The gibel carp is one of the most successful invasive fish species in Eurasia, thanks to strong adaptive capacity and high reproductive output that allow it to dominate nearly every kind of freshwater habitat. First reported in Georgia in 1985 and initially misidentified as the crucian carp, the species was later confirmed through morphological and genetic work to be C. gibelio. It has since spread rapidly through virtually all Georgian water bodies. Yet despite its ecological prominence, its parasite fauna in the country remained poorly documented, prompting the new investigation of the Georgian portion of Jandari Lake.

Jandari Lake, known historically also as Karaia and Gardabani Lake, sits at 290 meters above sea level in the Kvemo Kartli lowland within a temperate dry climate zone. Covering 10.6 square kilometers with a maximum depth of 7.2 meters, the lake is fed primarily by the Mtkvari River and is one of the most important eutrophic lakes in eastern Georgia’s Mtkvari River Basin. Eutrophication is ongoing, and dissolved oxygen levels are constrained by phytoplankton metabolism. The lake’s shores are largely swamped with reeds and bulrushes, and despite heavy organic pollution, the water body remains highly productive, supporting substantial zoobenthos biomass and serving as a stopover for up to 20,000 swimming birds during migration and wintering.

Between July 2014 and May 2018, the team collected young gibel carp — individuals up to four years old in three age groups (1+, 2+, and 3+) measuring 8 to 15 centimeters — using fishing nets across all seasons, examining at least 30 specimens per month. Each fish was immobilized, measured, weighed, and aged from its scales before standard parasitological dissection. Skin, fins, and gills were scraped and examined as mucus smears under the microscope for ectoparasites, while eyes and internal organs were inspected for endoparasites. Parasites were fixed and identified using established taxonomic keys, and infection levels were quantified as prevalence and intensity.

The results revealed that 20.3 percent of the examined fish — 86 individuals — harbored eleven parasite species belonging to five taxonomic groups: Protozoa, Monogenea, Trematoda, Nematoda, and Copepoda. Protozoa contributed three species (Trichodina sp., Ichthyophthirius multifiliis, and Dermocystidium sp.), Monogenea four (Dactylogyrus vastator, Gyrodactylus elegans, Ancyrocephalus sp., and Diplozoon paradoxum), Trematoda one (Diplostomum spathaceum), Nematoda two (Philometra sp. and Contracaecum sp.), and Copepoda one (Ergasilus sp.). Eight of these species — Ichthyophthirius multifiliis, Trichodina sp., Dermocystidium sp., Gyrodactylus elegans, Ancyrocephalus sp., Diplozoon paradoxum, Philometra sp., and Contracaecum sp. — were recorded for the first time in gibel carp from Georgian freshwater bodies.

Most infections were modest: 73.2 percent of infested fish carried a single parasite species, 16.2 percent carried two, and only 9.3 percent carried three simultaneously. The dominant taxa were the protozoan Trichodina sp. and the monogeneans Ancyrocephalus sp., Dactylogyrus vastator, and Gyrodactylus elegans. Notably, none of the parasites found were specific to gibel carp, and none posed any threat to human health. Differences in infection between female and male fish were negligible, with 37.5 percent of females and 35.8 percent of males infested — a pattern consistent with findings for the species in Croatia’s Lake Sakačaš.

Seasonal analysis showed that overall invasion was low throughout the study but peaked in spring, when 36.8 percent of sampled fish were infected, compared with 15.0 percent in autumn, 13.0 percent in winter, and just 8.3 percent in summer. Different parasites responded differently to the temperature regime: Trichodina invasion reached 16.5 percent in spring, monogeneans likewise peaked in spring (Ancyrocephalus at 14.2 percent, Gyrodactylus at 6.0 percent, and Dactylogyrus at 5.2 percent), while Contracaecum appeared only in winter (3.2 percent) and Dermocystidium was detected exclusively in winter (2.9 percent). Based on these patterns, the researchers grouped the parasites into those sensitive to seasonal temperature change — Trichodina, Dactylogyrus, Gyrodactylus, Ancyrocephalus, and Contracaecum — and those more uniform across the year, including Dermocystidium, Diplozoon, Philometra, and the copepods.

Age structure also shaped the parasite community. The youngest fish (age 1+) showed maximum invasion levels for Trichodina sp., Gyrodactylus elegans, Ancyrocephalus sp., and Philometra sp., all of which were detected across all three age categories. Dactylogyrus vastator occurred only in the 1+ and 2+ groups, while Diplostomum metacercariae appeared in minimal numbers in the youngest fish in spring but turned up in older individuals in autumn and winter. These findings align with classic ecological theory holding that young fish are primarily infested by monoxenic parasites — species with direct life cycles that require no intermediate host — while complex, multi-host parasites accumulate with age.

The most consequential story, however, is the long-term transformation of the lake’s parasite fauna. In the 1950s, 20 parasitic species were recorded in Jandari Lake’s fish, with the polyxenic cestode Ligula intestinalis — which requires multiple intermediate hosts — common among cyprinids. By the 1970s, dominant fish species still carried helminths of complex life cycles. Today, under strong eutrophication, the picture has changed completely: no ligulosis was detected at all, no acanthocephalans were found, and the community is dominated by ectoparasites of the gills and skin, namely Trichodina and monogeneans, which spread without intermediate hosts. Hydrobiological studies from the 2000s documented declining quality and quantity of zooplankton and zoobenthos due to organic pollution, and the researchers conclude that eutrophication has suppressed the copepod and amphipod intermediate hosts that heteroxenous helminths depend on. Resilient, directly developing parasites, by contrast, thrive under these regressive conditions.

The study also documented a striking synergistic association: in every spring and summer case, Trichodina appeared together with monogeneans on the gills, forming combined infections such as Trichodina plus Ancyrocephalus in 14 cases and Trichodina plus Dactylogyrus plus Gyrodactylus in 8 cases. Such co-occurrence of protozoans and monogeneans in the gill parasitocenosis has been noted before in Georgian cyprinids and appears to be a hallmark of freshwater systems under regressive change. Taken together, the results suggest that the invasive gibel carp is well adapted to Jandari Lake’s degraded ecosystem and may contribute to the spread of parasitic protozoa and helminths among commercial fish, while the lake’s eutrophication stands out as the single main ecological force determining which parasites persist — and which have vanished.

Subject of Research: Parasite ecology of invasive gibel carp in a eutrophic transboundary lake in Georgia

Article Title: Parasite ecology of Carassius gibelio (Bloch, 1782) in Jandari Lake (Georgia)

Article References: Murvanidze, L., Nikolaishvili, K., Lomidze, T., Japoshvili, B., Arabuli, L., & Asatiani, K. (2023). Parasite ecology of Carassius gibelio (Bloch, 1782) in Jandari Lake (Georgia). Caucasiana, 2, 15-23. https://doi.org/10.3897/caucasiana.2.e95777

Image Credits: AI Generated

DOI: 10.3897/caucasiana.2.e95777

Keywords: gibel carp, Carassius gibelio, parasites, eutrophication, Jandari Lake, Georgia, Monogenea, Protozoa, invasive species, freshwater ecology, fish health, seasonal dynamics

These sacs are called expelled food vacuoles, or EFVs. When a protozoan such as the ciliate Tetrahymena feeds, it engulfs bacteria into food vacuoles, compartments that normally mature into digestive vacuoles filled with the acidic enzymes and molecular machinery needed to dismantle their cargo. In many cases, that is exactly what happens, and the bacterium is destroyed. But a growing list of bacterial species has evolved the ability to resist this digestive programme. Rather than being killed, the survivors remain inside the vacuole, which the protozoan then expels intact. Earlier studies referred to these expelled structures by a variety of names, including expelled vesicles, faecal pellets and multilamellar bodies, and often regarded them as little more than the discarded refuse of protozoan digestion. The new review contends that this interpretation has understated their importance.

The central claim of the review is that EFVs should be understood as biologically generated reservoirs: protective containers that package viable pathogenic bacteria and release them into the environment already primed for persistence, dissemination and host colonisation. Far from being waste, these structures behave like miniature transport capsules, each one shielding its bacterial passengers from a hostile outside world. The authors describe this as a distinct environmental transmission state, a phase of a pathogen’s life in which it is neither free-swimming in water nor established inside a host, but travelling in a self-contained vehicle manufactured by another organism.

The evidence for this concept comes from a strikingly diverse set of pathogens. The review gathers findings from studies of Vibrio cholerae, the agent of cholera; Salmonella enterica, a leading cause of food poisoning; Legionella pneumophila, the cause of Legionnaires’ disease; Campylobacter jejuni, a common cause of gastroenteritis; Listeria monocytogenes, which can cause severe disease in pregnant people and the immunocompromised; Escherichia coli; the opportunistic pathogen Burkholderia cenocepacia; and the soil bacterium Mycobacterium smegmatis, a widely used model for tuberculosis research. Across these very different organisms, a common pattern emerges: bacteria that have passed through protozoa and emerged inside EFVs display measurable increases in resistance to stresses that would normally kill them.

The protective effects documented in the literature are broad. EFV-associated bacteria withstand acid exposure better than their free-living counterparts, an advantage that matters enormously when a waterborne pathogen confronts the acidic environment of the human stomach. They also survive starvation for longer periods, tolerate disinfectants and biocides, resist oxidative stress, and endure desiccation. In practical terms, a cholera bacterium packaged inside an expelled vacuole can persist in a drying pond, in treated drinking water, or on a contaminated surface for far longer than one floating alone, and it arrives at the next host in a hardier condition. The review also highlights reports of enhanced infectivity among EFV-associated pathogens, suggesting that the vacuolar packaging does not merely prolong survival but may actively prime the bacteria for subsequent colonisation of new hosts.

One of the most consequential findings relates to horizontal gene transfer, the process by which bacteria exchange genetic material. Because multiple bacteria can be concentrated inside a single small vesicle, EFVs create crowded micro-environments in which cells sit close together in fluid enclosed by a membrane. This physical intimacy facilitates the exchange of plasmids and other genetic elements, potentially accelerating the spread of antibiotic resistance genes through environmental microbial communities. In an era when antimicrobial resistance is recognised as one of the greatest threats to global public health, the possibility that protozoan predators inadvertently act as genetic mixing vessels for pathogens is a finding with far-reaching implications.

The review’s authors frame their synthesis around a conceptual shift. If EFVs are merely waste, then monitoring programmes that count free bacteria in water may be missing a hidden reservoir of infectious agents entirely. But if EFVs are vehicles of transmission, as the accumulated evidence suggests, then they represent a measurable and potentially targetable stage in the life cycle of environmental pathogens. The authors propose that EFV-associated pathogens could serve as useful targets for water surveillance and outbreak prediction. Detecting these vesicles in reservoirs, drinking water systems, or recreational waters could provide an early warning that a pathogen population has entered a particularly durable and transmissible state, before cases begin to appear in clinics.

This perspective also reframes the ecological relationship between protozoa and bacteria. The traditional view holds that protozoan grazing suppresses bacterial populations, acting as a check on microbial growth. The EFV concept reveals a paradox at the heart of that relationship: the predator that kills most bacteria may simultaneously serve as an incubator and distributor for the few that can resist digestion. In effect, protozoa may function as accidental selective agents, culling susceptible cells while packaging the resistant ones for wider dispersal. Over evolutionary time, this could help explain why so many environmental pathogens, including Legionella and various non-tuberculous mycobacteria, show an inherent ability to survive inside protozoa and, later, inside human cells. The machinery that lets a bacterium resist digestion in a Tetrahymena vacuole may be the same machinery that lets it resist killing by macrophages, the immune cells that patrol human tissue.

The authors are careful to present their synthesis as a framework rather than a settled conclusion, and they call for the field to move past the fragmented terminology of earlier decades and treat expelled food vacuoles as a unified object of study. Doing so, they argue, will allow researchers to connect environmental persistence with infection biology in a single mechanistic story: a pathogen that survives protozoan digestion exits the predator inside a protective vesicle, persists longer in the environment, resists treatment more effectively, exchanges genes more readily, and colonises new hosts more successfully. For public health authorities, the message is that the journey of a waterborne pathogen between hosts may be far more sophisticated than previously appreciated, and that the humble vacuole, long dismissed as refuse, may hold keys to predicting and preventing outbreaks. For microbiologists, the review opens a research agenda in which the ecology of single-celled predators and the epidemiology of human disease are inseparable parts of the same system.

Subject of Research: Expelled food vacuoles produced by protozoa as protective vehicles that promote bacterial persistence, stress resistance, gene transfer and transmission of pathogens

Article Title: Expelled Food Vacuoles as Protozoan-Derived Vehicles for Bacterial Persistence and Transmission

Article References: Faruque, S. N., Ponton, I. A., Moon, J. M., & Espinoza-Vergara, G. (2026). Expelled Food Vacuoles as Protozoan-Derived Vehicles for Bacterial Persistence and Transmission. Microbial Ecology. https://doi.org/10.1007/s00248-026-02880-6

Image Credits: AI Generated

DOI: 10.1007/s00248-026-02880-6

Keywords: protozoa, expelled food vacuoles, bacteria, Vibrio cholerae, Legionella pneumophila, Tetrahymena, water surveillance, antibiotic resistance, horizontal gene transfer, pathogen transmission, microbial ecology, environmental persistence