Our climate is changing and regardless of whether this change is due to a lack of wind turbines and electric cars, or simply an unavoidable natural event, most of us now accept that our warmer and wetter climate is here to stay. For the small animal veterinary surgeon this may not seem to present much change to the many clinical decisions that are made every day. However, from a parasitological point of view, a warmer and wetter climate will increase survival rates for some of the parasites affecting our companion animals and this will need to be considered when advising and formulating parasite strategies for our clients.
In 2018, ESCCAP UK and Ireland identified fleas, ticks (Ixodes spp.) and the canine lungworm (Angiostrongylus vasorum) as parasites that are likely to benefit from the milder winters and warmer summers (Wright, 2018) and therefore the risk of disease caused by these parasites should be reviewed. This short article will provide an overview of canine lungworm in the UK.
Since its discovery in France over 100 years ago, A. vasorum, also known as French heartworm, has been the focus of many headlines in the veterinary literature and press, and all for a good reason. The parasite is capable of causing severe disease in dogs, and more often than not, the pathology primarily affects the lung tissue. For this reason, A. vasorum is now known as canine lungworm in the UK.
Although other canine lungworm species exist, A. vasorum is more pathogenic than other lungworm species affecting the pulmonary system of dogs, such as Crenosoma vulpis and Oslerus osleri. The parasite, a metastrongyloid nematode, is also impossible to eradicate as it has an indirect life cycle in which slugs and snails act as intermediate hosts. Further to this, a large reservoir in the wild is provided by the red fox (Vulpes vulpes) and other wild canids acting as definitive hosts. Paratenic hosts include the common frog (Rana temporaria) and other more surprising hosts have been identified, such as captive red pandas in the UK (Patterson-Kane et al., 2009) and Denmark, and wild otters in Denmark.
The parasite’s life cycle begins with eggs hatching in the pulmonary blood vessels releasing L1 larvae, which are then carried by the blood vessels to the lung capillaries where they penetrate the alveolar and bronchial walls. The L1 larvae then migrate to the oropharynx where they are swallowed and excreted in the faeces. The further development from L1 to the infective L3 larval stage is facilitated by the intermediate host.
The L3 larvae are now ready to infect the canid, either directly by ingestion of the larvae from the environment or indirectly by ingestion of an intermediate or paratenic host. After ingestion by the definitive host, the parasite embarks on a migration from the gut wall to the abdominal lymph nodes, where it enters the portal circulation. It then migrates through the liver tissue on its way to the right ventricle and pulmonary arteries, where the final maturation and reproduction takes place. Once the eggs are hatched, L1 larvae are released and the life cycle is complete. The pre-patent period varies from 4 to 15 weeks.
A. vasorum has a near worldwide distribution, including Europe, Africa and North and South America. In Europe, A. vasorum is considered endemic within certain countries, including the UK, Ireland, France, Spain, the Netherlands, Germany and Italy, as well as countries with colder winters such as Denmark, Sweden and Poland. In some countries, “endemic hotspots” have been identified from which the parasite may have spread to other regions, possibly aided by the travelling of infected dogs to previously unaffected areas.
In the UK, since the first report in 1975 in a dog imported from Ireland, A. vasorum has spread and endemic hotspots are now identified in the south east and south west of England as well as in Wales. Comparative studies of the extent of infection in foxes in the UK, conducted eight years apart in 2005/06 and 2013/14, have shown a significant increase in prevalence, as well as a significantly higher prevalence in all regions except the south (Taylor et al., 2015).
Although an effect of short-term weather patterns and differences in mean temperatures on the results of this study cannot be completely ruled out, the survey clearly shows that A. vasorum is now present in all areas of the UK, including the north of England, Scotland and Northern Ireland, and as such the risk of infection is now higher than previously thought.
Disease caused by A. vasorum can occur in dogs of any age and studies have shown that dogs under two years old are more at risk. It has been reported that clinical signs are more likely to be seen in the winter and spring – possibly a reflection of an increased risk of infection in late summer when the intermediate hosts are more prevalent.
Clinical disease is due to the presence of adult worms and eggs in the pulmonary vessels and the larval migration through the lungs in particular. It is important to remember that mild infection with A. vasorum may be asymptomatic, whereas more severe infections can result in a wide range of symptoms reflecting pulmonary, cardiac, coagulative and occasionally neurological disorders due to bleeding in or around the central nervous system.
Not surprisingly, the list of clinical signs and symptoms is long and may include a cough (may be productive), dyspnoea, tachypnoea, exercise intolerance, anaemia, haematomas, melaena, heart murmurs, tachycardia, congestive heart failure and neurological defects, to name a few. No clinical sign is pathognomonic of angiostrongylosis.
Baermann’s test is a quick method of detection which can be performed at low cost in-house by trained staff. A newer in-house test is now available for the detection of circulating A. vasorum antigen using serum or plasma. A faecal smear test is less reliable. Negative results do not rule out angiostrongylosis as false negative results are possible. If clinical suspicion remains, the parasite should not be ruled out on the basis of a negative test and further investigation should be considered.
Considering the available research, it is clear that the risk of dogs being infected with A. vasorum is variable across the UK, but the risk is present, to some extent, in all areas.
To prevent infection, avoiding exposure to the intermediate hosts should be encouraged. Feeding outside should be avoided and walking dogs on the lead in the mornings and evenings as well as in damp weather, when the intermediate hosts are most likely to be encountered, may be a reasonable option for some owners. However, risk cannot be eliminated entirely by such practice.
Licensed preventative treatments with anthelmintics of the macrocyclic lactone group are today available in formulations containing moxidectin or milbemycin oxime (but note that due to P-glycoprotein interaction, products containing macrocyclic lactones should not be administered concurrently in dogs).
When these treatments are administered on a monthly basis, they will prevent established infections and the destructive migration of L1 larvae through the pulmonary tissues. Importantly, this treatment also prevents further spread of the parasite in the environment – the same environment that, due to global warming, seems to be changing and becoming more accommodating to this little, but potentially fatal, nematode.