Mortality Events and Emerging Pathogens Affecting Wild Fish Populations

Fogelson - AWHI 2026

why investgiate fish mortality

• Determine cause of death

• look for possible toxins

• understand dynamics of mortality

• confirm routes of transmission

• assess risk to other animals

• rule out zoonosis

one helath human concerns

• ingestion of contaminated foodstuffs

• longterm exposure to toxins and carcinogens

one health animal concerns

• mortality of wild and domestic animals

• bioaccumulation in aquatic animals

one health plant concerns

• bioaccumulation

• poor growth

• decreased survival

• decreased reproductive potential

how can veterinarians and researchers contribute to investigation?

• vigilant for emerging and zoonotic diseases and environmental toxicities

• investigate unusual events

• perform relevant testing

• educate clients / communityu

• report cases to CDC or state vet

history of wild fish populations

• species (how many, what kind)

• type of environment

• water quality

• time of day/year

• iatrogenic influence

• movement of dz / species in populations (e.g., salmon traveling, or home range cichlids)

• potential impacts of transmissible disease

what to look for in water quality

• O2 content

• biomass (live or dead)

• animaal waste productions

• chemical caontaminants

• ph

• alkalinity

• hardness

• salinity

• temp

• CO2

• environmental fluctuations

• species diversity (plankton, copepod, etc.)

• plant growth (algae growth?)

diagnostics in wild fish

• water testing

• autopsy (fresh dead or euth moribund)

• wet mount eval

• histopath

• bacterial culture

• PCR

• tissue culture

• other testing

main tool for wild fish pop medicine

histopathology

gill health is helpful for

environmental health

main causes of fish kill

• poor water quality

• DO depletion (algal blooms)

• nutrifications (N, P)

• Toxin exposure

• electricul currents

• pathogens

  • viral

  • bacterial

  • parasitic

  • fungal/oomycete

example of non-pathogen related fish kill

cyanobacteria - microcystis aeruginosa bloom

• associated with light, temp

cyanobacteria toxins classified by

• hepatotoxin

• dermatotoxin

• neurotoxin

when do cyanobacteria release toxin

• after death

• during life

cyanobacterial toxin can bio-accumulate in

fish tissues

when do you see fish kills from cyanobacteria

at night, low DO

consequences of fish kills from cyanobacterial blooms

• death of fish due to low O2

• deaht of exposed dogs and wildlife

• respiratory effects in humans

• possible carcinogenesis with long-term exposure

• continued research through HBOI

pathogen consideration in wild fish

• cross-over between wild and captive (e.g., aquaculture)

• bio-amplification of pathogen abundance

• drug resistance

• virulence

how do pathogens move

• human influence

  • transport of seafood products, feed, eggs, broodstock

• migration’

• feed contamination

• vector transmission

• density dep. transmission

• host susceptibility (appropraite receptors)

• virulence (asymptomatic spreader)

• pathogen specificity

salmon sea lice (Lepeophtheirus salmonis)

• arthropod ectoparasite of marine fish

• northern hemisphere

• atlantic and pacific ocean subspecies

• infects salmonids

• severity depends on host specificity AND immune repsonse

• feed on blood

• potential pathogen vector

gross findings of lepeophthirius

• attach to operculum and skin

• hemorrhage

• skin loss

• eorsions and ulcerations

• exposure of skeletal muscle

histo findings - leopeophthirius

• goblet cell hyperplasia

• eosinophilic granular immmune response

• epidermis hyperplasia

lepeophthirius diagnosis and tx

• gross observation

• potential to move through population at high rate

• tx limited

• drug resistance

• vaccine development underway (limited to aquaculture)

tilapia lave virus (TiLV)

• orthymyxo-like RNA virus

• enveloped, negative snese, ss

• farmed and wild tilapia

• species affects

  • hybrid tilapia, nile, red, wild

impact of Tilapia lake virus

• food source

lifestages affected by tilapia lake virus

• fingerlings

• eggs

• yolk sac larvae

• possibly fish >100g

mortality of tilalpia lake virus

10–90%

identification of tilapia lake virus

• PCR

• VI

• sequencing

mortality of tilapia lake virus

• often during hotter months

• water temps 22–32C

transmission of tilapia lake virus

• vertical, and

• horizontal

clinical signs / gross lesions of tilapia lake virus

• dark skin pigmentation

• pale gills

• skin erosions

• anorexia

• coelomic distension

• exophthalmia

• abnormal swimming

are tilapia lake virus clinical signs specific?

no, non-specific

histopathlogy of tilapia lake virus

• gliosis and perivascular cuffing in brain cortex

• endophthalmitis and lenticular degeneration

• hepatitis, hepatocyte necrosis, syncytial cell formation, intracytoplasmic inclusions

• interstitial renal hemorrhage

ichthyophonus hoferi

• mesomycetozoea (in kingdom of protozoa)

• mix of protozoa and fungi

  • chitonous walls

  • flagellated or amoeboid forms

• infects all halotype habitats

• coldwater species

  • epizootics in herring

lifecycle of ichthyophonus hoferi

• ingestion

• spores germinate in stomach

• branch hyphae

• enter vessels

• release uni/binucleate bodies oor amoeboid cells

• disseminate hematogenously

• multinucleate spores germinate in vital organs

diagnosis of ichthyophonus hoferi

• wet mount of organs

• histo

• PCR

• culture (MEM medium)

gross lesions of I. hoferi

• white nodules in organs and skeletal muscle

• possible hemorrhage in affectd organs

PAS stain of I. hoferi

bright pink wall

I. hoferi causes ____ inflammation

granulomatous

megalocytivirus

• iridoviridae

• ds DNA virus

• enveloped

types of fish infected with megalocytivirus

• aquaculture

• ornamental fish trade

• wild fish

• all FW and SW

another name of megalocytivirus

infectious spleen and kidney necrosis virus

all originated from one fish

clinical signs of megalocytivirus

• high mortality

• epidermal pallor or darkening

• pale gills

• labored respiration

• splenomegaly

• hemorrhage in multiple organs

histo of megalocytivirus

• widespread distinctive hypertrophied cells in perivascular conn. tissue

• large, basophilic, foamy to granular intracytoplasmic influcsions displacing nucleus peripherally

• most tissue damage resulting from vascular occlusion and ischemia

why is it important to ID megalocytivirus

can be reportable

what is pathognomonic for megalocytivirus

basophilic cytoplamsic inclusion body

how to dx megalocytivirus

• histo (looking for inclusions)

• PCR (important since some strains are reportable)

• viral culture

• FAT

• ELISA

• IHC

most pathogens have _____ pathogensis

unknown

megalocytivirus is _____, spreading through prenchymal epithelial cells

mesotheliotropic

megalocytivirus causes death via

• occlusion of vessels

• ischemia of vital organs