Outbreaks of a bacterial infection caused by Pasteurella multocida sporadically occur in staging areas where millions of waterfowl congregate during migration. During these periods where the density of the birds is very high, transmission of the bacteria can easily occur through close contact, splashing of bacteria laden water, and other mechanisms. Some outbreaks have resulted in deaths of thousands of ducks and geese. Some species are more sensitive to infection than others. For example in 1979, over 100,000 snow geese died near Hudson Bay due to avian cholera infection.
|
Beavers have gotten a bad rap for hosting and shedding in their feces a protozoan pathogen known as Giardia lamblia. Acquiring this gastrointestinal infection is associated with drinking untreated or unfiltered water from lakes and rivers even in remote areas such as the Boundary Waters Canoe Area in Minnesota. Although beavers get blamed for releasing this protozoan pathogen into the water, it is far from clear and certainly more complex; likely involving other wildlife species as well as domestic animals such as dogs. Even humans may have contributed to the prevalence of this pathogen in many pristine areas.
The rabies virus can infect a number of different animals including most notably raccoons, fox, coyotes, bats, skunks and others. The pathogenesis (disease progression) differs in each. The fact that there are animals in nature with antibodies to the virus in their body indicates that some will survive the infection. Raccoon rabies is a problem on the East coast where subdivisions of homes move into more wooded areas. Contact with rabid raccoons can occur. The general rule is that if you observe a raccoon wandering around in the middle of the day, there is a chance that it is rabid and is in the latter stages of the infection. The virus is transmitted through the bite of the infected animal's saliva. The infection in the brain causes hyper salivation in many animals. A vaccine program to decrease the number of rabid raccoons has been going for the last decade in the Southeastern states. Vaccine baited meat distributed by various means is being spread in raccoon habitats in hopes of immunizing the animals against the infection. Scientists hope to see a decrease in the number of rabid raccoons in the near future.
The cause of the decline in European honeybees known as Colony Collapse Disease (CCD) is unclear. Pesticides, mites, viruses and perhaps other chemicals may have contributed to the decline. In a recent article (Science 351:555, 2016), the authors focus on the host/vector relationship of the honeybee and a mite (Varroa destructor). The vector mite transmits a virus called deformed wing virus (DMV) to the honeybee that has been clearly linked to colony collapse. Factors driving CCD that have lead to a reemergence of DMV are thought to include human managed movement of bees, adaptation of this mite to a new host and change in the virus lethality. Future studies will need to continue to focus on how viruses spread to new hosts and adapt to new environments in light of the importance of the honeybee in pollination of agricultural crops.There is much that we do not know about the transmission of the Zika Virus which was first isolated from a non-human primate in the Zika Forest in Africa. Similar to other viruses such as West Nile Virus and Japanese Encephalitis Virus, these arboviruses utilize mosquitoes as vectors of transmission. That means when a mosquito takes a blood meal from an infected animal, the virus is taken into the mosquito's gut and starts replicating. Thus the mosquitoes are 'biological vectors'. When an infected mosquito bites a human, the virus could be transmitted and infect that human. What is not clear is whether humans can be involved in further transmission to other mosquitoes. For example, is there enough virus in the blood for a mosquito to pick up and continue the transmission to other hosts? With many other arboviruses, humans are considered 'dead end hosts' because there is not sufficient virus in the blood for other mosquitoes to become infected by and transmit to other hosts.
Because the virus utilizes the circulatory system as a means of dissemination to the target organ such as the central nervous system, exposure to blood from other sources could lead to infection. It has been reported that in some cases infection can lead to hematospermia (blood in the ejaculate) and this has resulted in transfer of the virus to the sexual partner. In fact the virus may be secreted in other fluids such as saliva and urine even without blood as has been shown with West Nile Virus present in urine. The natural hosts of this virus remain to identified. Natural hosts usually show no signs of disease and are important in maintaining the transmission cycle in the wild. Whether certain birds or small mammals are important in this natural cycle remain to be seen. Named after the Zika (translated overgrown) Forest in Uganda, by 2007 it had spread to Micronesia, Central and South America and the Caribbean. This is a Flavivirus carried by Aedes mosquitoes such as Aedes aegypti and Aedes albopictus thus it is referred to as an arbovirus. The RNA containing virus is similar to West Nile Virus, Yellow Fever Virus, and Dengue Fever. The natural (sylvatic) cycle involves hosts such as monkeys but identification of other host animals is far from complete. West Nile Virus is associated with many different bird hosts and the rapid dissemination of the virus in the United States starting in 2001 is partly due to these avian hosts. Some think the Zika Virus will not disseminate as quickly but worry that it will become established in the Southern US where these mosquito vectors are present.
The disease in humans is similar to Dengue Fever but mild and non-fatal and over 75% of infections are asymptomatic. However the main concern is for pregnant women where there is evidence the infected mother can pass the virus in utero to the newborn leading microcephaly and brain damage. Travel warnings may be forthcoming. The bacterium Yersinia pestis that is responsible for potentially severe infections sometimes referred to as the Black Death and responsible for the elimination of a large percentage of the population in Europe during the Middle Ages still exists in endemic areas such as prairie dog towns out west. In its natural (sylvatic) cycle, the bacterium is maintained in flea vectors and small rodents. The bacterium is often associated with prairie dog towns where transmission of the bacterium to prairie dogs by fleas results in amplification and further spread of the bacterium. The result is often death of prairie dogs which leads to infection of predator species such as the black-footed ferret. In addition with loss of the prairie dog, decline in both the ferret and prairie nesting birds such as the Mountain Plover and Meadowlarks often occur. Mountain plovers prefer dry habitat with short grass and bare ground for nesting. Loss of prairie dog colonies leads to a decline in nesting habitat for these birds. Chronic Wasting Disease (CWD) was confirmed in White-tailed Deer in Wisconsin in 2002. The disease is caused by an unusual infectious agent called a prion. A prion is a protein that all vertebrates have but can exist in different structural forms (sort of like the transformer toys). The ‘bad’ form’ of prions can convert ‘normal’ prion proteins into more ‘bad ‘ form and thus makes fibrils that can clog neurons and cause nervous systems signs and symptoms such as instability. The disease is not caused by a true infectious agent because the protein itself is not replicated. Therefore, we say that the disease is caused by a transmissible agent rather than an infectious agent. Other transmissible prion diseases includes Bovine Spongiform Encephalopathy (Mad Cow Disease) and a number of other animal and human diseases. Much remains to be learned about this agent and how it causes disease.
A big question on some peoples’ minds is whether one can get a prion disease from eating venison. No conclusive evidence exists, that CWD can be readily transmitted to humans, although some scientists think that many of the prion diseases are in fact somehow related through transmission over from one animal to another. Some of these transmission mechanisms have been artificial such as feeding animal by-products to other animals. Recent studies have focused on the transmissibility and stability of this agent in the deer environment. We have recently learned that prions can be released in urine, saliva, blood, and feces of affected deer. It can be associated with water and feed troughs at feeding stations. Prions can be found in gut piles and carcasses of killed deer. All of these examples provide modes of transmission from deer to deer in the contaminated environment. It obviously can be isolated from nervous tissue such as brain but also has been detected in muscle seemingly normal freshly killed deer. Prions are quite stable in the soil for 3 years or more. And because deer consume grams of soil for nutrients, normal grazing provides another way for them to acquire the agent. CWD has been shown to bind quite strongly to some minerals in the soil that may also provide protection for prion as it passes through the digestive tract of the deer. There are many different subtypes of Influenza A virus in nature. Surveys of waterfowl populations in the 1960’s and 1970’s showed that waterfowl including ducks harbor all of these different subtypes of the virus.
Ducks and geese are natural carriers or reservoir hosts of the influenza virus. A carrier usually means the animal does not appear ill, and remains unharmed despite the virus replicating in the animal. This natural resistance is genetic based and recent studies point to at least one candidate duck gene that helps prevent disease in the duck. The path to influenza virus infection in humans involves a number of steps including transmission of the virus from ducks to chickens, mutation of the virus, reassortment, and adaptation in humans. Chickens are infected, become ill and often die. They act as amplifying hosts and spread the virus to humans. Breaking this chain of events by making chickens more resistant to the virus has been recently demonstrated. Colony collapse disorder (CCD) is a relatively new phenomenon that affects many different species of bees. All flowering plants need bees to survive. Bumblebees (genus Bombus) are close relatives of honey bees (genus Apis) and many species of both bees are dying.
What is killing them? Everything from fungi, viruses, pesticides to cell phones have been implicated in their deaths which is often seen as an abandonment of the hive as bees seemingly go off to die. Specific causative agents include mites, the fungus Nosema apis, Israel acute paralysis virus, Invertebrate iridescent virus, the pesticide clothianidin, and cell phone radiation to name a few. While there may be multiple causes or perhaps combinational effects, it is unclear if there is any commonality among these deaths. None of the mentioned pathogens are infectious for humans. Make no mistake about it, without bees, it will be a less colorful and fruitful world. |
Archives
April 2016
Categories |
RSS Feed
