The subpopulation of Atlantic cod, Gadus morhua, in the eastern part of the Baltic Sea has experienced a significant increase in infections with anisakid nematode larvae of the species Contracaecum osculatum sensu lato (s.l.) since the year 2000. The life cycle of the parasite includes seals and especially the grey seal, Halichoerus grypus, as final hosts, carrying the adult nematodes in the stomach, crustaceans (copepods, amphipods) as first intermediate hosts and various fish species (clupeids, sandeel) including cod as second intermediate/paratenic hosts. Cod with a body length below 28 cm are generally non-infected but experience increasing infection levels when they switch to a piscine diet (infected intermediate/paratenic hosts). We present an overall frequency distribution analysis of worms in 166 cod (body length 30–49 cm) collected in the spawning area over the last 5 years. It shows a fit to the negative binomial distribution, a prevalence of infection of 89.8%, a mean intensity of 29.3 parasites per fish (range 1–377) and a variance/mean ratio of 59.2 (≫1), indicating overdispersion. We present measurements of the adult Contracaecum osculatum (s.l.) specimens in the seal stomach and show that the parasites reach a maximum length of 6.6 cm (females) and 5.8 cm (males). L3s in sprat have a total length from 1to 11 mm whereas the larvae in cod liver are 3–27 mm. A decreasing mean worm length associated with high worm densities in cod (number of nematodes per liver) was recorded. Possible explanations might include timing of feeding on infected intermediate/paratenic hosts, intraspecific competition (crowding) between larvae in cod and host responses (indicated by a significant antibody production in cod against C. osculatum (s.l.) antigens). A significant negative correlation between infection intensity and muscle mass of cod was found, suggesting parasite-induced down-regulation of growth factors in cod.

自 2000 年以来，波罗的海东部的大西洋鳕亚群Gadus morhua 被Contracaecum osculatum sensu lato (sl)种的异尖线虫幼虫感染显着增加。寄生虫的生命周期包括海豹，尤其是灰海豹Halichoerus grypus，作为最终宿主，在胃中携带成年线虫，甲壳类动物（桡足类、片脚类动物）作为第一中间宿主，各种鱼类（鲽科、沙鳗）包括鳕鱼作为第二中间/副宿主。体长低于 28 厘米的鳕鱼通常未受感染，但当它们转向鱼类饮食（受感染的中间宿主/副生宿主）时，感染水平会增加。我们对过去 5 年在产卵区收集的 166 条鳕鱼（体长 30-49 厘米）中的蠕虫进行了总体频率分布分析。它显示了对负二项分布的拟合，感染率为 89.8%，每条鱼的平均寄生虫强度为 29.3（范围 1-377），方差/平均比为 59.2 (≫1)，表明过度分散。我们提供了成人Contracaecum osculatum 的测量值(sl) 海豹胃中的标本，显示寄生虫的最大长度为 6.6 厘米（雌性）和 5.8 厘米（雄性）。鲱鱼中的 L3s 的总长度为 1 至 11 毫米，而鳕鱼肝中的幼虫为 3 至 27 毫米。记录了与鳕鱼中高蠕虫密度（每个肝脏的线虫数量）相关的平均蠕虫长度的减少。可能的解释可能包括以受感染的中间/副宿主为食的时间、鳕鱼幼虫和宿主反应之间的种内竞争（拥挤）（由鳕鱼中针对C. osculatum (sl) 抗原的显着抗体产生表明）。发现感染强度与鳕鱼肌肉量之间存在显着负相关，表明寄生虫诱导的鳕鱼生长因子下调。