Causes and Risk Factors of Strep Throat

Most sore throats, aka pharyngitis, are caused by viruses. Of the remaining infections, however, strep throat is most common. It is responsible for 15 to 30 percent of cases in children and 5 to 10 percent in adults.

While streptococcus infection is the definitive cause of strep throat, there are several factors that can make someone more susceptible to the disease. Knowing about them can help you to decrease your risk for infection.

There are four different types of streptococcal bacteria—A, B, C, and G. Group A Streptococcus (GAS), also known as Streptococcus pyogenes, is the bacteria responsible for strep throat. There are different strains of the bacteria, the most common of which lead to respiratory and skin infections.

Besides strep throat, other common infections caused by S. pyogenes include:

• Cellulitis
• Erysipelas
• Impetigo
• Otitis (ear infections)
• Scarlet fever 

Untreated strep throat can lead to recurrent episodes of strep throat or to the more serious but rare complication of rheumatic fever. ​Even if treated, strep throat can sometimes lead to post-streptococcal glomerulonephritis (PSG), a disease that causes inflammation in the kidneys. Most people completely recover from PSG without long-term complications.

How Streptococcal Bacteria Spreads

S. pyogenes is most commonly spread from person to person. Bacteria in salivary droplets or in nasal discharge can spread when you cough or sneeze. You could inhale these respiratory droplets directly. These droplets can also settle on surfaces. If you touch your mouth, nose, or eyes after touching something with these droplets on it, you could become infected.

The bacteria is less commonly transmitted through food or water. Since you are unlikely to get strep from animals, there is no need to worry about your family pets.

Incubation Period, Contagious Period, and Duration of Illness

The typical incubation period for strep throat is two to five days. This means, on average, it takes three days from the time you are exposed to the bacteria to the time you develop symptoms. If you know you have had contact with someone who has been diagnosed with strep throat, be on the lookout for symptoms during this time.

Strep throat usually lasts three to seven days with or without treatment. If you are treated with antibiotics, your symptoms are likely to improve within a day or two and you are not considered infectious 24 hours after your first dose.

Left untreated, however, you could be contagious from the time you are exposed to the bacteria until your symptoms resolve. Some resources claim that infectivity could last as long as a week afterward.

Active Infection vs. Carrier State

Not all S. pyogenes bacteria lead to active infection. Some people live with the bacteria in their pharynx and nasal passages and don't develop symptoms. These bacterial strains tend to be less virulent. These people are said to be colonized with the bacteria and are carriers of the disease. As many as 20 percent of school-aged children fall into this group.

Carriers are less likely to spread disease. It remains controversial whether they should be treated with antibiotics to eradicate the bacteria given the small chance that they could infect others. This may be a reasonable option if the carrier has frequent close contact with someone who has a weak immune system (e.g., someone on chemotherapy). It may also be a consideration if there are recurrent infections to other people within the same household.

Race and gender do not predispose you to infection, but there are other factors that increase the odds of getting strep throat.

Age

Strep throat is most common in children 5 to 15 years old. Younger children can also get infected but less frequently and often with atypical symptoms.

A meta-analysis of 29 articles in Pediatrics showed that among children of all ages who presented with sore throat, 37 percent were diagnosed with S. pyogenes but that prevalence decreased to only 24 percent for children younger than 5 years old. Adults are infected at a much lower rate of 5 to 10 percent.

Close Contact

Close quarters make it more likely that infection will spread from person to person. Schools and day care centers are notorious for this. People who live with someone who has strep throat are also at higher risk for infection.

Hygiene

Hygiene is a common culprit when it comes to the spread of strep infections. Children may cough into their hands or rub their noses without using tissues. Research shows that S. pyogenes can live on the hands up to three hours.

Hand washing is key to decreasing the spread of infection. When soap and water are not available, consider alcohol-based hand sanitizers. Also avoid sharing food, drinks, or utensils and for obvious reasons, kissing is a no-no during an infection.

Pollution or Smoke Exposure

Whether you smoke yourself or are exposed to second-hand smoke, your throat and airways are likely to be irritated by the particulate matter. This leaves the throat prone to infection not only from strep but from viruses too. Air pollution can do the same.

Time of Year

Strep throat can occur year-round, but it has seasonal variations. Infections are more common in late winter and early spring. This correlates with the academic school year.