Introduction

Pertussis, more commonly known as the whooping cough, is a bacterial infection caused by the bacterium Bordetella pertussis. It is characterised by its characteristic ‘whooping’ sound when coughing upon inhalation. Epidemiological studies conducted in multiple countries around the world have indicated an increase in pertussis incidence during the summer months

What is Pertussis?

Transmission

Pertussis is a bacterium transmitted via respiratory droplets. When someone who is infected coughs, sneezes, or speaks, they expel contaminated droplets into the air, which can be inhaled by others, leading to infection. These droplets are also able to settle on surfaces and pose a risk of infection when someone comes in contact with the contaminated surface and subsequently touch their mouth or nose, although this method of transmission is less common. 

How it causes disease 

Bordetella pertussis causes infection by attaching to cells present in the respiratory tract. This leads to inflammation, contributing to disease. The bacteria also produce pertussis toxins, which further intensify the infection.

Stages and symptoms 

There are three stages of pertussis infection. The first is the catarrhal stage, which is the most infectious phase and lasts for one to two weeks. It develops one to three weeks after exposure to the pathogen and causes symptoms commonly associated with respiratory infection. These include a fever, runny nose, fatigue, mild cough, and sometimes conjunctivitis. The next stage is the paroxysmal phase. During this phase, the fever will likely subside but the coughing will get worse and start presenting with the distinctive ‘whooping’ sound. Coughing bouts are also common and tend to be quite violent. This exhausting coughing may lead to other symptoms such as vomiting and fatigue. The last stage is the convalescent stage, which can last as long as several months. It most commonly involves a residual cough, which may often be triggered by other respiratory infections or irritants. 

Diagnosis

Whooping cough is usually diagnosed through a clinical assessment of signs and symptoms by a doctor. In some cases, the doctor may want to take a swab of the nose to send to a laboratory for testing. At the laboratory, the sample will be used for culturing on a plate or to run a polymerase chain reaction (PCR). PCR is more efficient and sensitive as it provides more rapid results and has higher sensitivity compared to the culture method, which requires three to seven days for growth and has a sensitivity of only 20% to 40%. 

Treatment

As pertussis is a bacterial infection, it is most commonly treated with a course of antibiotics prescribed by a doctor. It is also important to, in general, stay rested and hydrated to aid your body in its recovery. 

Prevention

The best way to prevent infection with whooping cough is by receiving the diphtheria, tetanus, and pertussis (DTaP) vaccine. This vaccine is usually administered in five doses when a child is two months, four months, six months, 15-18 months, and four to six years old. As immunity decreases with time, adults are encouraged to receive a booster called the Tdap vaccine to enhance their protection.^1,2

Seasonal Variation of Pertussis Incidence

Seasonal variation refers to a cyclic change in disease prevalence during certain time periods of the year. For example, for Bordetella pertussis, it is most prevalent during the summer months of the year. This was observed all over the globe as seen in studies done in The Netherlands, the United States, China, and Australia. For all the countries in the northern hemisphere, pertussis incidence was highest in the summer months of June to September, and lowest in the months of October and February. Conversely, the levels of whooping cough cases were highest in the summer months of December to February in the southern hemisphere.^3,4,5,6

Factors Contributing to Seasonal Variations

Climatic influences

The climate, including both temperature and humidity are the main contributors to a rise in pertussis cases during summer months, as the causative pathogen of the disease is believed to have enhanced survival at hotter temperatures. Furthermore, hotter temperatures have also been shown to increase the activity of the pertussis toxin, which is a major factor in disease progression. This may result in a more severe infection, exacerbating symptom presentation, leading to more people seeking medical attention, undergoing diagnostic testing, and getting diagnosed with whooping cough. 

Higher humidity has also been shown to accelerate Bordetella pertussis growth and spread. The bacterium was shown to grow faster on agar, a medium which provided more humid conditions. The humidity also allows for increased deposition of the bacterium on multiple surfaces, increasing transmission by the increase in likelihood of someone touching an infected surface. 

Human behaviour and activities 

During the summer months, transmission may be increased due to an increase in social gatherings and travel. Warmer temperatures encourage people to meet in bigger groups or to go to events, like festivals, which are prevalent during the summer period. This gathering of people allows for the bacteria to spread more easily, leading to an increase in cases. Travelling is another big factor as people move between countries, possibly contracting an infection in the place they are visiting and taking it back home, or vice versa. Travelling usually involves confined indoor spaces like planes, coaches, and trains, allowing for increased bacterial transmission. 

Biological factors 

Biological factors, which may lead to an increase in whooping cough cases, involve respiratory infection susceptibility. This susceptibility is often mediated by the temperature as hot weather tends to be correlated with an increase in respiratory diseases. Moreover, in the summer temperatures may vary drastically, as often seen in the United Kingdom. These variations may increase the workload on the lungs, resulting in people being more susceptible to respiratory infections like pertussis. 

The immune system may also be influenced during the summer, with increased levels of inflammatory mediators released by mast cells. Another contributing factor may be seasonal coincidence with other respiratory infections, exacerbated asthma, or allergies. As initial symptoms may coincide, this could lead to misdiagnosis leading to increased reported cases, and a higher assumed incidence.^4,7 

Implications for Public Health

Information regarding the seasonal variation may be of great use for multiple public health issues. One would be the timing of vaccination campaigns, as countries would be able to work proactively and administer vaccinations before the supposed peaks in cases. This was seen in the Netherlands where a higher incidence was seen in the summer months due to children succumbing to the infection near the end of their exhaustive year at school and transmitting the illness to their family members. Therefore, a pertussis acellular pre-school booster shot was introduced to decrease infection levels and introduce herd immunity.³ It may also guide disease surveillance, encouraging doctors to test for pertussis more during the summer months where symptoms indicate a possible pertussis infection.⁴ Finally, governing entities may be encouraged to draw awareness to the disease with knowledge of peak seasons of pertussis infection. This may be via public campaigns or posters, which would provide guidance to people as to how best to protect themselves from a disease that is likely to be circulating. 

Summary

Pertussis is a respiratory infection caused by the bacterium Bordetella pertussis. The infection has three stages, which in total could last months, presenting through various signs. Its most distinctive symptom is the ‘whooping’ sound occuring during inhalation at the end of a coughing fit. It is an infection most prevalent during the summer months, due to the hotter and more humid weather enhancing bacterial survival, increasing chance of transmission. Additionally, seasonal human behaviours during the summer, including large gatherings and increased travel, further contribute to the spread of pertussis. Moreover, higher reported cases during the summer could be a result of misdiagnosing the infection due to its similarity to other infections, allergies, or asthma exacerbations that are common in the summer. Being aware of the seasonal variation of pertussis may be positive in aiding public health by informing vaccination schedules, encouraging public health education, as well as optimising disease surveillance.