Scabies, who cares?

Doherty CSI: Clinical and Public Health Seminar

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.

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

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

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

Netherlands: 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

But…

During periods of high scabies prevalence:

  • Higher burden due to secondary infections
  • Increased pressure on healthcare systems
  • Negative economic impacts
  • Stigma-related delays in care-seeking

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

Adam Cole/NPR

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

What we know

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

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

Index-case-to-case (ICC) intervals (A-D) and comparison of serial interval estimates across studies (E). Red curves show fitted distributions.

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 the Netherlands, we estimated the annual growth rate

  • Estimated 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.

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 \(R_0 = 1.09\) \((95\% CI: 1.07, 1.11)\)

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.

Now what?

Policy Implications

Controlling 8% of secondary infections prevents epidemic growth

  • Educational campaign targeting high-risk groups
  • Enhanced GP training for rapid diagnosis
  • Voluntary screening
  • Systematic contact tracing
  • Targeted (voluntary) mass drug administration campaigns

This is only the beginning

There is still a lot that we don’t know

  • Future research should prioritize three key areas:
  • collecting transmission pair data
  • characterizing the delays between symptom onset and healthcare seeking
  • reproducing Mellanby’s 1944 incubation period estimates

Software

We developed an R package called mitey.

Wrapping up

Manuscript

  • Nature Communications 2025 (in press)
  • Preprint: Available at SSRN

Thank you

  • Jacco Wallinga
  • Mariette Hooiveld

Funding

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

Get in touch!

kylie.ainslie@unimelb.edu.au

linkedin.com/in/kylieainslie

@kylieainslie.bsky.social

github.com/kylieainslie

kylieainslie.github.io