Of all the parasites of dogs, Angiostrongylus vasorum has perhaps received the greatest publicity and media attention in the UK through advertising campaigns. This publicity has been driven largely by pharmaceutical companies as increasing numbers of products have been licensed for its prevention. As a result, veterinary professionals have reasonably asked how great a threat A. vasorum poses and to what extent it has been framed as “the bogeyworm”.
The raised awareness of this parasite among pet owners and veterinary professionals has been beneficial in reducing canine morbidity and mortality associated with infection where the parasite is endemic. This is especially true in parts of the country where it has previously been absent and awareness has historically been lower.
To establish which dogs require routine preventative treatment, the geographical spread of A. vasorum across the country and the lifestyle factors that increase the risk of exposure need to be considered.
What is Angiostrongylus vasorum and how is it transmitted?
Angiostrongylus vasorum is a metastrongyloid nematode that lives in the right side of the heart and pulmonary arteries. It is commonly referred to as “lungworm” because the most common signs seen with infection are respiratory, associated with larval invasion of the lungs.
The life cycle of A. vasorum is indirect. First-stage larvae (L1) pass out in the faeces of infected canids and require gastropod molluscs (slugs and snails) as intermediate hosts for further development. Infection occurs in canids when infective third-stage larvae (L3) are ingested. This occurs most commonly through deliberate or accidental consumption of infected slugs or snails; however, infection can also occur through the consumption of paratenic hosts such as frogs and other amphibians (Morgan et al., 2005). Under experimental conditions, infection has also been demonstrated to occur from the ingestion of larvae present in slime trails, but the significance of this in natural transmission is unclear (Conboy et al., 2015; Robbins et al., 2021).
Clinical signs
Although A. vasorum is technically a heartworm, the most common clinical signs and pathological changes in dogs are respiratory in nature. Another significant consequence of infection with A. vasorum that can occur is coagulopathy (Morgan et al., 2005). The mechanism by which this occurs is still not fully understood but can lead to potentially life-threatening complications, including anaemia secondary to blood loss, haematomas, neuropathies and prolonged bleeding associated with surgical procedures and trauma. These potentially severe outcomes make routine preventative treatments vital for dogs at high risk of exposure.
Although A. vasorum is technically a heartworm, the most common clinical signs and pathological changes in dogs are respiratory in nature
Where is the parasite in the UK?
A. vasorum has spread rapidly over the past two decades from endemic foci in Wales and the south-west and south-east of England across the whole of the UK. Increased reporting of cases has been seen in domestic dogs, with 20 percent of practices across the country reporting at least one case over a 12-month period in a 2014 survey (Kirk et al., 2014). As awareness of the parasite grew during this period, questions were raised about whether this number represented the genuine spread or the increased testing and reporting.
Genuine expansion of the parasite and increased prevalence in its fox wildlife reservoir were demonstrated by post-mortem surveys carried out on foxes in 2005 (Morgan et al., 2008) and 2014 (Taylor et al., 2015). The overall prevalence rose from 7 to 18 percent in foxes during this period and extended to regions previously clear of infection, including northern England and Scotland. Although these studies showed the spread of A. vasorum to be genuine, it has not occurred in a uniform fashion, with some focal areas of very high prevalence and other areas remaining free of the parasite.
Although there have been no prevalence studies of A. vasorum in UK domestic dogs, case reporting sites, such as the one run by Elanco, have been useful in mapping areas of high prevalence and surveillance for the introduction of A. vasorum into new areas. The distribution of A. vasorum infection and prevalence in any given area, however, remains very fluid, with slugs in horticultural products and infected dogs moving infection around the country. Sometimes the arrival of infection in a new area will establish new foci, where on other occasions infection will fail to take hold.
Is routine preventative treatment required to keep dogs free of A. vasorum infection?
The desire to avoid the use of anthelmintics in dogs where possible has led to control strategies other than routine preventative treatment being considered. Intermediate host avoidance strategies (such as keeping dog toys indoors, regularly cleaning outdoor water dishes and not walking dogs after heavy or prolonged rainfall) are likely to reduce but not eliminate the risk of exposure. They are, therefore, useful as an adjunct to preventative treatment with anthelmintics, but the ubiquitous nature of slug populations means they should not be used as a replacement.
Picking up and responsibly disposing of dog faeces is an important component of control for a range of parasites and has some use for A. vasorum control. This is because some species of slug are coprophagic and, ironically, may be accidentally ingested by dogs exhibiting coprophagic behaviour. The large fox reservoir of infection, however, means that the removal of dog faeces is unlikely to significantly reduce slug exposure to the parasite over time.
Monthly use of a licensed (monthly) moxidectin or milbemycin oxime product is […] still necessary to minimise the risk of disease in dogs at high risk of exposure
Monthly use of a licensed (monthly) moxidectin or milbemycin oxime product is, therefore, still necessary to minimise the risk of disease in dogs at high risk of exposure.
How do you assess dogs at high risk of exposure?
Dogs may be at high risk of infection with A. vasorum due to lifestyle factors and the presence of the parasite geographically. Lifestyle factors include the deliberate consumption of slugs and accidental ingestion through the consumption of grass and coprophagia. This can occur very easily as some slugs capable of harbouring the parasite are very small (Figure 1).
Some regions of the UK are known to be highly endemic for A. vasorum, including greater London where there is a very high prevalence in the local fox population. In areas known to be endemic foci for the parasite, monthly preventative treatment should be seen as routine, especially prior to surgery. However, in areas where endemic status is unknown or thought to be low, then testing dogs for the parasite is essential to establish if it is present locally.
The testing of dogs before surgery and of those with relevant clinical signs together with annual testing of young dogs will rapidly build up a picture of whether A. vasorum is present. Faecal testing by the Baermann apparatus (Figure 2) and blood antigen testing, such as the patient-side test Angiodetect, are useful for diagnostic screening. The latter is particularly useful as it is simple to run, has a sensitivity of approximately 85 percent and a specificity of close to 100 percent (Schnyder et al., 2014).
Faecal smears (Figure 3) are also beneficial for the initial screening of dogs with relevant clinical signs as although the sensitivity is low (54 to 61 percent), it will pick up approximately half of the positive cases very cheaply (Humm and Adamantos, 2010). Dogs with relevant clinical signs that have a negative faecal smear should always have follow-up Baermann or Angiodetect tests. Annual testing of dogs on preventative treatment is also beneficial to check that owner compliance is adequate and drug resistance is not developing.
What are the considerations when selecting a preventative product?
A range of tablets combining a macrocyclic lactone and an iso-oxazoline are now available, as well as milbemycin/praziquantel tablets and imidacloprid/moxidectin spot-on solutions. When choosing a product the following should be considered:
- Owner preference and compliance – if the owner prefers a tablet or spot-on preparation or feels more confident administrating one of them, then compliance and the chances of successful administration will be increased
- Previous/past drug reactions – whether reactions in an individual case are real, suspected or perceived, compliance will be reduced if the owner is concerned about possible adverse effects
- Does the pet frequently swim or is it frequently shampooed? – if this is the case, a tablet option may be preferable to a spot-on preparation containing moxidectin due to the increased chances of wash-off
- Other parasite prevention – depending on the parasite prevention requirements of the dog (eg ticks, tapeworms), a product can be chosen to help cover all of the potential parasites
Conclusion
A. vasorum is a parasite capable of causing severe disease in infected dogs, so preventative treatment needs to be considered for dogs at high risk of infection as part of an overall parasite control programme. Lifestyle and geographic risk factors need to be assessed to establish whether routine preventative treatment is required.
Knowing geographic and lifestyle factors will allow veterinary professionals to be able to discuss risk with owners so they can make an informed decision regarding preventative treatment
While testing dogs for the parasite is vital to establish whether it is present in a geographic area, the fluid nature of endemic foci must be remembered, so vets do not become complacent if they have not yet seen cases in their locale. Knowing geographic and lifestyle factors will allow veterinary professionals to be able to discuss risk with owners so they can make an informed decision regarding preventative treatment.