It Is Always a Bad Year for Ticks: Update on Ticks and Pathogens

Vector-borne diseases are important to the health of domestic animals, humans, and some wildlife species, and they are more than twice as likely to be emerging diseases (i.e., increasing in incidence in either new or existing populations) as non–vector-borne diseases.¹ Worldwide vector-borne diseases are increasing, and specifically in the United States, from 2004 to 2016, the number of reported human cases of tick-borne disease more than doubled.² Several factors are thought to contribute to the increasing incidence of diseases caused by vector-borne pathogens, including climate change; habitat changes, such as suburbanization and reforestation, that bring people, wildlife, domestic animals, and pathogens together; and an increase in certain wildlife species (e.g., rodents, deer) that support vectors and/or serve as reservoirs of infection. These factors are dynamic and, as a result, novel pathogens are expected to emerge and the incidence, prevalence, and spatial distribution of tick-borne diseases are expected to change, making monitoring essential.

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Editor’s Note: Since publication, the Companion Animal Parasite Council released its 2022 Pet Parasite Forecasts, which can be accessed here.

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Tick-Borne Pathogens Continue to Emerge

Until the early 1980s, Rocky Mountain spotted fever, caused by Rickettsia rickettsii, was the most commonly recognized human tick-borne disease in the U.S. However, since its initial description in the 1970s, Lyme disease, caused by Borrelia burgdorferi, has quickly become the most diagnosed tick-borne disease. Currently, approximately 30 000 human Lyme disease cases are confirmed annually in the U.S., but it is estimated that the actual number of cases could range from at least 300 000 to as many as 475 000.^3,4 Additionally, in the past several decades, numerous new human pathogens have been recognized (at least 9 since 2004). Many of these also infect dogs, which are commonly exposed to ticks; fewer infect cats and horses. 

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The diversity of tick-borne pathogens recognized in dogs has also been increasing (TABLE 1). Among the most common are B burgdorferi, Anaplasma phagocytophilum and Anaplasma platys (agents of anaplasmosis), and Ehrlichia species (agents of ehrlichiosis). There are 3 primary agents for canine ehrlichiosis in the U.S.: Ehrlichia canis, Ehrlichia ewingii, and Ehrlichia chaffeensis, but dogs are also susceptible to infection by Ehrlichia muris eauclairensis, believed to be transmitted by Ixodes scapularis, and the “Panola Mountain” Ehrlichia species, which is transmitted by Amblyomma americanum. In dogs, tick-borne pathogens generally cause nonspecific illness characterized by fever, lethargy, and inappetence; some may cause lameness, rash, and clinical abnormalities. 

As shown in TABLE 1, individual tick species can be associated with several pathogens, meaning that a person or dog infested with one species of tick may be at risk of several tick-borne diseases. For example, I scapularis is a competent vector for A phagocytophilum as well as B burgdorferi, while Rhipicephalus sanguineus can transmit E canis and is the presumed vector for A platys. Similarly, A americanum can transmit several important pathogens. The distributions of these pathogens follow those of the associated tick vectors (FIGURES 1–3).

While the specific agent may not be clinically significant (e.g., treatment for E chaffeensis and A phagocytophilum is the same), accurate diagnosis is important epidemiologically, as different vectors may be involved and some tick-borne pathogens (e.g., Babesia species, viruses) are not susceptible to the same treatment protocols as bacterial pathogens. Also, many species of ticks may coexist on the same dog; therefore, collection and identification of one species do not rule out the presence of another species that may be responsible for the presenting disease.

Ticks Are on the Move, and They Take Their Pathogens with Them

It is well established that several tick species are expanding their ranges and/or developing higher densities within their historical range. As ticks expand their range, they take their pathogens with them. This can cause diagnostic difficulties in areas where multiple pathogens may cause tick-borne diseases that present with similar signs. For example, anaplasmosis has long been endemic in dogs and humans in the northeastern U.S., but in recent years the lone star tick has expanded into the area and ehrlichiosis is now a differential in this region. 

In addition, an exotic tick species, Haemaphysalis longicornis (Asian longhorned tick), was first detected in the U.S. in 2017 and is now widespread in the eastern states.⁵ The potential for pathogen transmission by this exotic tick is a major concern. To date, there is limited evidence of natural infection of H longicornis with B burgdorferi, a variant of A phagocytophilum, and Rickettsia felis.⁶ Experimentally, this tick can transmit R rickettsii but was not successful in transmitting B burgdorferi or a human variant of A phagocytophilum.^7-9 Additional research is clearly needed to understand the pathogen risk of this new tick, keeping in mind that this risk may change over time as the tick spreads or uses novel hosts that may be infected with different pathogens. 

Prevalence Maps Provide Real-Time Data on Pathogens 

Mapping the distribution of ticks and their pathogens has obvious importance, but it is complicated by many factors. A significant limitation is the great expense and logistics of gathering real-time data on the presence and density of ticks across the potential range of each tick species. For pathogens, especially multi-host pathogens, it is even more difficult to determine distribution. However, the Companion Animal Parasite Council (CAPC) provides free, interactive parasite prevalence maps for a limited group of pathogens (capcvet.org). These maps provide a local, real-time quantification of risk, and they are now being used to develop annual forecasts to allow preventive care recommendations to be proactively strengthened. 

To create the maps, the CAPC receives monthly serologic data from the U.S. and Canada for several vector-borne pathogens, antigen data for heartworm (Dirofilaria immitis), fecal flotation data for intestinal parasites, and data on exposure to selected viruses in dogs and cats. The maps are updated monthly and data can be viewed at the national, state/province, or county level (for the U.S.) going back to January 2012, giving veterinarians expert insight into local pathogen prevalence with which to advise their clients. 

Some of the pathogens included in the mapping effort are zoonotic, meaning they can infect humans. In the past, several studies have suggested and investigated the utility of domestic dogs as sentinels for where human infections may occur. Dogs live in the same general environment as their owners, often spend more time outdoors, and may have increased exposure to ideal tick habitats; therefore, their owners may be exposed to ticks that infest them. Using the very large CAPC B burgdorferi dataset, it was found that the mean incidence of human Lyme disease case numbers increases with canine seroprevalence until the seroprevalence in dogs reaches approximately 30%.¹⁰ This finding reinforces the use of dogs as sentinels for human risk, especially with respect to identifying geographic areas of concern for potential human exposure.

Trends in Prevalence of Selected Tick-Borne Pathogens Show Changing Risk 

These data can also be used to look at trends in prevalence. Using B burgdorferi as an example,^11,12 modeling shows that areas with significant increases in canine B burgdorferi seroprevalence between January 2012 and December 2016 are the same states in which 95% of human cases occur: Maine, New Hampshire, Vermont, Massachusetts, New York, Rhode Island, Connecticut, Pennsylvania, New Jersey, Delaware, Maryland, Virginia, West Virginia, Minnesota, and Wisconsin (FIGURE 4). There is also a significant increase in the surrounding states: the upper peninsula and west coast of the lower peninsula of Michigan, eastern North Dakota, northeastern South Dakota, eastern Iowa, northern and eastern Illinois, Indiana, eastern Kentucky, northeastern Tennessee, eastern Ohio, North Carolina, northern California, and southern Oregon. The rate of increase in seroprevalence was highest within the high-incidence states, while seroprevalence in transitional zones immediately bordering the high-incidence regions is increasing at a slower rate. 

These kinds of data are important because they show that the seroprevalence in high-incidence regions is still increasing despite the availability of acaricides and anti-Borrelia vaccines, which suggests that current compliance with these preventive measures is inadequate. Veterinarians and pet owners in high-incidence regions need to recognize the growing risk of exposure and implement appropriate preventive measures. Additionally, veterinarians in the areas surrounding high-incidence regions need to recognize that the seroprevalence is rising and adjust screening and preventive care protocols accordingly. Similar studies on the trends of Anaplasma and Ehrlichia species have been conducted, and they show similar trends in certain regions of the U.S.¹³ When all of these pathogens are combined, it shows that dogs are at risk of a tick-borne disease in most regions of the U.S., and the risk is increasing in certain areas. 

These historical prevalence data can also be combined with climatological and ecological data to construct annual forecast maps that display the expected prevalence of each pathogen in dogs for the upcoming year. These forecasts are released annually at capcvet.org and at a more “client-friendly” site, petsandparasites.org. Monthly forecasts and alerts are available at petdiseasealerts.org. 

Conclusion

It seems veterinarians are constantly bombarded with annual warnings that “this year will be a bad tick year” or “Lyme disease cases are increasing” or “tick-borne diseases are on the rise,” but importantly, the data do support a concern that tick-borne diseases are getting worse in people and pets. However, an arsenal of tools exists to help. Pet owners and veterinarians should always practice appropriate care in preventing exposure to ticks (e.g., using tick preventives, performing thorough examinations for ticks, avoiding tick habitats if possible), especially in areas with high risk. 

Note: This article is adapted and updated from the NAVC’s 2020 VMX Conference Proceedings.

Recommended Reading

For more discussion on the pathogen exposure data used in the CAPC parasite prevalence maps and how they are developed, see: Self SCW, Liu Y, Nordone SK, et al. Canine vector-borne disease: mapping and the accuracy of forecasting using big data from the veterinary community. Anim Health Res Rev. 2019;20(1):47-60. doi: 10.1017/S1466252319000045