Haemaphysalis concinna (Acari: Ixodida): Persistent efficacy of doramectin in rabbits under laboratory conditions

Introduction

Haemaphysalis concinna (Acari: Ixodida) is an obligate hematophagous ectoparasite with a life cycle characterized by a period of starvation, which infests a wide range of mammals and birds. H. concinna is host to a number of zoonotic pathogens, many of which are life-threatening, such as Borrelia burgdorferi, tick-borne encephalitis virus, Borrelia garinii, Coxiella burnetii, Rickettsia spp., Ehrlichia spp. and Anaplasma spp. (Filippova 1990; Khazova & Iastrebov 2001; Sun & Xu 2003; Spitalská & Kocianová 2003; Shpynov et al. 2004b; Sréter-Lancz et al. 2006a). H. concinna is widely distributed in Germany, Italy, the Czech Republic, Austria, Russia, Kazakhstan, Hungary, Spain and Iran (Kahl et al. 1992; Iori & Di Paolo 1999; Spitalskáet et al. 2003; Khazova & Iastrebov 2001; Shpynov et al. 2004a; Sréter-Lancz et al. 2006b; Barandika et al. 2006; Rahbari et al. 2007). It is also widely distributed in north-east China, including Neimeng, Jilin, Shanxi, Ningxia, Gansu and Xinjiang provinces (Shen & Huang 2005). In 1999, there were some reports of H. concinna infestations in Bazhong county, Sichuan province (Xian et al. 2000, 2002a,b) that spread over 181.75 km² and resulted in 82 369 bites reported among the local populations. In addition, infestation rates in pigs, cattle, goats, rabbits, dogs, fowl, ducks and geese were 4.1% (3028/73 901), 84.0% (12 399/14 763), 100% (2012/2012), 15.8% (272/1725), 22.5% (1782/7906), 74.8% (85 899/114 902), 28.7% (37 363/13 131) and 99.5% (8512/8554), respectively. In 2005, H. concinna infestations emerged in Cangxi county, Sichuan province and spread over 94.7% districts in Cangxi. In humans, bites result in skin rashes, red swelling, itching and infested ulcerations of bite sites which, in some cases, result in pigmented spots that last for several months after wound healing. Tick infestations among domestic animals negatively impacted on husbandry production, which resulted in severe economic losses (Sun et al. 2006).

Doramectin (DOR) has been widely used to control ectoparasites and internal parasites (Lloyd et al. 1996; Kanbur et al. 2008; van der Merwe et al. 2008). In clinical trials, it was found to be highly effective, broad-spectrum and safe for animal (cattle, sheep, dogs and pigs) use (Yazwinski et al. 1997; Lavy et al. 2002; Kuzner et al. 2005; Bengone-Ndong et al. 2008; Zacharias et al. 2008).

As a new macrocyclic lactone anthelmintic, DOR offers advantages for flea, tick and mite control, as well as for some gastrointestinal nematodes. Economic analysis of return on investment indicated that DOR, even at low doses in heavy bodyweight cattle, was highly effective. The pattern of residue depletion from muscular tissue may differ according to its anatomical locations and/or physiological role. This should be considered in implementing residue control strategies in meat safety assurance for human consumption (MacGregor et al. 2001; Moreno et al. 2008).

In the present study, we report for the first time the effectiveness of DOR (200 µg/kg) on H. concinna in rabbits under laboratory conditions.

Materials and methods

Six- to eight-month-old naive rabbits (with no previous exposure to ectoparasitic drugs) weighing 2.9–3.2 kg from the experimental animal center of Sichuan Agricultural University (Ya’an, China) were used. Rabbits were selected on the basis of a satisfactory physical examination and a Specific Pathogen Free (SPF) status. Throughout the study, all rabbits were maintained in individual cages and provided with water and food daily.

The amphigenesis China strain of the tick H. concinna (Deng 1978) has been maintained for several generations in our laboratory since 2006 by feeding on rabbits. Ticks were maintained in glass flasks in an incubator (29 ± 3°C) and at 80 ± 4% relative humidity for molting or oviposition. There was some sandy soil in the glass flasks, and a piece of filter paper on the sandy soil was kept moistened.

Fifteen rabbits were allocated to three groups: larvae, nymph and adult groups (i.e. each group had five rabbits). On day zero, DOR (Revolution; Pfizer Inc, New York, USA) (200 µg/kg) was administered to four rabbits by subcutaneous injection (s.c.) once. One rabbit per group, the control, was injected without DOR. On days 1, 8, 15 and 22, one rabbit from each group was inoculated with 60 larvae, 20 nymphs or 20 adults on the surface of the ear. Each rabbit was inoculated with ticks only once. The ear of each rabbit was secured with a nylon ear cap (6 × 30 cm) to prevent ticks from escaping. Detached ticks (engorged ticks or dead ticks) were collected and counted daily.

All analyses were performed using Microsoft Excel 2003 and spss software for Windows, Release 13.0 (this spss product contains software developed by the Apache Software Foundation). The level of significance for analyses was set at P < 0.01.

Results

On days 1, 8, 15 and 22 post-inoculation of larvae, larvae were observed for 7 days, and the efficacy of DOR (dead ticks) was 100.0, 100.0, 86.7 and 61.7%, respectively. During days 15–21 and 22–28, the number of detached ticks was 51 out of 120 (42.5%), but 31 out of 120 (25.8%) did not molt. The mortality of larvae in untreated rabbits was 5.0%. The mortality of ticks was significantly higher (P < 0.01) for rabbits treated with DOR than for those inoculated without DOR treatment (control) (Table 1).

On days 1, 8, 15 and 22 post-inoculation of nymphs, nymphs were observed for 7 days, and the efficacy of DOR (dead ticks) was 100.0, 90.0, 75.0 and 65.0%, respectively. During days 15–21 and 22–28, the number of detached ticks was 33 (41.3%), but 17 of them (21.3%) did not molt. No dead ticks were found in untreated rabbits. The mortality of ticks was significantly higher (P < 0.01) for rabbits treated with DOR than for those inoculated without DOR treatment (control) (Table 2).

On days 1, 8, 15 and 22 post-inoculation of adults, adults were observed for 7 days, and the efficacy of DOR (dead ticks) was 85.0, 65.0, 55.0 and 55.0%, respectively. During days 1–7 and 8–14, the number of detached ticks was 10 (25.0%), but none of them oviposited. During days 15–21 and 22–28, the number of detached ticks was 18 (45.0%), but 13 of them (32.5%) did not oviposit. No dead ticks were found in untreated rabbits. The mortality of ticks was significantly higher (P < 0.01) for rabbits treated with DOR than for those inoculated without DOR treatment (control) (Table 3).

Discussion

Doramectin is a safe and approved treatment for selected farm animals when used at recommended doses. Injectable DOR at a dosage of 200 µg/kg has been shown to be safe in the treatment of cattle, pigs, sheep and rabbits (Phillips et al. 1996; Yazwinski et al. 1997; Kolar et al. 2006; Kanbur et al. 2008).

DOR is readily absorbed and hosts develop high plasma concentrations (Phillips et al. 1996; Yazwinski et al. 1997; Kolar et al. 2006). After injection, it results in high mortality of H. concinna.

The persistent efficacy of DOR in the treatment of rabbits inoculated with larvae, nymphs and adults of H. concinna was significant. In the present study, at time intervals of 14 days, the efficacy of DOR on feeding larvae, nymphs and ovipositing adults rapidly reduced. Based on the developmental stages of ticks affected and the duration of suppression (larvae > nymphs > adults), the efficacy of DOR in the treatment of rabbits infested with adults was lower than for larvae and nymphs, but effective control was demonstrated by reducing oviposition. While DOR achieved high levels of control 2 weeks after injection, continued injection at 2 week intervals would be necessary to maintain a high level of control. Further studies on other animals are needed to determine DOR efficacy for reducing tick populations. Of concern is the mass use of this drug and the effects on humans through food consumption. Zoonotic hosts may play a vital part in the maintenance of this tick in the environment.