23 May 2016
Ian Wright details ticks posing the most threat to UK dogs and their owners, and emphasises the veterinary profession’s role in limiting new disease outbreaks.
Ian Wright
Job Title
Figure 2. Owners should be instructed on how to remove ticks using a tick hook.
Ticks and tick-borne diseases remain a significant risk to UK dogs, both domestically and while travelling abroad.
As the incidence of Lyme disease and the frequency of pet travel increase, so do the risks of exposure to tick-borne diseases. The profiles of ticks and their pathogens have also been raised, with numerous reports on Lyme disease hitting the news in 2015, as well as an outbreak of canine babesiosis in Essex.
This article considers the ticks that pose the most significant threat to UK dogs and owners, their treatment and prevention.
Tick-borne diseases are an ongoing and potentially growing risk to UK dogs and their owners. This comes from both endemic ticks and those encountered in mainland Europe, where pet travel is increasing.
Dogs encountering ticks abroad may be exposed to pathogens not present in the UK. This, in turn, may lead to significant disease in the travelling pet, with exotic ticks and tick-borne diseases being brought back to the UK.
Ixodes species ticks, the vectors of Lyme disease, are the most common found on UK dogs (Smith et al, 2011). Dermacentor reticulatus is also present in endemic foci in England and Wales, presenting the opportunity for Babesia canis to establish in these areas. This possibility became reality in Essex, with an outbreak of canine babesiosis in Harlow.
Rhipicephalus sanguineus is capable of transmitting a variety of pathogens, including rickettsial diseases, some with zoonotic potential. It is not endemic in the UK, but increased pet travel makes the establishment of the endemic foci of Rhipicephalus and novel tick-borne diseases in the UK more likely.
This article provides a review of these ticks and the threat they pose to UK dogs and owners. The prevention of tick-borne diseases will be discussed, as well as the role veterinary professionals can play in preventing the establishment of novel tick-borne diseases in the UK.
Ixodes species are the predominant ticks in the UK and have a three-year life cycle in temperate climates, with each life cycle stage feeding once per year.
The larval stage, which feeds in the first year, requires small hosts, such as rodents and songbirds. Nymphs and adults will feed on a variety of mammals, including pets, livestock and people.
Sheep tick Ixodes ricinus is the most common tick found on dogs in the UK and is the most important vector of Borrelia burgdorferi, the cause of Lyme disease, throughout Europe. Ixodes hexagonus and Ixodes canisuga are also implicated in Lyme disease transmission in the UK, but they are not thought to be as significant vectors as I ricinus.
Lyme disease is maintained in endemic areas by reservoir hosts that act as subclinical carriers of infection and reproductive hosts that do not carry infection, but maintain populations of infected ticks. Small rodents and birds are the most significant reservoir hosts of Lyme disease, but, in the UK, deer – especially roe deer – act as important reproductive hosts, carrying Lyme disease around the country. This has been particularly true in Wales, where roe deer have facilitated spread of the disease though the Wye valley.
Ticks mostly become infected as larvae and remain infected as nymphs and adults. When the tick feeds, Borrelia species multiply in the gut and, over several days, penetrate the gut epithelium and migrate to the salivary glands, where they may be transmitted. As a result, it is thought a tick has to feed for at least 24 hours to 48 hours for transmission to occur.
Nymphs are thought to be more significant than adults in terms of overall transmission, as they are more common and, due to their small size, less likely to be groomed off and missed when pets or people are being examined.
The reported incidence of Lyme disease in people is increasing, with 0.5 cases per 100,000 people in England and Wales in 2001 rising to 1.73 cases per 100,000 in 2011.
Public Health England has published figures showing 421 of laboratory-confirmed cases of Lyme disease in the UK in the third quarter of 2015. In comparison, 300 were reported in the same period in 2014.
The growing number of reported cases is likely to be a combination of an increase in disease transmission, heightened public awareness and increased surveillance.
A number of high-profile cases reported in the British press (BBC, 2015; The Telegraph, 2015) is likely to have led to increased public concern and awareness of relevant clinical signs, but some real increase in transmission is also likely. This may be due to changes in human activity or increase in wild host reservoir or tick numbers.
It has been suggested dog owners might be at greater risk of Lyme disease infection than people without dogs, but studies have found no correlation between dog ownership and risk of infection, and infected dogs pose little or no direct risk to humans.
The incidence of Lyme disease in dogs is unknown, with little data on the prevalence of Borrelia species infection or the incidence of active disease. The PDSA has published figures suggesting a massive increase in reported cases at its surgeries over a seven-year period (PDSA, 2016; The Telegraph, 2016), but these were suspected as well as confirmed cases and should be interpreted in light of increased awareness and reporting.
A large study looked at the prevalence of B burgdorferi in Ixodes species ticks found on UK dogs (Smith et al, 2012). The prevalence of B burgdorferi was examined in 739 of the ticks collected using PCR, with 2.3% of the ticks testing positive – suggesting significant exposure of dogs to B burgdorferi.
Most infections with B burgdorferi are subclinical, with 5% to 10% of dogs exposed to infection developing clinical signs.
Where clinical signs develop, this is often after reinfection with an incubation period of two months to five months. The long incubation period means a recent history of tick bites may not exist before signs develop.
Dogs typically present with acute or subacute arthritis in one or more joints, with associated lameness and joint swelling. Other acute signs may then develop, including fever, anorexia, lethargy and lymphadenopathy. The acute forms may be transient and relapses may occur. The circular skin rash, erythema migrans (“bullseye lesion”) – often seen in human infection, is rarely seen in dogs.
Chronic disease is also uncommon, but may develop, especially in the absence of treatment. Chronic disease can consist of a non-erosive polyarthritis and glomerulonephritis, progressing rarely to kidney and heart failure.
None of the clinical signs seen in dogs are pathognomonic for Lyme disease. It should be considered as a differential in pets presenting with any of the signs described.
Serology can be used as a sensitive test, but only indicates exposure to the parasite and does not confirm infection. PCR will confirm infection, but is more sensitive from skin and synovial fluid samples than blood.
Treatment may often start with a strong index of suspicion rather than a definitive diagnosis and this is justified, as early intervention will greatly improve prognosis if Lyme disease is suspected.
Doxycycline at 10mg/kg orally, once daily, remains the mainstay of treatment, although amoxicillin may be used as an alternative. Treatment should be continued for at least four weeks, but relapses occasionally occur after cessation of therapy. Supportive treatment, such as angiotensin-converting enzyme (ACE) inhibitors, may be required for secondary complications, such as glomerulonephritis and cardiac disease.
Dermacentor ticks are vectors of Babesia canis subspecies canis, a cause of potentially fatal haemolytic anaemia in dogs.
Infection occurs when the parasite is transmitted in the saliva of a feeding tick a few days after the start of blood feeding (Matjila et al, 2004). Transmission has been reported through blood transfusions, but tick bites remain the primary source.
Distribution of B canis canis is, therefore, closely linked with its vector, D reticulatus. The presence of the vector in south-west and south-east England (Smith et al, 2012) has raised concerns about the possibility of B canis canis being introduced to the UK via infected dogs having travelled abroad, or dogs carrying infected Dermacentor ticks from continental Europe.
Concerns grew as D reticulatus increased its range and prevalence in France, a popular holiday destination for UK tourists taking their pets abroad. These fears were not thought to have become reality until February 2016, when B canis canis was reported in three dogs in Harlow, Essex (Swainsbury et al, 2016) and in a fourth dog from the same area in March 2016 (Woodmansey, 2016). The four dogs, from separate households, had not travelled abroad and not been in contact with dogs that had done so (Woodmansey, 2016). DNA barcode testing and PCR testing confirmed ticks found on the infected dogs to be D reticulatus infected with B canis canis (Phipps et al, 2016). This suggests there is an endemic focus of B canis canis in Essex and this pocket of infection is likely to spread.
Transovarial transmission from tick to tick has the potential to rapidly increase B canis canis prevalence in the tick population. In addition, movement of infected ticks on mammalian hosts, mechanically in vehicles and on clothes, may disseminate it to other D reticulatus populations in the country.
It has been suggested relaxation of the compulsory treatment for ticks before return to the UK under the Pet Travel Scheme (PETS) was, in some way, responsible for D reticulatus becoming endemic in the UK. This tick was present in the UK before the relaxation of the rules, but it is likely increased pet movements overall have led to the spread of B canis canis across Europe and to the UK.
The high levels of publicity in newspapers and television associated with this outbreak (BBC, 2016) has led to understandable concern from the public and veterinary professionals about potential spread and preventive measures in pets living in south-east England.
While increased vigilance is justified and B canis canis is likely to spread to adjoining counties, it is limited to a small number of cases, so public fears should be kept in perspective. However, dog owners in Essex – particularly in Harlow – and adjoining counties need to be aware of the potential for its transmission and the clinical signs of disease.
Infection can lead to red cell lysis and immune-mediated disease, resulting in anaemia, icterus, lymphadenopathy, pyrexia, secondary renal and hepatic disease, and, in severe cases, death. Diagnosis can be achieved by demonstration of the parasite in peripheral blood smears stained with Giemsa or Diff Quik. The parasite presents as piriform (pear-shaped) organisms, often paired, in the red blood cell (Figure 1). PCR on blood is a sensitive alternative and allows speciation of the parasite, but can take up to seven days.
Treatment consists of supportive treatment for anaemia and treatment for the parasite. Blood transfusion may be required if PCV drops below 15% and steroids may be indicated if PCV is low to respond to treatment, although the merits of their use is the subject of some debate.
Imidocarb given as two doses, 5mg/kg IM, 14 days apart, is the most effective drug for eliminating the parasite. Atropine may be required to counteract anticholinesterase side effects and frequently pain exists at the injection site. If this drug cannot be immediately obtained, clindamycin given at 12.5mg/kg orally twice daily will have some efficacy.
Either method of treatment is unlikely to completely eliminate the parasite, leaving infected dogs as low-level subclinical carriers. This needs to be borne in mind, as infected dogs may have subsequent relapses and act as potential reservoirs of infection.
Preventive products on dogs with previous infection and monitoring for ticks is important to reduce the risk of spreading further infection.
Rhipicephalus sanguineus is not endemic in the UK as the climate is not favourable for establishment of an endemic population.
However, the tick is thought to be moving north through Europe – including France – and the potential for establishment of endemic foci in the south of England is a significant concern.
It has the potential to complete its life cycle quicker than Ixodes species, often in one season. This allows it to potentially take advantage of increased temperatures in centrally heated homes and become established in households in a similar way to fleas.
Two cases of house infestation have been recorded in the south of England (Hansford et al, 2015). These cases were from dogs imported from Europe and at least one had been treated for ticks before admission. Nymphs of R sanguineus survived and, once established, took more than 12 months to eliminate despite effective tick treatment of pets and repeated fumigation of the house.
This demonstrates both the importance of checking dogs treated for ticks, and rapid reporting and identification of ticks found in the home.
In addition to the potential for house infestation, R sanguineus carries a number of rickettsial tick-borne diseases with potential pathogenicity for UK dogs:
These diseases have a high incidence in southern Europe and the Mediterranean basin, though cases of E canis have now been reported in France, Germany and Switzerland as R sanguineus has spread.
Examination for the organisms on blood smears can be difficult and time-consuming, due to the sometimes low circulating numbers of parasites in affected white and red blood cells, but also because of the ease they can be mistaken for artefact staining. Diagnosis is made easier by a combination of clinical signs, serology and PCR.
Doxycycline is the treatment of choice for three weeks at 10mg/kg daily orally. Imidocarb given as two doses of 5mg/kg IM, 14 days apart, is useful in refractive or chronic cases.
R sanguineus also acts as a vector for B canis subspecies vogeli. This parasite also causes clinical babesiosis in dogs, but is not as pathogenic as B canis canis.
Veterinary professionals play a vital role in giving accurate tick and tick-borne disease prevention advice, but also putting risks in perspective for the pet-owning public.
Although Ixodes ticks are found throughout the UK, their distribution across the country is not uniform, with high densities being found in south-west and south-east England (Smith et al, 2011). Dense foci are also found in the Lake District and Scottish Highlands.
Pockets of Dermacentor have been found in Wales and south-east England (Smith et al, 2011), but these may have spread and a Dermacentor tick was found in an untravelled dog in Nottingham (Nottingham Post, 2016).
The big tick project, organised by the MSD and the University of Bristol, will provide an updated distribution map from veterinary and public surveys in 2016.
Lifestyle also puts some dogs at greater risk of infestation, such as those walked in rural areas and pasture with tall grass or used by deer or ruminants. There may also be a history of previous tick attachment. Use of tick preventive measures should, therefore, be based on risk assessment, although the risk to all dogs in Essex should be considered to be high until more epidemiological data in the area is obtained.
Evidence suggests tick-borne pathogens endemic in the UK take at least 24 hours to transmit. Therefore, for dogs considered at risk of exposure to ticks and tick-borne diseases, a preventive product that will rapidly kill or repel ticks is recommended. No tick preventive product is 100% effective, but the isoxazolines and pyrethroid-containing products are highly efficacious for this purpose.
It is important for owners to be vigilant and check for ticks on their pets every day, removing any ticks with a tick hook (Figure 2). It is important owners are instructed on how to remove ticks without stressing them and without leaving the head and mouthparts in situ. A simple “twist and pull” action is required. This can also be performed with tweezers, but they should be fine-pointed and not blunt as crushing will stress the tick, causing it to regurgitate and increase the risk of disease transmission. Traditional techniques to loosen the tick, such as applying petroleum jellies, freezing or burning, will also increase this likelihood and are contraindicated.
Zoonotic risk comes from exposure of people to infected tick bites. Avoiding high tick and Lyme disease endemic areas is the most effective way of minimising risk, but these are also some of the most popular walking and outdoor recreational sport destinations in the country.
Sensible precautions when travelling in these areas include wearing long sleeves and ensuring the lower body is covered, the use of repellent sprays on the body or clothing and careful inspections for ticks on the body after periods spent outdoors.
Councils will often put up signs in high-risk areas, giving some or all of this advice (Figure 3). Any ticks found should be carefully removed with a tick hook.
Identification and recording of ticks across the country is important so their distribution can be mapped, therefore, helping to predict where tick-borne diseases are likely to spread. It is also helpful in identifying potential tick-borne diseases that may have been passed to infested dogs or owners.
Male ticks can be distinguished from females by differences in the size of their dorsal plate or scutum.
Ixodes species ticks are undecorated (Figure 4) and have a groove anterior to the anus. Dermacentor and Rhipicephalus ticks, in contrast, have a groove running posterior to the anus. In addition:
Public Health England is happy to receive ticks for identification. Ticks should be placed in a container with a secure lid and sent in an envelope marked “biological sample” to Tick Surveillance Scheme, Public Health England, Porton Down, Salisbury, Wiltshire SP4 0JG.
Samples should ideally be unfed, or only partially engorged, as features on the scutum and festoons may be partially obscured by engorgement. Tick surveillance forms and further information can be obtained from the gov.uk website under “tick surveillance”.
The European Scientific Counsel Companion Animal Parasites (ESCCAP) UK and Ireland website – www.esccapuk.org.uk – has news updates on Lyme disease and babesiosis, as well as tick-borne disease prevention advice.
Compulsory tick treatments under PETS have ceased for dogs abroad travelling to the UK. This is perceived by some in the veterinary profession to have been a factor in the establishment of R sanguineus in households in the south of England and in the introduction of babesiosis to Essex.
It is likely maintaining compulsory treatment would have slowed down the introduction of foreign ticks and tick-borne diseases affecting canids, but this is not why it was established. Like all compulsory PETS treatments, it was put in place to stop human disease entering the country, primarily Mediterranean spotted fever caused by Rickettsia conorii.
While this infection is still present in many European countries, it was felt, when PETS was renegotiated in 2012, human tick-borne disease did not pose a significant enough risk to the UK to continue compulsory treatment.
There would also have been an onus on the Government to continue to demonstrate the country was R sanguineus-free, which would have been expensive.
The removal of the compulsory treatment, however, while not related to canine disease, may have inadvertently sent a message to pet owners taking their dogs abroad the risk to their pets had reduced. This is not the case and it is vital veterinary professionals continue to stress the importance of owners using preventive measures while abroad.
Rickettsial parasites may be transmitted much quicker than Borrelia species or Babesia species, with transmission in a few hours of biting having been demonstrated (Fourie et al, 2013). A tick repellent product should, therefore, be applied a week before travel and cover maintained while abroad.
Because no product is 100% effective, owners should also check pets every 24 hours for ticks and remove any found with a tick hook. It is also useful for a vet or veterinary nurse to check for ticks on the pet’s return.
The ESCCAP UK and Ireland supports these recommendations and has further information and advice on its website.
Tick-borne diseases are a growing threat to UK dogs, both in terms of increased potential exposure to Lyme disease and from increased pet travel. Increasing awareness of these diseases, early diagnosis of infection, identification and mapping of ticks, and educating pet owners to the risks in a responsible way are all vital in limiting new outbreaks and risks to pets and owners.
Veterinary professionals and independent sources of advice and information, such as ESCCAP UK and Ireland, will be vital in helping to prevent future outbreaks and managing tick-borne diseases endemic in the UK.
