Streptococcus suis infection: causes, symptoms, diagnosis, and treatment

Streptococcus suis infection is rare in clinical practices and manifested by sepsis, meningitis, and large ecchymoses on the skin, which is similar to meningococcal infection.


Causes

The pathogen is streptococcus suis, which can infect swine and human. Most patients are infected by group R type II streptococcus suis, and few are infected by type I streptococcus suis. Streptococcus suis type II infection in humans is usually manifested by purulent meningitis and sepsis, as well as toxic shock syndrome in severe cases, leading to multiple organ dysfunction and death.

Different serogroups of type II streptococcus suis have different pathogenicity to different animal hosts, and virulence may be associated with capsular polysaccharide, muramidase-released protein (MRP), and extracellular factor (EF). Asymptomatic swine are often present after infection. Many swine are carriers of type II streptococcus suis.

Human infection with streptococcus suis is mostly related to direct contact with sick swine and sick pork. Therefore, streptococcus suis infection occurs mostly in slaughters, farm workers, raw pork processing and sales personnel, as well as individuals poaching wild boars. Pathogenic bacteria invade the human body through skin breaches, resulting to onset within few hours to several days, but human-to-human transmission has not been found.


Signs and Symptoms

After human infection with streptococcus suis, there are different clinical manifestations according to different portals of bacterial invasion. In most cases, systemic symptoms, such as high fever, general malaise, and dizziness, are present, and the incubation period is 4 hours to 3 days.

Septicemic streptococcus suis infection

Streptococcal toxic shock syndrome (STSS) often occurs. Septicemic streptococcus suis infection is characterized by acute onset, mostly sudden hyperthermia, petechiae and ecchymoses on the distal extremities, accompanied by gastrointestinal symptoms and shock in the early stage, quickly evolving to multiple organ dysfunction, such as acute respiratory distress syndrome (ARDS), heart failure, disseminated intravascular coagulation (DIC), and acute renal failure, with poor prognosis, with high mortality rate.

Meningitic streptococcus suis infection

The main clinical manifestations are headache, high fever, and positive meningeal irritation signs. Mild clinical manifestations, good prognosis, and low mortality rate are present, but sensorineural deafness and motor function disorder can occur when cranial nerves are invaded. Aspiration pneumonia and secondary cerebral hypoxia can be complicated.

In addition, bacteremia or sepsis caused by streptococcus suis infection can also cause invasions of the joints, eyes, and heart of humans, causing pyogenic arthritis, endophthalmitis, and endocarditis.

Peripheral leukocytes are elevated, and neutrophil ratio is increased. Streptococcus suis can be found in bacterial cultures in blood, cerebrospinal fluid, and skin lesions. Serological examination can determine the serotype of streptococcus suis, but only can use in retrospective diagnosis. DNA of streptococcus suis can be detected in PCR test. Suppurative meningitis can be found in cerebrospinal fluid examination.

The mortality rate of streptococcus suis type II infection is as high as 12% - 26%, the mortality rate of septicemic type is the highest, up to 75% - 80%, and most patients die within 1 - 3 days after onset. Immediate and effective antibiotics can help reduce the mortality rate.


Histopathology

Thromboses composed of fibrins and platelets in the blood vessels in the dermis can be seen. Vascular endothelial cell swelling, damaged blood vessel walls, and even necrosis are visible. Different sized hemorrhagic areas caused by numerous extravasated erythrocytes are present. In addition, severe vasculitis and many neutrophils and nuclear dusts can be seen in the dermis. Numerous positive cocci inside and outside endothelial cells and neutrophils can be seen in Gram stain.


Diagnosis

On the basis of a history of exposure to infected swine or pork, clinical manifestations such as sepsis or meningitis, and ecchymoses on the skin, and streptococcus suis found in bacterial culture, a definitive diagnosis can be provided.


Treatment

Early and sufficient use of effective broad-spectrum antibiotics is the key to reducing the mortality rate. Treatment regimen includes penicillin 3,200,000 - 4,800,000U  intravenously once every 8 hours for 10 to 14 days, ceftriaxone sodium 2.0g diluted with 100ml of 5% glucose solution intravenously once every 12 hours, or cefotaxime 2.0g diluted with 100ml of 5% glucose solution intravenously once every 8 hours. Medications can be adjusted according to the results of drug sensitivity tests.