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Cattle protected from onchocerciasis by ivermectin are highly susceptible to infection after drug withdrawal

Njongmeta, L.M., Nfon, C.K., Gilbert, J., Makepeace, Benjamin L., Tanya, V. N. and Trees, Alexander J. (2004) 'Cattle protected from onchocerciasis by ivermectin are highly susceptible to infection after drug withdrawal'. International Journal for Parasitology, Vol 34, Issue 9, pp. 1069-1074.

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Abstract

Ivermectin administration is now the major tool in the control of human onchocerciasis (caused by Onchocerca volvulus) based on its suppression of microfilariae and hence the prevention of disease. However, in Africa, transmission is not eliminated and treated populations continue to be exposed to infective larval (L-3) challenge, albeit at reduced levels. We have investigated whether protective immunity might develop under such conditions using the analogous host-parasite system Onchocerca ochengi in cattle, based on our previous findings in cattle exposed to challenge, that in vivo ivermectin attenuates the development of adult infections and that irradiation-attenuated L-3 induce significant protection. In a two-phase prospective study over 4 years, groups of cattle were exposed to severe natural challenge. In the first phase, 38/40 animals treated either with ivermectin or with moxidectin at either monthly or 3-monthly intervals had not developed detectable infections after 22 months of exposure whereas, in a non-treated control group (n = 14), nodule prevalence was 78.6% and the geometric mean (range) nodule load was 4.8 (0-33). In the second phase, all drug treatments were withdrawn, a new control group (n = 8) introduced, and exposure continued at the same site. After 24 months, all groups had developed patent infections, with geometric mean (range) nodule loads of 17.4 (4-99), 38.4 (10-111), 50.7 (26-86), 14.3 (0-69) and 14.7 (0-55) for the control, monthly-ivermectin, 3-monthly ivermectin, monthly moxidectin and 3-monthly moxidectin groups, respectively. There was no evidence of protection-indeed the 3-monthly ivermectin group was significantly (P < 0.05) hyper-susceptible. In addition, microfilarial densities and the rate of increase in microfilarial load were significantly higher (P < 0.05) in the ivermectin-treated groups than in control animals. These results have important implications for ivermectin-based control of human onchocerciasis and suggest that humans exposed to ongoing transmission in endemic areas whilst receiving ivermectin are unlikely to develop immunity and will be highly susceptible should drug distribution cease. (C) 2004 Australian Society for Parasitology Inc Published by Elsevier Ltd. All rights reserved.

Item Type: Article
Uncontrolled Keywords: onchocerciasis onchocerca ochengi cattle ivermectin chemoprophylaxis immunity chemically-abbreviated infections dirofilaria-immitis volvulus ochengi transmission responses immunity suggests program africa
Subjects: QV Pharmacology > QV 38 Drug action.
QV Pharmacology > Drug Standardization. Pharmacognosy. Medicinal Plants > QV 771 Standardization and evaluation of drugs
WC Communicable Diseases > Tropical and Parasitic Diseases > WC 885 Onchocerciasis
Faculty: Department: Groups (2002 - 2012) > Veterinary Parasitology Group (2002-2008)
Digital Object Identifer (DOI): https://doi.org/10.1016/j.ijpara.2004.04.011
Depositing User: Sarah Lewis-Newton
Date Deposited: 20 Mar 2012 15:28
Last Modified: 06 Feb 2018 13:03
URI: http://archive.lstmed.ac.uk/id/eprint/2164

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