Managing Ixodes scapularis and Lyme disease in a risk area

Tick encounters have become increasingly common in eastern Canada due to the northerly expansion of Ixodes scapularis (also known as the blacklegged tick, or deer tick).  A publication modeling range expansion for I. scapularis predicted a 46 km/year expansion between 2010-2020 (Leighton et al, 2012) and a recent study (Clow et al, 2017) finds that this projection is consistent with tick sampling in the field. Ixodes scapularis is now the most common tick submitted through passive surveillance in Ontario (Nelder et al, 2012) and is the vector for Borrelia burgdorferi, the causative agent of Lyme disease. Veterinarians should feel comfortable identifying and distinguishing between the main species of ticks found on companion animals in Canada (Ixodes scapularis, Dermacentor variabilis, Ixodes cookei, Amblyomma americanum, and Rhipicephalus sanguineus).  It is recommended that veterinary teams ensure clients are comfortable knowing the basics about ticks (relative size, appearance in the engorged and unengorged states, seasonality, habitat, etc).

Ixodes scapularis is a three-host tick, feeding once at each life stage (larva, nymph, adult), and lives for 2-4 years in order to complete this cycle. These ticks prefer a high- moisture environment such as leaf litter, preferably under hardwood forest canopy. Adult female ticks drop off their host after a blood meal and lay eggs in the late spring of the year. These eggs hatch in mid- to late-summer (July or August) as larval ticks. Larval ticks are not transovarially infected (i.e. they do not carry Borrelia burgdorferi) and instead may pick up B. burgdorferi during a blood meal.  After feeding successfully, the larval tick falls off the host and molts into a nymph, transstadially carrying B. burgdorferi.  This tick will feed in late spring or early summer (May-July) the following year. Nymphs prefer to feed on small mammal hosts or birds, but will bite humans and sometimes dogs.  After feeding as a nymph, the tick will fall off the host and molt into an adult, which will feed between the late fall (typically September-November) and early spring (typically March-May). Because the adults have fed twice (once as a larva, once as a nymph), they generally have twice as great a chance of carrying B. burgdorferi. For example, in one study in 2009-2011 in the Thousand Islands, 16% of the nymphs and 31% of the adults carried B. burgdorferi (Werden et al, 2014). 

It is important to note that adult I. scapularis will be active during the winter months on any day the temperature is >4˚C. Since 2010 in Ontario, only February 2015 failed to see ambient temperatures reach 4˚C and in Quebec, only January and February 2015 failed to reach this threshold. In Nova Scotia, all months since 2010 have seen at least 1 day >4˚C. Therefore, risk of exposure to I. scapularis is possible in all 12 months of the year in areas with established populations. 

Serological screening tests are available to detect exposure to Borrelia burgdorferi. Patients living in, or traveling to areas endemic for B. burgdorferi, or those with history of tick exposure should be screened for exposure following the recommendations in the ACVIM Consensus Update on Canine and Feline Lyme Borreliosis (Littman et al, 2006, 2018). Antibodies to B. burgdorferi typically take weeks to generate and testing is recommended 4-6 weeks after tick exposure. Treatment of positive dogs should be handled in accordance with the ACVIM Consensus Statement (Littman et al, 2006, 2018).

Lyme disease is preventable and focus in the veterinary practice should be on prevention rather than treatment of exposed patients. CPEP recommends the following threepronged approach to prevention: 1. Client education and regular tick checks, 2. Using a preventive with label claim against I. scapularis (potentially year-round if temperatures >4˚C) and 3. Vaccination. These steps should be instituted in order, based on escalating risk to the patient. The primary goal is to reduce (to as close to zero as possible) the number of ticks the animal is exposed to by performing tick checks and using a preventive. In areas where tick burdens are high, vaccination should be considered to protect against those ticks that evade tick checks and preventives. Dogs living in, or regularly traveling to, endemic areas should have tick checks performed daily, receive a preventive and receive a Lyme vaccine. 

There is little evidence to suggest that Lyme disease affects cats clinically. Cats will, however, seroconvert and can test positive on serological tests after exposure to Borrelia burgdorferi. There are laboratory studies that have demonstrated lameness and possible neurological signs in cats after exposure to B. burgdorferi, but naturally occurring clinical cases have not been reliably reported. 

Leighton PA, Koffi JK, Pelcat Y, et. al. (2012). Predicting the speed of tick invasion: an empirical model of range expansion for the Lyme disease vector Ixodes scapularis in Canada. J. App. Ecol, 49, 457-464.

Clow KM, Leighton PA, Ogden NH, et. al. (2017). Northward range expansion of Ixodes scapularis evident over a short timescale in Ontario, Canada. PLOS ONE. E0189393

Nelder MP, Russell C, Lindsay LR, et. al. (2014).  Population-Based Passive Tick Surveillance and Detection of Expanding Foci of Blacklegged Ticks Ixodes scapularis and the Lyme Disease Agent Borrelia burgdorferi in Ontario, Canada. PLOS ONE. E105358.

Werden L, Barker IK, Bowman J et. al. (2014). Geography, Deer and Host Biodiversity Shape the Pattern of Lyme Disease Emergence in the Thousand Islands Archipelago of Ontario, Canada. PLOS ONE. E85640

Littman MP, Goldstein RE, Labato MA, et. al (2006).  ACVIM Small Animal Consensus Statement on Lyme Disease in Dogs: Diagnosis, Treatment and Prevention. J Vet Intern Med, 20: 422:434.

Littman MP, Gerber B, Goldstein RE, Labato MA, Lappin MR, Moore GE. 2018 ACVIM consensus update on Lyme borreliosis in dogs and cats. Journal of Veterinary Internal Medicine; 32(3): 887-903.