The predominant streptococcal disease of swine is caused by Streptococcus suis. Other less common, largely sporadic, streptococcal infections are summarized as follows.
Sporadic infections occasionally are caused by Streptococcus equisimilis. This beta-hemolytic streptococcus and a few others sometimes cause arthritis, septicemia or meningitis in young pigs, complicate other disease processes, or may contribute to cases of vegetative, valvular endocarditis in older growing/finishing pigs. This organism is indigenous in the vagina and infects pigs at birth. The cases in young pigs are a result of navel infection or from breaks in the integument as a result of knee abrasions, or infection of skin wounds including those caused by tail docking, ear notching or clipping of needle teeth.
Streptococcus porcinus is the same as Lancefield group E streptococci associated with jowl abscess (feeder boils) in swine. This disease was once quite prevalent but is now rare, presumably because of improvements in husbandry and feeder design. It is occasionally isolated from pigs with septicemia or abscesses.
Additional streptococci believed to be pathogens have been isolated from the respiratory tract, mammary glands, localized skin lesions and subcutaneous abscesses, from the reproductive tract of aborting sows, and sows with fertility problems or agalactia. Their role in these conditions is less clear. Streptococcus durans has been associated with enteritis in suckling pigs.
This is the most important streptococcal infection of pigs and is usually seen in nursing or recently weaned piglets. The disease usually is characterized by septicemia, acute meningitis, polyarthritis, polyserositis, or bronchopneumonia.
Although S. suis infection occurs primarily in nursing or recently weaned pigs, all age groups occasionally are affected. The disease is found in all major swine-raising countries. It is relatively common, occurs throughout the year, and is prevalent in confinement-raised swine. Streptococcus suis infection also occurs sporadically in several domestic animals (cattle, sheep, goats, horses, dogs). It is zoonotic and occurs occasionally in people who work with pigs or handle meats (butchers, farmers, and veterinarians), causing localized infections, arthritis, septicemia and possibly death.
Outbreaks of streptococcal infection in swine occurred many years before the organism, S. suis, was identified in 1987 as a new pathogenic species. Outbreaks of S. suis infection now are reported frequently. Because S. suis colonizes piglets at birth, both segregated early weaning (SEW) and medicated early weaning (MEW) fail to prevent S. suis infections.
There has been much research on streptococci. Early research on S. suis largely concerned capsular serotypes and comparison of signs and lesions produced by different serotypes. Virulence factors have been the focus of more recent research. Infection models vary widely and likely contribute to the lack of agreement in findings reported in published studies.
Streptococcus suis is Gram-positive and easily grown on blood agar. Identification is based on biochemical reactions and capsular serotyping. There are about 35 capsular serotypes. Their prevalence varies in different geographic regions of the US with types 2, 3, 4 and 7 predominating in the Midwest. Type 2 may be most prevalent worldwide and is the most likely to express virulence. Virulence factors are not well understood but capsular polysaccharide (CPS) and two proteins, muramidase released protein (MRP) and an extracellular factor (EF) are thought to play important roles in the severity of disease caused by infection with type 2.
Many species of Streptococcus can be isolated from normal swine. There is no single, consistent predictor of possible pathogenicity. Environmental or other predisposing factors may influence pathogenicity. Most streptococci are readily destroyed by common disinfectants such as phenol, quaternary ammonium, formaldehyde, hypochlorite, chlorhexidine, or 3% iodine.
Streptococcus suis survives in dust and feces in the usual swine environment. It can be isolated from the nasal cavity and palatine tonsils of many normal pigs. It is present in the feces and nasal secretions of carriers. Transmission may be through ingestion, inhalation or nose-to-nose contact. Flies and rodents may play a role in mechanical spread. S. suis can survive on flies for at least 5 days.
Healthy carrier swine usually introduce infection. Sows infect piglets at or shortly after birth. Contaminated secretions, excretions and skin of infected pigs also expose other pigs. These factors may make early weaning (7-24 days) and MEW ineffective in preventing transmission to at least some piglets which may then serve as sources of infection to cohorts during lactation and nursery phases. Wounds also are a common route of entry for S. suis. Once a few piglets are infected, they are able to disseminate the bacteria to other litters or commingled pigs. Close contact favors spread.
Multiple serotypes may exist on the same farm. Control of a particular type of S. suis may lead to emergence of another serotype, frustrating long-term control strategies.
Pathogenesis is somewhat speculative and may differ among different serotypes. Management and husbandry stresses may predispose piglets to outbreaks. Also, preexisting injury to respiratory epithelium, especially rhinitis, may predispose to infection. Known causes of rhinitis include ammonia fumes, infection with Bordetella bronchiseptica and Pasteurella multocida, and inclusion body rhinitis. Interaction of S. suis with viruses (pseudorabies, swine influenza, porcine reproductive and respiratory syndrome [PRRS]) may trigger some outbreaks.
After oral or respiratory entry, streptococci pass to the crypts of the palatine tonsils. From there the organisms invade via lymphatics to the mandibular lymph nodes where they can remain localized or become septicemic. Some organisms may survive within phagocytes and gain access to cerebrospinal fluid, brain, meninges, lungs and joints where they localize and cause meningoencephalitis, arthritis or pneumonia. In pneumonic lungs, S. suis can be either a primary or secondary pathogen.
Signs and lesions are similar in all S. suis infections, regardless of serotype. Signs usually suggest septicemia and/or localization of systemic infection. Pigs with septicemia die after a short, acute course and may not have been observed as sick pigs. Young piglets with central nervous system (CNS) lesions are often found in lateral recumbency and paddling. Older pigs have a wider variety of CNS type signs that include ataxia, opisthotonus, incoordination, tremors, convulsions, blindness and deafness. Pigs with polyarthritis have swollen joints and are lame. In some outbreaks, respiratory signs associated with pneumonia may occur. Both morbidity and mortality vary widely and are affected by treatment. Streptococcus suis, much like Pasteurella multocida, is ubiquitous in swine and a frequent secondary contributor to pneumonia.
There often is reddening of the skin and modest enlargement of lymph nodes. Pigs that die suddenly often have fibrinopurulent meningitis microscopically, seen grossly as visible clouding of leptomeninges of the brain. In septicemic cases there may be slight enlargement of the spleen and petechial hemorrhages on the kidneys or other organs. With polyarthritis there is enlargement of joints that contain turbid synovial fluid. Streptococcus suis pneumonia, often secondary, has a bronchogenic distribution pattern. Predominance of any one of these lesions varies in outbreaks. Polyserositis is quite common. Other lesions seen in chronic cases include endocarditis (usually on the mitral valve), myocarditis and fibrinopurulent to fibrous pericarditis.
A tentative diagnosis often can be made on the basis of history, signs, lesions and the demonstration of Gram-positive cocci in the lesions. Confirmation should be made through culture and identification of the streptococci. Both pathogenic and nonpathogenic strains can be recovered from pigs on most farms so presence of the organism, especially in an unexpected area, in addition to presentation of a typical clinical picture is important in establishing the correct diagnosis. If convenient, the capsular type should be determined since that can assist in selecting an appropriate vaccine. The predominate capsular type may change over time within a farm so periodic typing of S. suis organisms recovered from affected pigs may have value as an epidemiologic tool. Outbreaks are frequently associated with a variety of infectious and non-infectious risk factors so a complete diagnostic evaluation of agents and the environment is usually warranted.
At present there are no highly effective measures for control of S. suis. Elimination of stresses, particularly overcrowding, poor ventilation, high humidity, and inadequate sanitation should be part of the strategy. The all in/all out system with emphasis on age segregation, multiple site rearing and strict sanitation of facilities and instruments used in processing piglets may be beneficial. Control of primary diseases (PRRS, influenza, pseudorabies, mycoplasmosis, etc.) that interact with S. suis is important. It may reduce incidence and improve response to treatment.
Affected pigs should be treated individually and as early as possible with injectable antibiotics. Systemically absorbed antibiotics are sometimes pulsed for a few days in feed or water to groups experiencing high morbidity. Some strains of S. suis are resistant to many antibiotics (particularly tetracyclines) so sensitivity tests are helpful in selecting a therapeutic agent. Because the type of S. suis may differ in consecutive outbreaks, sensitivity tests often need to be repeated.
Both commercial antisera and vaccines are available and are of value. The capsular type of the product used should be the same as the type causing the disease. Not all vaccines have been effective, perhaps because the product used was not appropriate for the serotype causing the problem or because of shift to another predominant serotype within the herd. Vaccination of sows prior to farrowing may be more effective and less costly than vaccinating the pigs for control in suckling piglets but is of less value for control of postweaning streptococcosis.
Streptococcus suis can be an important zoonotic pathogen with spread generally thought to be a result of direct inoculation through skin wounds. A large outbreak of Type 2 infection in humans was reported from China in 2005 that included a case fatality rate of approximately 18%.