Scabies, who cares?

North West Seminar Series of Mathematical Biology and Data Science

Kylie Ainslie

What even is scabies?

Sarcoptes scabiei

• Scabies is a disease caused by infestation of the skin with a microscopic mite.
  • a.k.a. the Itch or seven-year itch
  • From the Latin scabere, meaning ‘to scratch’.
• Symptoms are characterized by itchiness and rash at the site of infestation.

Scabies mite. Image Credit: Aliaksei Marozau / Shutterstock.com

How does scabies spread?

• Direct, extended, skin-to-skin contact
• Sharing clothing, towels, or bedding used by an infected person
  • Less common

Do people still get scabies?

Scabies Worldwide

• Scabies affects around 400 million people per year.
• Accounts for a large proportion of skin disease in many low- and middle-income countries

Karimkhani et al. 2017. Lancet Infect. Dis. 17(12):1247 - 1254

Scabies in Europe (then)

• Scabies infections were a common affliction in Europe in the late 1800s and early 1900s
  • Highest incidence seen in 1918 and 1945.

Savin 2005. J R Soc Med; 98(3):124–129.4

Scabies in Europe (now)

A rise in scabies cases has been observed through out Europe in recent years.

Reichert et al. 2021. Emerg Infect Dis.;27(6):1693–1696.

Scabies in Europe (now)

A rise in scabies cases has been observed through out Europe in recent years.

Lugović-Mihić et al. 2020. Zdravstveno Varstvo; 59(4):264-272.

Scabies in Europe (now)

A rise in scabies cases has been observed through out Europe in recent years.

Amato et al. 2019. Eurosurveillance; 24(23):pii=190020.

Scabies in Europe (now)

A rise in scabies cases has been observed through out Europe in recent years.

van Deursen et al. 2022. PLoS One; 17:e0268865

Ok, so what’s the big deal?

• The global disease burden of scabies is low compared to other infectious diseases

• Disease severity is also low - you’re not going to die

• BUT

• Secondary infections can lead to severe outcomes

• No simple diagnostic test for scabies

  • diagnoses rely on clinical assessment (more GP visits)

• scabies can have economic implications for both the patients and the healthcare system

What can we do about it?

What we need to know

Adapted from: Lehtinen et al. 2021. J. R. Soc. Interface.1820200756

What we need to know

• the incubation period: time from infection to symptom onset

Adapted from: Lehtinen et al. 2021. J. R. Soc. Interface.1820200756

What we need to know

• incubation period: time from infection to symptom onset
• generation time: time between infection of an index case and a secondary case

Adapted from: Lehtinen et al. 2021. J. R. Soc. Interface.1820200756

What we need to know

• incubation period: time from infection to symptom onset
• generation time: time between infection of an index case and a secondary case
• serial interval: the time from onset of symptoms in an index case to the time of symptom onset in a secondary case

Adapted from: Lehtinen et al. 2021. J. R. Soc. Interface.1820200756

What we need to know

• incubation period: time from infection to symptom onset
• generation time: time between infection of an index case and a secondary case
• serial interval: the time from onset of symptoms in an index case to the time of symptom onset in a secondary case
• growth rate: how quickly the numbers of infections are changing over time

Picheira-Brown and Bentancor. 2021. Epidemics; 37:100486

What we need to know

• incubation period: time from infection to symptom onset
• generation time: time between infection of an index case and a secondary case
• serial interval: the time from onset of symptoms in an index case to the time of symptom onset in a secondary case
• growth rate: how quickly the numbers of infections are changing over time
• reproduction number: the average number of secondary cases resulting from one index case

https://andreashandel.github.io/IDEMAbook/R0.html

What we know

• incubation period: time from infection to symptom onset
• generation time: time between infection of an index case and a secondary case
• serial interval: the time from onset of symptoms in an index case to the time of symptom onset in a secondary case
• growth rate: how quickly the numbers of infections are changing over time
• reproduction number: the average number of secondary cases resulting from one index case

What we know

• Incubation period
  • 4-6 weeks after primary infection
  • 24 hours after post-primary infection
• A series of studies in the 1940s form much of the basis of our current understanding of scabies transmission.

Let’s dive in!

Our Roadmap

• What is the timescale of a typical scabies transmission event?
• How are the number of scabies cases changing over time?
• How many people will a scabies case go on to infect?
• How does scabies transmission change over time?

Our Roadmap

• What is the timescale of a typical scabies transmission event?

Serial Interval

Adapted from: Lehtinen et al. 2021. J. R. Soc. Interface.1820200756

Date of symptom onset data from four scabies outbreaks.

Serial Interval

• We calculate the index case-to-case (ICC) interval for each person,
  • index case: the person with the greatest value for number of days since symptom onset.
  • “secondary” cases: have an ICC interval calculated as the number of days between their symptom onset day and the index case.

Serial Interval

• We use a maximum likelihood framework to determine how likely each ICC interval can be attributed to each transmission route - We then estimate serial interval using ICC intervals consistent with Primary-Secondary transmission

Vink et al. 2014. Am J Epidemiol; 180(9):865–875

Serial Interval

• We use a maximum likelihood framework to determine how likely each ICC interval can be attributed to each transmission route
  • We then estimate serial interval using ICC intervals consistent with Primary-Secondary transmission

Vink et al. 2014. Am J Epidemiol; 180(9):865–875

Serial Interval

Epidemic curves and estimated serial interval distributions from four scabies outbreaks. Red line indicates estimated serial interval density assuming an underlying normal distribution.

*unit is days

Serial Interval

• We performed a meta-analysis.
• Estimated pooled mean serial interval:
  • 123.24 days (95% credible interval: 91.44, 153.41 days)

Our Roadmap

• What is the timescale of a typical scabies transmission event?
• How are the number of scabies cases changing over time?

Growth Rate

• To determine how the number of scabies cases is changing each year in NL we estimated the annual growth rate
  • Fit GLM to the annual number of scabies diagnoses per 1000 people.
• Estimated an annual growth rate: 0.25 (95% CI: 0.2, 0.3)

Our Roadmap

• What is the timescale of a typical scabies transmission event?
• How are the number of scabies cases changing over time?
• How many people will a scabies case go on to infect?

Basic Reproduction Number \(R_0\)

• We can relate the growth rate to the basic reproduction number as \[R_0 = \exp(r*T – (1/2) r^2 s^2)\]
  • \(r\) is the annual growth rate,
  • \(T\) is the mean generation time (in years), and
  • \(s^2\) is the variance of the generation time distribution.

Assumption: nearly everyone exposed has not been previously infected.

Basic Reproduction Number \(R_0\)

• We can relate the growth rate to the basic reproduction number as \[R_0 = \exp(r*T – (1/2) r^2 s^2)\]
  • \(r\) is the annual growth rate,
  • \(T\) is the mean generation time serial interval (in years), and
  • \(s^2\) is the variance of the generation time serial interval distribution.

• This gives us an \(R_0 = 1.09\) \((95\% CI: 1.07, 1.11)\)

Assumption: nearly everyone exposed has not been previously infected.

Our Roadmap

• What is the timescale of a typical scabies transmission event?
• How are the number of scabies cases changing over time?
• How many people will a scabies case go on to infect?
• How does scabies transmission change over time?

Time-varying Reproduction Number \(R_t\)

Incidence of scabies infections per 1000 people in the Netherlands by date of diagnosis.

Time-varying Reproduction Number \(R_t\)

Time-varying Reproduction Number \(R_t\)

Now what?

Policy Implications

• Scabies cases are increasing in the Netherlands.
• Minimal control measures could decrease \(R_0<1\), such as
  • educate age groups (adolescents and young adults) with highest prevalence of scabies about the causes and symptoms of scabies infections,
  • require a scabies check for university students prior to the school year, or
  • inform GPs about the higher risk of scabies in certain groups
    • could lead to quicker diagnoses

https://www.informedhealth.org/scabies-how-can-you-prevent-infection.html

This is only the beginning

• The epidemiological quantities estimated here can be used to parameterise mathematical models of scabies spread
• There is still a lot that we don’t know
  • Future work should focus on obtaining more precise estimates of the underlying quantities that govern scabies spread.

Your turn

mitey package

Wrapping up

Thank you

• Jacco Wallinga
• Mariette Hooiveld

Funding

This work was financed by the Netherlands Ministry of Health, Welfare and Sport.

Contact

Email: kylie.ainslie@rivm.ml

Website: https://kylieainslie.github.io/

GitHub: https://github.com/kylieainslie

Bluesky: https://bsky.app/profile/kylieainslie.bsky.social

LinkedIn: https://www.linkedin.com/in/kylieainslie/