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An episode of respiratory disease developed in a group of 6-week-old pigs. A few pigs had signs of acute CNS disease, including ataxia, lateral recumbency, and paddling, and some were found dead. The veterinarian euthanatized 2 affected pigs and submitted them for necropsy.

Grossly, pulmonary consolidation was evident, and fibrin tags were found on the pleural and pericardial surfaces. Various tissues were fixed in 10% formalin, and fresh companion tissues from these same organs were obtained for microbiologic evaluation. Microscopic examination revealed meningitis, pericarditis, and pneumonia. Results of virologic testing were negative. Aerobic incubation of bovine blood agar plates inoculated with brain, lung, liver, and kidney yielded large numbers of mucoid, weakly hemolytic colonies. The same organism was isolated from the joints of one pig. Biochemical and immunologic tests were used to identify the organism as a member of Lancefield's streptococcal serogroup D and to identify a surface (capsular) antigen belonging to Streptococcus suis type 2.

After reading recent reports dealing with the infection, the veterinarian was able to answer the following questions from his client.^1-3

Q: Where did this disease come from?

A: Streptococcus suis type 2 is commonly found in the upper respiratory tract and tonsils of pigs in most areas of the world in which swine are raised.¹ The gilts brought in last spring might have been the source of infection. Within an infected herd, virtually all pigs carry the organism in their tonsilar crypts, and farrowing sows probably transmit the organism to young pigs through nasal contact.^4,5 Streptococcus suis also has been isolated from the genital tract of farrowing and aborting sows, making it possible for newborn pigs to become infected during delivery. Although some serotypes of S suis may cause peracute meningitis and septicemia in suckling pigs, signs of disease caused by S suis type 2 typically are not evident until a few weeks after weaning. The air was stagnant in the nursery about a week before this episode. Overcrowding and poor ventilation are 2 factors frequently associated with this disease.⁶

Q: What can I do about the presence of S suis in my herd?

A: Although eradication through depopulation has been suggested, most veterinarians recommend control by improving management practices and administering antibiotics when necessary. The organism is susceptible to penicillin in vitro, and antibiotic treatment is usually effective in acute cases, but is not necessarily favorable in chronic cases. In spite of a lack of supporting experimental data, autogenous bacterins have been used widely. It is well known, however, that pigs may carry S suis in tonsilar crypts even when specific antibodies or antibiotics are in the serum; therefore, carrier animals represent a constant threat to other susceptible pigs.⁶

Good management techniques should minimize the risks of S suis infection in swine herds. Avoid overcrowding, poor ventilation, and other stressful conditions in the pig houses, especially when young pigs are mixed and moved to the nursery. Attempt to use all-in/all-out rearing, and clean and disinfect the premises between groups. Finally, investigate all disease, and necropsy pigs that die.

Q: How is S suis type 2 transmitted among pigs?

A: The high rate of tonsilar and nasal carriage ensures a ready source for susceptible contact animals. Studies have shown that pigs are infected within 5 days after contact with carrier animals, and the large number of these carriers ensures a ready source for susceptible members of the herd. Although high attack rates may occur, a disease prevalence of less than 5% is more common.⁶ It is important to remember that S suis can survive in the environment for various periods. Cool temperatures and a moist medium encourage its survival. For example, the bacterium may survive in feces for a week and in decomposing carcasses for almost 2 weeks. Fortunately, this organism is highly susceptible to most cleaning and disinfecting agents.⁷

Q: Are there any health risks for my family or me?

A: Streptococcus suis is one of several microorganisms that can cause disease in human beings as well as in animals.⁸ Transmission occurs directly from infected animals, and most cases are seen in people whose work brings them in close contact with pigs, including swine producers, rendering-plant workers, pet-food processors, and veterinarians.^2,9 Home butchering must be approached with extreme care. Though sick pigs or their carcasses represent the greatest threat, the presence of S suis type 2 in otherwise normal carrier pigs extends this risk to various meat handlers. The organism is believed to enter the human body through skin cuts and abrasions. In a recent study, S suis type 2 was found in 16.1% of 413 randomly sampled pigs in an abattoir in southern Ontario.¹⁰ Based on the sampling technique, this was considered a conservative number. Of particular interest was the recovery of S suis type 2 from the knives of both an eviscerator and a butcher in the cut-up room. The former emphasizes the risk to workers handling tissues of the head and tonsils. The latter shows the survivability of S suis type 2 in meats through at least short-term cold storage.¹⁰

Q: How do I know if I've caught this disease?

A: Streptococcus suis becomes septicemic and may localize in a variety of tissues, including the joints and lymph nodes. The most prominent symptoms in human beings are a result of brain infection, particularly meningitis. Headache, fever, unsteadiness, and vertigo may progress to coma if medical assistance is not sought immediately. Most North American physicians and their laboratory support personnel are unfamiliar with this disease; therefore, people who develop these signs should inform their physician of their involvement in the swine industry and of the existence of this zoonotic disease.

Q: Can it be treated?

A: As in pigs, detection and early treatment is successful in reversing this life-threatening illness. However, in spite of treatment, some people may continue to suffer from vertigo, unsteadiness, and hearing deficit. Because deaths have been known to occur, this must be considered a serious health threat, particularly in the high-risk group.

Q: I have never heard of this disease in human beings--is it common?

A: There are no known reports of human S suis type 2 infection in the United States; however, more than 50 cases have been reported in western Europe and other areas of the world. Two cases were found in pig handlers from southern Ontario, establishing the potential virulence of North American isolates.² It is possible that the unfamiliarity of laboratory staff with this organism has allowed isolates from CSF to be reported as group D streptococci without further classification.

Discussion

The high correlation between isolation of S suis type 2 from human patients and their association with the swine industry suggests that this infection is an occupational hazard. However, a few cases have occurred in people with no apparent contact with pigs.^8,9 In many, but not all cases, contamination of wounds was the likely site of entry, emphasizing the need for strict hygienic measures and protective covering such as gloves for handling pig carcasses. The fact that subclinical or normal carrier pigs may be sources of infection increases the risk to those in close contact with these animals.⁸ The prerequisites for this infection in North American workers seem to be present, yet the reasons for the disparity in case numbers between Europe and this continent are not obvious.

Within the streptococci, S suis is probably the best example of a bacteria that causes a zoonotic infection by direct transmission to human beings; nevertheless, several others should be mentioned. Streptococci belonging to Lancefield's groups B, C, and G have been isolated from human beings and animals, and zoonotic transmission may occur, but separate reservoirs of infection probably exist in the respective species.¹¹ Group B streptococci (S agalactiae) are among the most common causes of neonatal sepsis in human beings and are a well recognized cause of bovine mastitis. The colonized birth canal of parturient women is the most likely source of infection for infants. Some group C streptococci (S equisimilis and S zooepidemicus) cause various suppurative infections in livestock and horses as well as man. Similarly, group G streptococci are commonly involved in urogenital, respiratory, and cutaneous infections in dogs and cats. Group G streptococci have been isolated from cows with mastitis and from human beings with pharyngitis.¹¹

Veterinarians frequently are asked to examine an animal, usually a dog or a cat, as a potential source of streptococcal infection in members of the family. It should be determined whether streptococci have been isolated from the human patient. If a group A streptococcus (S pyogenes) is found, it is highly unlikely that an animal reservoir was involved, as this organism has rarely been found in companion animals or livestock.

Though animals and human beings may harbor streptococci of the same serogroup and even the same species, careful microbiologic, serologic, and epidemiologic studies are required to establish a zoonotic connection. It is likely that streptococcal infections have been misidentified, leading to false assumptions about their zoonotic nature.¹² Still others maintain independent reservoirs of infection in separate host species. In the case of S suis type 2, epidemiologic and microbiologic evidence has confirmed its zoonotic potential.³

References

1. Clifton-Hadley FA. The epidemiology, diagnosis, treatment and control of Streptococcus suis type 2 infection, in Proceedings. Annu Meet Am Assoc Swine Pract 1986; 471-491.

2. Sanford SE, Tilker AME. Streptococcus suis type 2-associated diseases in swine: observations of a one-year study. J Am Vet Med Assoc 1982; 181:673-676.

3. Erickson ED, Doster AR, Pokorny TS. Isolation of Streptococcus suis from swine in Nebraska. J Am Vet Med Assoc 1984; 185:666-668.

4. Clifton-Hadley FA, Alexander TJL, Upton I, et al. Further studies on the subclinical carrier state of Streptococcus suis type 2 in pigs. Vet Rec 1984; 114:513-518.

5. Arends JP, Hartwig N, Rudolphy M, et al. Carrier rate of Streptococcus suis capsular type 2 in palatine tonsils of slaughtered pigs. J Clin Microbiol 1984; 20:945-947.

6. Clifton-Hadley FA. Studies of Streptococcus suis type 2 infection in pigs. Vet Res Commun 1984; 8:217-227.

7. Clifton-Hadley FA, Enright MR. Factors affecting the survival of Streptococcus suis type 2. Vet Rec 1984; 114:585-587.

8. Zanen HC, Engel HWB. Porcine streptococci causing meningitis and septicemia in man. Lancet 1975; 1:1286-1288.

9. Clifton-Hadley FA. Zoonoses in practice--Streptococcus suis type 2 infection. Br Vet J 1983; 139:1-5.

10. Breton J, Mitchell WR, Rosendal S. Streptococcus suis in slaughtered pigs and abattoir workers. Can J Vet Res 1986; 50:338-341.

11. Erickson ED. Streptococcosis. In: Steele JH, ed. Handbook series in zoonoses. Section A: Bacterial, rickettsial and mycotic diseases. Vol 2. Boca Raton, Fla: CRC Press Inc, 1980; 65-80.

12. Drusin LM, Ribble JC, Topf B. Group C streptococcal colonization in a newborn nursery. Am J Dis Child 1973; 125:829-821.

Addendum (1994)

Considerable work continues on the epidemiology¹ and molecular biology² of Streptococcus suis. The duration and degree of contact with pigs appear to be the most important factors in determining development of substantial serum titers to S suis surface antigens in human beings. Although as many as 29% of those in close contact with swine may carry antibody titer to S suis, clinical disease is still rare.¹

Isolates of S suis from swine in North America now include 30 serotypes; however, there appears to be some consistency from year to year in the serotypes most frequently isolated.³ The organism has been isolated from cattle,⁴ cats,⁵ and horses,⁵ as well as swine. In 1991, an isolate of S suis serotype 2 was recovered at the Sainte-Foy Hospital in Quebec, Canada, from a man with endocarditis.⁶ This patient had stuck himself with a hypodermic needle while vaccinating his pigs. Streptococcus suis was isolated earlier from human patients in Ontario, Canada.⁷ These appear to be the only reports of isolations made in North America, and add to the record of isolations from human beings in other countries.^1,8,9 Our understanding of the host range, clinical spectrum, and options for control of S suis infections will continue to unfold as basic and clinical research continue.

Streptococci have, in general, recaptured the attention of clinical and laboratory investigators in the past few years. Streptococcus pneumoniae and Enterococcus (Streptococcus) faecalis are serious problems in hospitalized, aged, and immunocompromised hosts because of the organisms' increasing resistance to chemotherapeutic agents.^10,11 Streptococcus pyogenes (group A) has received considerable attention recently, in the medical and nonprofessional presses, because of reports from England and North America of its several clinical manifestations, including a toxic shock-like syndrome and cases of necrotizing fasciitis.^12,13 The history of these syndromes represents trends that have received current attention under the umbrella of new and emerging diseases.

References

1. Robertson ID, Blackmore DK. Occupational exposure to Streptococcus suis type 2. Epidemiol Infect 1989; 103:157-164.

2. Jacobs AAC, Loeffen PLW, van den Berg AJG, et al. Identification, purification, and characterization of a thiol-activated hemolysin (suilysin) of Streptococcus suis. Infect Immun 1994; 62:1742-1748.

3. Higgins R, Gottschalk M. Distribution of Streptococcus suis capsular types in 1993. Can Vet J 1994; 35:245.

4. Hommez M, Wullepit P, Casimon F, et al. Streptococcus suis and other streptococcal species as a cause of extramammary infection in ruminants. Vet Rec 1988; 123:626-627.

5. Devriese LA, Sustronk B, Maenhout T, et al. Streptococcus suis in a horse. Vet Rec 1990; 127:68.

6. Trottier R, Higgins R, Brochu G, et al. A case of human endocarditis due to Streptococcus suis in North America. Rev Infect Dis 1991; 13:1251-1252.

7. Sanford SE, Tilker AME. Streptococcus suis type 2-associated diseases in swine: Observations of a one-year study. J Am Vet Med Assoc 1982; 181:673-676.

8. Chau PY, Huang CY, Kay R. Streptococcus suis meningitis. An important underdiagnosed disease in Hong Kong. Med J Aust 1983; 1:414-417.

9. Arends JP, Zanen HC. Meningitis caused by Streptococcus suis in humans. Rev Infect Dis 1988; 10:131-137.

10. Appelbaum PC. Antimicrobial resistance in Streptococcus pneumoniae: An overview. Clin Infect Dis 1992; 15:77-83.

11. Johnson AP, Uttley AHC, Woodford N, et al. Resistance to vancomycin and teicoplanin: An emerging clinical problem. Clin Microbiol Rev 1990; 3:280-291.

12. Demers B, Simor AE, Vellend H, et al. Severe invasive group A streptococcal infections in Ontario, Canada: 1987-1991; editorial response by DL Stevens. Clin Infect Dis 1993; 16:792-800 and 801-802.

13. Burge TS, Watson JD. Necrotizing fasciitis. Br Med J 1994; 308:1453-1454.