Co-infections are very common in nature and occur when hosts are infected by two or more different pathogens either by simultaneous or secondary infections so that two or more infectious agents are active together in the same host. Co-infections have a fundamental effect and can alter the course and the severity of different fish diseases. However, co-infection effect has still received limited scrutiny in aquatic animals like fish and available data on this subject is still scarce. The susceptibility of fish to different pathogens could be changed during mixed infections causing the appearance of sudden fish outbreaks. In this review, we focus on the synergistic and antagonistic interactions occurring during co-infections by homologous or heterologous pathogens. We present a concise summary about the present knowledge regarding co-infections in fish. More research is needed to better understand the immune response of fish during mixed infections as these could have an important impact on the development of new strategies for disease control programs and vaccination in fish.

Whirling disease of rainbow trout is caused by Myxobolus cerebralis, a myxozoan parasite possessing a life cycle well adapted to the natural environments where salmonid fish are found. Whirling disease was first described in Europe in 1898 among farmed rainbow trout but recent occurrences have been devastating to wild trout in North America. The disease is considered a major threat to survival of wild rainbow trout in the intermountain west of the United States. Difficulties in containing the spread and potentially eliminating the pathogen are tied to features of a complex life cycle involving two hosts, the salmonid fish and an aquatic oligochaete. Details of the morphologic development of the parasite have been described in each host but only now are we beginning to appreciate the breadth of interactions between these developmental forms and the sequential responses of the host. Fundamental mechanisms of the recognition and attachment of the parasite to the hosts, how host immunity is evaded and the unknown influences of environmental factors all contribute to a rather poor understanding of the biology of the parasite. Although the biology and ecology of the salmonid host are better known than for the oligochaete host, our knowledge is inadequate to interpret their complex interactions with the parasite. This uncertainty precludes the development of effective management activities designed to enhance the viability and productivity of wild trout populations in M. cerebralis-positive river systems. Improving our understanding of the hosts, the parasite and the environmental factors determining their interaction should provide for more focused and effective control methods for containing the spread and devastating effects whirling disease is causing to our wild trout populations.

Enteric redmouth disease (ERM) is a serious septicemic bacterial disease of salmonid fish species. It is caused by Yersinia ruckeri, a Gram-negative rod-shaped enterobacterium. It has a wide host range, broad geographical distribution, and causes significant economic losses in the fish aquaculture industry. The disease gets its name from the subcutaneous hemorrhages, it can cause at the corners of the mouth and in gums and tongue. Other clinical signs include exophthalmia, darkening of the skin, splenomegaly and inflammation of the lower intestine with accumulation of thick yellow fluid. The bacterium enters the fish via the secondary gill lamellae and from there it spreads to the blood and internal organs. Y. ruckeri can be detected by conventional biochemical, serological and molecular methods. Its genome is 3.7 Mb with 3406–3530 coding sequences. Several important virulence factors of Y. ruckeri have been discovered, including haemolyin YhlA and metalloprotease Yrp1. Both non-specific and specific immune responses of fish during the course of Y. ruckeri infection have been well characterized. Several methods of vaccination have been developed for controlling both biotype 1 and biotype 2 Y. ruckeri strains in fish. This review summarizes the current state of knowledge regarding enteric redmouth disease and Y. ruckeri: diagnosis, genome, virulence factors, interaction with the host immune responses, and the development of vaccines against this pathogen.

Tetracapsuloides bryosalmonae is a myxozoan parasite of salmonids and freshwater bryozoans, which causes proliferative kidney disease (PKD) in the fish host. To test which fish species are able to transmit T. bryosalmonae to bryozoans, an infection experiment was conducted with 5 PKD-sensitive fish species from different genera. Rainbow trout Oncorhynchus mykiss, brown trout Salmo trutta, brook trout Salvelinus fontinalis, grayling Thymallus thymallus and northern pike Esox lucius were cohabitated with T. bryosalmonae-infected Fredericella sultana colonies and then subsequently cohabitated with statoblast-reared parasite free Bryozoa. Statoblasts from infected colonies were tested by PCR to detect cryptic stages of T. bryosalmonae, which may indicate vertical transmission of the parasite. In this study, brown trout and brook trout were able to infect Bryozoa, while there was no evidence that rainbow trout and grayling were able to do so. Few interstitial kidney stages of the parasite were detected by immunohistochemistry in brown trout and brook trout, while rainbow trout and grayling showed marked proliferation of renal interstitial tissue and macrophages with numerous parasite cells. Intraluminal stages in the kidney tubules were only detected in brown trout and rainbow trout. In contrast to previous observations, pike was not susceptible to PKD in these trials according to the results of T. bryosalmonae-specific PCR. No DNA of T. bryosalmonae was detected in any statoblast.KEY WORDS: Tetracapsuloides bryosalmonae · Myxozoa · Malacosporea · Bryozoa · Proliferative kidney disease · PKD · Host specificity · Transmission experiment Resale or republication not permitted without written consent of the publisherDis Aquat Org 79: [133][134][135][136][137][138][139] 2008 the parasite using brown trout Salmo trutta, in the study of Tops et al. (2004), no infection of Bryozoa could be achieved with PKD-infected brown trout, rainbow trout Oncorhynchus mykiss or chinook salmon O. tschawytscha. It remains unknown in which fish host species T. bryosalmonae can form mature spores infective for Bryozoa. In the present study, different fish species were tested for the first time under laboratory conditions for their susceptibility to PKD and their subsequent capability to transmit T. bryosalmonae back to Bryozoa. We also took into account the possibility that the presence of cryptic parasite stages in bryozoan statoblasts could cause an overt infection when colonies start to grow. MATERIALS AND METHODSFish. Rainbow trout of the American Trout Lodge strain were raised from eggs in our specific pathogenic free (SPF) wet lab. Brown trout, brook trout and grayling were obtained as SPF fingerlings from 2 Bavarian hatcheries. Pike (10 to 15 cm) were obtained from a Bavarian hatchery. All fish were young of the year. Fish were maintained in aquaria with flowthrough dechlorinated tap water and fed on commercially available fish food or live fish (pike only) until they reached a size of 9 to 15 cm.Bryozoa. Infected bryozoa...

Koi sleepy disease (KSD), also known as carp edema virus (CEV), was first reported from juvenile colour carp in Japan in the 1970s. Recently, this pox virus was detected in several European countries, including Germany, France and the Netherlands. In England, in addition to colour carp, outbreaks in common carp are reported. KSD/CEV is an emerging infectious disease characterized by a typical sleepy behaviour, enophthalmia, generalized oedematous condition and gill necrosis, leading to hypoxia. High mortality, of up to 80-100%, is seen in juvenile koi collected from infected ponds. In Austria, this disease had not been detected until now. In spring 2014, diagnostic work revealed the disease in two unrelated cases. In one instance, a pond with adult koi was affected; in the other, the disease was diagnosed in adult common carp recently imported from the Czech Republic. A survey was carried out on recent cases (2013/2014), chosen from those with similar anamnestic and physical examination findings, revealing a total of 5/22 cases positive for KSD/CEV. In this study, two paradigmatic cases are presented in detail. Results together with molecular evidence shaped the pattern of the first diagnosis of KSD/CEV in fish from Austrian ponds. In the light of the positive cases detected from archived material, and the spread of the disease through live stock, imported from a neighbouring country, the need for epidemiological investigations in Austria and surrounding countries is emphasized.