There are almost 700 species of hard-bodied ticks worldwide, but fortunately only a minority of those found within the U.S. constitutes a disease threat to dogs and cats regarding transmission of pathogens or toxins (See Table 1). Still, the danger they pose is serious and can be lethal.
What makes a tick tick?
Protecting pets and keeping them healthy from tick-borne diseases require an understanding of the parasite’s basic biology. Ticks are tough and tolerant to adverse environmental conditions. Ticks are patient, questing day after day in search of a suitable host. Ticks are adaptable to a variety of habitats and travel with their repertoire of hosts across broad geographic ranges. Ticks feed on a variety of mammalian, avian, and reptilian hosts. Ticks are able to reproduce at astounding rates—females produce thousands of eggs per clutch. Many ticks spend the vast majority of their life cycles, which may extend over two years, within the microenvironment at the soil/vegetation interface, although there are exceptions. The brown dog tick can complete its life cycle indoors in as little as three months.
The variety of tick species in the U.S. and their phenology constitute a year-round risk to dogs and cats, justifying the recommendation of the Companion Animal Parasite Council (CAPC) to advise pet owners to, “Administer year-round broad-spectrum parasite control with efficacy against… ticks.” To quote a well-known veterinary parasitologist, “Your dog or cat might not need tick protection 365 days of the year, but it is certainly needed 12 months of the year.”
The colloquial description of the interaction of a tick with a host as a “bite” is both inaccurate and underrepresents the trauma with which tick mouthparts assault the host’s skin. Blood-feeding by ticks is not the surgically precise intervention that is the mosquito’s. A tick feeds with all the subtlety of a chainsaw. Ticks are acarids, not insects—they are relatives of mites, but not fleas and mosquitoes. When acarids diverged from insects, their mouthparts developed into an efficient cutting, penetrating, and anchoring structure called the hypostome and the chelicerae. The former is a flat plate with rows of spine-like denticles, while the latter comprises a pair of articulated structures armed with cutting edges and hook-like barbs, capable of telescoping and flexing. Together, they lacerate, saw, and penetrate the skin of their hosts to establish a hemorrhagic pool where they can remain anchored with the assistance of a cement-like substance secreted from the tick’s salivary glands. A feeding tube created by the confluence of the hypostome and chelicerae allows for exchange of host and tick fluids during the period of attachment.
Ticks require blood meals to survive, to grow, to develop to the next stage in their life cycle, and to reproduce. It is primarily, though not exclusively, through blood-feeding that a tick can harm its host by acquiring disease-causing organisms from one infected host and transmitting them to another host.
Ticks not only suck blood, they also spit. They concentrate their blood meals by taking blood from the hemorrhagic pool and returning waste, plasma, and lymph to the host via the salivary glands. Although the volume of ingested blood in a replete female tick may range from approximately 0.5 to 2.0 ml or more, this may represent a fraction of the total amount ingested. One expert believes, for example, that the female blacklegged tick, Ixodes scapularis, may feed on as much as 15 ml of host blood1 (See Table 2).
Tick feeding is a complex process. For adult females, feeding generally begins slowly after initial attachment, which provides the tick time to develop her reproductive organs, her salivary glands, and her cuticle in anticipation of the expansion of her body during the last rapid phase of engorgement called the “big sip.” The tick is feeding on blood and initially returning water to the host because she cannot accommodate the ingested volume until her cuticle and other organs are ready. She is also excreting feces, which include blood and digestive products. This can concentrate pathogens on the skin adjacent to the feeding site, which cannot be ruled out as a means of transmission. Adult male ticks usually take smaller blood meals between their quest for adult females with which to mate. Ticks in the larval and nymphal life stages have shorter feeding periods, so as to acquire nutrients for their next molt.
The tick needs to remain undetected and undisturbed while initiating and maintaining the invasive process of insertion of mouthparts and removal of blood, which is facilitated by the secretion of large quantities of saliva. Tick saliva is a cocktail of active substances that mitigate host responses and defenses (See Table 3, page 36). Saliva-assisted transmission (SAT) is a process by which tick-borne pathogens use saliva for their survival and multiplication in the tick, and subsequent transmission into the host. SAT substances enhance the pathogen’s ability to evade numerous host immune responses, as well as to proliferate and invade target organs. They also facilitate the infection of nearby feeding ticks, a process called “co-feeding transmission.”
A race against time
The successful transmission of a pathogen to a host depends on a multitude of factors—some are better understood than others. In addition to the duration of attachment and feeding, the concentration and location of the pathogen in the tick at the time of feeding is significant and varies among and even within species of pathogenic bacteria. Some pathogens require a period of growth and replication within the tick to expand to a critical mass. The process of pathogen transmission essentially begins with the insertion of the tick’s mouthparts—the longer it remains attached to the host, the greater the likelihood an infectious dose of pathogen can be transmitted.
Pathogen transmission by ticks is an under-researched field of science and little is known about minimum transmission times. Unfortunately, results of experimental models using laboratory animals and immature ticks may not extrapolate to real-life field conditions.
The risk of acquiring a tick-borne disease is a function of exposure to the threat of blood-feeding by an infected tick. To reduce the risk is to mitigate the threat. Countermeasures to the threat include partial or full avoidance of ticks, which is impractical. Inspection and removal of loose and wandering ticks, as well as those that are attached and feeding, is well-advised. In dogs and cats, application or administration of appropriate acaricides registered with the U.S. Environmental Protection Agency (EPA) or approved by the U.S. Food and Drug Administration (FDA) is the standard of care. In making a recommendation or prescribing, keep in mind that protection against tick-borne disease logically rests with either preventing blood-feeding or killing the tick as soon as possible after feeding begins.
Not a matter of extrapolation
A complete and thorough discussion of the full range of tick-borne disease in pets is beyond the scope of this article. As such, the focus will be on Lyme borreliosis, which is the most economically significant tick-borne disease in companion animal veterinary medicine. The causative agent (the bacterium, Borrelia burgdorferi) and the principal vector tick, the blacklegged tick (Ixodes scapularis), make up the most frequently discussed pathogen-vector pairing in science and media today. In the animal health industry, significant business segments have been built around Lyme borreliosis diagnosis and prevention.
Scientific and social interest in Lyme disease is driven by the fact large numbers of humans in some of the most populous areas of the U.S. are at risk for contracting it, with reported cases of infection steadily climbing year after year.
Although grateful for the scientific research driven by the desire to understand, diagnose, prevent, and cure Lyme disease in humans, it is important to differentiate this knowledge base from one focused on veterinary medicine. Although facts about disease and seasonal risk may be accurately represented, they do not necessarily apply to dogs and Lyme borreliosis. Unlike humans, dogs do not develop characteristic signs of disease soon after infection. Another distinction is that the time of year representing greater risk of tick-feeding and transmission for dogs may be the cooler months when adult blacklegged ticks are more prevalent compared to the warm weather months when nymphal ticks predominate and pose a risk for humans.
In any serious discussion about Lyme disease in humans or Lyme borreliosis in dogs, it becomes readily apparent the science is complicated and often the only agreement is disagreement. In the absence of information, opinion abounds. In veterinary circles, some argue the science we have is dated and interpretation of the results may be questionable. Yet, conclusions are cited often enough to the point where they become dogma. Controversy exists in terms of the minimum amount of time a blood-feeding tick needs to transmit B. burgdorferi.
The transmission of B. burgdorferi from its tick vector to a susceptible host has been thought to require a blood meal, which signals the organism in the tick to relocate from the midgut to the parasite’s salivary glands, facilitating movement via the blood-feeding process. The most frequently cited studies on the transmission dynamics of B. burgdorferi-infected blacklegged ticks are four articles published in journals from 1987 to 1998 using rodent models and nymphal ticks. The majority of test subjects were infected when uninterrupted feeding occurred for 36 to 72 hours. In one model, interrupted feeding resulted in a majority of test subjects being infected within 24 hours. Interrupted feeding assumes the bacteria have begun their relocation from midgut to salivary glands, thereby shortening the time required to move into the host during feeding. It should be noted the challenge models for current studies involving vaccines and diagnostics for dogs use adult ticks that feed in an undisturbed setting until repletion.
Recent studies using techniques unavailable to earlier researchers conclude that some species and strains of disease-producing Borreliae may preferentially locate themselves primarily in the tick’s salivary glands before a blood meal, which, in a rodent experimental model, allowed for transmission within 12 hours.
As is typical with interpreting results from scientific studies, certain assumptions need to be made. Regarding the brief review of the transmission dynamic studies of Borrelia species and Ixodes species, the assumption is that results and conclusions are applicable to the range of hosts, vectors, and bacterial species and strains.
This focus on feeding time required for transmission of B. burgdorferi from blacklegged ticks draws attention from the single most important aspect of prevention. If a blood meal is discouraged, the feeding time required for transmission becomes less relevant. Contact irritant repellents, such as permethrin, discourage tick attachment and possess anti-feeding substances.
It has been claimed most natural canine infections with B. burgdorferi may be nonpathogenic, meaning non-disease-producing. In seven studies published between 1993 and 2016, 86 percent of experimentally infected dogs had significant inflammation of the blood vessels in the tissues surrounding their joints. This pathogenic lesion is considered distinctive enough to be used as a marker in vaccine-efficacy studies reviewed by the United States Department of Agriculture (USDA). In light of 2019 CAPC seroprevalence data and an estimated U.S. dog population of 89.7 million, there is a potential for more than 3.9 million cases of joint inflammation caused by infection with B. burgdorferi.
Signs of infection
In addition to controversy regarding transmission times, there is also debate on the consequences of infection with B. burgdorferi for the dog. Most dogs infected with B. burgdorferi show no short-term clinical signs. Evidence of infection is first detectable three to five weeks post-feeding using the most common screening blood tests, while the onset of signs of clinical disease may not be for eight to 20 weeks after infection. The most typical clinical signs of joint inflammation, lameness, and swelling may be accompanied by fever and may be transient. Many dog owners take a wait-and-see approach to such an illness and as a result, an unknown, though perhaps significant, number of dogs fail to be diagnosed with clinical illness.
Due to significant gaps in clinical scientific studies, it is not uncommon to observe positive serologic screening test results being dismissed as having little significance other than as evidence of “exposure” to B. burgdorferi, which is an inaccurate and dangerously dismissive assessment.
Experimental studies2,3 suggest that once a dog is infected with B. burgdorferi, it may be infected for life, a consequence that has very little upside. When it comes to clinical disease, “Absence of evidence is not evidence for absence.”
The routine screening test performed by many veterinarians to detect antibodies to B. burgdorferi is not a test for disease. Rather, it is a test for past or present infection. A positive screening test (i.e. a “blue dot”) further requires a thorough history, a careful physical exam, and appropriate follow-up to establish a diagnosis of disease. The “blue dot” does unequivocally diagnose a failure of effective tick control and should be a call to action for a reassessment of the preventive strategies recommended for that patient.
A multimodal approach to prevention
An international panel of canine Lyme borreliosis experts recently convened to share their expertise and to review existing and newly published literature regarding pathogen transmission, epidemiology, and prevention. Their soon-to-be-released consensus document, GuideLyme, recommends a three-pronged, risk-management approach to prevent canine Lyme borreliosis, similar to what is recommended for heartworm disease.
With Lyme borreliosis, the first step is to prevent infection, which is accomplished through visual inspection, removal of embedded ticks, and the use of acaricides with properties that prevent and discourage attachment and blood-feeding by the vector tick. I. scapularis can be active and quest at temperatures above 40 F, which is why dogs in endemic areas should be provided tick protection 12 months of the year. The second is prevention of disease through the use of approved vaccine products. And finally, threat reduction places value on avoidance or minimizing exposure to ticks when feasible.
Ticks have evolved over the course of at least 390 million years to become the current threat they pose to people and pets. The danger is not just the tick itself, but the sum total of what it does, namely feed on blood and transmit toxins and pathogens. Modern countermeasures to the threat, such as the broad-spectrum repellent acaricide permethrin, have been in use for just over three decades, and the most recent advancements in systemic-acting acaricidal drugs have been in common use for a handful of years. A multimodal, risk-management approach is the best strategy to mitigate the threat of any tick-borne disease to companion animals.
Edward M. Wakem, DVM, is a veterinary services manager with Ceva Animal Health, and has extensive experience as a clinical practitioner, diagnostician, industry consultant, and military public health officer. Dr. Wakem received his BSc and DVM from Michigan State University and postgraduate education and training in veterinary anatomic pathology from the University of Connecticut. He has actively participated in organized veterinary medicine at a state and nation level throughout his career. Wakem is the representative for the American Veterinary Medical Association (AVMA) on the U.S. Environmental Protection Agency (EPA) Pesticide Program Dialogue Committee. He is an ex-officio member of the American Heartworm Society (AHS) executive board. Wakem can be contacted at firstname.lastname@example.org.
1 Barbour A. Lyme Disease 2015, Johns Hopkins University Press, page 53
2 Clinical manifestations, pathogenesis, and effect of antibiotic treatment on Lyme borreliosis in dogs. Straubinger RK1, Straubinger AF, Summers BA, Jacobson RH, Erb HN Wien Klin Wochenschr. 1998 Dec 23;110(24):874-81.
3 Persistence of Borrelia burgdorferi in experimentally infected dogs after antibiotic treatment. Straubinger RK1, Summers BA, Chang YF, Appel MJ J Clin Microbiol. 1997 Jan;35(1):111-6.