Focus on Upper and Lower Respiratory Diseases

Transcription

1 Proceedings of the American Association of Equine Practitioners - Focus Meeting Focus on Upper and Lower Respiratory Diseases Salt Lake City, UT, USA 2010 Next Focus Meetings: July 24-26, Focus on Colic Indianapolis, IN, USA September 18-20, 2011 Focus on Dentistry Location TBD Reprinted in the IVIS website with the permission of the AAEP

2 Caught in the Middle of a Strangles Outbreak Review of the Diagnostic and Management Procedures Dorothy M. Ainsworth, DVM, PhD, Diplomate ACVIM Author s address: Department of Clinical Sciences, Cornell University, Ithaca, NY 14853; e- mail: dma2@cornell.edu. Take Home Message The diagnosis of Strep equi infections can be enhanced by performing both microbial culture and polymerase chain reaction (PCR) analysis of guttural pouch lavage samples. Managing a farm outbreak requires identification and segregation of horses that are shedding the organism and implementation of strict hygiene protocols to restrict the exposure of non-infected horses. Vaccination of healthy non-infected horses during an outbreak has been advocated to stem the contagion but effective immune responses take at least 2 weeks. Introduction Strangles is a highly contagious disease of equids caused by the Gram positive bacterium, Streptococcus equi subspecies equi. The organism, which is not a normal inhabitant of oropharnygneal flora, is spread to susceptible horses by either direct contact with infected horses or indirectly, through fomites on contaminated halters, lead ropes, buckets, brushes or handlers. Most outbreaks are associated with the introduction of a horse that is either incubating the disease or has recently recovered clinically from the disease but is still shedding the organism. Limited data suggest that the organism does not survive for extended periods of time in the environment outside of the horse. 1 In typical cases of strangles, affected horses exhibit inappetence and a fever that precedes the development of a purulent pharyngitis and lymphadenitis of the mandibular and retropharyngeal lymph nodes by 7-10 days. In farm or stable outbreaks, morbidity rates are often high but case fatalities are low. However, complications from S. equi infections disseminated (metastatic) strangles, purpura hemorrhagica (PH) and myopathies may involve 20% of cases. 2 Because it may take months to eliminate Strep equi outbreaks at a facility, significant economic losses are incurred due to disruption of training and riding activities; loss of athletic condition; development of lethal sequelae and expenses associated with veterinary care. 3 While complete eradication of strangles is eventually feasible (but unlikely to occur in the immediate future), current efforts should be focused on rapidly identifying infected animals both symptomatic and asymptomatic carriers and implementing management protocols that reduce the spread of the infection and secondary complications. Diagnostic Methods - To Swab or Not to Swab! Experimental studies of intranasal (IN) inoculation of S. equi have provided insight into the kinetics of the infection. 4 Once inhaled, entry into the oro- and nasopharyngeal tonsillar tissue by 49

3 the bacteria is very rapid: Within 3 hours of exposure, small numbers of organisms are detected in the tonsillar crypts and sub-epithelial follicular tissue. By 48 hours post-inoculation, large numbers of cocci are found in the tonsils and lymph nodes. At the onset of fever (and in the absence of lymphadenopathy), mandibular and retropharyngeal lymph nodes become heavily infiltrated with neutrophils and organisms. Nasal shedding of S. equi is usually evident 2 to 3 days after the onset of fever and persists for 2 to 4 weeks and perhaps for as long as 6 weeks in some horses. Thus, the definitive diagnosis of strangles is dependent upon positive microbial culture results or by identification of bacterial DNA using PCR technology. Diagnostic samples can be obtained by nasal swabs, nasal washes, nasopharyngeal swabs and guttural pouch (GP) lavages. Although culture of nasopharyngeal swabs has traditionally been regarded as the gold standard for detecting S. equi, some investigators 5,6 have found that during strangles outbreaks, a higher percentage of culture positives (88%) are obtained from GP lavages compared to nasopharyngeal swabs (45% culture positives). The lower microbial yield from nasopharyngeal swabs may be the result of (1) swabbing too early in the course of the disease; (2) retrieving insufficient amount of secretions or (3) having overgrowth of other organisms (Strep zooepidemicus). Endoscope guided guttural pouch lavage also offers the advantage of identifying persistent long-term asymptomatic carriers that may have concurrent GP pathology. To perform a GP lavage, a 1 m endoscope is passed nasally into each guttural pouch following sedation of the horse. One should exercise caution when sedating affected horses as this procedure increases laxity of the pharyngeal muscles and further reduces the diameter of a compromised upper airway. Endoscopy allows visual inspection of the pouch for evidence of inflammation, empyema, chondroids or protruding lymph nodes. Then, a sterile polyethylene tubing is advanced through the biopsy port of the endoscope and 30 to 50 ml of sterile saline is gently instilled and aspirated. Blind lavage of the pouches may also be achieved using a Chamber s catheter. PCR analysis of samples is used as a supplementary technique to microbial culture for maximizing the likelihood of identifying S. equi, especially in persistent carriers. Indeed, it is considered to be 2-3 times more sensitive than culture alone in detecting the organism 2,5,6 but PCR is not without limitations. False negatives may occur in samples with heavy growths of S. equi and/or in samples with large amounts of purulent material that contain high concentrations of DNA polymerase inhibitors. False positives may occur when the organism has been killed but PCR technology still detects residual DNA. At our institution, we routinely use both microbial culture and PCR analysis of GP lavage samples to maximize detection of S. equi in suspected cases. Serological diagnosis of S. equi infections. A commercially available ELISA a enables detection of serum antibodies against the SeM protein produced by S. equi. In general, during natural infections serum IgG titers to SeM peak approximately 5 weeks post-exposure and remain high for at least 6 months or more. Responses to commercial extract vaccines peak around 2 weeks post IM vaccination and remain high for 6 months. However, the ELISA does not allow one to distinguish between immune responses to vaccine versus natural infection with S. equi. Overlapping titer breakpoints in normal and convalescent horses also complicate the interpretation of test results. 6 Furthermore, identification of animals with subclinical disease (early in the course of infection or those with persistent infection) is also not possible. 6,7 ELISA 50

4 results are reported as negative; weak positive (1:200-1:400); moderate positive (1:800-1:1,600); high positive (1:3,200-1:6,400); very high positive (> 1:12,800). The company suggests that the ELISA results may be useful for the detection of horses recently vaccinated as well as horses with purpura hemorrhagica, streptococcal myopathies and/or metastatic strangles. However, titers should be interpreted in context of clinical findings. Interestingly, a recommendation in the American College of Veterinary Internal Medicine (ACVIM) Consensus Statement 2 that horses with SeM titers > 1:1,600 or 1:3,200 not be vaccinated against S. equi (to deter the development of purpura hemorrhagic) has been suggested to be too conservative a recommendation. 8 For example, in a recent clinical study conducted by Boyle et al. (2009), 26 horses with SeM titers > 1:1,600 (5 with titers > 1:3,200) were re-vaccinated with an IN attenuated vaccine b and followed for at least one year post-vaccination. None of the horses developed signs of PH. However, whether this lack of risk for developing PH is unique to horses receiving the intranasal vaccine as compared to the intramuscular vaccine could not be addressed in that study. Additional numbers of re-vaccinates will need to be examined to determine if current vaccination guidelines or precautions need to be changed. Managing Strep equi Outbreaks - Identifying the Guilty Parties! The detection and isolation of the Strep equi carriers in conjunction with the implementation of strict hygiene measures is requisite in controlling outbreaks and enabling the premises to quickly return to normal activity. During outbreaks, it is useful to recall 5 (1) that not all affected horses will have discharging abscesses; (2) that some horses may have recurrence of disease several weeks later and (3) that no diagnostic sampling method is 100% sensitive. Some recommended control measures 2 include: (1) the cessation of all movement of horses onto and off the premises; (2) the monitoring of rectal temperatures of all horses at least once daily to detect, promptly segregate and treat new cases. Recall that horses spike fevers approximately 2 days before organisms are shed; (3) the isolation of infected horses in an area that is physically separated from other clean areas of premises where non-infectious horses are kept; (4) the assignment of a separate staff to care for the infectious horses. Such individuals should be provided with separate equipment (buckets, halters, ropes), disinfection facilities, protocols and information about the potential risks of contracting S. equi infections. Although rare, S. equi meningitis has been reported in the elderly, in the very young, and in individuals with prior health issues; 9-11 (5) the composting of manure and waste feed from infectious animals and resting pastures used to hold infectious animals for 4 weeks and (6) the daily disinfection of water troughs. During an outbreak, some clinicians advocate the vaccination of healthy, non-infectious horses that have not been exposed to or have had contact with infected horses in an effort to reduce the spread of the disease. However, effective immune responses following vaccination require days to be generated. There is also a potential risk of inducing purpura hemorrhagica in vaccinates that have previously been infected although this may be dependent upon the type of vaccine administered (extract versus intranasal). Treatment of Strep equi infections remains controversial. During the acute phase of the disease when the horse exhibits fever and depression, penicillin therapy may prevent abscess formation. 51

5 However, once antimicrobial therapy is stopped, if protective immunity has not been established, the horse will be susceptible to re-infection. When external lymphadenopathy is evident and the horse remains bright and alert, it is recommended not to treat with penicillin but to simply hot pack the abscesses and allow their maturation and eventual rupture. In cases with retropharyngeal lymphadenopathy and guttural pouch empyema, attention should be given to ensuring that the horse has an adequate airway and is able to eat and drink normally without risk of aspiration. It is recommended that a tracheostomy be performed if stridor and/or respiratory distress are evident. In horses with GP empyema or erupted retropharyngeal lymph nodes, GP lavage (2-3 liters of 0.9% saline or polyionic solutions) followed by instillation of a penicillingelatin mixture is recommended. 12 Although lavage may remove small chondroids, physical removal by endoscopic-guided grabbing forceps, a basket snare or a memory-helical polyp retrieval basket c may be required. 12 For depressed horses, for horses with potential aspiration pneumonia (from upper airway dysfunction) or for horses with metastatic strangles, systemic penicillin therapy, potentially coupled with other antibiotics to broaden the spectrum, is indicated. Horses with purpura hemorrhagic and infarctive myositis which can be fatal complications of S. equi require intensive management and benefit by being referred to a tertiary care center. These cases are treated with intravenous crystalloids (50-60 ml/kg/day), sodium or potassium penicillin (22,000 IU/kg q6 IV) and dexamethasone (0.1 mg/kg q24 IV) therapy. A feeding tube is placed so that enteral nutrition or a slurry of hay pellets may be given. Complete resolution of the outbreak may take months. It is generally recommended that in order to declare a horse free of infection, at least 3 negative culture/pcr results obtained at weekly intervals, be obtained. Prophylactic strategies include preventing newly acquired and potentially infected animals from co-mingling with resident horses. Whenever possible, all new acquisitions should be isolated for at least 3 weeks and screened for S. equi by repeated swabs or lavages at weekly intervals. The author prefers to obtain bacterial culture samples endoscopically following lavage of the GP of newly acquired horses. Because the majority of subclinical long-term carriage of S. equi occurs in the GPs of recovered horses, endoscopy of the GP (with lavage) may improve detection of these horses. Vaccination of resident horses on the farm is another useful prophylactic approach depending upon the extent of horse movement onto and off of the farm. The basis and duration of protective immunity following natural infection is not fully understood but in the majority of horses that recover, immunity is believed to last of > 5 years. 6 Thus, in horses recovering from an outbreak, vaccination may not be required for some time. Following intranasal vaccination with an attenuated strain of S. equi, b protective immunity takes approximately 2 weeks. The typical schedule requires 2 doses at 2-3 week intervals with an annual booster. Adverse reactions intramuscular abscesses have been reported. It is generally recommended that other intranasal vaccines not be given simultaneously with this vaccine and that other intramuscular vaccines not be given following IN vaccination. The occasional report of lack of efficacy of the IN vaccine may reflect failure of the vaccine to reach the tonsil; the action of blocking antibody or the existence of a non-responder horse. 4 A promising multi-component recombinant subunit vaccine is under investigation that provided nearly 100% protection against the development of 52

6 strangles in experimentally challenged ponies 13 several years away. but its commercial availability is probably References and Footnotes 1. Weese JS, Jarlot C, Morley PS. Survival of Streptococus equi on surfaces in an outdoor environment. Can Vet J 2009; 50: Sweeney CR, Timoney JF, Newton R et al. Streptococcus equi infections in horses: guidelines for treatment, control and prevention of strangles. J Vet Intern Med 2005; 19: Prescott JF, Timoney JF. Could we eradicate strangles in equids? (Commentary). J Am Vet Med Assoc 2007; 231: Timoney JF, Kumar P. Early pathogenesis of equine Streptococcus equi infection (strangles). Equine Vet J 2008; 40: Newton JR, Verheyen K, Talbot NC et al. Control of strangles outbreaks by isolation of guttural pouch carriers identified using PCR and cultures of Streptococcus equi. Equine Vet J 2000; 32: Waller AS, Jolley KA. Getting a grip on strangles: recent progress towards improved diagnostics and vaccines. Vet J 2007; 173: Davidson A, Traub-Dargatz JL, Magnuson R et al. Lack of correlation between antibody titers to fibrinogen-binding protein of Streptococcus equi and persistent carriers of strangles. J Vet Diagn Invest 2008; 20: Boyle AG, Sweeney CR, Kristula M et al. Factors associated with likelihood of horses having a high serum Streptococcus equi SeM-specific antibody titer. J Am Vet Med Assoc 2009; 235: Popescu G, Fuerea R, Benea E. Meningitis due to an unusual human pathogen: Streptococcus equi subspecies equi. So Med Assoc 2006; 99: Elsayed S, Hammerberg O, Massey V et al. Streptococcus equi subspecies equi (Lancefield group C) meningitis in a child. Clin Microbiol Infect 2003; 9: Breiman RF, Silverblatt FJ. Systemic Streptococcus equi infection in a horse handler a case of human strangles. West J Med 1986: 145: Verheyen K, Newton JR, Talbot NC et al. Elimination of guttural pouch infection and inflammation in asymptomatic carriers of Streptococcus equi. Equine Vet J 2000; 32: Guss B, Flock M, Frykberg L et al. Getting to grips with strangles: an effective multicomponent recombinant vaccine for the protection of horses from Streptococcus equi infection. PLoS Pathog 2009; 5(9):e Doi: /journal.ppat a. IDEXX Streptococcus equi ELISA test ; IDEXX, Westbrook, ME. b. Pinnacle IN; Pfizer Animal Health, NY, NY. c. Cook Inc., Bloomington, IN. 53