A study examining the role of wildlife species as sentinels for rat lungworm suggests an expanded distribution of the parasite, and the need for pet owners and wildlife carers to take precautions in order to minimise transmission of the disease to animals in care.
Rat lungworm, or Angiostrongylus cantonensis, was the most common cause of neurological disease in tawny frogmouths (Podargus strigoides) in the Sydney region, and is an emerging disease in brushtail possums (Trichosurus vulpecula).
Previous histological surveys, also conducted in Sydney and published in 1993 and 1995, failed to detect evidence of the parasite in either species.
Rat lungworm is known to cause neurological disease in a range of species including dogs, wallabies, foxes and birds, and is the most common cause of eosinophilic meningitis in humans. Adult worms dwell in the pulmonary arteries and right ventricle of rats. Larvae are shed in rat faeces where they are ingested by intermediate hosts – most notably slugs and snails. They moult twice within the intermediate hosts to the L3 stage which are infectious to rats and other hosts.
Once ingested, the L3 larvae penetrate the intestinal wall of the host, spreading haematogenously and via the lymphatic system to the brain and spinal cord. Here the combination of migration and associated inflammation causes neurological disease, often characterised by marked pain in people.
“The parasite originated in south eastern China and has been expanding its range worldwide and within invaded countries since World War II,” Sydney University veterinary pathologist and study co-author Derek Spielman said.
“As it originated from a tropical to semi-tropical environment and first established in Australia in semi-tropical south eastern Queensland, climate change may be facilitating its range increase.”
Because the parasite does not rely on the establishment of exotic molluscs for success, it is able to infect a wide range of intermediate hosts.
Key clinical signs of rat lungworm in tawny frogmouths include severe paresis or paralysis of the legs followed by progressive weakness of the wings and subdued mentation.
“Frogmouths that are rescued early in the disease show weak legs so can’t land or stand and can be picked up by members of the public early,” Spielman said. “These birds tend to have normal mentation (they threaten with wide gapes and vocalisations), good condition (good fat deposits and musculature) and may flap weakly but generally can’t fly and can’t right themselves if placed on their backs. Placing your finger in their claws does not illicit clenching which it normally would.”
Severely affected birds may be moribund on presentation.
Affected brushtail possums may exhibit hindlimb paresis or paralysis without any indication or history of trauma.
“Other neurological signs are not as indicative as the signs seen in tawny frogmouths as brushtail possums suffer from a number of neurological syndromes that are yet to be resolved aetiologically such as wobbly possum syndrome,” Spielman said.
According to Spielman, the current findings have implications for public health, pet owners and anyone involved in the care of native wildlife.
“Parents should be alert to the possibility of young children finding and eating snails and slugs (or frogs) in the garden or even pet food left in bowls,” he said. “About two children per year in Sydney are affected [by rat lungworm] and occasionally this results in significant brain damage or death.”
Fruit and vegetables from gardens frequented by snails and slugs should be washed thoroughly before consumption. Spielman said this is particularly important with back garden and organically grown produce where molluscicides are not used.
Dog and cat food bowls should not be left out where they can be exposed to snails or slugs.
“Eating slugs in food bowls is a common way puppies are infected,” Spielman said.
“Carers must be careful not to feed contaminated fruit or vegetables to possums or any other animal in care which means washing all produce thoroughly and not leaving food in bowls overnight outside when snails and slugs are active,” he said.
While this is contrary to the natural nocturnal feeding behaviour of these species, it is an important means of preventing disease transmission.
“Perhaps removing the food bowls before you go to sleep will give them time to eat before the food is removed.
Symptomatic treatment is often successful in people and dogs where the disease is picked up early, but the prognosis for affected wildlife species is grave.
“Wildlife invariably are severely affected with permanent neurological damage by the time they come in to care,” Spielman said. “This means even if they don’t die they will not be able to survive in the wild so are euthanised.”
Control of definitive hosts (the introduced black rat, Rattus rattus, and the Norway rat, Rattus norwegicus) reduces the risks of angiostrongylosis.
The research team plans to expand their study to investigating the prevalence of rat lungworm in wild rats, slugs and snails. In the near future, they will be calling veterinarians and members of the public to submit snails and slugs to determine whether the parasite is present.
Ma G, Dennis M, Rose K, Spratt D and Spielman D (2013) Tawny frogmouths and brushtail possums as sentinels for Angiostrongylus cantonensis, the rat lungworm. Veterinary Parasitology 192:158-165.