ABSTRACT: There are some UK diseases with zoonotic potential that remain under the public radar; but there are others, such as toxocariasis and toxoplasmosis, that have gained a degree of fame and notoriety. Lyme disease fits very much into the latter category and this article discusses the epidemiology, diagnosis, treatment and prevention of the disease. It also highlights the zoonotic risks and misconceptions that surround it.

Anyone working in veterinary practice can expect to field questions from the General Public about canine and human Lyme disease. It is important that nurses are well informed about disease prevention to enable them to dispel myths about the disease.

Life cycle and epidemiology

Lyme disease is caused by a Gram-negative spirochete, Borrelia burgdorferi. This was thought originally to be one species of bacterium, but is now known to consist of at least 11 subspecies, three of which are known to be pathogenic, Borrelia burgdorferi sensu stricto (a sub group of burdorferi), B. garinii and B. afzelli.

Pet dogs and, less commonly, cats may be infected by tick bites. It is a significant zoonosis, with approximately one case per 100,000 people per year in the UK. Infection is transmitted by Ixodes spp. ticks that commonly have a three-year life cycle in the UK (Figure 1).

Figure 1: Ixodes tick

Eggs are laid in ‘year one’ and hatch to larvae, which overwinter. The following spring, these larvae become active and feed on small wildlife hosts. After feeding, the larvae detach from their host, moult into the nymphal stage and hibernate until the following spring. The nymphs become active again and feed on small mammals or larger hosts, such as rabbits. They then drop off the host and moult to the adult stage.

The adults will then feed on another host. This can be any one of a variety of mammals, commonly deer, livestock, pets or people. Mating takes place on the host and the female drops off to lay eggs, before dying. The tick may acquire or transmit Borrelia spp. through any of these blood meals. Although livestock are frequently fed upon by Ixodes spp. ticks, they are rarely found to be infected with Borrelia spp.; but wildlife, such as deer, rabbits, rodents and birds are known to act as significant reservoirs of infection.

Three species of tick are known to transmit Lyme disease in the UK.

Ixodes ricinus – This is the most common species of tick in UK and the commonest vector of Lyme disease. It typically inhabits moorland and rough pasture.

Ixodes canisuga (the ‘dog tick’) – This tick is adapted to live in lairs or dens. As a result outbreaks are sometimes seen in kennels or breeding establishments and this limits exposure to wildlife reservoirs of Lyme disease. 

Ixodes hexagonus (the 'hedgehog tick’) – This tick is found commonly in burrows, kennels and urban environments. It is a significant vector of Lyme disease and found in cats more commonly in the UK than I. ricinus.

Ixodes spp. are ‘hard’ ticks with a scutum, and identification can be confirmed by an anal groove that loops around the anus anteriorly. Speciation is more complex, but is not normally required in practice as all Ixodes spp. ticks can transmit Lyme disease. If clients are concerned about Lyme disease and ticks are found on their pets (or on themselves!) then it is important for nurses to stress to them that many ticks are not infected with Lyme disease. Prevalence in ticks is rarely higher than 50 per cent, even in high prevalence areas.

Although Lyme disease can be carried in the mouthparts of the ticks, the majority of transmissible infection is in the stomach. This means, in most cases, a minimum of 24 hours attachment is required for ticks to transmit the disease; so ticks are rarely required to be removed as an emergency in order to prevent transmission.

‘Hot spot’ areas in the UK, known to have significant reservoirs of Lyme disease include Exmoor, the New Forest, South Downs, Thetford Forest, the Lake District, Yorkshire Moors and the Scottish Highlands. This list is not exhaustive, however, and no part of mainland UK can be considered to be Lyme disease-free.

Clinical signs

Cases of borreliosis can remain subclinical, or present with a variety of clinical signs. These include polyarthritis, degenerative joint disease, fever, depression, anorexia, lymphadenopathy, renal failure and cardiac disease. Erythema migrans, the circular red skin rash associated with Lyme disease, is a common early sign in people, but is not seen in its classic form in cats and dogs. Any of these clinical signs can develop months after initial exposure, so there may not be a recent history of tick bites.

Diagnosis

None of the signs described are pathognomic for Lyme disease, although it should be considered as a differential diagnosis in pets presenting with these signs and that have had known, or potential, exposure to Ixodes spp. ticks. This is especially true of dogs having visited known areas of high endemicity.

Some clients – whose pets have visited these areas and developed any of these presenting signs – will be convinced that they have been caused by Lyme disease. It is important for nurses to explain that, although this is a possibility, many other causes of disease may lead to these signs and further diagnostic tests will be required.

The following tests are available for the diagnosis of Lyme disease:

Blood film examination

Borrelici spp. can be detected on smears of peripheral blood, or synovial joint fluid, stained with Giemsa stain. The organisms appear as extra-cellular spirochetes with a wavy elongated appearance.

This test has the advantage that it can be performed in ‘in-house’ laboratories; but the presence of the parasite in joints and the peripheral circulation is intermittent and it may only be present in low numbers. As a result, this is a highly specific – but very insensitive – test.

Serology

Serology by enzyme-linked immunosorbent assay (ELISA) or immunofluorescence antibody testing (IFAT) is commercially available. The presence of antibodies in blood or synovial fluid samples indicates exposure to Borrelia spp., but does not confirm current infection or that the organism is the cause of clinical signs.

Equally, the absence of antibodies does not rule out infection, because clinical signs can develop months after initial infection and does make recent exposure less likely.

Polymerase chain reaction (PCR)

This is a highly sensitive method for diagnosing current active infection from blood and synovial fluid samples. A positive test, in combination with relevant clinical signs and a history of tick exposure, is highly suggestive of Lyme disease.

Treatment

Elimination of the parasite can be achieved thro
ugh antibiosis; however, long courses of treatment are necessary. Doxycycline is effective at 10mg/kg for a minimum of four weeks. Penicillins are also effective, but doxycycline is advantageous in cases where there is renal compromise as it is excreted in the faeces.

Supportive treatment may also be required for cardiac and renal complications. Intravenous fluid therapy is indicated in renal and pyrexic patients. Renal disease may occur months or years after the infection has been cleared, so regular urine testing for specific gravity is useful in pets previously infected with Borrelia spp..

Prevention

Prevention of the spread of Lyme disease hinges predominantly on preventing tick attachment to a new host for more than 24 hours. In the UK, it is impractical to control the numbers of Ixodes spp. ticks in the environment with insecticides because of the large numbers of different wildlife reservoirs and the dense vegetation in which they live.

Chemical prophylaxis with spot-on formulations containing fipronil, mataflumizone/amitraz, pyriprole or imidacloprid/permethrin are all effective in reducing the numbers of ticks on dogs; but none is 100 per cent effective. It is, therefore, important to advise clients to check themselves and their pets for ticks at least every 24 hours when walking in rural areas, especially recognised hot spot areas.

Local County Councils often put up signs in these areas to remind people to do this (Figure 2), but it should also be emphasised in veterinary practices. Removal should be performed by twisting and pulling the tick by the neck with a tick hook (Figure 3). The use of tweezers, burning and freezing of ticks should be avoided, as this will lead to regurgitation of the tick’s stomach contents and increase the risk of Lyme disease transmission. 

Figure 2: Local County Councils often put up signs to remind people to check their pets and themselves for ticks

Figure 3: Tick hooks

Author

Ian Wright BVMS BSc MSc MRCVS

After completing a Masters Degree in Veterinary Parasitology at the Liverpool School of Tropical Medicine in 1997, Ian went on to qualify as a veterinary surgeon from Glasgow in 2002. Since then, he has continued research in parasitology while practising companion animal medicine and giving lectures and short talks to vets and vet nurses on a variety of parasite-related topics.

To cite this article use either

DOI: 10.1111/j.2045-0648.2012.00155.x or Veterinary Nursing Journal Vol 27 pp 103-105

Veterinary Nursing Journal • VOL 27 • March 2012 •