RISK OF EXPOSURE TO VECTOR-BORNE PATHOGENS: WHAT TO EXPECT IN 2024

Pathogens that cause heartworm disease, Lyme disease, ehrlichiosis, and anaplasmosis continue to increase and spread throughout the United States. The risk of vector-borne disease is complex and involves the interaction of vectors, pathogens, hosts, host behaviors, and habitats. Land use and climate change, human population growth, urbanization, changes in wildlife host and vector densities and distributions, and increasing international trade and travel are all factors that directly impact vector-borne diseases. An interdisciplinary approach between human, animal, and ecosystem health, referred to as One Health, recognizes the diversity of factors involved in vector-borne disease risks.

Risks have increased due to rehoming of pets, changes in distribution and prevalence of vector populations, habitat changes, changes in wildlife populations and increased interactions with them in newly developed and reclaimed areas, and the short and long-term changes in climatic conditions.

Ticks and mosquitoes remain the principal transmitters of pet and human vector-borne pathogens. The recent discovery of the invasive Asian long-horned tick (Haemaphysalis longicornis) in the eastern United States, and the continuing recovery of H. longicornis southward and westward of the original site, adds to our concern about disease agents that might be transmitted by this tick. According to the CDC, between 2004 and 2016, mosquito-borne and tick-borne disease incidence in people in the United States tripled, with much of this increase due to tick-borne pathogens which were reported in higher numbers and across a larger geographic area. The CAPC 2024 forecasts for vector-borne diseases in dogs, supported by ongoing research, continued to highlight areas where we can do more to lower the risk of exposure of companion animals to vectors of concern. The best preventive measures that veterinarians and their clients can take is to prevent contact between companion animals and these vectors. The foundation of these prevention strategies are the recommended use of products that kill and/or repel mosquitoes and ticks, administration of approved vaccines, and the compliant use of year-round preventives.

Lyme Forecast

The tick vector (Ixodes scapularis) of Lyme disease (Borrelia burgdorferi) continues to expand its geographic range. These ticks (called black-legged ticks) also transmit other important agents such as Anaplasma phagocytophilum, Babesia spp., Borrelia miyamotoi, Ehrlichia muris eauclarensis, several viruses, and possibly Bartonella spp. On-going research is increasing the spectrum of disease agents transmitted by black-legged ticks. Lyme disease is an important One Health pathogen that impacts the health of both humans, dogs, and horses.

• The 2024 forecast for Lyme disease in dogs is similar to the 2023 forecast with the geographic distribution of Lyme disease continuing to expand southward and westward outside of the historically high-risk areas (Northeast and Upper Midwest).
• We expect minor increases beyond last year’s forecasts in eastern Tennessee, Indiana, and North Dakota. High-risk “hot-spots” are again predicted in northern and southwestern Lower Michigan, and southern Indiana.
• High risks of Lyme disease persist in all portions of the Northeast, the upper mid-western states of Wisconsin, Minnesota, and the upper peninsula of Michigan. A higher than normal risk is expected to continue in North Dakota, northeastern South Dakota, southeastern Iowa, Illinois, and eastern Kentucky. For the first time, Lyme forecasts include eastern Montana.
• The southward movement of Borrelia burgdorferi is also evident in the increasing risk in North Carolina and Tennessee. Higher risk areas in parts of coastal North Carolina continue as was reported in the 2022 and 2023 forecasts. The 2024 Lyme prevalence maps, indicate that one northern North Carolina county has a prevalence rate of 44% (72 positive of 163 tested). Although this number represents a small number of dogs tested to date, the emerging data validates and emphasizes our reminders that pets and pet owners do not have to travel to the traditional endemic areas of the Northeast to encounter substantial Lyme disease risks.
• Veterinarians in regions of historically high prevalence and in forecasted regions of increased risk should reinforce their recommendations of aggressive tick control. Keep in mind that potential spread of Lyme Borreliosis can occur anywhere the tick vector is present. Always consider vaccinating high-risk patients. Annual testing can aid in assessing risk for dogs in hyper-endemic areas.

Heartworm Forecast

• We expect the high risk of heartworm infection to continue along the Mississippi river, throughout the southern portions of the interior Midwest, and along the Atlantic coast north into Virginia and southern New Jersey. The prevalence of heartworm continues to increase in the mid-Atlantic region, pushing northward into the densely populated regions of the northeast. The number and diversity of mosquitoes teamed with the population densities of the region support that heartworm infections are more likely to affect the health of increasing numbers of dogs in those areas.
• Veterinarians in states with historically lower prevalence are again cautioned about the increasing risk of heartworm infection and are encouraged to have a discussion with their clients about the changing prevalence. This is particularly important in southern Indiana, central and southern Illinois, southern Iowa, Kansas, and Lower Michigan and Ohio in the Great Lakes region.
• Additional areas likely to experience increased risk include the southwest (New Mexico), large portions of Colorado, and the northern Great Plains.
• Forecast of increased risk continues in northern California, with an additional increased likelihood of infection in western North Dakota, eastern Montana, and northwest South Dakota.
• We expect that much of the upper Midwest and the Northeast will have little change, perhaps indicating that increased emphasis on vector control and compliant heartworm prevention are having an impact in these regions. It is important to remember that areas of greater local prevalence and increased risk are likely to remain in some areas within these regions.
• The risk of heartworm infection reported during the previous years in much of central and southern Florida remains. However, the predicted prevalence in southcentral Florida decreased slightly. Perhaps increased post-pandemic veterinary visits and compliance has helped to lessen what was once a severe emerging risk.
• Very few regions are expected to have lower than average prevalence. As mentioned previously, it is important to remind clients that changes in local environments could create risks that might not be evident in the broader regional forecasts.

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

Nationwide, the high prevalence areas for ehrlichiosis are more wide-spread and less well-defined compared to other vector-borne pathogens. The maps represent multiples species of genus Ehrlichia that have different geographical ranges and vectors; however, all of them cause disease in dogs so the maps do represent disease risk for dogs. In general, the 2024 forecast is similar to 2023’s forecast which noted an expected increase in prevalence for the majority of the United States with some interesting emerging risk areas.

• The forecasted risks of Ehrlichia spp. in dogs remain high throughout the southeast, southwest, southcentral, and coastal Atlantic states.
• Compared to 2023, we expect increasing numbers of seropositive dogs in central California, northern Idaho, western Montana, western and central Washington, and parts of the Northeast (e.g., Vermont, New Hampshire, Connecticut). Risks also continue in much of Colorado and many parts of Wyoming. The relatively recent emergence of Ehrlichia muris eauclarensis in parts of Wisconsin and Minnesota emphasizes the need for testing and prevention in these areas.
• Considering the widespread prevalence of Ehrlichia spp. throughout much of the United States, the continued use of acaricides, routine examination of pets for the presence of ticks, and prompt removal remains imperative. Testing of symptomatic dogs can help monitor for ehrlichiosis and will also help reinforce the use of available tick control products.

Anaplasma spp. Forecast

Similar to Ehrlichia spp., the maps include data from multiple species of Anaplasma spp. in dogs. One of them, A. platys, is transmitted by brown dog ticks and is most commonly found in the southern and western United States. The other species, A. phagocytophilum, uses the same vector as Lyme bacteria so it is most common in the upper Midwest and Northeastern United States.

• The seroprevalence of Anaplasma phagocytophilum in dogs continues to follow the expanding range of Ixodes scapularis and Borrelia burgdorferi. Major risks for dogs continue in the Northeast and upper Midwest. As was pointed out for Lyme borreliosis, the prevalence of A. phagocytophilum continues to spread southward and westward. Forecasts predict an increase in prevalence in eastern Kentucky, eastern Tennessee and northwestern North Carolina. Increased prevalence is also expected in western North Dakota and eastern Montana. Veterinarians throughout these regions should reinforce the recommendations to their clients on the year-round use of tick preventives and encourage owners to check their dogs for ticks and remove them promptly. Annual testing can aid in assessing risk for dogs in hyper-endemic areas.
• Prevalence of Anaplasma phagocytophilum is expected to remain high throughout the Northeast. An increase is also expected in the western regions of the lower peninsula of Michigan.
• Regions of increase are also predicted in California, Texas, and the great plains, areas where A. platys is more common.
• Because many areas are expected to have increased prevalence, veterinarians and clients should remain particularly vigilant. Many of these areas are also at high risk for Lyme. As mentioned for Ehrlichia, the presence of more than one species of Anaplasma in some areas can create confusion, particularly in asymptomatic pets. Aggressive and compliant tick control can help eliminate some of the confusion.

Year-Round Protection and Annual Testing

The best way to protect your patients is to advise owners of the importance of year-round prevention, even during the winter months. 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 critical 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.