G. parasuis has been increasingly documented globally, with resistance to tetracyclines, sulfonamides, and trimethoprim-sulfamethoxazole most commonly reported. Resistance to beta-lactams is less prevalent but has been identified in some field isolates, emphasizing the importance of culture and sensitivity testing when treatment failures occur. Multi-drug-resistant isolates have been reported from intensive production systems, particularly in regions with high antimicrobial use pressure. Routine susceptibility testing of clinical isolates, combined with antimicrobial stewardship practices, helps ensure that treatment protocols remain effective and aligns with regulatory and industry expectations for responsible antimicrobial use.

The extraordinary serovar diversity of Glaesserella parasuis is one of the defining challenges in managing Glasser's disease through vaccination and underscores the importance of accurate diagnostic characterization of isolates from clinical cases. The 15 recognized serovars, defined by differences in capsular and lipopolysaccharide antigens, represent only a portion of the antigenic diversity in field populations, as a substantial proportion of clinical isolates (estimated at 20-40% in some surveys) are non-typeable by conventional serological methods. This antigenic heterogeneity means that no single vaccine formulation can be expected to provide universal protection against all G. parasuis strains.

Cross-protection between serovars has been evaluated in experimental vaccination-challenge studies with variable results. Some degree of cross-protection has been demonstrated between certain serovar pairs, but the protection is generally incomplete and unpredictable. For example, serovar 5 vaccination may provide partial protection against challenge with serovars 4 or 13 in some studies, but this cross-protection is not consistently reproducible across different experimental conditions and animal populations. The lack of reliable broad cross-protection is the primary reason that autogenous vaccines tailored to farm-specific isolates are often preferred over commercial products with fixed serovar compositions.

Diagnostic isolation and serotyping of G. parasuis from clinical cases provide the foundation for rational vaccine selection and treatment planning. The organism is fastidious and requires enriched media containing NAD (factor V) for primary isolation, though it does not require factor X (hemin), distinguishing it from true Haemophilus species. Isolation should be attempted from affected serosal surfaces, joint fluid, meningeal swabs, or cardiac blood collected aseptically from acutely affected or freshly dead pigs. Prior antimicrobial treatment can reduce isolation rates, making collection before treatment initiation ideal when possible.

Molecular diagnostic methods have increasingly supplemented traditional culture-based approaches for G. parasuis identification and characterization. PCR assays targeting species-specific gene sequences can confirm the presence of G. parasuis in clinical samples, including formalin-fixed tissues, with higher sensitivity and faster turnaround times than traditional culture. Multilocus sequence typing (MLST) and whole-genome sequencing provide high-resolution strain characterization that goes beyond serovar classification, enabling epidemiological tracking of strains within and between herds and informing autogenous vaccine strain selection. These advanced molecular tools are becoming more accessible to diagnostic laboratories and are reshaping the approach to G. parasuis surveillance and management.

Serological monitoring of herd immunity to G. parasuis can be performed using ELISA-based assays that detect antibodies against whole-cell or specific antigenic fractions of the organism. Serum profiling of pigs at different ages (cross-sectional serological surveys) reveals the kinetics of maternal antibody decline and the timing of active seroconversion, informing optimal vaccination timing decisions. A characteristic pattern showing high antibody levels in young pigs (maternal), declining titers through the nursery period (waning maternal immunity), and rising titers in the growing-finishing phase (active seroconversion from natural exposure) indicates endemic circulation. The nadir between declining maternal and rising active antibodies defines the window of maximum susceptibility, which is the critical period that vaccination protocols must bridge.

G. parasuis bacterin vaccines produce a range of local and systemic reactions consistent with the general side effect profile of gram-negative killed bacterial vaccines in swine. The inactivated whole-cell nature of bacterin vaccines, combined with the adjuvants used to enhance immunogenicity, creates a relatively reactogenic immunological stimulus compared to some other vaccine types. Understanding the expected side effect profile helps producers and veterinarians distinguish normal post-vaccination responses from unusual reactions requiring attention.

Local injection site reactions are the most commonly observed adverse effects following G. parasuis vaccination. Swelling, firmness, and transient soreness at the intramuscular injection site occur in a variable proportion of vaccinated pigs and sows. These local reactions result from the inflammatory response to adjuvant compounds and bacterial cell components and typically resolve over 1-3 weeks without treatment. The severity of injection site reactions can vary among vaccine products and manufacturers due to differences in adjuvant formulations. Autogenous vaccines, which may use different adjuvant systems than commercial products, can produce more or less severe local reactions depending on the specific formulation.

Systemic reactions following G. parasuis vaccination may include transient fever, lethargy, and reduced feed intake for 24-48 hours after vaccination. These responses reflect the systemic immune activation stimulated by the vaccine and are generally self-limiting. In young piglets, post-vaccination malaise can temporarily reduce nursing and growth, though this effect is typically minor and short-lived. Sows vaccinated in late gestation occasionally show mild systemic reactions that should be distinguished from signs of concurrent illness. Monitoring pigs for 30-60 minutes after vaccination allows early identification of more serious reactions.

Anaphylactic reactions to G. parasuis bacterins are rare but can occur, particularly with gram-negative whole-cell vaccines that contain endotoxin (lipopolysaccharide, LPS). The LPS component of killed G. parasuis organisms is a potent immune stimulant that can trigger acute hypersensitivity reactions in sensitized animals. Signs of anaphylaxis in swine include acute respiratory distress, cyanosis, vomiting, urticaria, collapse, and death. Epinephrine (adrenaline) should be available during vaccination events for emergency treatment of anaphylactic reactions. Animals that have experienced anaphylaxis to a G. parasuis vaccine should not receive the same product again without veterinary assessment and possible pre-treatment with antihistamines or corticosteroids.

Abortion and reproductive effects from vaccination of pregnant sows are uncommon when vaccines are administered according to label directions but have been anecdotally reported following the use of some bacterin products, particularly autogenous vaccines with high endotoxin content. Vaccinating sows during the first trimester of gestation is generally avoided when possible to minimize any theoretical risk to embryonic survival. If adverse reproductive outcomes are temporally associated with vaccination, the vaccine product, timing, and handling should be reviewed, and the manufacturer should be consulted. Reproductive safety testing is a standard component of USDA licensing for commercial vaccines but is not required for autogenous biologics.

Effective control of Glasser's disease requires an integrated approach that combines vaccination with management practices designed to reduce the impact of predisposing factors, minimize stress-related immunosuppression, and optimize the development of natural herd immunity. Vaccination alone is often insufficient to eliminate Glasser's disease from a swine operation, particularly in multi-site production systems where mixing of pigs from different sources creates ongoing opportunities for exposure to novel G. parasuis serovars.

All-in/all-out (AIAO) production flow at the room or barn level is one of the most effective management tools for reducing Glasser's disease incidence. AIAO flow prevents continuous exposure of incoming naive pigs to G. parasuis shed by older, carrier pigs, reducing infectious pressure during the vulnerable nursery period. Between groups, thorough cleaning and disinfection of rooms removes residual bacterial contamination and allows a defined break in the chain of transmission. Continuous-flow nurseries, in which pigs of different ages are commingled in the same airspace, experience higher rates of Glasser's disease and other endemic bacterial infections due to sustained pathogen cycling.

Reduction of stressors known to predispose pigs to Glasser's disease is a critical management component. Early weaning before 21 days of age is associated with increased Glasser's disease incidence, as pigs weaned at younger ages have less mature immune systems and often lower maternal antibody levels. Minimizing transport stress through gentle handling, adequate temperature control during transit, and prompt access to feed and water upon arrival at nursery facilities supports immune competence during the high-risk transition period. Optimizing nursery environment quality, including ventilation, temperature, humidity, and stocking density, reduces the overall disease pressure on recently weaned pigs.

Controlling concurrent immunosuppressive infections, particularly PRRSV and PCV2, is fundamental to Glasser's disease management. These viral infections compromise systemic and mucosal immune responses, creating conditions that allow commensal G. parasuis to invade systemically. Herds that achieve stable PRRSV status or eliminate PRRSV circulation consistently experience reduced incidence of secondary bacterial diseases, including Glasser's disease. PCV2 vaccination is now standard in most commercial swine operations and contributes to improved overall immune competence in the nursery and growing-finishing phases.

Feedback or controlled exposure programs, in which gilts and sows are intentionally exposed to farm-specific G. parasuis strains before farrowing, represent an alternative or adjunctive approach to vaccination. These programs aim to stimulate robust maternal immunity that provides colostral protection to piglets during the vulnerable early weeks of life. Feedback can involve exposure to nasal secretions, placental tissues, or live bacteria from the farm's own pig population. However, uncontrolled feedback carries biosecurity risks, including the potential introduction of other pathogens, and should be carefully managed under veterinary supervision. Vaccination with farm-specific autogenous bacterins provides a controlled alternative to live-organism feedback that achieves similar goals without the biosecurity risks.

Genetic and sourcing considerations influence Glasser's disease susceptibility at the herd level. High-health herds maintained through strict biosecurity and limited external introductions may have pigs with minimal prior exposure to virulent G. parasuis serovars, making them highly susceptible when introduced to conventional environments. Gilt acclimation programs that include G. parasuis vaccination as part of incoming gilt processing help ensure that replacement breeding animals develop immunity before their first parity, supporting colostral protection for their piglets. Sourcing pigs from a single genetic source reduces the serovar mixing that occurs when pigs from multiple origins are commingled.

Withdrawal time compliance for products used in Glasser's disease prevention and treatment encompasses both the vaccines administered preventively and the antimicrobial drugs used therapeutically. Each product carries specific regulatory requirements that must be observed to ensure food safety compliance when treated pigs enter the food supply. The diversity of products potentially used in managing Glasser's disease, from bacterin vaccines to injectable antibiotics to in-feed antimicrobials, requires careful record-keeping and treatment documentation.

G. parasuis bacterin vaccines, both commercial and autogenous, generally carry short or zero-day slaughter withdrawal periods because they contain only killed bacterial proteins and adjuvant components that do not persist as violative residues in tissues. Specific withdrawal times should be verified on the product label, as they can vary among manufacturers and formulations. Autogenous vaccines produced under 9 CFR 113.113 carry withdrawal times assigned by the manufacturer, typically 21 days, though this can vary. The withdrawal clock begins from the date of the last vaccine administration.

Antimicrobial withdrawal times are the primary regulatory concern in Glasser's disease management, as the antimicrobials used for treatment and metaphylaxis carry established tissue depletion periods. Ceftiofur sodium (Naxcel) carries a zero-day slaughter withdrawal in swine, while ceftiofur crystalline free acid (Excede) has a 14-day withdrawal following a single injection in swine. Penicillin products carry species-specific and formulation-specific withdrawal periods, and producers must consult the label of the specific product administered. Tetracycline products used in-feed or in-water carry their own withdrawal requirements, and VFD compliance documentation must be maintained.

Extra-label antimicrobial use in swine for Glasser's disease treatment is governed by AMDUCA regulations and requires a valid veterinarian-client-patient relationship (VCPR). When antimicrobials are used at doses, frequencies, routes, or in species not specified on the label, the prescribing veterinarian establishes an extended withdrawal period based on FARAD guidance or professional judgment. All extra-label antimicrobial use must be documented, and the extended withdrawal period must be communicated to the producer and recorded in treatment logs. Certain antimicrobials, including fluoroquinolones and certain others, are prohibited from extra-label use in food animals by FDA regulation.

Record-keeping requirements for Glasser's disease management integrate with broader food safety and quality assurance programs. Pork Quality Assurance Plus (PQA Plus) protocols require documentation of all medications administered to swine, including vaccines and antimicrobials, with product identification, dates of treatment, individual or group identification, withdrawal periods, and the name of the administering or prescribing person. These records support traceback investigations in the event of residue findings at slaughter and demonstrate the producer's commitment to responsible medication use. Electronic record-keeping systems integrated with production management software are increasingly used to automate withdrawal date calculations and prevent inadvertent marketing of animals with unexpired withdrawal periods.

Research into Glasser's disease prevention and treatment continues to advance on multiple fronts, driven by the limitations of current vaccine technologies, the challenge of serovar diversity, growing concerns about antimicrobial resistance, and the development of new molecular tools for pathogen characterization. These research directions hold promise for improving the management of this economically important swine disease and may reshape prevention strategies in the coming years.

Next-generation vaccine approaches aim to overcome the serovar-specific limitations of current whole-cell bacterins by targeting conserved antigens shared across multiple G. parasuis serovars. Outer membrane protein (OMP) subunit vaccines, recombinant protein vaccines based on conserved virulence factors such as transferrin-binding proteins and autotransporter adhesins, and live attenuated vaccine candidates are under investigation. These approaches seek to stimulate broadly protective immune responses that are not restricted to the serovar-specific capsular and LPS antigens that dominate the immune response to killed whole-cell vaccines. Early-stage research has identified several candidate antigens that show promise in experimental models, though commercialization of next-generation G. parasuis vaccines remains a work in progress.

Genomic and proteomic studies of G. parasuis are providing deeper understanding of the molecular determinants of virulence and the genetic basis for serovar diversity. Whole-genome comparisons between virulent and commensal isolates have identified pathogenicity islands, secretion system components, and surface-exposed proteins that distinguish disease-causing strains from harmless commensals. These findings inform both vaccine antigen selection and diagnostic marker development. The application of whole-genome sequencing to routine diagnostic isolates is enabling high-resolution epidemiological surveillance of G. parasuis populations within and between herds, production systems, and geographic regions.

Immunological research is clarifying the host immune mechanisms that determine the outcome of G. parasuis infection and the response to vaccination. Studies examining the roles of mucosal immunity, systemic humoral responses, and cell-mediated immunity in protection against Glasser's disease are informing vaccine design and delivery strategies. The development of mucosal vaccine delivery systems that stimulate local respiratory immunity at the primary site of G. parasuis colonization represents an intriguing approach that could complement or eventually replace systemic injection-based vaccination.

Antimicrobial alternatives for Glasser's disease management are being explored in response to increasing pressure to reduce antimicrobial use in livestock production. Phage therapy using bacteriophages specific to G. parasuis, antimicrobial peptides, immunomodulatory compounds, and probiotic-based approaches to competitive exclusion of pathogenic strains from the upper respiratory tract are all under investigation at various stages of development. While none of these alternatives has yet reached commercial availability or demonstrated field efficacy comparable to conventional antimicrobials, the growing body of research in these areas reflects the swine industry's proactive engagement with antimicrobial stewardship goals and the search for sustainable disease management strategies.