Parasitology Notes

Parasites

• Parasites benefit at the host's expense; restrictive definitions require living in/on the host and not immediately killing it.

Tick Feeding Strategies

• Hard Ticks (Ixodidae):

  • Attach for days/months, may change hosts when molting. Subtypes: one-host, two-host, three-host (most).

• Soft Ticks (Argasidae):

  • Attach for minutes/hour, may feed on multiple hosts between molts.

Parasite Types

• Microparasites (pathogens):

  • Smaller than host ($< 10 \mu m$), short generation times, large numbers in infected hosts.

  • Examples: SARS-Cov-2, Coccidioides immitis, Mycobacterium tuberculosis

• Macroparasites:

  • Larger than host ($> 100 \mu m$), generation time comparable to host, complete life cycle outside host, modest numbers in infected hosts.

  • Examples: Schistosoma mansoni, Eutrombicula alfreduggesi

• Demodex folliculorum + D. brevis:

  • Obligate human commensals in hair follicles/sebaceous glands; implicated in skin disorders.

• Dermatobia hominis (human bot fly):

  • Attaches eggs to other arthropods; larva burrows into host's skin, causing myiasis.

• Parasitoids:

  • Juvenile stage develops on/in host, fatal to the host

• Parasitic castrators:

  • Sterilize hosts, diverting resources to the parasite

  • A parasitic barnacle: Sacculina carcini

• Parasitic plants:

  • Hemiparasites: derive some resources from host, capable of photosynthesis.

  • Holoparasites: derive all fixed carbon from host plant.

• Social parasites:

  • Exploit the social structure of the host

  • Brood parasites and Inquilines

Parasite Species

• Estimated 30-60% of eukaryotic species are parasites.

• Estimated $10^7 - 10^9$ virus species, mostly bacteriophages.

• Approximately 70% of all described Hymenoptera are parasitoids.

Viruses and Carbon Cycle

• Viruses eliminate 20-40% of bacterial cells daily, influencing the global carbon cycle.

Human Parasites

• Over 2382 parasite species infect humans (viruses, bacteria, fungi, protists, helminths).

• Infectious diseases caused approximately 13.7 million deaths in 2019.

Burden of infectious diseases

• The burden of infectious disease is disproportionately borne by impoverished communities in tropical areas.

• GNI per capita vs DALYS lost due to communicable and non-communicable diseases

Virulence Evolution

• Virulence measures harm caused to host; usually measured via fitness proxies (mortality, fertility, body condition).

• Virulence is influenced by:

  • parasite genotype

  • host genotype

  • host condition

  • host immunity

  • co-infections

• Virulence can result from processes that are either beneficial or harmful to the parasite.

Myxoma Virus

• Myxoma virus in European rabbits: virus evolved to be less virulent, while rabbits evolved resistance.

Avirulence Theory

• Rejected theory that parasites evolve towards low virulence due to competition.

Parasite Fitness

• Parasites must survive/reproduce within host and transmit to new host.

Basic Reproductive Number ($R_0$)

• Mean number of secondary infections from one infected individual in a susceptible population.

• Larger $R_0$ values mean faster spread.

• Can be used as a proxy for parasite fitness.

SIR Model

• Describes microparasite dynamics in a homogeneously mixing population.

• Assumptions include:

  • hosts are susceptible, infected, or recovered

  • individuals are born susceptible at rate $b$, uninfected individuals die at rate $\mu$

  • susceptible individuals are infected at rate $\beta$ through contacts with infected individuals

  • infected individuals die at an elevated rate of $\mu + \alpha$ ($\alpha$ is virulence)

  • infected individuals recover at rate $\rho$

SIR Model Equations

• $\frac{dS}{dt} = b - \mu S - \beta SI$

• $\frac{dI}{dt} = \beta SI - (\mu + \alpha)I - \rho I$

• $\frac{dR}{dt} = \rho I - \mu R$

$R_0$ for the SIR Model

• $R_0 = \frac{\beta N}{\mu + \alpha + \rho}$

• Increasing Function:

  • transmission rate $\beta$

  • equilibrium population density $N$

• Decreasing Function

  • baseline death rate $\mu$

  • excess death rate due to infection $\alpha$

  • recovery rate $\rho$

• Parasite invasion occurs if $R_0 > 1$.

Virulence Tradeoff Theory

• Parasite virulence covaries with other parameters (transmission, recovery rate).

• $R_0 = \frac{\beta N}{\mu + \alpha + \rho}$

Virulence-Transmission Tradeoffs

• Transmission rate is positively correlated with parasite virulence: $\beta = \varphi(\alpha)$

• $R_0 = \frac{\varphi(\alpha)N}{\mu + \alpha + \rho}$

• Optimal level: transmission rate is a decelerating function of the virulence.

Host Background Mortality and Virulence

• Higher virulence is favored in hosts with higher background mortality.

Vaccination and Virulence

• Leaky vaccines (reduce growth/mortality without blocking infection/transmission) select for increased virulence.

• Vaccines reducing infection/transmission select for reduced virulence.

Multihost Parasites

• Exhibit source-sink dynamics; reservoir hosts maintain parasite, spillover hosts experience high virulence.