Bullet-Shaped: I've blogged on multiple occasions (most recently here) about the filovirus family and a couple of the deadly hemorrhagic fevers its members cause, Ebola Virus and Marburg. The filoviruses are related to rhabdoviruses, which are a curious mix of plant, insect, and vertebrate pathogens. Only one of them commonly preys upon man--rabies. But due to the length of this post, rabies has to wait until next week.
Rhabdovirus consists of six or seven genera with a total of over 200 species of rod-shaped viruses (listing here). The majority are transmitted by insect vectors, and several cause significant economic damage to fish, crops, and herds. Here's info on a few of them.
Fishy Letters: Many salmon and trout fishermen here in the Northwest are familiar with the acronym IHNV, which is mercifully short for infectious hematopoietic necrosis virus (hematopoietic has to do with the formation of blood). This virus tends to prey upon the young in spring and early summer, with mortality rates of up to 60 percent in Chinooks and 90 percent in Sockeyes.
The clinical signs may include abdominal distension, exophthalmia, darkening of the skin, anemia, and fading of the gills. Long, semi-transparent fecal casts often trail from the anus. Hemorrhages are common at the base of the pectoral fins, the mouth, the skin posterior to the skull above the lateral line, the muscles near the anus, and the yolk sac in sac fry. In sac fry, the yolk sac often swells with fluid. Diseased fish move slowly, float with the current, and occasionally act as if they were suffering from cramps. In later stages, fish tend to float on the surface, turn over, and occasionally swim frantically. Surviving fish often have scoliosis.
Folks with aquariums refer to exophthalmia as "popeye." This virus is readily shed in various fish excretions, including those of asymptomatic carriers (which is what some survivors become). Plus it can be spread via contaminated feed...and there may be insect vectors. Prevention is the key.
Yet More Letters: Here are some other rhabdoviruses that prey upon fish.
- The redundantly-named viral hemorrhagic septicemia virus (VHSV or Egtved disease) has a growing list of strains that until recently were known for attacking salmonids in Europe (where it's a scourge of rainbow trout) as well as the Pacific Rim. Now it's also becoming infamous for killing a large variety of species in the Great Lakes and the region's fish farms. After a number of ugly symptoms, the fish generally die from internal bleeding. Man is definitely helping to spread VHSV via contaminated water, eggs, and bait fish. Plus again, there can be asymptomatic carriers. With the Great Lakes strain now being found in channel catfish, farmers in the South see it as an impending threat.
- Spring Viremia of Carp Virus (SVCV) was an Old World problem for carp and goldfish until earlier this decade, when it was found in parts of the Americas (including Washington). Its mortality rate is high in cold water (up to 90 percent), but drops off considerably in warmer conditions.
The clinical signs may include darkening of the body, sluggish breathing, tilting to one side, abdominal distension, exophthalmia, anemia, hemorrhagic spots on the gills, and reddening and swelling of the anus. Diseased fish tend to gather at the water inlet. Some recovered fish may become virus carriers.
- And a couple of others...Snakehead rhabdovirus (SHRV) strikes its namesake fish family, members of which are farmed in parts of Southeast Asia. It can also kill zebrafish--the danios of aquarium fame. Hirame rhabdovirus is a type hematopoietic necrosis suffered by flounder, causing problems for those who farm them. But, there is a vaccine for this virus. FYI, hirame is Japanese for flat fish.
Blisters of the Mouth: A genus of rhabdoviruses that can strike both fish and animals is Vesiculovirus. It's most famous member causes vesicular stomatitus (VSV), a disease primarily of the Americas that...
...primarily affects cattle, horses, and swine. The virus that causes vesicular stomatitis has a wide host range. This disease also occasionally affects sheep and goats. Many species of wild animals, including deer, bobcats, goats, raccoons, and monkeys, have been found to be susceptible hosts.
It's considered one of the "big four" livestock diseases. Plus, it occasionally strikes people who come into direct contact with diseased animals. In people, VSV is like a mild flu that lasts for 4-7 days and includes a fever, head and muscle aches, and what resemble cold sores. Continuing from the above quote...
In affected livestock, vesicular stomatitis causes blisterlike lesions to form in the mouth and on the dental pad, tongue, lips, nostrils, hooves, and teats. These blisters swell and break, leaving raw tissue that is so painful that infected animals generally refuse to eat or drink and show signs of lameness. Severe weight loss usually follows, and in dairy cows, a severe drop in milk production commonly occurs. Affected dairy cattle can appear to be normal and will continue to eat about half of their feed intake.
That probably sounds like another of the big four, foot & mouth disease. However, that was eradicated from the U.S. in 1929. VSV is hardest on horses, but fortunately it rarely kills its victims.
The morbidity rate for vesicular stomatitis varies considerably within species. For example, about 5 to 10 percent of affected herds generally show clinical signs of the disease. Up to 80 percent of dairy cattle herds have become affected by vesicular stomatitis. If there are no complications such as secondary infections, then affected animals recover in about 2 weeks.
The disease is generally spread directly between animals, via shared objects (like a salt lick), and in some instances by sand flies, black flies, and possibly mosquitoes. In the U.S., it tends to strike for 2-3 years and then disappear for about a decade...which is why the Southwest should be due pretty soon. Curiously, research may be finding that VSV is actually a plant virus that can also strike some animals. There's also a line of thought that similar plant and animal viruses evolved from the same insect virus.
Three Day Sickness: Bovine ephemeral fever (BEF) virus afflicts cattle and water buffalos in the temperate through tropical areas of the Old World plus Australia. The disease, which is probably transmitted via mosquitoes, doesn't last long, but it can hit hard.
Infected cattle usually develop a biphasic or triphasic fever, with temperature peaks approximately 12-18 hours apart. During the first fever, milk production in lactating cows often drops dramatically, but other clinical signs are mild. During the second fever, the symptoms are more severe. Animals may have an increased heart rate, tachypnea, depression, anorexia, ruminal atony, serous or mucoid discharges from the nose and eyes, salivation, muscle twitching, waves of shivering, joint pain, stiffness, and shifting lameness. There may also be submandibular edema, or patchy edema on the head. Many animals become recumbent for eight hours to days. Some may temporarily lose their reflexes and be unable to rise. These clinical signs can be exacerbated by severe environmental stress or forced exercise.
Tachypnea is "abnormally fast breathing," and ruminal atony is essentially indigestion related to the first compartment of the stomach.
BEF outbreaks can sicken up to 80 percent of cattle, typically killing 1-2 percent of them. Strangely, the malady is more severe in robust animals...it can kill over 30 percent of "very fat cattle."
Plant Problems: There are more than 70 plant viruses in the rhabdovirus family. The common names of these and other viruses are often descriptive of a key problem or appearance for which the virus is responsible. There are almost certainly a large number of plant viruses that haven't been discovered or identified yet...research often tends to follow monetary damage. In most instances, viruses weaken their victims, causing reduced yields, unattractive flowers, etc. Here are a few of the plant rhabdoviruses:
- Beet leaf curl virus attacks beets (including chards & sugar beets) and spinach in parts of Eurasia. This pathogen is controlled by going after its vector--the beet bug--and by quarantine. The virus used to be a major problem in Central Europe, reducing sugar beet yields by as much as 75 percent. But in recent years, the problem has waned significantly.
- Citrus leprosis virus was originally observed in Florida in about 1860. It causes spotting on fruits, leaves, and twigs, and can eventually kill the trees. Leprosis had spread to 17 nations by 1925, by which time it had been eradicated in the U.S. The keys were planting in new locations and controlling flat mites, the vector. Sulfur works well on the mites, but it can kill citrus trees. The virus remains firmly established in South America, where it causes millions in damage. A recent outbreak in Panama has growers in Florida fearing what they now consider to be an exotic disease.
- Euonymus fasciation virus is best explained by defining fasciation, which is also known as cresting:
Fasciation is a condition that may randomly affect a diverse range of plants. Round stems are flattened, suggesting many stems have fused together. True fasciation is the product of a single, normally dome-shaped growing point that has become abnormally broadened and flattened. Any sideshoots usually remain small and undeveloped.
Several things can cause fasciation, including genetic disruption, bacterial infection, frost, insects, fungi, plant damage, and sometimes viruses. In a few plants, fasciation is a desirable trait (cactus photos here). A few very decorative plant variegations are also caused by viruses. However, viruses which cause color breaks in flowers are generally not appreciated.
- Raspberry vein chlorosis virus is a native of Eurasia that's been introduced to both New Zealand and Canada. It afflicts the red raspberry (Rubus idaeus), though it can infect woodland strawberries and has been grafted into loganberries. It's spread by aphids.
- Strawberry crinkle virus was first reported here in Oregon, but it lives in much of the world. The virus, which is also spread by aphids, causes different symptoms in the various types of strawberries. In the woodland strawberry, its calling cards are "chlorotic and necrotic spotting of veins, epinasty, deformation, local lesions in petioles and stolons, streaking and deformation of petals." FYI, epinasty is a downward bending of leaves driven by the tops of the leaves growing faster than the bottoms.
- Wheat American striate mosaic virus is found in the northern Great Plains. This virus affects some varieties of wheat more than others (and can infect a few other types of grasses), but it rarely seems to impact a large or concentrated portion of a wheat field. It's spread by leafhoppers, but curiously, moulting rids leafhoppers of this particular virus.
And again, rabies (which is shaped like a plant virus) comes next week.
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