One of the larger flukes familiar to veterinarians is the giant liver fluke of ungulates - Fascioloides magna, native to North America. It is also variously known as the deer fluke and large American liver fluke. The adult flukes may measure upto 10 cm, properly earning the species name magna.
The major definitive host for this parasite is the white tailed deer (Odocoileus virginianus), but it can also infect black tailed deer (Odocoileus hemionus), wapitis (Cervus canadensis) and caribou (Rangifer tarandus) in North America. The fluke was introduced into Europe accidentally in the 20th century and has been spreading, becoming common in some countries of eastern europe. In Europe, hosts infected include the red deer (Capreolus elaphus), roe deer (Capreolus capreolus) and fallow deer (Dama dama).
Unlike the more popular and zoonotic Fasciola hepatica, adults of which live in the bile ducts, Fascioloides magna adults live in the parenchyma of the liver and are typically found in pseudo-fibrous cysts. These cysts are open/patent in deer but are not patent in domestic cattle, sheep to goats. Upto 4000 eggs may be shed through the patent openings in deer, pass into bile and from there into the feces. Due to the non-patent nature of infections in domestic ruminants, eggs cannot found in the feces in these hosts, unlike in deer.
Inside the eggs, a miracidium develops in the environment in 4-7 weeks, hatches by secreting proteolytic enzymes, swims in water to find a suitable snail intermediate host - of the genus Lymnaea. In the snail, the miracidium develops through the sporocyst (in the snail's pulmonary sac) and redia (in the snail's hepatopancreas) stages, eventually leaving the snail as a ceracaria in 6-9 weeks. The cercariae encyst onto grass, become meta-ceracariae and are eventually eaten by a susceptible host, ideally a deer. Inside the deer, the metacercaria migrate around, eventually entering the liver, where their movement provokes the formation of pseudocysts. Pseudocysts typically have more than one adult fluke. The prepatent period is variable, ranging between 3 and 7 months.
The presence of the flukes do not affect deer to a large extent, which carry the small infections without clinical disease. Deer hunters may find the adult flukes in hunted deer. My PhD advisor shared a story of a hunter that he met in a small Wisconsin pub, who had just been imbibing after a deer hunt. The hunter appears to have been quite a foodie with an appetite for the extraordinary, evidenced by their description of how they absolutely loved the taste of the little butterflies in the liver of deer!
Just like with other parasites, the devil is in the numbers. Death can occur if there are hundreds of flukes. Egg production and the occurrence of the flukes as large foreign bodies in the liver are likely not good for the deer host. However, while severe clinical signs may not occur in North American wild ungulates (perhaps for lack of observation), clinical signs in european deer include typical non-specific signs such as reduced weight gain, poor ruts and poor antler quality.
It is of interest to note that non-cervid ungulates get classified into aberrant and dead-end hosts for F. magna infections. Infection in aberrant hosts (domesticated sheep, goats and wild sheep) typically result in poor prognosis for these hosts due to liver damage, aberrant wanderings into other splanchnic organs, which may prove fatal. In dead-end large herbivore hosts such as cattle, bison, camelids, moose, pseudocysts form without an outlet, resulting in typical non-patent infections, unless the pseudocysts rupture.
From a ecological stand point, it is to be noted that the fluke picks and chooses among several cervid options available on the continents as definitive hosts, and also picks specific species of snail hosts within the genus Lymnaea to use as first intermediate hosts, all in all contributing to its weirdness!
Ref:
Malcicka, Miriama. "Life history and biology of Fascioloides magna (Trematoda) and its native and exotic hosts." Ecology and evolution 5.7 (2015): 1381-1397.
