Background information and definitions
High parasite loads may reduce the fitness of marine and freshwater mammals and lead to higher levels of mortality (Aznar et al. 2001). Drugs are available to reduce the infestation levels of some parasites. Attempts to treat wild marine and freshwater mammals are most likely to be made when a species is highly threatened or when there are public health risks or economic costs associated with not treating. In such cases, mammals may be captured, treated, and released with the aim of improving the health of wild populations. For studies that treated mammals for parasites in captivity as part of a rehabilitation programme, see Rehabilitate and release injured, sick or weak marine and freshwater mammals.
Aznar F.J., Balbuena J.A., Fernández M. & Raga J.A. (2001) Living together: the parasites of marine mammals. Pages 385–423 in: Evans P. G. H. & Raga J. A. (eds.) Marine Mammals: Biology and Conservation. Springer US, Boston, MA.
Supporting evidence from individual studies
A controlled, before-and-after study in 2006 on an island in the North Pacific Ocean, off the coast of California, USA (DeLong et al. 2009) found that northern fur seal Callorhinus ursinus pups treated with an anti-parasitic drug (ivermectin) had reduced hookworm Uncinaria lucasi infections, lower mortality rates and greater growth rates than untreated pups. The number of treated pups with hookworm infections decreased from 24% (36 of 151 pups) to 6% (2 of 34 pups) 19–34 days after treatment with ivermectin. In comparison, the number of infected untreated pups increased from 24% (36 of 149 pups) to 67% (20 of 30 pups). Mortality rates were lower for pups treated with ivermectin (10 of 149 pups died, 7%) than untreated pups (50 of 151 pups died, 33%), and growth rates were greater (treated: 0.06 kg/day; untreated: 0.04 kg/day). In July 2006, seal pups were captured, tagged and alternately assigned to a treatment group (injected with ivermectin; 151 pups) or untreated control group (injected with saline solution; 149 pups). Hookworm eggs were counted in faecal samples in July (all of 300 pups) and August 2006 (34 treated pups, 30 untreated pups). Pups were weighed in July (all of 300 pups) and September 2006 (number not reported). Mortality surveys were carried out every 3–20 days in July–December 2006.Study and other actions tested
A controlled study in 2009–2010 on an island in the North Pacific Ocean, Hawaii, USA (Gobush et al. 2011) found that Hawaiian monk seal Monachus schauinslandi pups treated with an anti-parasitic drug (praziquantel) had similar parasite loads and survival rates to untreated pups. The number of faecal samples containing parasitic worms (cestodes Diphyllobothrium spp.) did not differ significantly between treated pups (44 of 46 samples, 96%) and untreated pups (43 of 44 samples, 98%). Survival rates also did not differ significantly between treated pups (20 of 23 pups survived, 87%) and untreated pups (19 of 20 pups survived, 95%). Forty-three tagged seal pups (<2 years old) were randomly assigned to a treatment group (injected with praziquantel; 23 pups) or an untreated control group (20 pups). Each of 43 pups was captured, weighed, measured, injected (treatment group only) and had faeces sampled up to four times, 8–16 weeks apart, between December 2009 and May 2010.Study and other actions tested