Fast-spreading killers: how Ebola compares with other diseases

Symptoms of the flu can start within a day or two of being infected and last for about a week. But virus shedding begins before the symptoms and one-third of cases show no symptoms at all.

The virus also readily mutates, so past infection does not necessarily provide future protection.

The fatality ratio of seasonal influenza is low, usually claiming one in 1,000 lives, so it is often regarded as inconsequential. But a virus shift can cause a pandemic and increased rates of death.

Influenza: a profile

  • Region: Unknown. The first well described influenza -like outbreak was recorded in 1580, but it is thought people have suffered influenza for thousands of years.
  • Origin: Animals. Humans may have acquired influenza when domestication of animals first began.
  • Infection agent: Virus (two types: A and B)
  • Transmission medium: Inhalation of infectious droplets. The airborne nature of influenza makes it relatively easy to spread among people.
  • Incubation: 2-3 days, but anywhere from 1 to 7 days.
  • Transmission rate: R0 of 1 to 3 (Ro[basic reproduction number] is an approximate measure of how many new infections one person will generate during their infectious period). Different outbreaks have different R0 values, and the “Spanish flu” of 1918 may have had a much higher basic reproduction number.
  • Fatality ratio: Usually around 0.1%, but the Spanish flu had a fatality ratio of more than 2.5%
  • Death toll: Uncertain. Each year, an estimated 3 to 5 million people suffer severe cases of influenza and about 250,000 to 500.000 of them die.
  • Medication status: Various flu vaccines exist

Source: WHO & CDC

Bird flu
Of greater concern is the avian influenza reservoir in parts of Asia. Transmission of H5N1 and H7N9 influenza A from birds to humans has resulted in a fatality ratio of 50 percent.

If these viruses were to mutate and become easily transmitted between humans, public health services would be severely challenged by the resulting pandemic.

The basic reproduction number of an infection, usually expressed as R0, is defined as the expected number of cases that would arise from a typical primary case in a susceptible population.

If R0 is bigger than one, then the number of cases will increase until limited by control measures, behavior change or the exhaustion of the supply of susceptibles. If R0 is less than one, then the incidence of infection will decrease and the outbreak will fizzle.

Influenza typically has values of R0 between one and three, so vaccinating 67% of the population would prevent epidemics.

Avian Flu or Bird Flu (H7N9, H5N1): a profile

  • Region: China
  • First diagnosed with the death of six people in Hong Kong in 1997.
  • Infection agent: Virus
  • Transmission medium: Handling of infected birds. Bird flu is not thought to be airborne, so person -to-person transmission is considered rare.
  • Incubation: Anywhere from 2-8 days, but can be as long as 17 days.
  • Transmission rate: Rc of approximately 0.7 (Ro([basic reproduction number] is an approximate measure of how many new infections one person will generate during their infectious period). Avian flu has a low 1Rc (basic reproduction number), so transmission between people is less likely. In 2012, researchers created a highly infectious hybrid virus by merging bird flu with “swine flu.”
  • Fatality ratio: Approximately 60%
  • Death toll: Approx. 393 deaths since 2003
  • Medication status: No cure. Candidate vaccines to prevent H5N1 infection have been developed, but are not ready for widespread use.

Source: WHO & CDC

SARS
Severe acute respiratory syndrome (SARS) is likely to have emerged from a bat reservoir in China, and spread to a number of other countries in 2003. Its infectious agent is a coronavirus, from a family often implicated in the common cold.

Its mode of transmission was similar to that of influenza but its basic reproduction number, at over three, was twice that of the flu.

With no vaccine available and a fatality ratio around 10 percent, SARS presented a scary prospect: it caused an estimated US$40 billion of economic damage and severely disrupted international travel.

But SARS had an Achilles heel. The long (four to five-day) incubation period made it amenable to contact tracing, enabling the isolation of contacts before they became infectious.

It was largely these non-clinical interventions that succeeded in reducing the contact rate, and therefore the reproduction number, below one. The pandemic was brought to a halt and the infection eliminated, with the loss of 774 lives.

However, another coronavirus is now responsible for 837 cases of Middle East respiratory syndrome (MERS) and 291 deaths.

SARS: a profile

  • Region: China
  • The first known outbreak was in China, 2003.
  • Origin: Bats
  • Infection agent: Virus
  • Transmission medium: airborne droplets. SARS most readily spreads through close person -to-person contact through respiratory droplets.
  • Incubation: 4-5 days. The 4-5 day incubation period allows relatively easy tracking and isolation of people who have come in contact with infected people.
  • Transmission rate: Rc Of 3 (Ro([basic reproduction number] is an approximate measure of how many new infections one person will generate during their infectious period).
  • Fatality ratio: 10%
  • Death toll: About 775 deaths
  • Medication status: No vaccine

Source: WHO & CDC

Ebola
Ebola is not an airborne infection; it is spread by bodily fluids, which means that more intimate contact is required for transmission. The basic reproduction number is usually estimated to be around two, although a recent estimate for hospital settings in Africa puts the value of R0 over four. Because SARS can be transmitted through airborne droplets, it has a relatively high R.

Research suggests around 70 percent of those infected in the latest Ebola outbreak will die of the disease. However, according to the World Health Organization, past fatality ratios have ranged from 25 percent to 90 percent.

It seems that transmission only occurs while the patient is symptomatic, so contact rates must be low even though the probability of transmission given contact is high. The eight to ten-day average incubation period also suggests that contact tracing and isolation should be successful in halting the spread of infection.

Ebola (H2N): a profile

  • Region: Africa
  • First diagnosed in what is now the Democratic Republic of the Congo in 1976.
  • Infection agent: Virus
  • Transmission medium: Bodily fluids. Ebola is not airborne. It is transmitted via direct contact with the blood or secretions of an infected person.
  • Incubation: Anywhere from 2-21 days, but average incubation period is 8-10 days. The relatively long incubation period of Ebola should make it easier to track and isolate people who have had contact with infected people.
  • Transmission rate: Ro Of 2 (Ro([basic reproduction number] is an approximate measure of how many new infections one person will generate during their infectious period), but the latest outbreak’s Rc could be as high as 4.
  • Fatality ratio: Average around 50%. Case fatality ratios have varied from 25% to 90% in past outbreaks.
  • Death toll: Over 6,400 so far
  • Medication status: No vaccine or cure. Two possible drugs are currently being trialed.

Source: WHO & CDC

HIV/AIDS
While not a fast spreader, another infectious disease that originated in Africa and is transmitted by bodily fluids is HIV/AIDS (human immunodeficiency virus infection and acquired immune deficiency syndrome).

HIV was transmitted to humans in the 1920s from a chimpanzee, but remained largely confined to (what is now) the Democratic Republic of Congo until undergoing an epidemiological transition in the 1960s. The resulting global HIV pandemic has infected nearly seventy-five million people.

While transmission of HIV and Ebola both rely on the transfer of bodily fluids, there are major differences. A HIV patient can be infectious for many years while not exhibiting any symptoms, and may be unaware of their status.

In contrast, for Ebola, the time from being infected to infecting others is measured in days and, although not as infectious as influenza, it is more readily transmitted than HIV.

HIV/AIDS: a profile

  • Region: Africa
  • Has been traced back to the 1920s in the Democratic Republic of the Congo.
  • Infection agent: Virus
  • Transmission medium: Bodily fluids. HIV/AIDS is not airborne and is transmitted via contact with bodily fluids. Sexual contact and the sharing of needles are two common modes of transmission.
  • Incubation: 3 to 8 weeks to develop HIV antibodies. An HIV patient can be infectious for many years while not exhibiting any symptoms.
  • Transmission rate: R5 , is difficult to define (Ro([basic reproduction number] is an approximate measure of how many new infections one person will generate during their infectious period). For HIV. R0 varies greatly with location and mode of transmission.
  • Fatality ratio: less than 5% with treatment, but the ratio jumps to about 80% without treatment.
  • Death toll: Over 35 million deaths
  • Medication status: No vaccine or cure. Antiretroviral drugs are used for treatment

Source: WHO & CDC

Emergence of infectious diseases
While bubonic plague is bacterial, all of the other infections mentioned above are viral. Yellow fever was the first human virus to be discovered in 1901. Since then more than 200 have been recognized.

Most “new” infections result in “species jumps”, mainly from mammals and birds. The ability of viruses to adapt should make us wary of the avian influenza viruses and their potential for human-to-human transmission.

Ebola has all the epidemiological characteristics of a containable infection, but it is only now spreading to developed countries that have no experience of dealing with it. The desirable strategy, from global health and humanitarian perspectives, is to eliminate the epidemic at its source.

Mick Roberts is Professor in Mathematical Biology at Massey University. This story is published courtesy of The Conversation (under Creative Commons-Attribution/No derivatives).