Verification of Intraovum Transmission of a Microsporidium of Vertebrates: Pseudoloma neurophilia Infecting the Zebrafish, Danio rerio

Abstract

Direct transmission from parents to offspring, referred to as vertical transmission, occurs within essentially all major groups of pathogens. Several microsporidia (Phylum Microsporidia) that infect arthropods employ this mode of transmission, and various lines of evidence have suggested this might occur with certain fish microsporidia. The microsporidium, Pseudoloma neurophilia, is a common pathogen of the laboratory zebrafish, Danio rerio. We previously verified that this parasite is easily transmitted horizontally, but previous studies also indicated that maternal transmission occurs. We report here direct observation of Pseudoloma neurophilia in the progeny of infected zebrafish that were reared in isolation, including microscopic visualization of the parasite in all major stages of development. Histological examination of larval fish reared in isolation from a group spawn showed microsporidian spores in the resorbing yolk sac of a fish. Infections were also observed in three of 36 juvenile fish. Eggs from a second group spawn of 30 infected fish were examined using a stereomicroscope and the infection was observed from 4 to 48 hours post-fertilization in two embryos. Intraovum infections were detected in embryos from 4 of 27 pairs of infected fish that were spawned based on qPCR detection of P. neurophilia DNA. The prevalence of intraovum infections from the four spawns containing infected embryos was low (∼1%) based on calculation of prevalence using a maximum likelihood analysis for pooled samples. Parasite DNA was detected in the water following spawning of 11 of the infected pairs, suggesting there was also potential for extraovum transmission in these spawning events. Our study represents the first direct observation of vertical transmission within a developing embryo of a microsporidian parasite in a vertebrate. The low prevalence of vertical transmission in embryos is consistent with observations of some other fish pathogens that are also readily transmitted by both vertical and horizontal routes.

Expansion of the Known Host Range of the Microsporidium, Pseudoloma neurophilia

“…17 Infection by P. hyphessobryconis often results in clinical signs such as grossly visible lesions and high levels of mortality. In contrast, infections by P. neurophilia are often subclinical and the ability for this parasite to be transmitted vertically within the developing embryo, 18 at least in D. rerio, provides another route leading to the persistence of this parasite in both laboratory colonies and pet stores.…”

Distribution, prevalence, and pathology of a microsporidian infecting freshwater sculpins

“…However, we do not know whether infected eggs would have developed into fry or whether those fish would have developed infections. It is notable that in 2 other fish-infecting microsporidia (Ovipleistophora ovariae and Pseudoloma neurophilia), initial molecular and observational evidence for vertical transmission were doubted, but more sophisticated techniques confirmed that these parasites are vertically transmitted (Phelps & Goodwin 2008, Sanders et al 2013.…”

Zebrafish Embryo Disinfection with Povidone–Iodine: Evaluating an Alternative to Chlorine Bleach

“…Finally, as previously discussed, 9,12 these methods contribute to good biosecurity and, as such, are likely to reduce the spread of microorganisms. However, they are less likely to prevent the spread of intraovum pathogens (e.g., Pseudoloma neurophilia) 23 and a holistic approach, including additional disease prevention and control measures, should be taken in addition to embryo disinfection.…”