Parasites Ento 5041 DA Andow Types of parasites

Parasites Ento 5041 DA Andow

Types of parasites in relation to arthropods • Microparasites • Parasites of arthropods (insect pathogens) • Parasites transmitted by arthropods (insect vectors) (animal disease, plant disease) • Macroparasites • Arthropods as parasites – traditional view (hematophagous arthropods) • Arthropods as parasites – expanded view (some herbivores)

Fuxa and Tanada 1987, p. 163

Nuclear Polyhedrosis Virus Bacillus thuringiensis https: //wimastergardener. org/article/bacillus-thuringiensis-bt/

Beauvaria bassiana

Protozoa, Microsporidia, Nosematidae, Nosema bombycis Control

Brown planthopper, Nilaparvata lugens Brachypter Macropter Agamermis unka, Nematoda, Mermithidae

Disease epidemiology • Central role of horizontal transmission • necessary for persistence –need to colonize new hosts to persist • because host is the environment, transmission occurs by “contact” between infective hosts and susceptible hosts • Vertical transmission • disease cannot persist indefinitely – lineages ultimately die out • therefore some kind of horizontal transmission must occur (or disease is transient)

Disease epidemiology • Density dependent horizontal transmission (SI/SIR model of disease dynamics) • S, Susceptible (never been infected, not immune) • I, Infected and infective • R, Resistant (have developed immunity) • Transmission is proportional to βSI • Depends on the number of infected individuals (“contact process”) • Frequency-dependent horizontal transmission • Sexually transmitted diseases- depends on proportion of the opposite sex that is infected

Disease epidemiology • Density dependent horizontal transmission • S, Susceptible (never been infected, not immune) • I, Infected and infective • Threshold host density (SI model) • Transmission is proportional to βSI • R 0 = net reproductive rate of the disease (per capita) • a = disease-induced host mortality • b = natural host mortality • γ = host recovery from disease • R 0 ≥ 1 for disease to persist; set R 0 = 1 and rearrange to find S • NT = threshold density (minimum density of susceptible hosts for the disease to persist

Disease epidemiology • Frequency-dependent horizontal transmission • No threshold host density • Transmission is proportional to βS I/(S+I) • R 0 = net reproductive rate of the disease (per capita) • a = disease-induced host mortality • b = natural host mortality • γ = host recovery from disease • R 0 ≥ 1 for disease to persist; hard to rearrange to find S. Trick: suppose there is no disease in the population and disease is introduced. Then (S+I) = S, and R 0 is independent of host density! • No threshold!

Evolution of virulence (disease-induced mortality rate) • Myxomatosis/mosquito/rabbit (Fenner 1983, Levin & Pimentel 1981) • Evolution of reduced virulence • Current view: Balance between virulence and transmission (NOT a general evolutionary tendency to become benign)

Some Definitions Disease – signs or symptoms of infection Pathogen (parasite) – disease-causing agents Prevalence - number or proportion of hosts in a defined population that are afflicted with a disease at a given point in time. It depends on both the proportion of hosts that become afflicted with the disease and the duration of the disease. Incidence - number of new cases of disease in a defined population during a given interval of time. Mortality rates: Gross death rate - number of deaths from all causes during a specified unit of time divided by the total number of individuals in the population. Cause-specific mortality rate - number of deaths from a specified cause (such as disease, etc. ) during a specified unit of time divided by the total number of individuals in the population. Age-specific death rate - number of deaths among individuals of a specific age divided by the total number of individuals of that age (see life tables, qx). Case-fatality rate - number of individuals dying from a particular disease divided by the total number of individuals that have the disease.

Some More Definitions Transmission - the process(es) by which a pathogen or parasite is passed from a source of infection to a new host (includes movement and infection processes). Direct- pathogen transfers without the intervention of any other living agent Indirect- one or more intermediate hosts or vectors is involved Vertical- pathogen is transmitted from parent to offspring Horizontal- pathogen is transmitted from one individual to another, but not from parent to offspring Infection rate (R) - rate of disease increase per unit of infectious host per unit of time. Apparent infection rate (r) - rate of disease increase per unit of infected host per unit of time (n. B. , not all infected hosts are infectious). R is much more difficult to estimate than r. Virulence - aka disease-induced mortality (plant pathologists call this “infectivity”) Pathogenicity - the ability of a group or species or strain or isolate of pathogen to invade and injure host tissue (disease-producing power). This is often measured as a response in a population of hosts to a given inoculum of pathogen or parasite (e. g. , LD 50, LC 50 or LT 50).

Dietz, K 1987 The population dynamics of onchoceriasis. In RM Anderson, ed. , The population dynamics of diseases. Chapman & Hall, London, pp. 209 -241. Dobson, AP & PJ Hudson 1986 Parasites, disease and the structure of ecological communities. TREE 1: 11 -15. Fuxa, JR and Y Tananda (eds) 1987 Epizootiology of insect diseases. Wiley, NY References Cited on Disease Transmission Anderson, R. M. & R. M. May. 1981. The population dynamics of microparasites and their invertebrate hosts. Proc. Royal Soc. London 291 B: 451 -524. Antonovics, J, Y Iwasa and MP Hassell. 1995. A generalized model of parasitoid, venereal and vector-based transmission processes. American Naturalist 145: 661 -675. Bailey, N. T. J. 1975. The mathematical theory of infectious diseases and its application (Griffin: London). Fenner, F. 1983. Biological control, as exemplified by smallpox eradication and myxomatosis (The 1983 Florey Lecture). Proc. Royal Soc. London 218 B: 259 -285. Getz, WM and J Pickering. 1983. Epidemic models: Thresholds and population regulation. American Naturalist 121: 892 -898. Levin, S. A. & D. Pimentel. 1981. Selection of intermediate rates of increase in parasite-host systems. American Naturalist 117: 308 -315. Kermack, W. O. & A. G. Mc. Kendrick. 1927. A contribution to the mathematical theory of epidemics. Proc. Royal Soc. London A 115: 700 -721.