HARD SCIENCE ON LYME: Removal of Western fence lizard decreases number of nymphal ticks and number of infected nymphal ticks.
A group of researchers, (Swei, Ostfeld, Lane, and Briggs), did a study on the western fence lizard recently that produced some interesting results. The life cycle of the tick involves three stages and three blood meals. Ticks are born without the Lyme disease bacteria. However, they may acquire it during their first blood meal (as larval ticks), their second blood meal (as nymphal ticks) or their third blood meal (as adult ticks). Lizards host up to 90% of larval and nymphal I. pacificus ticks. Western fence lizards (as well as the southern alligator lizard) are not competent hosts for Lyme disease bacteria. This means they cannot transmit Lyme disease to feeding ticks. But even better, their immune system actively kills the Lyme spirochete in ticks that feed on them. So a tick that feeds on one of these lizards will take its next blood meal without the bacteria that transmits Lyme disease. This eliminates the risk of people contracting Lyme disease from the ticks that have previously fed on the lizard and reduces the infection rate of the ticks generally in the area. You would think that the number of infected ticks would go up in the absence of the lizard that kills the Lyme bacteria. Not so. . . Find out why after the leap.
This study was done in California on larval ticks and that critical first blood meal that determines whether the nymphal ticks responsible for most human infection will acquire the Lyme bacteria. So what happens to larval ticks when you take some lizards out of an area? Do infection rates in nymphs go up or down? Well, as you might expect, there are a lot of hungry larvae looking for a meal ticket. But lizards aren’t the only diner in town, they can also get their blood meal from mice and other rodents that harbor the Lyme bacteria. Surprisingly, it turns out that roughly 95% of the larval ticks did not switch to mice for a blood meal! Why this is so is not known. Most larval ticks prefer lizards for their blood meal but it is also possible that it was harder for them to find and feed on mice. Larval ticks that don’t eat don’t survive. Other rodents are competent reservoirs for the Lyme disease bacteria. So the 5% who fed on these rodents would be expected to have a higher infection rate.
The net result was that there was a lower number of nymphal ticks the following season and a lower number of infected nymphal ticks, but the infection rate remained the same. The reasons aren’t known. The researchers speculated that some of the ticks may have switched to the southern alligator lizard (which also kills Lyme bacteria) or that infectivity rates were impacted by the higher burden of larvae on the mice. These results suggest that the environmental factors affecting infection rates of nymphal ticks are more complex than assumed.
Although not a part of this study, the reduction in nymphal ticks should mean fewer adults the following season. Fewer adults having fewer offspring should reduce the tick population over all. The determination of this would require a multiyear study—but could provide a clue on how to reduce the overall population of ticks.
In a nutshell:
When there are a lot of western fence lizards, there are a lot of nymphal ticks.
When there aren’t a lot of western fence lizards, there are far fewer nymphal ticks.
The nymphal ticks in the areas where the western fence lizard is removed have fewer infected nymphal ticks.
Here’s the citation to the article and the abstract: Swei, A, Ostfeld, R., Lane, R.S., Briggs, C.J., Impact of the experimental removal of lizards on Lyme disease, http://rspb.royalsocietypublishing.org/content/early/2011/02/10/rspb.2010.2402
Prior studies have suggested that the net effect of lizards is to reduce risk of human exposure to Lyme disease, a hypothesis that we tested experimentally. Following experimental removal of lizards, we documented incomplete host switching by larval ticks (5.19%) from lizards to other hosts. Larval tick burdens increased on woodrats, a competent reservoir, but not on deer mice, a less competent pathogen reservoir. However, most larvae failed to find an alternate host. This resulted in significantly lower densities of nymphal ticks the following year. Unexpectedly, the removal of reservoir-incompetent lizards did not cause an increase in nymphal tick infection prevalence. The net result of lizard removal was a decrease in the density of infected nymphal ticks, and therefore a decreased risk to humans of Lyme disease. Our results indicate that an incompetent reservoir for a pathogen may, in fact, increase disease risk through the maintenance of higher vector density and therefore, higher density of infected vectors.
You can contact Lorraine Johnson, JD, MBA at firstname.lastname@example.org.