Abstract

The efficacy and safety of avermectin B1 sa against the adult hookworm of young Northern fur seals was examined after subcutaneous administration at dose ranges of 100 ug/kg body weight and 200 ug/kg body weight. Percent clearance, determined by comparing worms shed to those remaining at postmortem examination, was 100% at either dose range. Introduction

Introduction

Within the last few years, a new family of antiparasitic compounds, the avermectins, have been derived from a newly characterized species of Actinomycete, Streptomyces avermitilis. Ivermectin (Merck, Sharpe, & Dohme Research Lab, Rahway, NJ), a synthetic derivative of avermectin 6, has been shown effective against a number of nematode and arthropod parasites in both a wild and domestic animals (1,2,3,4,5,6). Safety has been assured due to Ivermectin's unique mode of action. Nematodes, in particular, are paralyzed and eliminated as a result of action on inhibitory neurons between the parasite's ventral nerve cord and motor neurons. The effect of the neurotransmitter gamma amino benzoic acid (GABA) is potentiated at the synapse by stimulating pre-synaptic of GABA and enhancing its binding to post-synaptic receptors. The result is induction of the resting stage within the motor neuron caused by an influx of chloride ion. This occurs even as sodium should be causing stimulation. This non­recognition of signals by the receptor cells results in paralysis and death for the parasite and its elimination. Mammalian systems are unaffected, as GABA is restricted to the central nervous systems in mammals.

This investigation was concerned with evaluating the use of Ivermectin clinically to eluninate the adult hookworm (Uncinaria lucasi) from Northern fur seal pups (Callorhinus ursinus) on St. Paul Island, Alaska.

Fur seal pups acquire infective third stage larvae from the dam's milk during the first day of life on the rookery (7). In about two weeks, the larvae have matured to adults and can cause severe intestinal hemorrhage and associated blood-loss anemia. Keyes has estimated a 12% death rate due to uncinariosis as well as citing, it as a contributing factor in a like number of deaths (8). Hookworms are eliminated in pups that approach the third month of age. Although the exact mechanism of this elimination is unclear, it is probably immunologic.

Hookworm eggs are passed with the pup's stool and hatch in late August or September. Free-living larvae infect seals returning to the rookery the following year, via percutaneous migration or ingestion, with largest concentrations occurring in subcutaneous blubber and mammary glands, thus completing the cycle (9).

Materials and Methods

Ten Northern fur seal pups were collected in mid-July, 1983, from Northeast Point Rookery. Previous investigators compiled a virtual 100% incidence of infection among pups at that rookery.

All pups were weighed and placed in individual labeled cages with a bottom of heavy brown paper from which all excreta could be collected. Pups were assigned to one of four groups. After 24 hours for acclimatization, all pups were given medication according to group assignments.

Group I animals were given Ivermectin at a dosage of 100 ug/kg body weight.

Group 2 animals were given Ivermectin at 200 ug/kg body weight. Group 3 animals received vehicle only (60% propylene glycol, 40% glycerol formal). Group 4 received no medication.

All animals received a total dosage of 1.0 ml, the volume differences made up by addition of appropriate amounts of vehicle. All injections were administered subcutaneously in the axillary region after first scrubbing the area with a povidine iodine scrub and application of povidine iodine solution. These precautions were taken because of the reported incidence of clostridial infections after injections of Ivermectin in other species. Post-injection, all pups were observed for any adverse reactions.

Fecal material was collected from paper on cage bottoms every 12 hours for the duration of the trial. Feces were preserved in 10% formalin and were later washed through a #60 mesh seive to recover hookworms, which were then observed with a dissecting microscope and counted. 72-96 hours post-injection, each pup was sacrificed by intracardial injection of sodium pentobarbitol. Each was examined for presence of concurrent disease and all were found to be grossly in good health.

Entire intestinal tracts were removed from the pups. Each was opened and its contents washed into large bottles. Contents of bottles were strained through a #60 mesh sieve. The resultant debris was formalized and placed in petri dishes for observation under the dissecting microscope. Hookworms remaining in the bowel at necropsy were thus counted and recorded.

Percent removal of hookworms for each pup was calculated by dividing the number of hookworms removed post-injection by the total number of hookworms (shed and remaining at necropsy), and expressing that number as a percentage (10,11):

Hookworms removed x 100 
Hookworms shed & hookworms remaining

Results

Total number of hookworms ranged from 0-432. Data from each of the groups are presented in the following tables:

No adverse reaction to the injection was noted, although one pup exhibited mild pruritis at the injection site for approximately 5 minutes post-injection. Throughout the trial, animals exhibited no abnormal behavior. Two animals, which showed hematochizia prior to injection, had normal stool within 24 hours of injection.

Discussion

Ivermectin, at concentrations of either 100 ug/kg body weight or 200 ug/kg body weight, appears to be highly effective in elimination of adult hookworms from Northern fur seal pups when administered subcutaneously. In addition, no adverse reactions were noted. Empirically, ectoparasites of the seals were also eliminated, although this reaction was not quantitated.

Two pups in the trial were found to harbor no adult hookworms. This was unexpected, due to the previously mentioned 100% incidence of hookworms at NEP rookery. However, a number of other pups from NEP, also examined during the same period, showed no hookworms.

Control animals showed some elimination of larvae, probably due to immunologic reasons. Lack of infection of two animals may have been due to pre-patent infections, however, the 9 kg weight of one animal suggested that the animal was old enough to harbor adul ts. Failure to nurse might be another reason, however, the animal would not be expected to survive for the duration of the experiment had they not initially nursed.

Acknowledgements

The author wishes to thank Mr. J. Scordino and Ms. M. Tomita for assistance in securing animals, Dr. Lee Seward and the Merck Company for supplying products and vehicle, and to Ms. R. Silva for typing the manuscript.

References

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