Smallpox and Measles

Did you know that smallpox is the only infectious disease to be considered officially eradicated by the World Health Organisation (WHO)? And that one person with measles can infect 12-18 people, making it the most infectious disease? Both smallpox and measles are two very contagious viral diseases that display very similar symptoms despite being caused by two different viruses. Both illnesses have caused a lot of fatalities throughout human history, but neither is now among the leading causes of death because of successful widespread vaccination campaigns.

The definition of smallpox

Smallpox is an ancient and feared viral infectious disease caused by the variola virus. Smallpox is thought to have existed for 3000 years before it was eradicated by a very successful WHO smallpox immunisation program that started in the 1960s.

The last reported smallpox natural case happened in 1977 in Somalia. After three years without any natural cases, this infectious disease was considered the first and only time in human history to have been eradicated by the WHO in 1980. This achievement remains one of the most relevant public health success stories in medical history.

The definition of measles

Measles, like smallpox, is a highly contagious human viral infectious disease. A virus of the paramyxovirus family causes measles. Despite not being a leading cause of death anymore like other more deadly infectious diseases such as HIV, malaria or tuberculosis (TB), measles still kills many people a year. According to 2018 WHO statistics, more than 140 000 people died of this illness that year. Millions were infected, particularly where measles remains a major disease.

Measles fatalities have been reduced significantly in the last decades due to the development of a safe and cost-effective vaccine and its widespread immunisation effort throughout the world. The WHO data shows a 73% drop in measles deaths due to vaccinations in this century, avoiding an estimated 20 million deaths.

The causes of smallpox and measles

In most instances, smallpox disease is caused by inhaling the variola virus. This infectious disease is usually transmitted through direct personal contact via infective respiratory droplets released in a contagious person’s breath while talking, coughing or sneezing. Smallpox can also be transmitted through contaminated bedding or clothing in less common instances. After exposure, the variola virus incubates for 7 to 17 days before a person becomes infectious.

Fig. 1: Smallpox and measles transmission.

The beginning of the infective stage of smallpox is marked by the development of a fever followed by a typical rash two to three days after. This initial first week of illness period is often the most infectious. However, infected smallpox patients primarily infect their household members and not the general public. This is because contagious people are already sick and most likely on bed rest and semi-isolated from the overall community. In most cases, people with smallpox infection recovered, but up to 30% of the time, they passed away due to systemic shock and toxinemia (toxins in the blood).

As for measles, this infectious disease is also caused by breathing in virus droplets. The viral infectious particles are released through coughing or sneezing of an infected individual and can remain in suspension in the air and on surfaces for 2 hours. Once it enters the body, the measles virus targets cells of the respiratory tract, starting its incubation period of up to 14 days with no obvious visible symptoms. Shortly before symptom onset, an infected individual starts to become contagious. On most occasions, the measles illness fades out after several days without complications. The appearance of complications is associated with severe measles, which can often lead to death, especially in certain risk groups.

The symptoms of smallpox and measles

Smallpox progresses slowly. The initial symptoms after the incubation period include:

• a high fever,

• backaches,

• headaches,

• fatigue,

• even vomiting, and abdominal pain.

After a few days, a characteristic rash appeared first on the face and then spread to other body parts. This rash included red spots filled initially with clear liquid and then thick pus. These pustules turn into a crust that dries and falls off, severely disfiguring and potentially blinding infected people. Ulcerating lesions also appear in the mucous membrane of the mouth and nose.

Measles tends to happen quickly and affects humans for a shorter period. After exposure to the virus, it begins its incubation in human cells for up to 2 weeks and only then do symptoms appear. Initial symptoms are very similar to smallpox, like high fever and a rash that spreads throughout the body. Other symptoms include:

• coughing,

• runny nose,

• conjunctivitis

• and the appearance of small white spots on the inside of the cheeks.

In cases of severe measles illness, additional serious symptoms may develop. These complications may include:

• severe diarrhoea and dehydration,

• pneumonia,

• blindness,

• ear and sinus infections,

• convulsions,

• and encephalitis (brain-swelling infection).

These clinical features can eventually lead to death, especially in young children.

The treatment of smallpox and measles

Smallpox, unfortunately, does not have a cure. The only known successful control measure against the variola-induced disease is vaccination.

Vaccination was crucial in eradicating the disease, but it is only effective if given up to four days after variola exposure. After identifying possible smallpox infections, the infected person’s household, along with the 30 surrounding households, would be vaccinated in a ring vaccination strategy. The ring vaccination and surveillance programs were the two successful strategies the WHO smallpox eradication program employed to stop outbreaks and end this infectious disease.

Fig. 2: Ring vaccination strategy.

There are now new antiviral drugs developed for other diseases that might be successful in treating smallpox, making it less severe, but have never been tried because the disease has been eradicated. The same is not true for measles, which currently does not have any specific antiviral or other drug therapeutic treatments available.

Measles treatment typically only involves bed rest and medication to lower fever. The disease usually then clears away after a few weeks of symptom onset. In the rare case of severe measles, additional medication and treatments can be used to mitigate some of the most severe clinical features. These strategies usually include:

• fluid intake for dehydration treatment,

• antibiotics to treat pneumonia and ear infections,

• vitamin A supplements, and other nutritional supportive care for young children with measles infections.

Much like smallpox, the only effective preventive measure to stop and eradicate measles is vaccination. Measles is a completely preventable disease. A two-dose inexpensive measles vaccine has been around for nearly 60 years and is the main tool required to eradicate this disease. Immunisation programs have steadily increased the percentage of people vaccinated for the past few decades.

Differences between smallpox, chickenpox and measles

Smallpox, chickenpox and measles are all three viral infectious diseases. As discussed before, smallpox is caused by the variola virus and measles by a virus of the paramyxovirus family.

Chickenpox, like measles, is still present and has not been eradicated. Despite being caused by different viruses, they cause similar rash and fever-like symptoms, often making them hard to distinguish from each other, leading to misdiagnosis.

Chickenpox and measles are both very contagious childhood infections and can be prevented through vaccination. Chickenpox is, however, far more common, infecting millions every year in developed countries. Both diseases result in the development of a rash, but they are different in appearance and growth. Chickenpox rash initially begins in the chest, face and back spreading from there, while measles’ rash typically starts in the forehead spreading downward. The rash itself is also different. Chickenpox rash is composed of papules (elevated red bumps), while measles has more flat-like spots.

Smallpox no longer occurs naturally, so many of its symptoms are now almost certainly always of another illness. Smallpox diagnosis is generally straightforward, and its symptoms are easy to recognise. Severe chickenpox however was sometimes misdiagnosed as smallpox before its eradication, but there are also slight differences between the rashes.