ABSTRACT: Angiostrongylus vasorum (also known as 'lungworm' or French heartworm') is a potentially life-threatening parasite of dogs and foxes, which is reported to have been on the increase in the UK over the past few years.' Dogs can present with a range of clinical signs, and spotting the disease is not always easy, so maintaining a degree of vigilance for its presence plays a crucial role in diagnosis. This article focuses on the mode of transmission, clinical signs and treatment of this parasitic infection, and discusses preventative measures that may be undertaken to reduce the likelihood of dogs acquiring the disease.
Introduction
Angiostrongylus vasorum is a metastrongylid nematode (a member of the lungworm family) that resides in the heart and pulmonary arterial circulation of infected dogs and foxes. Clinical signs are very variable and dogs may present anywhere on the spectrum of mild disease, through to severe signs that may prove fatal.
In the UK, the parasite has traditionally been seen to cause disease in specific areas of the country, and has long been recognised in areas such as South Wales, Cornwall and Surrey. It is only in the past few years that cases have been reported further afield, including the North of England and Scotland, indicating that the parasite has spread beyond its previously known ‘hot spots’. 2'3
The precise reason for the geographical spread of infection is not known, but factors such as climate change and the rise in the urban fox population have been implicated as possible factors. It has been suggested that most of the UK is suitable for transmission of the nematode;1 and it is, therefore, important for practices across the whole of the UK to be aware of the parasite, and the disease it can cause.
Life cycle
The life cycle of A. vasorum is indirect, with dogs becoming infected on ingestion of the intermediate host – the slug or * snail (Figure 1).4 The adult worm resides in the right side of the heart and pulmonary arteries of infected dogs and foxes. The female worm releases eggs, which are carried in the bloodstream to the pulmonary capillaries, where they develop into first stage (LI) larvae and hatch.
Figure 1: The life cycle of Angiostrongylus vasorum. (Image courtesy of Bayer Animal Health)
LI larvae penetrate through the capillary and alveolar walls, and from there are eventually coughed up and swallowed, and passed out into the environment with the faeces. Slugs or snails crawling over an infected animal’s faeces become infected with LI larvae, and development to the third larval stage (L3) takes place within the mollusc.
Dogs and foxes become infected by eating infected slugs or snails. The L3 is released into the dog’s intestine upon digestion of the mollusc, where it penetrates the intestinal wall and undergoes further development in the mesenteric lymph nodes to the fifth larval stage (L5).
The life cycle is completed when the L5 larvae leave the lymph nodes and migrate via the hepatic portal vein, liver and caudal vena cava, to the right side of the heart and pulmonary arteries, where development into a new adult worm takes place (Figure 2).
Figure 2: An adult Angiostrongylus vasorum [x 200). Ilmage courtesy of Bayer Animal Health)
The pre-patent period of A. vasorum averages 38-57 days.
Signalment
Dogs of all ages and breeds can become infected with A. vasorum. The disease is more often seen in younger dogs – in a study at the University of Copenhagen in 2002, over half of the cases affected were under one year old, but affected animals ranged from three months to 14 years.5 The apparent increased prevalence in younger dogs may be a consequence of their less discriminate eating habits, which then increase their chances of ingesting the intermediate mollusc host.
Not just known slug and snail eaters
It's worth remembering that slugs can be extremely small, and it may not be the case that the owner (or even the dog) is aware of ingestion. Dogs that eat grass may ingest small slugs that are attached to the grass blades; slugs can fall into water bowls or be attached to a dog toy in the garden.
Therefore, even though an owner may say that he or she is not aware of their dog eating a slug or snail, the disease cannot be ruled out on this basis.
Clinical signs
One of the key features of this condition is that clinical signs can be extremely varied, and it is difficult to either definitively determine that a dog has the condition, or to conclusively rule it out, based on clinical signs alone. A. vasorum may, therefore, need to be considered for a wide variety of clinical presentations.
Typically three syndromes have been recognised, which can occur in isolation, or in combination, in any one patient.
Cardiorespiratory
Cardiorespiratory signs are the most common manifestation of angiostrongylosis, classically being caused by the migration of first stage larvae through the lung tissue. Signs may include coughing, exercise intolerance, gagging, and respiratory distress. It is important to realise, however, that dogs with angiostrongylosis do not always present with respiratory signs – one study demonstrating that 68.1 per cent of affected dogs presented with a cough.5
Bleeding disorders
Bleeding disorders represent a second possible manifestation of A. vasorum infection, with several studies citing that approximately 15 per cent of infected dogs may present with this symptom – either alone or in combination with other signs.5-6
The bleeding abnormality may present in a variety of ways, including internal bleeding, bleeding from the nose, bleeds into the eye, coughing up blood and postoperative haemorrhage, among others. The exact mechanism by which the parasite is able to disrupt the dog’s normal clotting ability is not yet fully understood. Angiostrongylosis should be considered a possible diagnosis in any dog presenting with an unexplained bleeding disorder.
Neurological signs
Neurological signs have also bee
n reported in a small proportion of dogs infected with A. vasorum. Signs may include paralysis, ataxia, circling, behavioural changes and seizures.
Other non-specific signs, including inappetance, weight loss, depression, lethargy, and gastrointestinal signs such as vomiting and diarrhoea, have also been reported.
It is also worth noting that a small number of dogs with A. vasorum may demonstrate no signs at all – this figure being around seven per cent in one study.5 These animals represent a potential risk to the dog population if they are shedding larvae into the environment undetected; however, the true significance of this has not been determined
Diagnosis
Diagnosis usually involves analysis of the dog’s faeces, using the modified Baermann technique, where demonstrating the presence of first stage (LI) larvae of A. vasorum in the faeces confirms the disease.
Ideally, faecal samples should be collected over three consecutive days and analysed separately to increase the likelihood of finding larvae, because the larval output can vary each day and only examining a single sample may result in larvae being missed.7 A definitive diagnosis of the condition can still be difficult, so presumptive diagnosis and treatment may be necessary in some cases.
A blood test to diagnose A. vasorum is not currently available at the time of writing, but such tests are likely to be available in the near future.
Treatment
As angiostrongylosis is potentially life threatening, it is vital to administer effective anthelmintic treatment to all cases.
A spot-on containing a combination of imidacloprid and moxidectin is the only product licensed as both a treatment and preventive for A. vasorum. The treatment regimen consists of administration of a single spot-on dose, followed by a repeat examination 30 days later, because some animals may require a second treatment.
Milbemycin oxime/praziquantel tablets are licensed for ‘reduction of the level of infection’ of A. vasorum, but require a specific treatment schedule to be adopted (use of milbemycin oxime once a week for four weeks).
Alongside the anthelmintic treatment, supportive treatment may also be needed depending on the animal’s initial presenting signs.
Prevention
In addition to being an effective treatment for A. vasorum, the spot-on containing imidacloprid/moxidectin is the only licensed preventive medication for this parasite, when used as a spot-on on a monthly basis.
While it is impossible to stop a dog ever eating an infected mollusc, if a dog on regular monthly imidacloprid/moxidectin should ingest an infected slug or snail, the product has been shown to be highly effective at killing the parasite at the L4 larval stage. If killed at this larval stage, the parasite is prevented from completing its life cycle and, therefore, from reaching the heart of the dog, such that patent infection becomes established.
Outcome
Outcome partly depends on the severity of the initial presenting signs – the condition can be fatal in severe cases. However, in many instances, where appropriate therapy is initiated early enough, dogs will go on to make a full recovery.
It is worth considering that, even after successful treatment, re-infection is possible, because the dog will be returning home to the same local environment containing infected intermediate hosts (Figure 3). Ongoing preventive measures should, therefore, be considered.
Figure 3: Even after successful treatment, re-infection is possible [Image courtesy of Bayer Animal Health)
Conclusion
A. vasorum is an important emerging infectious disease that can be fatal to dogs. Owing to the wide array of presenting signs, a high level of awareness of the disease plays a vital role in its detection.
As professionals, both vets and nurses have a responsibility to educate the pet- owning public about the risks of this disease, and to consider implementing preventive measures, where appropriate.
While A. vasorum can prove fatal, with prompt recognition and use of effective treatments, the prognosis for complete recovery is very good. 3
Author
Lucy Smith BVSC MRCVS
Lucy graduated from Liverpool University in 2005, and spent five years working as a small animal vet in a variety of practices in West Yorkshire. In 2010, she joined the technical team at Bayer Animal Health, where she works as a field-based veterinary adviser.
To cite this article use either
DOI 10.1111/j.2045-0648.2012.00164.x or Veterinary Nursing Journal Vol 27 pp 138-140
References
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• VOL 27 • April 2012 • Veterinary Nursing Journal