RISK OF EXPOSURE TO VECTOR-BORNE PATHOGENS: WHAT TO EXPECT IN 2025

The Companion Animal Parasite Council has put out annual forecasts for canine Lyme, ehrlichiosis, anaplasmosis, and heartworm disease since 2012. The forecasts help highlight the dynamic nature of these vector-borne diseases and are meant to help veterinarians determine the risk of the diseases in their local and surrounding areas.

The 2025 forecasts are created to specifically anticipate the expected results from in-clinic and laboratory testing of dogs for these pathogens. Throughout the year, CAPC adds true results onto our parasite prevalence maps, which can be found at www.capcvet.org. Historically, the forecasts have been >94% accurate in predicting the true prevalence of these four diseases across the United States.

There are many factors that affect the risk of vector-borne diseases including the complex interactions between the host, vector, pathogen, and environment. Because of this complexity, the changes in risk of vector-borne diseases are not always linear or straightforward as a myriad of human-centered factors and natural factors interplay. On the human side, changes in land use (e.g. reforestation efforts) and animal transport can significantly alter the risk in a short period of time. Likewise, climate change and wildlife densities and ranges can also shift the landscape drastically. Thus, it is imperative that we continue to closely monitor many vector-borne diseases of human and veterinary importance.

Mosquitoes and ticks are the primary vectors of vector-borne diseases of human and veterinary importance. Recent years have shown us that diligent surveillance at the host and vector level is needed to accurately calculate the risk of specific diseases in the United States. In 2023, the United States had multiple cases of locally-acquired human malaria. It had been nearly 20 years since there had been a case of human malaria that was not acquired while traveling, which shocked the medical community but did not surprise those who study the ever-changing nature of vector-borne diseases. To truly understand the risk of vector-borne diseases, surveillance cannot be limited to check-ins every 10 years but should be routine, widespread, and across different seasons. With more than 10 million tests reported to CAPC for each disease each year across the United States, these forecasts are strongly rooted in evidence for where dogs are testing positive for Lyme, ehrlichiosis, anaplasmosis, and heartworm, respectively. That is what makes the canine surveillance efforts for these four diseases so powerful! While we can never fully account for travel or transport, the vast majority of pet owners select a veterinarian near where they live, work, and play, so the major trends seen should be taken seriously.

Overall, we hope these forecasts spark a conversation on where and how we can do more to lower the risk of vector-borne diseases through educational or medical interventions. While the continued use of routine diagnostic testing for vector-borne diseases is the best way to evaluate the overall risk in a specific clinic, veterinarians can use these maps to evaluate where their parasite control program is successful or lacking, what might be on the horizon, or to evaluate other risks throughout the country for those clients who travel. The basis of the vector-borne disease control programs is limiting the access of vectors to our pets and then killing or repelling the vectors before any pathogens can be transmitted. Therefore, the compliant, year-round use of products that kill and/or repel ticks and mosquitoes is a baseline to all parasite control programs.

Lyme Forecast

The tick vector (Ixodes scapularis) of Lyme disease (Borrelia burgdorferi) continues to expand its geographic range. While these ticks (called black-legged ticks) are found across much of the eastern half of the United States, the populations in the North appear to pose a higher risk of transmitting Lyme disease to humans and dogs. Unfortunately, we are seeing these populations spread South and West in the United States as well as northward into new areas of Canada.

This highlights the complexity of tick-transmitted diseases and their interaction with the environment. It is not always as simple as “Warmer temperatures = northward expansion of a disease”. Lyme disease is an important One Health pathogen that impacts the health of both humans, dogs, and horses. Multiple studies have shown that areas of high risk for dogs are also the same areas that humans are most likely to test positive for Lyme as well. These forecast efforts help provide valuable information not only for the risk of Lyme in dogs, but also a variety of blacklegged tick-transmitted pathogens of humans such as B. miyamotoi, Babesia microti, Anaplasma phagocytophilum, and Powassan virus.

• The Upper Midwest and Northeast continue to be the high-risk areas for Lyme disease although those two regions increasingly becoming connected as the risk increases across Indiana, Illinois, Michigan, and Ohio.
• There is also a southward expansion of the risk of Lyme into eastern Tennessee and northern North Carolina. Current control options for Lyme disease include tick control and vaccines. However, these controls must be administered prior to tick exposure to be efficacious. Therefore, veterinarians in these areas should evaluate their Lyme preventive programs and implement them before seeing major increases in positive cases in their clinic.
• A higher-than-normal risk is expected to continue in North Dakota, northeastern South Dakota, southeastern Iowa, spreading from the heavily endemic Upper Midwest region. Given that blacklegged tick populations are dependent on both deer populations and broadleaf forests, it is reasonable that the westward expansion of Lyme would likely track along major rivers in these predominantly agricultural states.
• Veterinarians in regions of historically high prevalence and in forecasted regions of increased risk should reinforce their recommendations of aggressive and proactive tick control. Studies indicate that in high-endemic areas, vaccinating dogs for Lyme reduces both the number of positive cases and the severity of disease seen in a clinic. As the Lyme vaccine is considered a risk-based vaccine, veterinarians in emerging or non-endemic areas are encouraged to assess each patient’s risk individually. Annual testing can aid in assessing regional risk for dogs, but lifestyle factors including travel to Lyme endemic areas or prolonged time spent in the woods (e.g. hunting dogs) may also warrant additional protection compared to the rest of your patient population.

Heartworm Forecast

The causative agent of heartworm disease, Dirofilaria immitis, is transmitted by a variety of mosquito species. This includes several mosquito species, like the Asian tiger mosquito (Aedes albopictus) and the yellow fever mosquito (A. aegypti), both of which have undergone rapid population expansions northward in the United States, bringing with them an increased risk of heartworm to dogs in those previously non-endemic areas.

• We expect the highest risk of heartworm infection to be in the southeastern United States with a consistent push northward along the Mississippi river as well as along the Atlantic coast. This puts much of the middle of the United States, from Kansas across to Maryland, at a heightened risk for heartworm disease as well.
• While we have not seen that northward expansion significantly reach the Upper Midwest (e.g. Minnesota and Wisconsin) or the Northeast, transport of animals from the South to larger cities in the North presents a real threat. Veterinarians in these cities should be just as prepared to diagnose, prevent, and treat heartworm as veterinarians in known high-risk areas. In this capacity, veterinarians play a crucial role in slowing the expansion, as a heartworm-positive dog in a neighborhood can significantly increase the risk to other animals in that neighborhood. Accurate diagnosis and treatment of any positive dogs can help prevent the disease from setting up in new areas.
• There are consistent areas of increased risk in New Mexico, Colorado, Wyoming, Montana, and North Dakota. Although these areas fall outside of the range many consider to be endemic for heartworm, veterinarians are diagnosing this disease at a higher-than-expected rate. This highlights the need for routine heartworm testing for all dogs to determine the true prevalence/risk in an area.
• Northern California continues to have an increasing risk of heartworm disease from the northern coast inland into the central valley. This region has an abundance of mosquito species that are competent vectors for heartworm as well as increasing wild canid (e.g. coyote) populations which may serve as reservoirs. Given the challenges of reducing mosquito populations of heartworm in wild canids, veterinarians should focus on the individual pets and redouble their efforts to get all pets on year-round heartworm prevention.
• It is important to remind clients that changes in local environments could create risks that might not be evident in the broader regional forecasts. We encourage veterinarians to adopt the mentality that “LOW risk is not NO risk” especially for a preventable, fatal disease such as heartworm.

Year-round use of preventive products remains the best means of providing comprehensive internal and external parasite control. Annual testing is recommended to monitor compliance and preventive efficacy.

Ehrlichia spp. Forecast

Ehrlichiosis remains a pathogen of concern in dogs across the United States. The data used to create these forecasts combines several Ehrlichia spp. that can infect dogs, including E. chaffeensis and E. ewingii transmitted by the lone star tick (Amblyomma americanum) and E. canis transmitted by the brown dog tick (Rhipicephalus sanguineus). Therefore, multiple patterns arise on these maps, each potentially associated with different ticks and Ehrlichia spp. One particular pattern across the middle of the US from Oklahoma and Kansas to North Carolina and Virginia mirrors the CDC reports of human cases of ehrlichiosis, which is also transmitted by the lone star tick. This again highlights the intertwined risk of these diseases to humans and pets as we share spaces and lifestyles.

• The forecasted risks of Ehrlichia spp. in dogs remain high throughout the southeast, southwest, southcentral, and coastal Atlantic states.
• Compared to 2024, we expect increasing numbers of seropositive dogs in many western states including Colorado, Utah, and Wyoming
• We are also continuing to see the progressive northward expansion of the lone star tick, which brings with it increased risk of ehrlichiosis into historically non-endemic states in the Upper Midwest (e.g., Minnesota and Wisconsin) and the New England region (e.g., New York, Massachusetts, Vermont, New Hampshire, and Maine).
• Given that multiple tick species can transmit ehrlichiosis to dogs, the risk to dogs is widespread across the US and the seasonality of the risk is year-round in many places. This highlights the need for routine testing of dogs as well as year-round tick control regardless of region.
• Perhaps more so than the other diseases forecasted, dogs that test positive for antibodies to Ehrlichia spp. may remain non-clinically affected. At a minimum positive test does indicate tick exposure to some level, and veterinarians should use this as further evidence to support year-round tick control for the dogs in their clinic.

Anaplasma spp. Forecast

Similar to Ehrlichia spp., the maps include data from multiple species of Anaplasma spp. in dogs, including A. platys transmitted by the brown dog tick (Rhipicephalus sanguineus) and A. phagocytophilum transmitted by the blacklegged tick (Ixodes scapularis). The distinct geographic ranges of each specific disease highlight the importance of knowing the tick and tick-borne disease risk respective to your local area.

• The seroprevalence of Anaplasma phagocytophilum in dogs continues to follow the south and westward expansion of the range of Ixodes scapularis.
• The major risks for dogs remain in the Northeast and upper Midwest, where the numbers of dogs testing positive for anaplasmosis can even surpass those testing positive for Lyme in some practices.
• Forecasts continue to predict and increase risk for dogs in eastern Ohio, Kentucky, and Tennessee, as well as much of Virginia and North Carolina.
• Anaplasma phagocytophilum also poses a risk to dogs on the West coast. Increasing numbers of Ixodes pacificus from northern California up to Washington bring with them a growing risk of infection to dogs.
• While the plains and their lack of trees often limits the westward expansion of the blacklegged tick, the risk of anaplasmosis to dogs in those upper plains states remains moderate. Whether that is from encroachment of the tick or eastward travel into higher prevalence states like Minnesota or Wisconsin, veterinarians should be prepared to provide appropriate tick control and diagnostic testing for their clients in those areas.
• The blacklegged tick has a protracted seasonality, with the juvenile stages active during the summer months and the adults active in the fall – winter depending on region. This has allowed the tick and its diseases to spread into more regions, varying its peak questing behavior to fit the new local climate.
• The higher risk areas in the southwest, from Texas across to Arizona, likely represent dogs that are at risk of infection with A. platys. This brown dog tick-transmitted disease tracks alongside E. canis, both of which are showing an increased risk of expansion northward through Colorado, Utah, and Wyoming. Co-infections are known to occur and can significantly increase the severity of disease.
• While these maps specifically focus on the prevalence of anaplasmosis, we can extrapolate some of these risks to other diseases known to be transmitted by the brown dog tick, specifically Rocky Mountain Spotted Fever (RMSF). The CDC reports Arizona as one of the states with the highest incidence of RMSF in humans, and those cases are specifically linked to high numbers of brown dog ticks. Therefore, tick control in those areas may serve to not only reduce the risk of anaplasmosis or RMSF in dogs but also in humans as well.
• We continue to highlight the interconnected risks of tickborne diseases for humans and dogs, with the greatest concern being those transmitted by the brown dog tick which utilizes the dog as its primary host and spends the majority of its life indoors. Highly efficacious tick control for dogs has been shown to be the key to reducing the risk of vector-borne pathogens like RMSF in humans. Veterinarians are urged to remember the public health role we play in protecting pets and their families!

Year-Round Protection and Annual Testing

The best way to protect your patients is to advise owners of the importance of year-round parasite prevention, even during the winter months, and routine testing. You can use the CAPC Parasite Prevalence Maps to support your recommendation by underscoring the risks in your area and in regions of the country your clients may travel with their pets. It is also important to emphasize the importance of compliance and using products correctly.

CAPC Parasite Prevalence maps and Forecast maps are validated tools for increasing client willingness to engage in parasite prevention. Sign up to receive local alerts on parasite testing results down to the county level today by visiting the CAPC Parasite Prevalence Maps and selecting "Get Updates".

Monthly Pet Parasite Forecasts

Veterinary professionals and pet owners who want to monitor parasite activity in their county throughout the year, can also access 30-Day Parasite Forecast Maps at www.petdiseasealerts.org. These maps, developed exclusively by CAPC, provide a local forecast for every county in the continental United States on a monthly basis.

Flea Forecasts

CAPC offers a daily Flea Forecast at www.petdiseasealerts.org that displays flea activity across the United States, based on environmental conditions. The Flea Forecasts also offer a unique look at the historical movement of fleas through a video animation, showing changes in flea activity over the previous 12-month period.

The Flea Forecasts are a strong reminder to pet owners to protect pets year-round with flea control products, limiting infestations on pets and preventing establishment of flea populations in the home.

The Science Behind the Forecasts

Vector-borne disease is dynamic and ever changing, driven by multiple factors that affect the development of arthropod vectors and the pathogens they carry. Leading parasitologists work in collaboration with a team of statisticians to identify regions of the country that may experience higher parasite incidence in the months ahead. Numerous factors are analyzed, including the number of positive tests and the influence of weather patterns, vegetation indices, and human population density. Using this multi-disciplinary approach, we are leveraging everyone’s expertise to focus on a single common interest: forecasting the risk of exposure to vector-borne pathogens. While these forecasts predict the potential risk of a dog testing positive, they do not necessarily reflect the occurrence of clinical disease.

To learn more about the science behind the maps, a free review can be read here and full access to all of our manuscripts describing the methodology and fidelity of our forecasts can be found here.