CANINE LYME DISEASE

Lyme Disease has been recognized in humans since 1975 when it was associated with an outbreak of arthritis in children in Lyme, Connecticut.  The disease was first reported in dogs in 1984 and the reported annual incidence has been rising rapidly in recent years.  In the state of Pennsylvania there are three areas that are heavily infested with the ticks that carry Lyme disease:  1) the southeastern portion of Pennsylvania; 2) the north central counties of Elk, northern Clearfield, and Cameron; and 3) Presque Isle in Erie County.  Centre, Mifflin and Huntingdon counties are believed to have a lower population of ticks and thus a lower risk of acquiring Lyme disease.  The purpose of this handout is to try to summarize the most recent information about Lyme disease in dogs.  Armed with this information, pet owners can then make better educated decisions about prevention and treatment of this disease in their animals.

THE ORGANISM

The causative agent of Lyme disease, Borrelia burgdorferi, is a host-associated spirochete, with alternate arthropod (ticks) and vertebrate hosts.  They do not live in water or soil and are not transmitted by aerosols or fecal contamination.

THE VECTORS

Deer ticks, hard-shelled ticks of the genus Ixodes, transmit the bacteria by attaching and feeding on various hosts.  Other blood-sucking insects including other types of ticks, biting flies and some species of mosquitoes may be involved, but the evidence indicates they are of minor importance as vectors.

Ixodes ticks require three hosts and four different developmental steps to complete their 2-year life cycle.  Each female tick lays up to 2000 eggs in the spring which hatch into tiny larvae.  The primary host of the larvae in the northeastern U.S. is the white footed mouse, which serve as a reservoir for the disease and can be infected for their lifetime without developing disease.  After feeding on the mice the larvae drop off and enter a resting phase through the next winter.  The larvae molt into still very small nymphs the following spring.  The nymphs attach to a new host, again, most commonly, the white-footed mouse or any of a wide range of animals including dogs and humans.  Transmission of the organism to the host occurs during the nymph's 4 day feeding period.  Infection of the host takes place only after 2 days of feeding as it takes this long for the organism to multiply and move from the tick's mid-gut to its salivary glands and then into the new host via secretions.  In the fall of the second year the nymphs molt to the adult stage.  This stage is the most important source of infection for dogs. As long as the temperature stays above 35 degrees, adult ticks can be found on shrubs where they gain access to white tailed deer and other large animals. Adult ticks feed for 5 to 7 days, again several days of feeding are usually required to transmit the organism to the new host.  Thus, early tick detection and detachment of ticks is an important factor in disease prevention.  After engorgement the adult female ticks drop off and reside under fallen leaves for the winter.  The following spring they lay their eggs and start a new cycle.

PREVALENCE OF INFECTION

The proportion of dogs that develop disease in an endemic area is relatively small.  In hyperendemic areas, where more than 75% of the dogs are exposed to infected ticks, and where 50% or more of clinically normal dogs will have positive serologic titres (detectable antibodies in their blood), only about 5% of the exposed dogs actually develop signs that may be attributable to Lyme disease.  Infection risks are greatest at times when tick activity is the highest (spring and early summer for nymphs, fall for adults).

CLINICAL SIGNS IN DOGS

The onset of clinical manifestations of disease, particularly lameness, did not appear in experimental dogs subjected to tick-induced infection for 2 to 5 months after exposure, indicating a long latent period.  The skin rash (ECM - erythema chronica migrans) seen as the first stage of human Lyme disease is rarely seen in dogs.

There are several distinct syndromes of musculoskeletal disease seen in dogs.  In the first syndrome, affected dogs have a sudden onset of lameness, lethargy, loss of appetite, with or without fever.  Swelling of one or more joints is common.  The carpus (wrist) or tarsus (hock) are the most commonly affected joints, and pain on manipulation can be severe. Many dogs also have swollen regional lymph nodes.

The second, less common, musculoskeletal syndrome involves sudden onset of severe pain which is not localized to the joints.  Loss of appetite and severe physical depression are present, fever is high (104-105.5 F), a total reluctance to move is demonstrated and manipulation of the head and neck produces pain. These animals may also have swollen lymph nodes.  These signs are similar to those seen in humans with Lyme meningitis (infection of the spinal canal and central nervous system).

A third group of dogs has a less acute history including some of the following signs: intermittent nonspecific lameness, normal to slightly increased temperature, slight decrease in appetite and slight lethargy.  Physical exam usually reveals no localized limb/joint pain or abnormality.  This last group is usually diagnosed as having Lyme disease on the basis of exclusion of other causes of disease, history of possible tick exposure, positive serology (antibodies to the organism in the blood) and response to antibiotic therapy alone.

In dogs, second-stage signs of carditis (heart involvement), nephritis (kidney involvement) and neuroborreliosis (nervous system involvement) are far less common than the musculoskeletal syndromes described above.  We have seen several cases of serious kidney damage caused by Lyme disease in Laborador and Golden Retrievers.  The severely debilitating chronic arthritis syndrome seen in humans is extremely rare in dogs regardless of how long they have been in high incidence areas or how long they have been infected.

Interestingly, in Beagle dogs experimentally infected by placing many infected ticks on them (15-100 ticks), the resulting arthritis, lethargy and loss of appetite lasted about 4 days, and resolved completely without antibiotics!  One or two episodes of recurrent lameness occurred in some dogs at intervals of several weeks or months with apparent full recovery after each episode without antibiotic therapy.  Thus it appears the disease is self-limiting without therapy in many dogs.

DIAGNOSIS

Four diagnostic criteria should be considered as important factors to establish a diagnosis of Lyme disease in dogs. These include: (1) history of exposure to Ixodes ticks in an endemic area, (2) typical clinical signs, (3) a positive serology, (4) and a prompt response to antibiotic therapy.

DIAGNOSTIC TESTS

Following experimental exposure to infected ticks, antibodies begin to appear in dogs within 4 to 6 weeks.  The level of antibodies increases for several weeks to high levels and remains constant for at least 18 months.  It must be pointed out that a positive serologic test alone, without any other signs of disease, in a Lyme endemic area is NOT sufficient for a diagnosis of Lyme disease.   Vaccination of dogs elicits antibodies that react in the Lyme antibody tests run at most laboratories.  Therefore routine serology in vaccinated dogs is not useful.  Recently a new test using Western Blot technology, has been developed that can differentiate between antibodies generated in a tick-induced infection, versus antibodies that develop following vaccination.  It can also tell when a dog has experienced both an infection and vaccination.

Changes in antibody titres were evaluated in a group of dogs treated for acute limb/joint Lyme disease to see if there was any value in reassessing to evaluate therapeutic success.  ANTIBODY TITRES WERE FOUND TO HAVE NO CORRELATION WITH CLINICAL CONDITION. With successful therapy some dogs had decreasing titres, some had increasing titres and some stayed the same!

TREATMENT

Antibiotics are the treatment of choice for Lyme disease in dogs.  Doxycycline appears to be the drug of choice at 22 mg/lb twice a day for 21 days.  Other antibiotics that appear to be effective include Amoxicillin at 10 mg/lb twice a day for 21-28 days or tetracycline at 10 mg/lb three times a day for 21-28 days.  Response to antibiotics is rapid in almost all cases.  In animals that show no improvement in 3 to 4 days on antibiotics alone, other diagnoses should be considered.  Analgesic/non-steroidal anti-inflammatory medication may be used, but their effect will obscure the response to antibiotic therapy.  Steroids should not be used.  Tetracycline should not be given to young dogs as it may cause damage and discoloration to their permanent teeth.  Dogs with recurrent episodes normally respond well to the same antibiotic doses used to treat the primary episode.

In most cases it is not recommended to treat asymptomatic, seropositive dogs with antibiotics.  Remember, the vast majority of previously infected dogs with antibodies in their serum never develop Lyme disease!  In addition, dogs are at extremely low risk of developing the chronic arthritis or other serious second-stage signs of Lyme disease seen in humans. 

RISKS TO HUMANS WITH INFECTED DOGS

The risk to humans in contact with infected dogs is extremely small.  It is also unlikely that dogs would transport ticks into the house that could then fall off and become attached to humans. This is because Ixodes ticks are thermotropic (seek heat) and once on the dogs hair will quickly go to the skin and start embedding.  Once a tick starts feeding on the dog, it will feed until full.  None of the 3 life stages of the tick are intermittent feeders.

PREVENTION

Tick engorgement on dogs may be prevented by controlling the tick population around the home and the use of tick repellents when going into high risk areas.  Daily grooming and removal of adult ticks before engorgement will prevent transmission of the organism, but this is difficult due to the small size of even the larger adult ticks.  Cutting brush and mowing grass in animal areas will make the environment less hospitable for ticks and the small mammals which are the reservoir for the bacteria.  At Centre Animal Hospital we recommend the use of the several products for tick control.  We have found the PREVENTIC collar to be highly effective for all types of ticks.  Please do not leave these collars on dogs that are housed in groups where one dog is likely to chew off and ingest the collar, as toxicity could result.  We also highly recommend a topically applied product called Frontline Plus.  It is applied every 30 days and will prevent both flea and tick problems.  The insect repellent diethyltoluamide (DEET) will repel ticks, but is rapidly absorbed through the skin and in combination with certain insecticides has been associated with severe toxicity including death (i.e. Hartz Blockade).  DEET products are effective when applied to clothing to prevent tick infestation on humans. We recommend using a flea/tick preventative year-round due to the mild winters we have been experiencing in our area.

VACCINATION

There are several vaccine products on the market now to immunize dogs against Lyme Disease.  There is much controversy as to the effectiveness of these vaccines under clinical conditions.  Vaccination will lead to positive blood tests for Lyme disease for months to years later.  We do not routinely recommend Lyme vaccination for all dogs but reserve vaccination as an elective procedure for high risk dogs (outdoor, hunting, and field trial dogs) in tick infested areas, especially when there is a high prevalence of Lyme disease in their region.

In Centre County, Pennsylvania, we have a low risk of exposure based on the low number of CONFIRMED human and animal cases that were believed to have been infected within the county.  Even if a local animal were exposed, the risk of ever developing clinical disease is low (5%).  When a dog is affected clinically, the disease is rarely fatal, is probably self-limiting in the majority of cases, and is effectively treated with antibiotics even among dogs that have recurrent episodes of Lyme disease.