Introduction

In this workbook, we will focus on streptococcal infections in general as well as 3 major pathogenic streptococci, namely S. equi subsp. equi, S. suis and S. canis. Note that the streptococci causing mastitis will be dealt with in the chapter on mastitis.

Below is the taxonomic position of the streptococci and enterococci. It is shown as related bacteria will have similar characteristics.

Identification of the streptococci

Streptococci are gram positive, non-motile, non-spore forming, facultatively anaerobic cocci that form chains. Unlike staphylococci, they form chains, are catalase negative and tend not to develop resistance against antibiotics. They are part of the lactic acid group of bacteria which includes the genus Enterococcus and Lactobacillus. With a few exceptions, pathogenic streptococci show beta-haemolysis (seen as the complete clearing of the blood agar due to the destruction of red blood cells by bacterial haemolysins which are porin toxins) on blood agar.  Some streptococci such as the oral streptococci and S. pneumoniae are alpha-haemolytic (greening on blood agar due to the reduction of iron in the haemoglobin in the red blood cells).

Diseases caused by the streptococci

Beta-haemolytic streptococci can be grouped into Lancefield types based on antibodies to carbohydrates (C-substance) in the streptococcal cell outer membrane. An antigen detection test known as the latex agglutination test is used. Clumping of the blue latex particles indicates that agglutinating antibodies have attached to the outer membrane of a homologous antigen.

A list of diseases caused by the pathogenic streptococci is below. Those in bold should be learned.

Be able to identify potential sources of pathogenic streptococci AND ENTEROCOCCI

Streptococci are normal flora or the skin and mucous membranes of the mouth, nose and throat as well as external genitalia. Some pathogenic streptococci have specific niches on the body. For example, Streptococcus equi subsp. equi, the agent of strangles, is found in the nasal cavity or guttural pouch of carrier horses, and Streptococcus uberis is a commensal within the teat canal of cows. Under ideal conditions i.e. kept moist in mucus secretions, streptococci can last 7 to 9 weeks in the environment. Enterococci are commensals within the intestinal tract.

streptococcal VIRULENCE FACTORS

Streptococci cause pyogenic (pus-causing) infections of the skin, udder, ear, upper and lower respiratory tract and urogenital tract. (Pus is an exudate consisting of necrotic cells, neutrophils and other inflammatory cells, inflammatory products and if infectious, bacteria). An accumulation of pus in an enclosed tissue space (usually a fibrous capsule) is known as an abscess, whereas a visible collection of pus within or beneath the epidermis is known as a pustule). Strains of pathogenic streptococci will produce some but not all of the toxins listed in the Table and picture below. Included is the bacterial capsule, which is not an exotoxin but is a virulence factor.

  antibiotics that are effective against Streptococci

Resistance to antibiotics is uncommon.

The most effective antibiotic, especially if administered early in the disease, is penicillin G. For oral therapy, amoxicillin is effective. This genus does not produce beta-lactamases, so there is no benefit in using amoxycillin plus clavulanic acid. For animals with long-standing infections or where there are well-formed abscesses that cannot be surgically accessed, the more lipid soluble TMS (trimethoprim sulphonamides), lincosamides and macrolides are generally effective. Note that the aminoglycosides and fluoroquinolones are less effective against this genus. The topical drug polymixin B is ineffective. It is also contraindicated to treat animals with known streptococcal infections with enrofloxacin. This is because there is a higher risk that these animals will develop toxic shock syndrome.

Antibiotic resistance is much more common in Enterococcus species. It is recommended that antibiotic susceptibility testing be done for this genus. For serious infections, a combination of a beta-lactam drug such as amoxycillin and an aminoglycoside such as gentamicin is used. These two drugs are synergistic. Avoid the use of vancomycin in veterinary practice. In humans resistant infections are usually treated with vancomycin.

STRANGLES IN HORSES

Strangles is a notifiable disease ONLY in Victoria under the Livestock Disease Control Act 1994, which provides for the monitoring and control of livestock diseases in Victoria, and must be reported within seven days of diagnosis.

Horses are infected either directly from inhalation of sneezed bacteria (droplet) or nasal/abscess contact with infected horses or indirectly by contact with contaminated stables, equipment, water, food or people = fomites. Morbidity is almost 100% in horses in a horse yard that have never been exposed to the bacterium before. Mortality is 1 to 2%. The disease is characterised by pyrexia (fever) followed by profuse nasal discharge and abscessation of the lymphoid tissue of the head and neck. The swelling of the retropharyngeal lymph nodes may, in severe cases, restrict the airway and it is this clinical feature that gave ‘strangles’ its name.

The incubation period of strangles is 3 to 14 days with full elimination of the bacteria in 2 to 6 weeks.

Pathogenesis of strangles

SEE enters the nasal cavity or mouth by direct contact or ingestion of contaminated water or feed. Within 3 hours, it attaches to tonsillar crypt and follicular cells of the soft palate through surface lipotechoic acids and SeM binding proteins. Unlike S. equi subsp.  zooepidemicus (SEZ – the most common cause of pyogenic infections in horses, including endometritis), SEE produces a capsule and a SeM protein which actively binds fibrinogen and IgG and inhibits deposition of C3b on the bacterial surface resulting in an antiphagocytic action similar to that of the M-proteins of Group A streptococci.  It then divides and spreads to the other lymph nodes of the head including the mandibular and retropharyngeal lymph nodes. The dividing bacteria stimulates an inflammatory response resulting in fever (3 to 14 days post-infection) and abscessation of the lymph nodes.

Thus the first sign of strangles is pyrexia (rectal temperature of >39.5C) – this occurs 1 to 18 days after transmission and is also when the horse becomes infectious for other horses. The horse will develop a purulent nasal discharge and the lymph nodes of the head will develop abscesses (3 to 5 days post-infection). the abscesses once ripe will rupture, either to the outside or through the nasal cavity leading to a purulent nasal discharge.  Shedding of bacteria usually lasts for 2 to 3 weeks.

In cases where the inflammation is severe, there will be marked swelling of the neck region leading to breathing difficulties. This is how strangles got its name.

Rupture of the retropharyngeal abscesses can lead to bronchopneumonia or guttural pouch empyema. The latter complication can result in persistent carrier horses. The bacteria can disseminate in the bloodstream and lodge in other tissues of the body, causing metastatic abscessation also called “bastard strangles”. This condition requires long-term antibiotic therapy and often results in death.

Three to 4 weeks after infection SEE may trigger an immune complex disease (Type III hypersensitivity) known as purpura haemorrhagica.  Horses with high antibody titers to SEE (>1:1,6000), either from natural infection or vaccine-induced are more likely to be affected.  Hypersensitised horses produce high quantities of IgA which result in generalised vasculitis leading to oedema and haemorrhages. It is fatal if left untreated. A horse suffering from ventral oedema due to generalised vasculitis is shown in the picture below. Immune-mediated myositis and myocarditis have also been reported. This could be in part due to  phage-associated bacterial superantigens such as SePE-I that are not found in S. zooepidemicus.

Laboratory Diagnosis and Differential Diagnosis of Strangles

qPCR (preferred) or bacteriological culture of nasal swabs or abscess exudates are used to diagnose clinical cases. Serology is used to determine exposure with high titers found in carrier horses (not that predictive) and those at risk of developing immune-mediated complications. 

Strangles must be distinguished from viral infections caused by equine influenza virus (highly contagious upper respiratory disease – not in Australia), equine rhinoviruses, equine herpesvirus 1 and equine herpesvirus 4; bacterial infections caused by Burkholderia mallei (Glanders – not in Australia), Streptococcus equi subsp. zooepidemicus (usually noncontagious), Streptococcus pneumoniae, Streptococcus pyogenes, Rhodococcus equi (rattles in foals), Actinobacillus equuli, Klebsiella pneumoniae and Mycoplasma felis (usually causes pleuropneumonia); and nasal mycoses usually caused by Aspergillus fumigatus (older, individual stabled animals).

Detection of Carrier animals

About 10% of infected animals, usually those with retained pus in the guttural pouch can become long-term carriers, intermittently shedding the bacterium in their nasal secretions or by coughing. It is believed that abscessed retropharyngeal lymph nodes rupture and drain pus into the guttural pouches causing infection. The diagram below illustrates the development of guttural pouch empyema. It is sourced from: Equine Veterinary Journal, Volume 49, Issue 2, pages 141-145, 8 FEB 2017 DOI: 10.1111/evj.12659, http://onlinelibrary.wiley.com/doi/10.1111/evj.12659/full#evj12659-fig-0002

About 50% of carrier animals will have a chronic cough. Pus, if not drained out from the guttural pouches or their sinuses, will dry and harden resulting in the formation of chondroids that can remain there for several years. Interestingly, SEE localised in the guttural pouch will eventually decrease in virulence.

Due to reluctance to scope and then lavage guttural pouches (takes time, endoscopes have to be disinfected after each animal) most diagnoses of guttural pouch empyema are carried on nasal fluids. The latter is less sensitive. Serology has also been used to detect carriers. Serology, however, has a poor specificity and sensitivity.

Control Measures

The following control measures are recommended in an infected horse yard:

Quarantine the yard until 21 days after the last case. In temperate environments or during rainy weather, this period may have to be increased to 2 months as the bacteria can survive for 7 to 9 weeks in the environment under these conditions.

1. Isolate known infected horses for a minimum period of 21 days (In colour code red yard). Horses in contact with infected horses can move to an orange zone and those with no known contact to a green zone.
2. Sick horses should never be transported off the property.
3. Twice daily rectal temperatures of all horses taken to detect a fever > 38.5°C. This clinical sign occurs 24-48 hours before evidence of a nasal discharge or swollen lymph nodes – so before bacteria are shed.
4. The use of antibiotics is controversial as they can interfere with immunity development in early infections and delay recovery in those animals with abscesses. Penicillin, used early in the disease i.e. when fever is the only clinical sign , will prevent the development of abscesses. Horses with severe clinical signs and internal abscesses should be treated with lipophilic antibiotics.
5. The daily application of warm compresses to abscesses will promote their ripening, once ripened they can be lanced, the exudate removed and destroyed and the abscess flushed daily with dilute povidone iodine or another disinfectant until healing starts. Non-steroidal anti-inflammatory drugs are administered to reduce pain and fever.
6. Feed and water horses off the ground to encourage pus drainage from the retropharyngeal area and guttural pouches.
7. People can transmit the agent between horses. So always treat sick horses last and wear different protective clothing for sick and healthy horses. Practice good hand hygiene and ensure that there is no sharing of equipment.
8. Once all horses have recovered, test for the presence of persistent carriers. This is done by testing for antibody titres in horses that were either “infected (red zone)” or “in-contact (orange zone)”. Any horses with antibody titres should be tested for the presence of guttural pouch infection, either via guttural pouch endoscopy or qPCR on a nasopharyngeal lavage. The chrondroids should be physically removed and the guttural pouch flushed with penicillin
9. Once all horses have recovered, clean and disinfect stables and equipment.

In unaffected horse yards:

Control is dependent on the level of risk – greater risk when there is a lot of horse movement to and from the yards and when most of the animals are young. All new animals should be kept apart from other horses for at least 3 weeks and serology carried out on days 0 and 14. If negative they can enter the yard, if positive, they should be treated. Animals should be sourced from uninfected premises.

A bacterin vaccine (Equivac S Zoetis) and one included with a tetanus toxoid (Equivac 2 in 1 Zoetis) is administered to at-risk horses from the age of 12 weeks either alone or in combination with a tetanus toxoid. It will provide some protection from disease but not infection. It is initially administered 3 times two weeks apart and thereafter annually. Infected horses should not be vaccinated i.e. don’t vaccinate in the face of an outbreak. Horses recovered from strangles develop a protective immunity for >5 years.

(Strangvac Intervacc) a multicomponent subunit vaccine has recently been developed that shows promise. It is also considered be a DIVA vaccine – it is registered for use in the European Union and UK but not currently registered in Australia. It has been shown to cover the SEE strains in Australia. Registration is in progress.

At sales, shows, racing, competitions etc

This risk is high that horses will become infected. As much as possible stable horses in a way to avoid contact with other horses. Don’t share any water bowls, food or equipment. Ensure that you wear different clothes and that footwear is cleaned. If possible, house horses that travel separately from those that don’t.

Streptococcal infections in pigs

Thirty-five capsular serotypes of Streptococcus suis have been recognised. In pigs S. suis serotype 1 is associated with neonatal septicaemia, polyarthritis and meningitis in 10-14 day-old piglets. S. suis serotype 2 infections are more common and causes pneumonia and sepsis in weaned and grower pigs (4-12 weeks). S. suis serotype 2 is also zoonotic (see the following paragraph). Disease in pigs is sporadic with sudden, explosive outbreaks that also stop suddenly. The bacterium is carried on the tonsils of pigs and the external genitalia of sows. Therefore, transmission of the agent is usually by droplets from the oral and respiratory secretions of carrier pigs or during birth. Flies and humans can mechanically transmit S. suis to neighbouring pig houses or farms. Disease in pigs is usually precipitated by stress i.e. weaning, mixing of age groups, overcrowding. Other bacterial and viral infections such as Bordetella bronchiseptica and Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) (not in Australia) can enhance susceptibility to S. suis infection.

Although several possible virulence factors have been recognised, the only difference found to date between virulent and less virulent strains, was that virulent strains were able to survive in the phagolysosome of macrophages.

Many other streptococcal species can cause pyogenic infections in pigs including the non-groupable Streptococcus porcinus, the cause of jowl abscess in pigs, and Group C streptococci.

Animals respond well to penicillin therapy.

For more information: Gottschalk  M.  (2013) Streptococcus suis infections in pigs, The Pig Site, accessed 1 January, 2023.

Streptococcus suis as a zoonosis

Streptococcus suis serotype 2  can infect humans causing meningitis and in some cases sepsis and death. In people it is predominantly an occupational disease where farmers, slaughterers and cooks that handle animals and uncooked meat are most at risk.

Describe Streptococcus canis as an opportunistic pathogen in dogs, cats and cows

A Lancefield Group G streptococcus known as Streptococcus canis is a normal commensal of the mucosa in dogs. It is also the third most common opportunistic pathogen in dogs and cats. More common is Staphylococcus pseudintermedius and Escherichia coli, both opportunistic pathogens.  Streptococcus canis produces a unique M-protein that is antiphagocytic and causes a variety of localised infections including abortions, vaginitis, cystitis and bronchopneumonia. It can disseminate from the site of infection to the rest of the body to cause septicaemia. In dogs it is identified in multibacterial infections of the external ear and skin. Rare strains of S. canis are infected with bacteriophage DNA that encodes for superantigens.  Dogs and cats infected with these strains may develop streptococcal toxic shock syndrome and necrotising fasciitis. Fluoroquinolones have been found to upregulate the expression of this gene in infected animals.

It is an unusual cause of mastitis in cows. There are rare recorded instances of sepsis developing in elderly human patients after Streptococcus canis had gained entry through the skin.

Streptococcal sepsis in fish

The most important bacterial pathogen of farmed tropical freshwater and marine fish globally is Streptococcus iniae. It affects a wide range of fish species, including barramundi. Fish usually become infected after eating (cannibalism) affected fish. It causes bacterial septicaemia and meningoencephalitis resulting in loss of orientation, lethargy, anorexia, ulcers, exophthalmia, and erratic swimming. New outbreaks in immunologically naive populations can result in up to 50% mortality. It has been known to cause sepsis, toxic shock syndrome and localised infections in fishermen or fish farmworkers when they have skin injuries.

S. iniae is a beta-haemolytic when grown anaerobically, coccus that does not group in the Lancefield grouping scheme and is CAMP positive. It is most closely related to Streptococcus porcinus, the cause of jowl abscess in pigs. A streptolysin  S-like toxin is thought to be the prime toxin responsible for cell destruction in numerous tissues. Streptococcus agalactiae (Group B) can also cause a similar septicaemic disease in fish.

High water temperatures increase the risk of disease. Stressed fish are especially prone to streptococcosis. Thus control measures include decreasing the feed to decrease bacteria ingestion, reducing the water temperature and decreasing stocking density. Decreasing stocking density also decreases cannibalism, a common way fish become infected.  Farmed fish, especially barramundi, are vaccinated by bath immersion or by intraperitoneal injection with an autogenous bacterin vaccine against S. iniae or S. agalactiae. This vaccine affords a 6 months protection against the bacterium. (There are commercial vaccines in the global market, however, they must contain the serotype causing streptococcosis on a farm to be effective). Streptococci are susceptible to the penicillin antibiotics.

Enterococcus species AS AN OPPORTUNISTIC PATHOGEN

Enterococcus species are gram-positive cocci that resemble Streptococcus species in that they form short chains, are catalase-negative and group as a Group D with the Lancefield grouping system. However, most enterococci are bile tolerant and unlike streptococci will grow as minute dark-pink (lactose-fermenting) colonies on MacConkey agar without crystal violet. They are common commensals of the intestinal tract of animals. Although they are never considered primary pathogens, they can cause a wide range, often life-threatening (sepsis), opportunistic infections in animals and humans. Common infections in animals include cystitis in dogs and environmental mastitis in cows.

Unlike the streptococci, bacteria in this family, especially E. faecalis and E. faecium, are usually resistant to commonly used antibiotics. Thus, it is recommend that antimicrobial susceptibility tests are carried out on Enterococcus isolates from clinical cases.

Vancomycin-resistant enterococci were at one time considered to be zoonotic. This was because at that time the in-food antibiotic growth promoter “avoparcin” which produced cross-resistance to vancomycin as they are both glycopeptide antibiotics. Antibiotic growth promoters are placed at sub-therapeutic concentrations in the feed to improve the growth  of livestock. Avoparcin is banned and no longer produced in all countries.  Only a few antibiotic growth promoters are allowed in Australia, one being Avilamycin in poultry.

Note that this bacterium can, in the presence of the beta-lactam drugs, stop producing a peptidoglycan outer membrane and survive for a limited time as a bacterial L-form. I have seen this happen in chronic urinary tract infections in dogs.