Preventing meningitis caused by parasites

[AlohaSteven]

Good news: two very experienced researchers in Australia have commented in some detail on the A. cantonensis outbreak in Puna.

Dr. Paul Prociv and Dr. Melissa Carlisle have been in touch with me via email and telephone. They have watched the webcast of Puna’s meeting on rat lungworm (though the sound was not completely audible for them) and looked through this Punaweb strand on the topic. Dr. Prociv is a physician and medical researcher who has decades of experience dealing directly with human incidence of “Angio” disease, including conducting autopsies and experiments with rats, slugs, and snails. Dr. Carlisle is a veterinarian and veterinary science researcher; she originally identified the rat lungworm in cockatoos and tawny frogmouth birds.

They write [shared with their permission]:

What you’re experiencing in Hawaii with the Angio “epidemic” is what we’ve been thru here before, several times. Angio is extremely common here in QLD (the state of Queensland), and over the years we both have been intimately involved with it from clinical (vet and human) and epidemiological angles.

I have just viewed those websites you suggested and am familiar with the concerns and responses of individuals. We’d be very happy to answer any specific questions you’d care to pass on. There are several points worth noting:

The parasite is very common in most endemic areas, e.g., where we lived in inner suburban Brisbane I used to regularly trap rats – R. rattus - in our backyard and consistently found 80% to be infected, some very heavily. Yet, they seemed to suffer no outer ill effects from their large parasite burdens (in contrast to lab rats, R. norvegicus, the white Wistars we often used in experiments). The lab rats died sometimes from as few as 30 Angio larvae. Wild rats stay infected for life, which is often as long as several years, unlike molluscs (slugs and snails) that have relatively short lives.

The third larval stage of the rat lungworm (L3) passed in rat faeces don’t survive long if they dry out or are exposed to sunlight (maybe a few minutes is enough to kill them), but enmeshed in a moist rat faecal pellet they can survive perhaps for weeks. Our slugs and snails just love rat-****; in the lab, I’d drop pellets into a box, and the snails would spend hours gobbling them up. While Achatina fulica (Giant African Snail) in Hawaii and SE Asia is implicated in many human cases, it is probably not important in the life-cycle; we suspect the smaller, short-lived slugs and snails are critical, as they’re what rats prefer to eat. Giant African snails are too big and tough to be a preferred food of rats, but rats love small tender slugs & snails and will snaffle them up immediately if available.

The surprising thing here is how few people acquire the infection (maybe 5-10 diagnosed cases a year, in a population of 4 million), although those who are stricken by Angio have spectacular and sometimes tragic presentations, making great media fodder and scaring the hell out of the public. Apart from the odd case of intentionally ingesting molluscs, most cases seem to involve eating salad vegetables almost certainly containing a small slug or snail; the story about mucus carrying the L3 is highly suspect and dubious. However, a person need ingest perhaps only 1 or 2 L3s to get a clinical infection, as each of these larvae then makes its way into the Central Nervous System (CNS) where they move about for 1-2 weeks, growing in size hugely -from memory I think they’re about 0.5 mm long when entering the CNS and 1-2 cms long when leaving. The clinical presentation depends entirely upon the pathways taken by the burrowing larvae, which seems completely random (apart from being directed from the arterial to the venous system). Of course, should you eat lots of L3s then you’re in big trouble. We were involved in a case about 11 years ago where an infant ingested thousands of L3, maybe all from one slug/snail; our infant here died after being in a coma for a month.

How to avoid it? That’s the big question – but keep it in perspective: you have a much greater chance of dying in a traffic accident on the roadway! Trying to completely wipe out all snails and slugs is a waste of time and money, as well as potentially damaging the local ecology in a big way (e.g. copper salts are highly toxic to plant and animal life, and can poison humans).

Rats are not directly involved--in the sense you can’t catch it from them--but of course they’re critical to maintaining the parasite in the environment and being relatively long-lived and pooping out thousands of L3s daily, so rats should be the target of control. Molluscs are ubiquitous, short-lived, usually lightly infected individually, and important in the local ecology so attacking them while ignoring the rats is not as useful, even tho the slugs and snails are who we directly get Angio from.

Other animals also get Angio, especially mammals. We’ve seen lots of cases in dogs, who eat lots of slugs n snails (probably accidentally, from food bowls left out overnight); fruit bats and horses are well documented, and no doubt probably most mammals which are exposed will take up L3s, meaning the parasite is not fussy about whom to invade (which makes sense evolutionarily), although as far as we know the L3s will develop into fertile worms only in rats (they get pretty close in people, too). Birds are probably extremely rarely infected; a PhD project in my old dept. (University of Qld) from the 1970s says chickens are refractory (altho I can’t vouch for the reliability of that limited study), and our case reports of infection in a few native birds just show how rare the infection is in them; this could reflect on the relative size of the avian CNS and hence its limited blood supply.

Anyway, there’s lots more to this, but I’ll leave it there, and await further questions. But I do sympathize with the comment about “the price you pay for living in paradise”. There’s nothing that public health authorities can do about this, apart from educating the public (responsible media would help, but is probably too much to ask for). It also hurts that clinicians can’t do much once a case is established, for there is no effective specific treatment; in fact, killing the worm during its CNS migratory phase would be counter-productive, as live ones eventually crawl out of the brain, while dead ones decay and trigger massive immune/inflammatory reactions there, which do the host-killing. And while clinicians always are tempted to administer corticosteroids in such cases, the evidence now is pretty strongly against this practice (in humans anyway; dogs seem to benefit!).

We hope this helps you a bit, but would be happy to discuss it further.

All the best,
Paul & Melissa Prociv-Carlisle


In conversation with Dr. Prociv he noted wild rats can bear a huge burden of rat lungworm without any apparent ill effect on their overall health, activity level, or how robustly they attempt to evade predation. In his opinion if there are sick-acting rats being sighted in Puna then look for sources to explain it other than the rat lungworms, as they just do not make wild rats sick.

In his backyard where he found eighty percent (80%) of rats to be carrying the rat lungworm, he also found only five percent (5%) of the slugs and snails to be infected. Since humans do not catch the disease from rats, only from the intermediate hosts (such as slugs and snails) or --more rarely-- from contaminated water where slugs or snails have died and shed L3 into the water, if this 80%:5% ratio of rat:mollusc infection holds, then reducing the rat population could significantly reduce incidence in slugs and snails ...and therefore reduce the potential hazard to human and animal health.

He stressed the tiny little slugs and snails, 1mm to 2mm long, are indeed those to be most concerned about eliminating. It is unlikely anyone will accidentally eat a large slug or snail, but even tiny little slugs and snails can carry many L3 stage A. cantonensis. Salt brine will indeed cause most tiny slugs and snails to drop off of a leafy surface if immersed in such a solution; but this should be tested with local species to determine actual efficacy in practice.

Produce at stores (and, therefore, subsequently in restaurants) has been demonstrated to contain tiny slugs and snails. Just because lettuce or whatever is purchased at a store or restaurant means nothing in regard to its potential for harboring small slugs and snails.

Most L3 consumed by a human will probably not survive the mastication and stomach acids, but even one surviving L3 in a human can cause distinctly problematic symptoms (headache, double vision, tingling and “pins & needles” sensations). Several L3 can cause devastating neurological damage. The L3 stage are only a few hundred microns long when they enter tissues but within 2 weeks they can attain lengths of 2 c.m. Hundreds to thousands (and it is possible to ingest that many, unknowingly, from a small number of very tiny slugs or snails if they are heavily infected) are enough to kill a human. The infant Dr. Prociv mentioned, on autopsy, was found to have approximately 2,000 L4 to L5, two-cm long rat lungworm larvae, embedded in the spinal cord and brain. The infant had been observed grubbing around in the dirt of a potted plant alongside an outdoor swimming pool, putting his hands in his mouth, the same number of days before he died as is consistent with the subsequent growth of the worms as judged by their development at autopsy. Undoubtedly the tot ingested one or several small but heavily infected slugs from that source.

The beginning of a wet spell in the weather is often followed by outbreaks of infection in pets and farm animals, as slugs and snails come out of hiding and are ingested. Dogs developing CNS disease in Queensland near the beginning of each rainy season are subsequently brought in to vets, this owing to slugs and snails on dogfood and drowning in water dishes. Horses can also have severe problems due to rat lungworm infection.

Fruit bats were found to be infected in Australia, which seemed odd, but then it was observed that the bats sometimes descend to the ground and flop around there eating fallen figs. Also, one species of local slug is a tree-climber and probably being ingested with fruit up in the trees by the bats.

L3 become very active as soon as a slug or snail dies. The L3 fight free of the dead slug or snail and remain active in water seeking a viable host for several weeks. Water containing L3, if a human drinks it, can infect. Trees (such as slug-attracting papaya) overhanging roofs should probably be trimmed back to prevent slugs and snails from dropping off the trees onto the roof and washing into water catchment tanks. Filter plates are recommended, as is filtration between the tank and the faucet.

Dr.s Prociv and Carlisle consider it unlikely coqui frogs are becoming infected in leaf axle pools by L3 liberated from the tissues of drowned molluscs. The L3 are seeking a mammalian host and would probably not zero in on the amphibians, is their guess. Chickens seem to be naturally resistant, but this might also be a function of the relatively small blood flow to the brain in chickens versus humans. The L3 are pumped passively in the blood until they are caught in tissues and begin burrowing. While 20% of the human cardiac output supplies the brain only a much smaller volume supplies avian brains. There may also be a different sort of blood-brain barrier in chickens; more research is needed but the experiment should be relatively inexpensive to conduct and conclusive if positive on autopsy for A. cantonensis.

In unpublished research, water about half as salty as seawater has been found to paralyze the L3. Paralyzed L3 removed from half-seawater-strength saline solution regain their motility and can become infective again in freshwater. It is highly unlikely A. cantonensis larvae of any stage can survive in seawater or in marine molluscs such as oysters.

Dr.s Prociv and Carlisle generously offered to make themselves available to answer further questions.

The big bottom-line take home points I find from this informative communication are to really focus eradication efforts on the rats and to take the issue seriously but not panic over it either. Running a very tight and tidy growing operation employing raised beds and using reasonable and environmentally sane deterrents has been demonstrated to lower slug and snail presence in produce grown in such controlled and sanitary conditions.



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A pleasant slideshow: http://www.thejoymovie.com

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