Repeated high doses of avermectins cause prolonged sterilisation, but do not kill, Onchocerca ochengi adult worms in African cattle

Effects of an injectable long-acting formulation of ivermectin on Onchocerca ochengi in zebu cattle

The availability of a safe macrofilaricidal drug would help to accelerate onchocerciasis elimination. A trial was conducted in Cameroon to evaluate the effects of a subcutaneous injectable long-acting formulation of ivermectin (LAFI) on the microfilariae (mf) and adult stages of Onchocerca ochengi. Ten zebu cattle naturally infected with the parasite were injected subcutaneously with either 500 mg (group A, N = 4), or 1000 mg long-acting ivermectin (group B, N = 4) or the vehicle (group C, N = 2). Skin samples were collected from each animal before, and 6, 12, and 24 months after treatment to measure microfilarial densities (MFDs). Nodules excised before, and 6 and 12 months after treatment were examined histologically to assess the adult worms’ viability and reproductive status. Blood samples were collected at pre-determined time-points to obtain pharmacokinetic data. Before treatment, the average O. ochengi MFDs were similar in the three groups. Six months after treatment, all animals in groups A and B were free of skin mf, whereas those in group C still showed high MFDs (mean = 324.5 mf/g). Only one ivermectin-treated animal (belonging to group A) had skin mf 12 months after treatment (0.9 mf/g). At 24 months, another animal in group A showed skin mf (10.0 mf/g). The histologic examination of nodules at 6 and 12 months showed that LAFI was not macrofilaricidal but had a strong effect on embryogenesis. The new LAFI regimen might be an additional tool to accelerate the elimination of human onchocerciasis in specific settings.

Examining the role of macrolides and host immunity in combatting filarial parasites

Macrocyclic lactones (MLs), specifically the avermectins and milbemycins, are known for their effectiveness against a broad spectrum of disease-causing nematodes and arthropods in humans and animals. In most nematodes, drugs in this class induce paralysis, resulting in starvation, impaired ability to remain associated with their anatomical environment, and death of all life stages. Initially, this was also thought to be the ML mode of action against filarial nematodes, but researchers have not been able to validate these characteristic effects of immobilization/starvation of MLs in vitro, even at higher doses than are possible in vivo. Relatively recently, ML receptor sites exclusively located proximate to the excretory-secretory (ES) apparatus were identified in Brugia malayi microfilaria and an ML-induced suppression of secretory protein release by B. malayi microfilariae was demonstrated in vitro. It is hypothesized here that suppression of these ES proteins prevents the filarial worm from interfering with the host's complement cascade, reducing the ability of the parasite to evade the immune system. Live microfilariae and/or larvae, thus exposed, are attacked and presented to the host's innate immune mechanisms and are ultimately killed by the immune response, not the ML drug. These live, exposed filarial worms stimulate development of innate, cellular and humoral immune responses that when properly stimulated, are capable of clearing all larvae or microfilariae present in the host, regardless of their individual sensitivity to MLs. Additional research in this area can be expected to improve our understanding of the relationships among filarial worms, MLs, and the host immune system, which likely would have implications in filarial disease management in humans and animals.

25 Years of the Onchocerca ochengi Model

Although of limited veterinary significance, Onchocerca ochengi has become famous as a natural model or 'analogue' of human onchocerciasis (river blindness), which is caused by Onchocerca volvulus. On the basis of both morphological and molecular criteria, O. ochengi is the closest extant relative of O. volvulus and shares several key natural history traits with the human pathogen. These include exploitation of the same group of insect vectors (blackflies of the Simulium damnosum complex) and formation of collagenous nodules with a similar histological structure to human nodules. Here, we review the contribution of this natural system to drug and vaccine discovery efforts, as well as to our basic biological understanding of Onchocerca spp., over the past quarter-century.

Immunotherapy with mutated onchocystatin fails to enhance the efficacy of a sub-lethal oxytetracycline regimen against Onchocerca ochengi

Human onchocerciasis (river blindness), caused by the filarial nematode Onchocerca volvulus, has been successfully controlled by a single drug, ivermectin, for over 25 years. Ivermectin prevents the disease symptoms of severe itching and visual impairment by killing the microfilarial stage, but does not eliminate the adult parasites, necessitating repeated annual treatments. Mass drug administration with ivermectin does not always break transmission in forest zones and is contraindicated in individuals heavily co-infected with Loa loa, while reports of reduced drug efficacy in Ghana and Cameroon may signal the development of resistance. An alternative treatment for onchocerciasis involves targeting the essential Wolbachia symbiont with tetracycline or its derivatives, which are adulticidal. However, implementation of antibiotic therapy has not occurred on a wide scale due to the prolonged treatment regimen required (several weeks). In the bovine Onchocerca ochengi system, it has been shown previously that prolonged oxytetracycline therapy increases eosinophil counts in intradermal nodules, which kill the adult worms by degranulating on their surface. Here, in an "immunochemotherapeutic" approach, we sought to enhance the efficacy of a short, sub-lethal antibiotic regimen against O. ochengi by prior immunotherapy targeting onchocystatin, an immunomodulatory protein located in the adult female worm cuticle. A key asparagine residue in onchocystatin was mutated to ablate immunomodulatory activity, which has been demonstrated previously to markedly improve the protective efficacy of this vaccine candidate when used as an immunoprophylactic. The immunochemotherapeutic regimen was compared with sub-lethal oxytetracycline therapy alone; onchocystatin immunotherapy alone; a gold-standard prolonged, intermittent oxytetracycline regimen; and no treatment (negative control) in naturally infected Cameroonian cattle. Readouts were collected over one year and comprised adult worm viability, dermal microfilarial density, anti-onchocystatin IgG in sera, and eosinophil counts in nodules. Only the gold-standard antibiotic regimen achieved significant killing of adult worms, a profound reduction in microfilarial load, and a sustained increase in local tissue eosinophilia. A small but statistically significant elevation in anti-onchocystatin IgG was observed for several weeks after immunisation in the immunotherapy-only group, but the antibody response in the immunochemotherapy group was more variable. At 12 weeks post-treatment, only a transient and non-significant increase in eosinophil counts was apparent in the immunochemotherapy group. We conclude that the addition of onchocystatin immunotherapy to a sub-lethal antibiotic regimen is insufficient to induce adulticidal activity, although with booster immunisations or the targeting of additional filarial immunomodulatory proteins, the efficacy of this strategy could be strengthened.

A repurposing strategy for Hsp90 inhibitors demonstrates their potency against filarial nematodes

Novel drugs are required for the elimination of infections caused by filarial worms, as most commonly used drugs largely target the microfilariae or first stage larvae of these infections. Previous studies, conducted in vitro, have shown that inhibition of Hsp90 kills adult Brugia pahangi. As numerous small molecule inhibitors of Hsp90 have been developed for use in cancer chemotherapy, we tested the activity of several novel Hsp90 inhibitors in a fluorescence polarization assay and against microfilariae and adult worms of Brugia in vitro. The results from all three assays correlated reasonably well and one particular compound, NVP-AUY922, was shown to be particularly active, inhibiting Mf output from female worms at concentrations as low as 5.0 nanomolar after 6 days exposure to drug. NVP-AUY922 was also active on adult worms after a short 24 h exposure to drug. Based on these in vitro data, NVP-AUY922 was tested in vivo in a mouse model and was shown to significantly reduce the recovery of both adult worms and microfilariae. These studies provide proof of principle that the repurposing of currently available Hsp90 inhibitors may have potential for the development of novel agents with macrofilaricidal properties.

Adult of Dirofilaria repens in a dog with recurrent multiple subcutaneous nodular lesions

Subcutaneous dirofilariosis is a zoonosis parasitizing the dog, transmitted by Culicidae mosquitoes. An increasing number of new cases have been reported both in dogs and in humans all over the world. The aim of this work was to describe an uncommon case of recurrent multiple subcutaneous nodules by Dirofilaria repens in a 2-year-old dog. Surgical excision of two nodules on both hind limbs revealed a single thread-like live nematode and multiple parasite fragments, respectively. The parasite was morphologically identified as an adult female of D. repens. Haematoxylin-eosin tissue sections of nodules revealed a diffuse pyogranulomatous angiocentric dermatitis with an intense eosinophilic infiltrate throughout the dermis. One month post-surgery, a new nodular lesion was detected on the right thigh and resolved spontaneously in a few weeks. During the following 3 months, two other nodules were briefly noted both on the right hip and on the left side of the chest. The dog was periodically treated with milbemycin oxime and praziquantel, starting a week before the appearance of the first nodule. Microfilariae were not detected neither in the bloodstream nor in fine-needle aspirates at any time, and the diagnosis was made only after surgery. Practitioners should consider dirofilariosis in the differential diagnosis of subcutaneous swellings, especially if they have a recurrent behaviour, previously described in humans but not in dogs.

Efficacy of three-week oxytetracycline or rifampin monotherapy compared with a combination regimen against the filarial nematode Onchocerca ochengi

Onchocerciasis (river blindness), caused by the filarial nematode Onchocerca volvulus, is a major cause of visual impairment and dermatitis in sub-Saharan Africa. As O. volvulus contains an obligatory bacterial symbiont (Wolbachia), it is susceptible to antibiotic chemotherapy, although current regimens are considered too prolonged for community-level control programs. The aim of this study was to compare the efficacies of oxytetracycline and rifampin, administered separately or in combination, against a close relative of O. volvulus (Onchocerca ochengi) in cattle. Six animals per group were treated with continuous or intermittent oxytetracycline regimens, and effects on adult worm viability, dermal microfilarial loads, and Wolbachia density in worm tissues were assessed. Subsequently, the efficacies of 3-week regimens of oxytetracycline and rifampin alone and a combination regimen were compared, and rifampin levels in plasma and skin were quantified. A 6-month regimen of oxytetracycline with monthly dosing was strongly adulticidal, while 3-week and 6-week regimens exhibited weaker adulticidal effects. However, all three regimens achieved >2-log reductions in microfilarial load. In contrast, rifampin monotherapy and oxytetracycline-rifampin duotherapy failed to induce substantive reductions in either adult worm burden or microfilarial load, although a borderline effect on Wolbachia density was observed following duotherapy. Dermal rifampin levels were maintained above the MIC for >24 h after a single intravenous dose. We conclude that oxytetracycline-rifampin duotherapy is less efficacious against O. ochengi than oxytetracycline alone. Further studies will be required to determine whether rifampin reduces oxytetracycline bioavailability in this system, as suggested by human studies using other tetracycline-rifampin combinations.

Of mice, cattle, and humans: the immunology and treatment of river blindness

River blindness is a seriously debilitating disease caused by the filarial parasite Onchocerca volvulus, which infects millions in Africa as well as in South and Central America. Research has been hampered by a lack of good animal models, as the parasite can only develop fully in humans and some primates. This review highlights the development of two animal model systems that have allowed significant advances in recent years and hold promise for the future. Experimental findings with Litomosoides sigmodontis in mice and Onchocerca ochengi in cattle are placed in the context of how these models can advance our ability to control the human disease.

Molecular phylogeny of the filaria genus Onchocerca with special emphasis on Afrotropical human and bovine parasites

Filarial parasites of the genus Onchocerca are found in a broad spectrum of ungulate hosts. One species, O. volvulus, is a human parasite that can cause severe disease (onchocerciasis or 'river blindness'). The phylogenetic relationships and the bionomics of many of the nearly 30 known species remain dubious. Here, the phylogeny of 11 species representing most major lineages of the genus is investigated by analysing DNA sequences from three mitochondrial genes (ND5, 12S and 16S rRNA) and portions of the intergenic spacer of the nuclear 5s rRNA. Special emphasis is given to a clade containing a yet unassigned specimen from Uganda (O. sp. 'Siisa'), which appears to be intermediate between O. volvulus and O. ochengi. While the latter can be differentiated by the O-150 tandem repeat commonly used for molecular diagnostics, O. volvulus and O. sp.'Siisa' cannot be differentiated by this marker. In addition, a worm specimen from an African bushbuck appears to be closely related to the bovine O. dukei and represents the basal taxon of the human/bovine clade. At the base of the genus, our data suggest O. flexuosa (red deer), O. ramachandrini (warthog) and O. armillata (cow) to be the representatives of ancient lineages. The results provide better insight into the evolution and zoogeography of Onchocerca. They also have epidemiological and taxonomic implications by providing a framework for more accurate molecular diagnosis of filarial larvae in vectors.

Evaluation of three 3ABC ELISAs for foot-and-mouth disease non-structural antibodies using latent class analysis

BACKGROUND

Foot-and-mouth disease (FMD) is a highly contagious viral disease of even-toed ungulates. Serological diagnosis/surveillance of FMD presents several problems as there are seven serotypes worldwide and in the event of vaccination it may be necessary to be able to identify FMD infected/exposed animals irrespective of their vaccination status. The recent development of non-structural 3ABC protein (NSP) ELISA tests has greatly advanced sero-diagnosis/surveillance as these tests detect exposure to live virus for any of the seven serotypes of FMD, even in vaccinated populations. This paper analyses the performance of three NSP tests using a Bayesian formulation of the Hui-Walter latent class model to estimate test sensitivity and specificity in the absence of a "gold-standard" test, using sera from a well described cattle population in Cameroon with endemic FMD.

RESULTS

The analysis found a high sensitivity and specificity for both the Danish C-ELISA and the World Organisation for Animal Health (O.I.E.) recommended South American I-ELISA. However, the commercial CHEKIT kit, though having high specificity, has very low sensitivity. The results of the study suggests that for NSP ELISAs, latent class models are a useful alternative to the traditional approach of evaluating diagnostic tests against a known "gold-standard" test as imperfections in the "gold-standard" may give biased test characteristics.

CONCLUSION

This study demonstrates that when applied to naturally infected zebu cattle managed under extensive rangeland conditions, the FMD ELISAs may not give the same parameter estimates as those generated from experimental studies. The Bayesian approach allows for full posterior probabilities and capture of the uncertainty in the estimates. The implications of an imperfect specificity are important for the design and interpretation of sero-surveillance data and may result in excessive numbers of false positives in low prevalence situations unless a follow-up confirmatory test such as the enzyme linked immunoelectrotransfer blot (EITB) is used.

Rate of elimination of Wolbachia pipientis by doxycycline in vitro increases following drug withdrawal

Wolbachia pipientis is an obligate intracellular bacterium within the family Anaplasmataceae that infects many terrestrial arthropods and arthropod-transmitted nematodes (filariae). Several filarial species are major human pathogens, and antibiotics with activity against Wolbachia offer a promising new therapeutic approach, since the adult worms are relatively refractory to conventional anthelmintics but depend on Wolbachia for reproduction and viability. In a natural filarial parasite of cattle, Onchocerca ochengi, intermittent chemotherapy is adulticidal whereas the equivalent dose administered as a continuous treatment is not. To investigate this further and to aid the design of efficacious regimens for human therapy, we used Wolbachia-infected Aedes albopictus mosquito cells in vitro. Here, we describe for the first time the accelerated depletion of bacteria after antibiotic withdrawal relative to the rate of elimination in the continuous presence of the drug. Mosquito cells were incubated with doxycycline while changes in 16S (bacterial) and 18S (host) rRNA and rRNA genes were determined by quantitative PCR assays. In cultures treated for 7 or 14 days followed by 7 days of drug withdrawal, the Wolbachia-to-Aedes rRNA ratio declined by approximately 6 log, whereas immediately after 14 or 21 days of continuous treatment, the reduction was only approximately 4 log (P < 0.05). However, low levels of 16S rRNA remained after 21 days of treatment, irrespective of whether doxycycline was withdrawn. Application of similar methodology to related intracellular bacteria may reveal that this posttreatment effect is not restricted to Wolbachia and could have wider implications for the design of intermittent regimens for antibiotic chemotherapy.

Control of Onchocerciasis

Onchocerciasis is a filarial infection which causes blindness and debilitating skin lesions. The disease occurs in 37 countries, of which 30 are found in Africa (the most affected in terms of the distribution and the severity of the clinical manifestations of the disease), six in the Americas and one in the Arabian Peninsula. The latest WHO Expert Committee on Onchocerciasis estimated that in 1995 around 17.7 million persons were infected, about 270,000 of whom were blind and another 500,000 severely visually impaired. The disease is responsible for 1 million DALYs. Eye disease from onchocerciasis accounts for 40% of DALYs annually although severe skin disease is also recognized as of public health significance. Great progress has been made in the last thirty years in the control of onchocerciasis, both in Africa and the Americas, and this progress has been due largely to international public-private partnerships, sustained funding regional programmes, and new tools and technology. Landmarks in the global control of river blindness include the significant success of the Onchocerciasis Control Programme of West Africa (1975-2002), and the donation of ivermectin (Mectizan) by Merck & Co. Inc., in 1988, a medicine that is distributed to millions free of charge each year. Future major technical challenges of onchocerciasis control include ivermectin mass administration in areas co-endemic for the parasite Loa loa in the light of possible severe adverse reactions, ivermectin treatment in hypoendemic areas hitherto excluded from African control programmes, sustainability of ivermectin distribution, post-control surveillance for recrudescence detection, surveillance for emergence of resistance, and decisions of when to stop mass ivermectin treatments. There is the need to develop the appropriate information systems and diagnostic tools to help in accomplishing many of these tasks. A search for a second-line treatment or as an additional drug to ivermectin as well as a search for a macrofilaricide are issues that need to be addressed in the future.