Effect of snow cover on the off-host survival of Dermacentor variabilis (Acari: Ixodidae) larvae.

The geographical range of the American dog tick, Dermacentor variabilis (Say), in Canada continues to expand northwards into areas with colder winter temperatures. Understanding what influences the off-host survival of D. variabilis over winter is important for predicting the northern distributional limit of this species. A field experiment was conducted to determine the effect of snow cover on the off-host survival of unfed D. variabilis larvae from a population situated near the northern distributional limit in Saskatchewan, Canada. Ticks were placed in tubes within 1 of 9 field enclosures (tickaria) in early fall. The 9 tickaria were divided into 3 equal treatment groups: no snow cover, natural snow cover (maximum depth of 25 cm) and extra snow cover (maximum depth of 32 cm). Tick survival was measured in mid-winter and in early spring (i.e., ~17 and ~26 wk after start of experiment). The results showed that snow cover had a significant impact on the relative humidity, but not temperature, in the tickaria. Larvae in tickaria with no snow cover had a higher mortality rate (27%) than larvae in tickaria with natural snow cover (6%) or extra snow cover (4%). Although snow cover enhanced off-host survival, many individuals were able to survive in sub-zero temperatures under leaf litter in the absence of continuous snow cover for a period of 108 days. These findings have implications for the construction of predictive models to determine the northern distributional limits of D. variabilis, a species that is an important vector of several pathogens.

Comparison of the supercooling points of questing Dermacentor variabilis adults in two populations on the Canadian prairies and implications for overwinter survival.

A comparison was made of the supercooling points (SCPs) of questing Dermacentor variabilis adults from two populations located ca. 800 km apart on the Canadian prairies. This is also the first study to examine whether there are seasonal fluctuations in the SCP of questing D. variabilis adults. The SCPs of adult ticks from Lizard Lake Community Pasture, a recently established population in west-central Saskatchewan, varied over spring and summer, with the median SCP warming over time. In addition, the SCPs of ticks from Lizard Lake Community Pasture were significantly higher than those of adult ticks collected from Sandy Hook in Manitoba, a population that has been established for decades. The off-host survival of adults from Sandy Hook between summer and spring has been shown previously to be significantly greater than that of adults from Lizard Lake Community Pasture. The findings of the present study suggest that there may be geographical variation in the SCPs of D. variabilis adults which may be associated with differences in overwinter survival. The relatively low SCPs of questing D. variabilis adults, and the ability of some adults to survive off-host during winter, may be factors contributing to the range expansion of this tick species in Canada.

Off-host survival of Dermacentor variabilis (Acari: Ixodidae) adults near their northern distributional limit in Saskatchewan, Canada.

Dermacentor variabilis (Say) is expanding its range northwards in Saskatchewan and Manitoba (Canada). Knowledge of the relative ability of different life cycle stages to survive off-host over winter is important for assessing the potential for further range expansion and risk of pathogen transmission. In the present study, the off-host survival of D. variabilis adults was determined in outdoor terraria within a field plot close to Lizard Lake Community Pasture (LLCP) situated near the northern distributional limit in Saskatchewan. Only a small proportion (7%) of adult ticks in this relatively recently established population survived from mid-summer to late winter. Off-host survival had declined further to <4% by mid-spring. A significantly greater proportion of female ticks (6%) survived from mid-summer to mid-spring than did male ticks (1%). The supercooling points (SCPs), the temperature below 0 °C when body fluids spontaneously freeze, of the 17 ticks that survived through to mid-spring did not differ significantly from the SCPs of questing ticks in the previous summer, suggesting adult D. variabilis do not undergo cold hardening during winter diapause. The off-host survival of D. variabilis adults near LLCP was also much lower than that previously reported for adults from Sandy Hook in Manitoba, a population that has been established for decades. This finding suggests that there is geographical variation in the ability of D. variabilis in southern Canada to survive from mid-summer to the subsequent spring. However, the ecological and genetic factors influencing the survival of northern populations of D. variabilis requires further investigation.

Reproductive output and larval survival of American dog ticks (Dermacentor variabilis) from a population at the northern distributional limit.

Female reproductive output and larval survival were determined for American dog ticks, Dermacentor variabilis (Say), from a recently established population near the northern distributional limit in Saskatchewan (Canada). Oviposition took 10-21 days at 25 °C and 95% relative humidity (RH). Temperature and relative humidity had a marked effect on egg development time and larval survival. Unfed larvae survived more than 100 days at 32 °C (with 95% RH) and 25 and 5 °C (with ≥ 85% RH). However, survival times declined markedly at lower relative humidities. In addition, 95% of the larvae placed in field enclosures survived for 140 days over winter during which they were exposed to sub-zero temperatures and 95-100% RH, while covered with snow. The median survival times (LT50) of unfed larvae submerged underwater was 68 days. These results show that D. variabilis larvae in populations near the periphery of the northern distributional limit are adapted to cope with sub-zero temperatures in winter, and can survive in the temporary pools of water created by the spring snow melt.

Supercooling Points of Adult Dermacentor variabilis (Acari: Ixodidae) From a Population Near the Northern Distribution Limit.

The northern distributional limit of Dermacentor variabilis Say, the American dog tick, is expanding in Saskatchewan and Manitoba (western Canada). The ability of D. variabilis to continue to expand its range northwards will depend upon the ability of individuals within populations at the species distributional edge to withstand very low temperatures during winter. One component of cold hardiness is the supercooling point (SCP), the temperature below 0°C at which an individual freezes. In this study, the SCP was determined for 94 questing D. variabilis adults (44 females and 50 males) from an established population near Blackstrap Provincial Park in Saskatchewan. SCP values ranged from -18.2 to -6.7°C, with a median of -13.3°C. This suggests that host-seeking D. variabilis adults differ in their ability to survive exposure to subzero temperatures, for at least a short period of time, without freezing. The distribution of SCPs was bimodal, but there was no significant difference in SCP values between female and male ticks, and no relationship between SCP and tick body weight. It remains to be determined what factors contribute to the variation in SCP values among questing D. variabilis adults.

Active surveillance of Anaplasma marginale in populations of arthropod vectors (Acari: Ixodidae; Diptera: Tabanidae) during and after an outbreak of bovine anaplasmosis in southern Manitoba, Canada.

Bovine anaplasmosis is the disease caused by the bacterium Anaplasma marginale. It can cause production loss and death in cattle and bison. This was a reportable disease in Canada until April 2014. Before then, infected herds were quarantined and culled, removing infected animals. In North America, A. marginale is biologically vectored by hard ticks (Acari: Ixodidae), Dermacentor variabilis and D. andersoni. Biting flies, particularly horse flies (Diptera: Tabanidae), can also act as mechanical vectors. An outbreak of bovine anaplasmosis, consisting of 14 herds, was detected in southern Manitoba in 2008. This outbreak lasted multiple rounds of testing and culling before eradication in 2011, suggesting local maintenance of the pathogen was occurring. We applied novel approaches to examine the vector ecology of this disease in this region. We did not detect A. marginale by screening of 2056 D. variabilis (2011 and 2012) and 520 horse flies (2011) using polymerase chain reaction (PCR).

L'anaplasmose bovine est une maladie causée par la bactérie Anaplasma marginale. Elle peut être responsable pour la perte de production et entrainer la mort du bétail et des bisons. La maladie devait être obligatoirement déclarée jusqu'en avril 2014. Avant cette date, les troupeaux infectés étaient mis en quarantaine et abattus pour éliminer les animaux contaminés. En Amérique du Nord, A. marginale est transmise par des vecteurs biologiques; les tiques dures (Acari: Ixodidae), Dermacentor variabilis et D. andersoni. Les mouches piquantes, en particulier les mouches à cheval (Diptera: Tabanidae), peuvent aussi agir comme vecteurs mécaniques. Une épidémie d'anaplasmose bovine touchant 14 troupeaux, a été détectée au Sud du Manitoba en 2008. Suite à une série de tests et d'abattages, cette épidémie fut éradiquée en 2011, suggérant qu'il se produisait localement un entretien du pathogène. Nous avons appliqué de nouvelles approches pour examiner l'écologie du vecteur de cette maladie dans cette région. Lors des tests de dépistage par réaction en chaîne pas polymérase (PCR), nous n'avons pas détecté A. marginale sur 2056 D. variabilis (2011 et 2012) et 520 mouches du cheval (2011).(Traduit par Dre Florence Huby-Chilton).

Ability of Unfed Dermacentor variabilis (Acari: Ixodidae) to Survive a Second Winter as Adults in Manitoba, Canada, Near the Northern Limit of Their Range.

One thousand seven hundred unfed field-collected adult Dermacentor variabilis (Say) were overwintered in 34 outdoor enclosures near the northern limit of their distribution in Manitoba, Canada. At the northern limits of the range of D. variabilis, it had always been assumed that unfed adult ticks questing in spring succumbed before the next winter and were not part of the population observed in the following year. Survival of the collected ticks was assessed on two occasions. In midwinter, an average 39.4% (SE ± 2.50) of the ticks were still alive, while an average 19.9% (SE ± 1.14) survived to April. Female ticks had significantly higher survivorship than males. The ability to survive an additional winter allows ticks to act in a greater capacity as reservoirs for tick-associated pathogens in this region.

Assessment of prevalence and distribution of spotted fever group rickettsiae in Manitoba, Canada, in the American dog tick, Dermacentor variabilis (Acari: Ixodidae).

Little is known about the distribution and prevalence of the spotted fever group rickettsiae in Canada. We conducted active surveillance for tick-associated rickettsiae in 10 localities in Manitoba. A total of 1044 adult American dog ticks, Dermacentor variabilis (Acari: Ixodidae), were collected and screened for spotted fever group rickettsiae. Rickettsia montanensis was the only species of rickettsia detected. The mean prevalence of infection was 9.8% (range, 0.00-21.74% among localities). The proportion of infected male and female ticks was not significantly different; however, tick populations near the northern limit of D. variabilis distribution in Manitoba had a lower prevalence of infection compared to tick populations from more southern localities in the province.