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Heterogeneity in Parasitism for Lyme Disease

Have you ever wondered why some individuals get a disease while others don’t? Could it be related to their genetics, immune system, behavior, or something else? Different pathogens express different heterogeneity in parasitism. This blog will therefore discuss parasite heterogeneity in Lyme disease.

A principal risk factor for contracting Lyme disease is exposure to wooded areas. This may be due to recreational activities (hiking, camping), outdoor occupations (landscaping, forestry), or residential proximity to a wooded area (especially in suburban/rural areas) (Halsey and Abramson, 2000). For example, outdoor workers are significantly more likely to contract Lyme disease than people who work primarily indoors (Schwartz and Goldstein, 1990). Behavior of the individual is therefore tightly linked to contracting Lyme disease. Interestingly, enhancing people’s knowledge of Lyme disease does not lead to significantly greater precautionary behaviors (Shadick et al. 1997).

All people are thought to be equally susceptible to Lyme disease, although some studies found that certain age ranges have the highest reported rates of Lyme disease: children from 2 to 15 and adults from 30 to 55 (Dennis 1998). Additionally, the reported incidence for men is generally higher than that of women, especially for individuals in the 5 to 19 and 60+ year age ranges (Orloski et al. 2000). These findings may be due to increased exposure of these individuals to Lyme-infected ticks, decreased use of protective measures against ticks, behavior associated with these age ranges and sex, or a result of reporting bias.


Parasitism heterogeneity also occurs in the other hosts of Borrelia burgdorferi sensu lato. For example, avian hosts are competent hosts for some Borrelia strains/species, but not for others. The same applies to rodents. “Lucky” for us living in the United States, Borrelia burgdorferi sensu stricto can invade both avian and rodent hosts (Kurtenbach et al. 2002). This host selectivity is due to the bacterium’s genetics, which is responsible for expressing specific outer surface proteins (Osp) that can suppress/evade the host’s complement pathway (Brisson and Dykhuizen, 2004). A similar situation exists for infecting ticks, as different Borrelia strains infect different Ixodid tick species (Burkot et al. 2001). The bacterium’s genetics therefore plays a big role in parasitism heterogeneity.


Seasonality can also affect the heterogeneity seen in Lyme disease. Hatched larvae typically feed in the summer and early fall months, nymphs feed in late spring and early summer, and adults mainly feed in late fall and early spring (CDC 2016). Larvae only feed on small-sized hosts (ex. White-footed mouse), while adults typically feed on larger hosts (ex. Humans). Nymphs can feed on both small and large hosts. Most states warn people that the deer tick nymph season typically starts early June, peaks in early July, and ends in August. Due to the nymph’s small size, they play a larger role in Lyme-transmission to humans than adults. For example, Piesman et al (1987) found that the risk of contracting Lyme disease in Massachusetts is greatest in late spring/early summer, especially May and July (nymph season). The adult tick season usually peaks at the end of October, but are more easily detected due to their larger size. They can still play a big role in transmitting Lyme disease to our pets, especially long-haired pets, since they are a lot less visible when buried under their hairy coat.

Lastly, abiotic factors can influence parasitism of ticks. Although adult ticks are present from fall to spring, they are typically less active during freezing temperatures. There are two different threshold temperatures for Ixodid ticks: uncoordinated activity threshold temperature and activity threshold temperature (Clark 1995). The first is the temperature below which the tick is no longer able to seek a host in a coordinated manner, while the latter refers to the temperature at which all tick activity ceases. Clark (1995) found that the average uncoordinated activity threshold temperature was 9.2 ± 4.1°C for females and 11.2 ± 3.4°C for males. The mean activity threshold temperature, on the other hand, was 6.2 ± 3.6°C for females and 8.5 ± 3.0°C for males. Ticks are therefore a lot less active during the cold winter months, thereby reducing the overall risk of contracting Lyme disease during this time (unless you live in a place where temperatures never get this cold!). Another interesting abiotic limitation for ticks is elevation, as ticks tend to avoid higher elevations. Bunnell et al (2003), for example, found no Ixodes scapularis ticks above 530 meters (1,739 feet) in elevation. Humidity is also important, as ticks prefer areas that have greater moisture (Stafford 1994). This is especially important for larvae, who have a small water mass and permeable cuticle.


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References

Brisson, D., Dykhuizen, D.E. 2004. ospC diversity in Borrelia burgdorferi. Genetics 168: 713-722.

Bunnell, J.E., Price, S.D., Das, A., Shields, T.M., Glass, G.E. 2003. Geographic information systems and spatial analysis of adult Ixodes scapularis (Acari: Ixodidae) in the Middle Atlantic region of the U.S.A. Journal of Medical Entomology 40: 570-576.

Burkot, T.R., Maupin, G.O., Schneider, B.S., Denatale, C., Happ, C.M., Rutherford, J.S., Zeidner, N.S. 2001. Use of a sentinel host system to study the questing behavior of Ixodes spinipalpis and its role in the transmission of Borrelia bissettii, human granulocytic ehrlichiosis, and Babesia microti. The American Journal of Tropical Medicine and Hygiene 65: 293-299.

Center for Disease Control and Prevention (CDC). 2016. Lyme disease: what you need to know. U.S. Department of Health and Human Services, Lyme disease brochure. Available at: https://www.cdc.gov/lyme/resources/brochure/lymediseasebrochure.pdf. Accessed 22 September, 2016.

Clark, D.D. 1995. Lower temperature limits for activity of several Ixodid ticks (Acari: Ixodidae): effects of body size and rate of temperature change. Journal of Medical Entomology 32: 449-452.

Dennis, D.T. 1998. Epidemiology, ecology, and prevention of Lyme disease. Page 34 in Rahn, D.W., Evans, J. (editors). Lyme Disease. American College of Physicians: Philadelphia, PA, USA.

Halsey, N.A., Abramson, J.S. 2000. Prevention of Lyme disease. Pediatrics 105: 142-147.

Kurtenbach, K., De Michelis, S., Etti, S., Schafer, S.M., Sewell, H.S., Brade, V., Kraiczy, P. 2002. Host association of Borrelia burgdorferi sensu lato – the key role of host complement. Trends in Microbiology 10: 74-79.

Orloski, K.A., Hayes, E.B., Campbell, G.L., Dennis, D.T. 2000. Surveillance for Lyme disease – United States, 1992-1998. Surveillance Summaries 49: 1-11.

Piesman, J., Mather, T.N., Dammin, G.J., Telford III, S.R., Lastavica, C.C., Spielman, A. 1987. Seasonal variation of transmission risk of Lyme disease and human babesiosis. American Journal of Epidemiology 126: 1187-1189.

Schwartz, B.S., Goldstein, M.D. 1990. Lyme disease in outdoor workers: risk factors, preventive measures, and tick removal methods. American Journal of Epidemiology 131: 877-885.

Shadick, N.A., Daltroy, L.H., Phillips, C.B., Liang, U.S., Liang, M.H. 1997. Determinants of tick-avoidance behaviors in and endemic area for Lyme disease. American Journal of Preventive Medicine 13: 265-270.

Stafford III, K.C. 1994. Survival of immature Ixodes scapularis (Acari: Ixodidae) at different relative humidities. Journal of Medical Entomology 31: 310-314.