Efficacy of doramectin in the protection of neonatal calves and post-parturient cows against field strikes of Cochliomyia hominivorax

Repurposing of Doramectin as a New Anti-Zika Virus Agent

Zika virus (ZIKV), belonging to the Flavivirus family and mainly transmitted by mosquitoes, causes a variety of adverse outcomes, including Guillain-Barré syndrome, microcephaly, and meningoencephalitis. However, there are no approved vaccines or drugs available for ZIKV. The discovery and research on drugs for ZIKV are still essential. In this study, we identified doramectin, an approved veterinary antiparasitic drug, as a novel anti-ZIKV agent (EC₅₀ value from 0.85 μM to 3.00 μM) with low cytotoxicity (CC₅₀ > 50 μM) in multiple cellular models. The expression of ZIKV proteins also decreased significantly under the treatment of doramectin. Further study showed that doramectin directly interacted with the key enzyme for ZIKV genome replication, RNA-dependent RNA polymerase (RdRp), with a stronger affinity (K_d = 16.9 μM), which may be related to the effect on ZIKV replication. These results suggested that doramectin might serve as a promising drug candidate for anti-ZIKV.

New World screwworm (Cochliomyia hominivorax) myiasis in feral swine of Uruguay: One Health and transboundary disease implications

Background

Feral swine (Sus scrofa) are highly invasive and threaten animal and human health in the Americas. The screwworm (Cochliomyia hominivorax) is listed by the World Organization for Animal Health as a notifiable infestation because myiasis cases affect livestock, wildlife, and humans in endemic areas, and outbreaks can have major socioeconomic consequences in regions where the screwworm has been eradicated. However, a knowledge gap exists on screwworm infestation of feral swine in South America, where the screwworm is endemic. Here, we report screwworm infestation of feral swine harvested in Artigas Department (Uruguay), where the Republic of Uruguay shares borders with Brazil and Argentina.

Methods

Myiasis caused by the larvae of screwworm were identified in feral swine with the support and collaboration of members of a local feral swine hunting club over a 3-year period in the Department of Artigas. Harvested feral swine were examined for the presence of lesions where maggots causing the myiasis could be sampled and processed for taxonomic identification. The sites of myiasis on the body of infested feral swine and geospatial data for each case were recorded. The sex and relative size of each feral swine were also recorded. Temperature and precipitation profiles for the region were obtained from public sources.

Results

Myiases caused by screwworms were recorded in 27 of 618 the feral swine harvested. Cases detected in males weighing > 40 kg were associated with wounds that, due to their location, were likely caused by aggressive dominance behavior between adult males. The overall prevalence of screwworm infestation in the harvested feral swine was associated with ambient temperature, but not precipitation. Case numbers peaked in the warmer spring and summer months.

Conclusions

This is the first report on myiasis in feral swine caused by screwworm in South America. In contrast to myiasis in cattle, which can reach deep into host tissues, screwworms in feral swine tended to cause superficial infestation. The presence of feral swine in screwworm endemic areas represents a challenge to screwworm management in those areas. Screwworm populations maintained by feral swine may contribute to human cases in rural areas of Uruguay, which highlights the importance of the One Health approach to the study of this invasive host species–ectoparasite interaction.

An Overview of the Components of AW-IPM Campaigns against the New World Screwworm

The New World Screwworm, Cochliomyia hominivorax (Coquerel), is one of the most damaging parasites of livestock, causing millions of dollars in annual losses to producers. The fly is an obligate parasite of warm-blooded animals, including humans. After a successful 50-year eradication campaign, C. hominivorax has been eradicated from the USA, Mexico and Central America by an area-wide integrated pest management approach. Recently, Caribbean and South American countries have expressed an interest in this approach. Aiming to support forthcoming projects in these countries, this review describes the main technical components of past and ongoing AW-IPM campaigns against C. hominivorax.

Current status of the New World screwworm Cochliomyia hominivorax in Venezuela

The geographical distribution and seasonality of the New World screwworm (NWS), Cochliomyia hominivorax Coquerel (Diptera: Calliphoridae), were monitored through the use of sentinel animals as part of a co-ordinated programme involving veterinarians and farmers, as well as undergraduate students and teachers from veterinary colleges in Venezuela. This surveillance activity made it possible to collect NWS egg masses or larvae from all 23 states in the country and to determine that the rainy season has a strong positive influence on the number of cases of myiasis caused by C. hominivorax in dogs. In addition, efforts were made to obtain the co-operation of the public health service in order to document the extent of human myiasis in the western-central region of Venezuela. Preliminary results revealed 241 cases over a 7-year period, with cases reported in infants as well as in elderly people. Larvae causing myiasis, other than C. hominivorax, were collected from primary myiasis in rabbit (Lucilia eximia [Wiedemann]), dog (an unidentified sarcophagid species), birds (Philornis sp.) and wild mice (Cuterebra sp.). The economic impact of NWS in Venezuela has not been calculated in terms of loss of milk and meat production, damage to hides or death of animals. Control costs (e.g. cost of larvicides) have been estimated at US$ 2 m per year. Control of myiasis in animals is achieved through the use of chemical compounds, mainly organophosphorus (OP) compounds, macrocyclic lactones and, more recently, a foamy spray based on spinosad. Concerns about insecticide resistance to OP compounds have been raised.

Efficacy of macrocyclic lactones for the control of larvae of the Old World screw-worm fly (Chrysomya bezziana)

OBJECTIVE

To assess the efficacy of four macrocyclic lactones for the control of larvae of the Old World Screw-worm Fly (OWS), Chrysomya bezziana, and to examine the effects of excreted residues on the dung fauna.

ANIMALS

100 heifers were divided into five groups of 20 animals. One group remained untreated, whereas the other groups were treated respectively with pour-on formulations of moxidectin, eprinomectin or doramectin, or a sustained-release bolus of ivermectin.

PROCEDURES

At intervals of 1 to 15 weeks after treatment, five cattle from each group were challenged with newly-laid eggs of OWS. The efficacy of each treatment was determined 48 h later by comparing the number of myiases in the treated and untreated groups. Abundance of fly larvae in naturally-voided dung pads and the survival of a species of dung beetle, Onthophagus sagittarius, were used to assess the effects of drug residues on the dung fauna.

RESULTS AND CONCLUSIONS

Moxidectin showed no activity against larvae of OWS during the first 14 days after treatment. Eprinomectin provided protection for 3 days after dosing, but failed at days 7 and 14, whereas doramectin was effective at day 7, but not at days 14 or 21. In contrast, no myiases were established on bolus-treated cattle from 14 to 102 days after treatment. Faecal residues of moxidectin had no effect on the survival of larvae of dung-feeding flies, whereas those of eprinomectin and doramectin reduced survival for 1 to 2 weeks. Dung voided by bolus-treated cattle inhibited fly breeding and had adverse effects on the development and survival of O sagittarius for up to 15 weeks after treatment.

Persistent activity of doramectin and ivermectin in the prevention of cutaneous myiasis in cattle experimentally infested with Cochliomyia hominivorax

A study was conducted to evaluate the activity of a single administration of doramectin or ivermectin against severe, induced infestations of Cochliomyia hominivorax. Twenty-four Holstein bull calves were allocated to four groups of six animals each and treated either with saline, doramectin 1%, or either one of two formulations of ivermectin 1% at a dose rate of 200 microg/kg. On Day 12 after treatment, each calf was anesthetized and two wounds were created on the left side of the shoulder and rump of each calf and 2 h later, each wound was implanted with 100 newly hatched larvae of C. hominivorax. On Day 15 after treatment, the procedure was repeated on the right side of each calf. Wounds were examined daily for 5 days and evidence of live larvae was recorded. Doramectin provided reduction in myiasis of 90.9 and 83.3% at 12 and 15 days after treatment, respectively, compared to the saline control treatment (P < 0.0001). In contrast, there were no significant differences in the number of calves with myiasis between those treated with either of the ivermectin formulations and the saline control.

Protective efficacy of doramectin and ivermectin against Cochliomyia hominivorax

Two studies were conducted in Brazil using induced infestations of the New World screwworm, Cochliomyia hominivorax, to investigate: a) the comparative prophylactic efficacy of doramectin and ivermectin at their recommended use levels (200 micrograms kg-1 s.c.), and b) the duration of protection of a single injection of doramectin. In the comparative efficacy study, two groups of six animals each were treated with ivermectin or doramectin. Two hours after treatment, four incisions were made. Each incision was infested with 30 first instar C. hominivorax larvae and their status evaluated daily for 7 days post-treatment (p.t.). Doramectin treatment was 100% effective in prevention of C. hominivorax infestations whereas ivermectin efficacy was incomplete. First instar larvae were eliminated in doramectin-treated calves by 48 h p.t., while in the ivermectin group, C. hominivorax developed in over 29% of the incisions. Healing began in wounds of doramectin-treated animals at 24 h p.t. and was in progress in 100% of all wounds at 2 days p.t., while 50% of ivermectin-treated calves showed incisions with active lesions. In the duration of protection study, 24 calves were allocated to six groups (T1-T6) of four animals each. Three groups (T1, T3 and T5) were treated with saline and three groups (T2, T4 and T6) with doramectin. Animals were infested as described previously according to the following schedule: T1 and T2 at day 14, T3 and T4 at day 21, and T5 and T6 at day 28 p.t. Incisions were evaluated daily for 8 days post-infestation. Screwworm infestations and viable third-instar larvae developed of all incisions of saline-treated calves, while doramectin was 100% effective preventing development of C. hominivorax for 21 days p.t. and showed partial activity at 28 days p.t.

Efficacy of injectable doramectin in the protection of castrated cattle against field infestations of Cochliomyia hominivorax

Three studies were conducted in Latin America--one in Venezuela, one in Argentina and one in Brazil--using a common protocol to investigate the efficacy of a single subcutaneous injection of doramectin in the prevention and control of Cochliomyia hominivorax infestations in castrated cattle. In each study, two groups of 20-28 animals each were allocated to a treated (T1) or to a control (T2) group on the basis of body weights. Animals of T1 received doramectin at 200 micrograms kg-1 (1 ml per 50 kg) and animals of T2 received saline solution at 1 ml per 50 kg of live weight. After treatment all cattle were castrated surgically. Animals were examined on treatment day and at 2, 4, 6 and 12 days post-treatment. At each observation day, the presence of C. hominivorax infestations was recorded. Doramectin was 100% effective in the prevention and control of screwworm strikes in castrated cattle exposed to continuous field infestations of C. hominivorax in tropical and subtropical areas of Latin America. Over the 12 day duration of the studies, 85%, 60% and 65% of animals in the control groups had infested wounds in Venezuela, Argentina and Brazil, respectively. Affected animals required repeated therapeutic treatment, whereas none of the doramectin-treated cattle were infested (P < 0.0001). A high proportion of the castration wounds in doramectin-treated cattle had the presence of characteristic C. hominivorax eggs but none developed into larvae. There were no clinical signs of adverse reactions to treatment in any of the three studies.