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This most recent article was sent out in May, 2001, by Dr. David Anderson from Ohio State University.
"This article is from Dr. Cliff Monahan, our parasitologist researching camelids. He says some thought provoking things! (Cliff Monahan, DVM, PhD; Dept. Veterinary Preventive Medicine; Ohio State University College of Veterinary Medicine)".

"Parelaphostrongylus tenuis is a very real concern in areas of the east where white-tail deer are prevalent. My talk will focus on the biology of the parasite, the epidemiology of the disease seen in camelids within the Ohio River Valley, and control programs that may be more relevant than the monthly treatments presently employed. These monthly treatments as a preventive program have led to drug resistance in the nematodes normally infecting several susceptible species. This highlights the need to have alternatives to the intensive anthelmintic prevention program used today.

Life Cycle: Parelaphostrongylus tenuis utilizes the white-tail deer as its definitive host and has an indirect life cycle, meaning there is an obligatory developmental stage in snails or slugs. The disease CANNOT be passed without the ingestion of an infected snail or slug. First-stage larvae are passed in the feces of P. tenuis infected WTD and these must be ingested by gastropods for development into the infective 3rd stage larvae. Once ingested by snails or slugs, the 1st stage larvae require warmth to develop, thus the rate of development will depend on the ambient temperature. Continued cold weather slows development. Next the snail or slug must be ingested by a susceptible WTD, or in the case of aberrant infections, ingested by a susceptible sympatric ruminant. Without ingestion of the gastropod carrying infective 3rd stage larvae, the infection is not transmitted. Many snails and slugs are consumed inadvertently by grazing or browsing animals, but only during parts of the year conducive to snails and slugs. Gastropods will be less active during cold weather, will hibernate during freezing weather, and will estavate during hot, dry weather.

Clinical cases of meningeal worm affecting camelids have followed a distinct pattern of disease here at the Ohio State University Veterinary Teaching Hospital -- This pattern may not exist in your area!!!! Two major peaks of disease are seen by Dave Anderson here at Ohio State, the major peak being Sept/Oct, the second during Jan/Feb. This implies that there are 2 peak seasons of transmission to correspond with the peaks of disease. During studies conducted at the Wilds in southeastern Ohio, we found that there were no snails or slugs present during freezing temperatures, and the numbers of gastropods increased in the spring as weather warmed, but it remained moist and relatively cool. The heat and dryness of mid summer drove the snails and slugs into estavation, then they reappeared when temperatures cooled again in late summer, early autumn.

Based on the 2 peaks of disease here at Ohio State, the necessity of gastropods for transmission of P. tenuis and the 2 peaks of gastropods on pasture, we speculate that the most important times for meningeal worm prophylaxis are these 2 times when gastropods are most prevalent.

This is relevant here in the Ohio River Valley. YOUR area may have some variation on the prevalence of gastropods, thus you MUST adapt these findings to your own area. Further north there may be shorter periods, and further south longer periods.

These recommendations are made in light of the relative risks of P. tenuis transmission and the very likely risk of developing drug resistant llama and alpaca parasites secondary to overuse of ivermectin or other macrocyclic lactones.

Please pay attention to what I am really saying:

1) Overuse of the avermectins (as I see regularly within the camelid industry because of meningeal worm prevention programs) is destined to create more problems than P. tenuis. If these practices create drug resistant camelid parasites by the monthly use of ivermectin, these parasites will be resistant to doramectin and moxidectin as well. The industry will be better served to avoid this eventuality.

2) Due to the seasonality of snails and slugs in our area, I recommend that camelid owners consider using drugs for prophylaxis during the peak risk timepoints, and I recommend that they do not use ivermectin year round for this purpose. This program does not give you 100% protection, but I can tell you all that the creation of drug resistant camelid parasites will be much more a problem.

3) My theoretical position is that camelid owners can use a long-acting macrocyclic lactone, such as doramectin, and this will reduce the number of treatments needed for protection. By reducing the overall number of treatments, you will delay or avoid the development of drug resistance.

IF YOU USE THESE LONG-ACTING DRUGS IN THE SAME FASHION THAT YOU USE IVERMECTIN, YOU DEFEAT THE PURPOSE AND ENHANCE THE DEVELOPMENT OF DRUG RESISTANCE!!!

4) Theoretically, you could inject an animal on May 1st and this would kill any migrating larvae on-board since April 1st. As our preliminary research shows us that there are not many infected snails or slugs present in April, this risk seems reasonable. The long-acting effect of doramectin in cattle kill any infective larvae ingested on pasture for approximately 28 days. I am speculating that the same level of protection will be provided to camelids, thus you do not need to treat again for almost 60 days.

Why 60 days? Theoretically, no ingested larvae will survive while the long-acting drugs are at these levels. After the 28 days of protection have elapsed, you should have ~ 30 day period when any ingested larvae are migrating and susceptible to treatments. I admit this must be verified with experiments in camelids, but this is a rational expectation. So, your injection on May 1st is good until May 29th based on residual activity, then you may add another 30 days before you need to treat again. This means 1 treatment every 60 days instead of every 21 days that I hear regularly."