Persistence of Trichinella spiralis muscle larvae in natural decaying mice.

The influence of natural weather conditions on the viability and reproductive capability of Trichinella spiralis muscle larvae in mouse corpses exposed to summer and winter conditions in the Buenos Aires Province, Argentina, was studied. For this purpose, a total of 49 mouse corpses harbouring muscle larvae of T. spiralis were exposed for a period of 1, 2, 4 and 6 weeks in each of the seasons. Control corpses maintained at 8°C were also included. In summer, T. spiralis muscle larvae were recovered from corpses exposed up to 1 week only. The viability of these larvae was 54.2%, and the reproductive capability index in mice (RCI) was 13.1 and significantly lower than the control (p<0.0005). Morphologic deterioration and reduction in the glycogen content of cysts and larvae were observed at the second week of exposition. By week 4, larval stages of Dermestes maculatus were observed inside corpses, and 22 live muscle larvae of T. spiralis were obtained by artificial digestion of their bodies. In winter, T. spiralis muscle larvae were always recovered, the viability being almost 100% except for a significant reduction by week 6 of exposition (p<0.0001). For this season, the RCI were 50.5, 46.9, 59.7 and 45.2 for the periods of 1, 2, 4 and 6 weeks of exposition, respectively. The morphology of cysts and larvae did not show alterations, and no variations were observed as well in glycogen reserves during the 6-week period of exposition. RCI of non-exposed muscle larvae were always significantly higher that any of those recorded from muscle larvae that belonged to exposed corpses (p=0.0005). The present results demonstrate that muscle larvae of T. spiralis are able to survive in nature and keep infective for a 1-week period in summer and at least for 6 weeks in winter, becoming an important source of infection for scavengers. In summer, larvae stages of D. maculatus, and probably other insects, may play an important role in the survival and transmission of T. spiralis in the sylvatic cycle.

Comparative efficacy of trichlorphon and trichlorphon/ivermectin combination treatment against anthelmintic-resistant cattle nematodes in Argentina.

Twenty-four Holando Argentino male calves were treated orally with 50 mg/kg body weight trichlorphon (TCF); 0.2 mg/kg body weight subcutaneous ivermectin (IVM); a combination of TCF+IVM at the same doses and administration routes; or remained untreated (control group). All calves were necropsied at day 14 post treatment for counting and identification of worms from abomasum, small and large intestines and lungs to determine the absolute efficacy (controlled efficacy test) for each treatment. Using the faecal egg count reduction test, the efficacy was 63.7 % for TCF, 72.3 % for IVM and 99.2 % for TCF+IVM. The absolute efficacy of IVM and IVM+TCF was 100 % against Haemonchus placei, Trichostrongylus axei and Ostertagia ostertagi (p < 0.05). TCF showed a similar level of efficacy except against O. ostertagi (84.7 %). Efficacy of the treatments against Cooperia oncophora/pectinata/mcmasteri was 80.4 % for IVM, 95.7 % for TCF and 99.6 % for TCF+IVM; against Trichostrongylus colubriformis was 79 % for IVM, 86.2 % for TCF and 94.1 % for TCF+IVM; against Nematodirus helvetianus was 0 % for IVM, 100 % for TCF and 93.8 % for TCF+IVM. The efficacies of TCF, IVM and TCF+IVM were 100 % against Oesophagostomum radiatum and Trichuris spp. The efficacy of TCF against Dictyocaulus viviparus was 52 % and 100 % for IVM and IVM+TCF, respectively. This is the first report of Trichostrongylus colubriformis and Nematodirus helvetianus resistant to ivermectin treatment in cattle of Argentina. The TCF+IVM combination could be an alternative for the control and treatment of nematode infections including IVM-resistant strains.

The efficacy of trichlorphon and naphthalophos against multiple anthelmintic-resistant nematodes of naturally infected sheep in Argentina.

An anthelmintic efficacy trial was conducted in sheep harbouring anthelmintic-resistant worms in Argentina. Seventy lambs were selected from a flock that had been grazed on pastures infected with trichostrongyles previously shown to be resistant to the main anthelmintic groups. Lambs were allocated to comparable groups of ten animals each and treated with trichlorphon (50 mg/kg body weight (b.w.) orally); naphthalophos (50 mg/kg b.w. orally); ivermectin (0.2 mg/kg b.w. subcutaneously); fenbendazole (5 mg/kg b.w. orally); levamisole (8 mg/kg b.w. subcutaneously) and closantel (10 mg/kg b.w. orally). There was also an untreated group. The dose selection was based on manufacturer's recommendations.Faecal samples were collected 0 and 10 days post treatment to estimate efficacy (faecal egg count reduction). Six animals from each group were necropsied at day 10 for enumeration/identification of worms from the abomasum, small and large intestines to determine the absolute efficacy of each agent (controlled efficacy test). Trichlorphon and naphthalophos were effective (> 99 %) against Haemonchus contortus (p < 0.05).Naphthalophos also showed efficacy against Trichostrongylus axei (99.3 %), Teladorsagia circumcincta (97.8 %), Trichostrongylus colubriformis (99.2 %), Cooperia punctata/curticei/pectinata (90.4 %), Nematodirus spathiger (89.2 %) and Oesophagostomum venulosum/columbianum (93.7 %). Fenbendazole and levamisole showed efficacy (> 95 %) against all nematodes except T. colubriformis. The efficacy of ivermectin was low against H. contortus (23 %) and Cooperia spp. (46.3 %). Closantel showed low efficacy against T. axei (64.4 %), H. contortus (80.6 %) and T. colubriformis (59.5 %).When anthelmintic resistance is widespread, trichlorphon treatment is appropriate if H. contortus is present; however, naphthalophos represents an effective therapeutic alternative for incorporation into worm control programmes.