Insects and Other Animals

As a veterinarian, a large area of my professional training and focus is in zoonotic disease, not only with regard to animal-to-human transmission but occasionally concerning the reverse, as well. Viruses will often mutate and jump from one species to another; bacteria are notoriously promiscuous and readily exchange DNA that will allow them to propagate more rapidly, often at the expense of host health; and parasites, though often more host-specific, can occasionally infect multiple species or can cause serious disease when they find their way into aberrant hosts.

Some animals can be very useful to humans, such as honeybees and other crop pollinators. Others are nuisance species and can cause serious damage to food crops, either by causing physical damage or by spreading viral or fungal plant pathogens.  Conversely, human activities can be damaging to animal populations, from habitat destruction to overuse of nonselective pesticides.

Because this topic covers such an enormous scope, I am going to focus primarily on areas in which I have particular knowledge and interest. First and foremost of these is rabies, which is one of the oldest recognized zoonoses. For the entirety of my career as a practicing veterinarian, I have lived and worked in rabies-endemic regions, principally the Atlantic seaboard states. The virus is primarily maintained in wildlife reservoirs and incidence fluctuates on a fairly regular cycle, as host populations wax and wane.

Because the disease is one of the earliest for which a vaccine was developed, vaccine programs are well recognized as the most efficacious means of prevention and standard protocols are well established. All fifty US states require vaccination of domestic pets, and approved vaccine products exist for livestock species, as well. As a result, human deaths from the disease are extremely low in the US, as well as in other countries which have adopted mandatory animal vaccination programs (i.e., essentially all developed nations). 

Worldwide, however, over 55,000 people are estimated to die each year from the disease, with the largest numbers of deaths occurring in Africa and in the Indian subcontinent (http://www.rabiescontrol.net).

In the US, the majority of human cases over the last couple of decades have been strains of the virus associated with bats. Mistakenly identified by Moeller as rodents (they are chiropterans), bats are a very important group of species ecologically, both as pollinators and seed dispersers and as consumers of insect pests. That said, they are also capable of finding their way into human dwellings through very small openings, and their bites are often not recognized, due to their small sharp teeth and the potential for victims to be bitten while they are sleeping or otherwise unaware of the bat’s presence (http://www.cdc.gov/rabies/bats/index.html).

Living in northern New England, tickborne disease are also a significant concern, especially Lyme disease, caused by the spirochete Borrelia burgdorferi. Lyme disease is not spread by direct transmission between affected species, but primarily by the tick species Ixodes scapularis. In clinical practice, I see the disease and identify the tick vector extremely frequently. There are some differences between the clinical syndromes in dogs and humans, but both can potentially experience polyarthritis, fever, and neuropathies. In dogs, we occasionally see immune-mediated damage to the kidneys, which generally carries a very poor prognosis. And although the organism is treatable, it is not considered to be a curable disease and recrudescence is possible.

When I lived in North Carolina, ticks were vectors for Rocky Mountain Spotted Fever, babesiosis, ehrlichiosis, anaplasmosis, and numerous other diseases. A study published one of my colleagues at NC State in 1999 showed high levels of coinfection in both dogs and humans at one kennel in the eastern part of the state (Kordick, et al., 1999).

Mosquitoes are also an important vector for disease spread. Worldwide, malaria is probably the most well-known mosquito-borne disease. Many other disease, including dengue fever and yellow fever, are also transmitted by mosquito species. In the late 1990s to early 2000s in the eastern US, mosquitoes were the vector for West Nile virus infection, which had spread to North America via migratory birds. Although the virus is host-adapted and rarely causes serious disease in definitive host species, in aberrant hosts, such as corvid birds and humans, it can potentially cause serious neurologic disease and even death.

Although syndromically similar, the western, eastern, and Venezuelan equine encephalitis viruses (WEE, EEE, and VEE, respectively), are genetically unrelated viruses which are also transmitted from birds to aberrant mammalian hosts via mosquito vectors. As their names suggest, they can cause neurologic disease similar to West Nile and can be fatal. My husband and I own alpacas, and the farm where most of our animals were boarding was one of several in southeast New Hampshire affected by an outbreak of EEE in the summer of 2005 (Nolen-Watson, et al., 2007). During that same period, a total of eleven human cases were identified in Massachusetts and New Hampshire, four of whom died from the disease (MMWR, 2006).

Clearly, control of vector and other pest species is important in managing these problems, but achieving this is not always straightforward. For some insect species, release of sterile males can help reduce population numbers, but given the short life cycle of most insect species, this requires sustained effort for long-term impact. The use of pesticides is problematic because of the risk of damaging non-target species – either beneficial insects or species higher up the food chain, such as occurred with DDT. In the case of rodent species, physical traps are not always humane or effective and poisonous baits can be consumed by non-target species – I frequently see rodenticide poisonings in pets.

That said, the challenges are not insurmountable. In malaria-endemic regions, selective spraying of DDT and increased use of mosquito netting has helped to impact the disease, which remains incurable. In agricultural settings, increased of integrated pest management strategies, such as promotion of predatory insect species, has helped to reduce the use of harmful pesticides. As long as we exist as part of the Earth’s ecosphere, though, we will have to continue finding a balance between our impact on other species and their impact on us.

REFERENCES

Moeller, D.W. Environmental Health, Fourth Edition. Harvard University Press. 2011.

Kordick, S.K., et al. “Coinfection with multiple tick-borne pathogens in a walker hound kennel in North Carolina”. Journal of Clinical Microbiology. 37(8): 2631-2638. 1999.

Nolen-Watson, R., et al. “Eastern equine encephalitis in 9 South American camelids”. Journal of Veterinary Internal Medicine. 21(4): 846-852. 2007.

MMWR. “Eastern equine encephalitis – New Hampshire and Massachusetts, August-September, 2005”. 55(25): 697-700. 30 Jun 2006.