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Streptococcus suis (S. suis) is one of the main bacterial pathogens involved in intensive pig production worldwide. In addition to the economic losses resulting from mortality, growth retardation, and treatment costs, this microorganism is also of zoonotic concern, as some serotypes can affect humans. In recent years, clinical conditions associated with S. suis have become more prevalent in pig production, partly due to reduced prophylactic use of antimicrobials.
Although multiple serotypes have been described, serotype 2 continues to be considered one of the most virulent and is frequently associated with septicemia, meningitis, and systemic diseases in piglets. The disease typically manifests during the nursery period, between 5 and 10 weeks of age, coinciding with periods of stress, animal mixing, and environmental changes.
The appearance of clinical signs caused by S. suis depends not only on the presence of the agent, but also on numerous predisposing factors, including viral coinfections, high animal density, environmental problems, and immunosuppression.
This clinical case describes an outbreak of meningitis caused by S. suis serotype 2 on a multi-site pig farm in Spain, along with the diagnostic approach taken and the measures applied for its control.
Farm description
The case was reported on a 2,500-sow farm in Spain, organized as a multi-site system with weekly batch management. Piglets were sent to environmentally controlled, heated nursery facilities. They were weaned at approximately 26 days of age and sorted by size before entering the nursery.
From a health standpoint, the farm was consistently positive for porcine reproductive and respiratory syndrome virus (PRRSV) and negative for Mycoplasma hyopneumoniae and Aujeszky's disease. Both sows and piglets followed routine vaccination programs adapted to the farm's health status.
Appearance of clinical signs
During the winter months, a progressive increase in mortality was observed in the nursery period, doubling from the usual values, rising from approximately 5% to 10%. The problem mainly affected 5- to 6-week-old piglets.
The affected piglets exhibited neurological signs consistent with bacterial meningitis. Clinically, ataxia, incoordination, inability to stand, pedaling movements, and opisthotonus were observed. Some animals died suddenly without any prior apparent symptoms. Rectal temperatures exceeding 41.5°C were recorded in clinically affected animals.
Photo 1. Neurological signs: head tilt.
Photo 2. Piglet with clinical signs compatible with meningitis.
Initial differential diagnosis
Initially, the differential diagnosis focused on meningitis caused by S. suis or Glaesserella parasuis (G. parasuis), since both agents can produce similar clinical signs in weaned piglets. Early individual treatments with amoxicillin showed a favorable clinical response, reinforcing the suspicion of a beta-lactam-sensitive bacterial infection.
Photo 3. Macroscopic image of meningitis.
Diagnostic research
With the aim of confirming the etiological agent involved and evaluating possible predisposing factors, a complete diagnostic study was carried out that included bacteriological analysis, molecular techniques, histopathological evaluation, and review of management conditions.
Bacteriological and molecular diagnosis
Cerebrospinal fluid samples were taken from five piglets with neurological symptoms for analysis by quantitative real-time PCR. The samples were positive for S. suis with a Ct of 23, and they were negative for G. parasuis.
Subsequently, serotyping was performed using PCR, which gave a result compatible with S. suis serotype 1/2-2. To definitively confirm the serotype involved, bacterial isolation was carried out from the cerebrospinal fluid, followed by typing by agglutination, confirming S. suis serotype 2.
The detection of the microorganism in cerebrospinal fluid, along with the observed clinical signs, clearly linked the agent to the neurological symptoms detected on the farm. Gloves were used during sample collection as a protective measure against the zoonotic potential of S. suis.
Evaluation of coinfections
Since different viral infections can favor the appearance of clinical disease caused by S. suis, the possible involvement of PRRSV, porcine circovirus type 2 (PCV-2), and swine influenza was investigated.
Health monitoring performed on animals aged 3, 6, and 9 weeks ruled out the involvement of PRRSV and PCV-2 in the observed clinical picture.
To assess the potential role of swine influenza in the outbreak, nasal swabs were taken from febrile piglets in both the farrowing and nursery areas. Twelve pooled swabs were analyzed from 4-week-old piglets, and three of four tested positive by RT-PCR. Subtyping confirmed the circulation of the H1avN2 subtype. Similarly, the same subtype was also detected in three of the five pools analyzed from 6-week-old piglets.
The temporal coincidence between the circulation of influenza and the appearance of the neurological signs suggested a possible predisposing role of viral infection in the development of systemic disease caused by S. suis.
Several experimental studies suggest that influenza infection may favor the appearance of clinical conditions associated with S. suis and increase their severity.
Management and environment assessment
The review of management conditions during the nursery period identified several factors that likely contributed to the outbreak. Key findings included:
- High animal density
- Temperature variations during the first few days after weaning
- Mixing of piglets from different rooms
The combination of all these stressful factors probably contributed to favoring bacterial transmission and the appearance of the observed clinical picture.
Histopathological findings
To complete the diagnostic study, two piglets with neurological symptoms were submitted for histopathological analysis. Microscopically, suppurative meningitis was observed, characterized by a neutrophilic inflammatory infiltrate, the presence of fibrin, edema, and erythrocyte extravasation located mainly in the subarachnoid space.
Both the clinical signs observed and the detection of S. suis in cerebrospinal fluid, bacterial isolation, serotyping, and histopathological lesions confirmed the diagnosis of meningitis caused by Streptococcus suis serotype 2.
Measures implemented
Once the diagnosis was confirmed, a comprehensive control plan was established, focused on both treating piglets with clinical symptoms and preventing new cases.
Initially, the affected animals continued to be treated with amoxicillin, as the clinical response observed during the first few days was favorable. Although an antibiogram was not performed at the beginning of the outbreak, the susceptibility of S. suis to beta-lactams described in the literature supported this therapeutic choice. Even so, prolonged metaphylactic treatments were avoided, prioritizing prudent use of antimicrobials.
Since no suitable commercial vaccines against the implicated serotype were available at the time, an autogenous vaccine was developed using strains isolated from cerebrospinal fluid. The bacterin was administered to sows before farrowing to enhance the transfer of maternal immunity to piglets. The use of strains from a systemic outbreak was considered particularly important to maximize the vaccine's immunogenic efficacy.
Due to confirmed circulation of the H1avN2 influenza subtype, influenza vaccination was implemented in sows to reduce infection pressure. Several management measures were also recommended, including reduced stocking density, improved heat control, and optimized environmental conditions at the start of the nursery period.
Evolution and discussion
Following the combined implementation of therapeutic, vaccination, and management measures, mortality in the nursery period gradually decreased until it returned to the farm's usual levels. The reduction in clinical cases was particularly evident after the introduction of the autogenous vaccine and influenza vaccination in sows.
This case reflects the multifactorial nature of the disease associated with S. suis. Although the microorganism can be part of the normal microbiota of pigs, factors such as coinfections, stress, or certain management problems can favor the appearance of systemic clinical conditions.
The simultaneous detection of swine influenza in the same age groups affected by the neurological symptoms supports the possibility of interaction between the two agents.
Furthermore, this case illustrates the usefulness of autogenous vaccines as a practical tool in situations where commercial vaccines against circulating serotypes are unavailable.
Therefore, this case reinforces the importance of combining accurate diagnosis, improved management, vaccination, and the judicious use of antimicrobials.
Conclusions
Meningitis caused by Streptococcus suis serotype 2 was the primary cause of the increased mortality observed during the nursery period on this farm.
The combination of clinical findings, positive PCR in cerebrospinal fluid, bacterial isolation, serotyping, and histopathological lesions consistent with bacterial infection allowed for a definitive diagnosis.
The concurrent circulation of swine influenza, along with predisposing management factors, probably played a significant role in the emergence of the clinical outbreak.
The combination of prudent antibiotic use, the use of an autogenous vaccine against the specific serotype of S. suis, influenza vaccination, and the correction of various management factors allowed the outbreak to be controlled and mortality to be reduced during the nursery period.
This case highlights the need for a multifactorial approach to the control of complex bacterial diseases in swine, combining diagnosis, vaccination, improved management and prudent use of antimicrobials, all under the One Health strategy approach.
