Preventing transmission of Lyme disease

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Caused by the spirochete bacteria Borrelia burgdorferi, Lyme disease is transmitted by a tick bite. The primary vector is the ixodes tick species, but other ticks are also involved in transmission of Lyme disease.

In the Northeast, Midwest and Southeastern United States, Ixodes scapularis is the most common source. Ixodes pacificus is the primary culprit on the West Coast.

Tick larvae become infected in the summer with their first blood meal from deer and other mammals. The bacteria winters in the midgut of the larva. Transmission of the bacteria in ticks is mostly considered transstadial but some transovarial transfer does occur, meaning that it goes from larva to nymph to adult vs adult to egg. A tick carrying B burgdorferi bites a susceptible animal and injects the spirochete bacteria into the skin via its saliva. Transmission of the spirochete bacteria occurs 36 to 48 hours after tick attachment, which is important when considering tick control.

Controlling exposure

The most important part of Lyme disease prevention is to control exposure of susceptible mammals to ticks. This involves frequent tick checks of all pets and people and rapid removal of the entire tick.

Ixodes ticks are less than 3 mm in size and like environments in the fallen leaves and debris of hardwood trees. Controlling the environment whenever possible, such as the yard and areas where people and pets frequent, is important. This involves keeping lawns short, cleaning out dead leaves and debris, and using wood chips or mulch in garden beds. There are innumerable chemicals that can be used to chemically treat areas as well.

Parasite preventives

There are topical and oral products available for tick control. In endemic areas, it is recommended that tick preventives be administered to pets year-round because these insects become active at temperatures above 40 ºF.

Topical products that prevent attachment such as amitraz (Preventic; Virbac) and permethrin for dogs (K9 Advantix II; Elanco), and fast-kill isoxazoline products in topical and oral formulations, are most common. Most of these products attack the tick nervous system by disrupting pathways found in the insect but not in mammals. This causes hyper-excitement and tick death.

There are a multitude of isoxazoline products such as fluralaner (Bravecto; Merck Animal Health), which acts by inhibiting ligand- gated chloride channels, including GABA-receptor and glutamate-receptor.¹ Afoxolaner (NexGard; Boehringer Ingelheim) is another one that inhibits acarine ligand-gated chloride channels, including GABA, causing hyper-stimulation of the tick nervous system.² Sarolaner (Simparia/Simparica Trio; Zoetis) acts to inhibit GABA channels and glutamate receptors at the neuromuscular junction in ticks and causes uncontrolled neuromuscular activity.³

Many other available products act similarly. However, these products have varying lengths of effectiveness and frequencies of administration requirements. There are also differences in the size and age at which these products can be administered.

Adverse effects of these products vary and are often neurological in nature but include vomiting, diarrhea, and lethargy. Topical products have also been known to cause reactions at the site of application. For the isoxazoline compounds, the tick needs to bite the host and have a blood meal.

Vaccinating against Lyme disease

There has historically been a lot of discussion about the efficacy of Lyme vaccination. In the American College of Veterinary Internal Medicine consensus statement update from March 2018, 3 of 6 panelists recommended Lyme vaccination in non-seropositive or healthy, nonclinical, seropositive nonproteinuric patients.⁴ This consensus or lack thereof is nothing new.

The Lyme vaccine is available as whole cell bacterins and recombinant versions and as such there is disagreement among veterinary professionals as to which, if either, are better. To understand how the vaccines are supposed to work, a brief discussion of the relationship between the bacteria and the tick is helpful.

There are 2 important proteins on the outer surface of the spirochete that are required for the bacteria to stay in the tick nymph midgut and to help the bacteria get into the tick saliva, allowing introduction to the host via a blood meal. They are outer surface protein (OSP) A (OSPA) and OSP C (OSPC), respectively. The OSPC binds to proteins in the tick salivary glands. The OSPC increases during a blood meal and leads to infection of the host. The tick saliva has been reported to protect the OSPC from being detected by the host immune system. Once the tick bites a susceptible host, the bacteria enter the skin and proliferate. Most traditional Lyme vaccines stimulate production of anti-OSPA antibodies, which are ingested by the tick during a blood meal. The antibodies attack the bacteria in the tick’s midgut and prevent transmission of the disease.

The concern about the OSPA antibodies is that they are short-lived and are not produced with natural infection. There are, however, bivalent vaccines available that stimulate antibodies to OSPA and OSPC. Unlike the OSPA antibodies, the OSPC antibodies are stimulated by natural infection and
can eliminate bacteria with OSPC, and immunity may last longer.

The efficacy of the vaccine is still under debate, with some sources recommending boosters at 6-month intervals in endemic areas. Vaccine reactions do occur and, historically, there was some thought that immunocomplexes caused by the vaccine were the cause of some cases of Lyme-induced glomerulonephritis, although there is little to no proof to support this. One point that many veterinary professionals can agree on is that vaccines by themselves need to be accompanied by some form of tick preventive.

Conclusion

In preventing Lyme disease, the most important step is to decrease exposure to ticks that carry the B burgdorferi spirochete. This is accomplished both environmentally and by routine administration of tick prevention year-round medication, especially in endemic areas. The role of vaccination is still up for debate but is not effective enough to be used by itself.

Marcy L. Rose, DVM, MS, is an associate veterinarian in the emergency department at Mount Laurel Animal Hospital in Mount Laurel, New Jersey.

References

1. Bravecto. Product label. Merck Animal Health; 2022. Accessed April 25, 2023. https://merckusa.cvpservice.com/ product/basic/view/1047512
2. NexGard Chewables. Product label. FrontlineVet Labs. 2020. Accessed April 25, 2023. https://nexgardforpets.com/sites/ default/files/2022-07/2022_NexGard_PI.pdf
3. Simparica Chewables. Product label. Zoetis. 2020. Accessed April 25, 2023. https://www.zoetisus.com/content/_assets/ docs/Petcare/simparica-pi.pdf
4. Littman MP, Gerber B, Goldstein RE, Labato MA, Lappin MR, Moore GE. ACVIM consensus update on Lyme borreliosis in dogs and cats. J Vet Intern Med. 2018;32(3):887-903. doi: 10.1111/jvim.15085